corrosion engineering notes (p.e-409)

Prepared by: Sheikh Mubin Ashraf (B.E in Petroleum NED Batch: 2010-11)

What is Corrosion? It is the degradation of materials properties due to the interaction with their environments. It is the deterioration of a metal as a result of chemical reactions between it & the surrounding. It is the spontaneous reaction between a material and its environment which results in the degradation

of that material. Metallic corrosion is the oxidization of the metal at metal/ environment interface which subsequently

results in a deterioration of the mechanical properties of the metal. Corrosion Attacks:

Corrosion occurs on an exposed surface. It can be concentrated locally to form a pit or a crack. It can extend across a wide area, more or less uniformly corroding the surface.

Corrosion Rate Determination

In order to compare the corrosion resistance of each metal & non-metal, the rate of attack of each metal must be expressed quantitatively. Corrosion rate is the speed at which metals undergo deterioration within a particular environment. This rate depends on environmental conditions and the condition or type of metal. In the U.S., corrosion rate is typically computed using Mils Penetration per Year (MPY). MPY is a unit of measurement equal to one thousandth of an inch. Corrosion can be expressed as following:

Percentage Weight Loss (%W) MPY mg/cm2.day g/in2.hr.

To calculate corrosion rate, the following must be determined: Weight loss (reduction in weight during reference time), [W in mg] Area (initial surface area), [A in sq. in] Time (length of reference time), [T in hr.] Density, [D in g/cm3]

MPY =

This measurement can be used to measure the thickness of items such as: Paint Paper Wires Foil Plastic sheets

Mils can be very useful in terms of manufacturing tolerances and dimensions, such as in the production of vehicle engines' head gaskets or the amount of material to remove from the automobile head in order to make adjustments to the cylinders compression ratio.


Polarization

Polarization is a deviation of the electrochemical process from equilibrium due to an electric current passing through the galvanic cell that causes a change in the electrode potential. It is referred as a potential shift away from the open circuit potential of the system. The deviation from equilibrium causes an electrical potential difference between the polarized & equilibrium (un-polarized) potential known as over-potential.

Examples: Zinc in air-free hydrochloric acid, Magnesium in neutral water, Zinc in aerated hydrochloric acid occur over a single metallic surface

Equilibrium State [Non Polarized State] At the equilibrium (non-polarized) state the rates of oxidation and reduction reactions proceeding at any electrode are equal. For example the ions of Cu are receiving electrons on the electrode surface and transfer from the electrolyte to the copper deposit. In parallel, the same number of copper atoms gives up their electrons and dissolves in the electrolyte.

Reduction: Oxidation: Cu++ + 2e Cu Cu Cu++ + 2e

The processes produce two equal electric currents in opposite directions. The current passing through the electrode surface in the equilibrium (non-polarized) state at any direction is called exchange current.

The electrode potential may be calculated according to the Nernst equation: E = E0 -

(Cion)

Processes occurring at Electrochemical Interface: Mass transport to a surface is governed by three forces

1. Diffusion, 2. Migration and 3. Convection

In the absence of an electrical field, the migration term, that only affects charged ionic species, is negligible while the convection force disappears in stagnant conditions while the diffusion of the ions through the layers controls the electrochemical reaction (corrosion, Electroplating).

Types of Polarization: Polarization may occur either at the cathode (cathodic polarization) or at the anode (anodic polarization). It is said to be Anodic when the anodic processes on the electrode are accelerated by changing the specimen potential in the positive (noble) direction and Cathodic when the cathodic processes are accelerated by moving the potential in the negative (active) direction. There are three distinct types of polarization.

Activation polarization Concentration polarization Resistance polarization


Concentration Polarization It is the polarization component that is caused by the

concentration changes in the environment adjacent to the surface that results in the loss of voltage.

The chemical species participating in a corrosion process is in short supply. Hence the cathodic processes depend on the reduction of dissolved oxygen since it is usually in low concentration (ppm).

It is associated with the concentration of ions in solution which shields the metal, thereby causing a decrease in the electrical potential of the cell.

Concentration polarization of an electrode is a result of formation of a Diffusion layer adjacent to the electrode surface due to the gradient of the ion concentration.

It may be lowered by increasing agitation and rising the electrolyte temperature.

Activation Polarization Activation polarization is the formation of a layer containing absorbed hydrogen atoms that block the

metal's surface from the corrosion process. It is associated with high concentration of active species.

It is the over-potential (change of the electrode potential) caused by overcoming the energy barrier of the slowest step of the electrochemical reaction.

Example Phenomenon: Common cause of cathodic activation polarization is the reaction of Hydrogen formation and evolution at the cathode surface:

Step 1: Reduction of the hydrogen ions resulted in formation of atomic hydrogen on the cathode surface.

H+ + e [H]

Step 2: Formation of molecules of gaseous hydrogen. 2[H] H2

Step 3: Formation of hydrogen bubbles. H2 + H2 + H2 +H2 + ... nH2

The cathode is polarized by the hydrogen atoms producing a film covering the cathode surface. The film affects the process kinetic: it slows down the reaction between the electrons and hydrogen ions dissolved in the electrolyte resulting in Activation Polarization.

Resistance polarization [Ohmic or IR Drop] Resistance polarization refers to the potential drop due to either the high resistivity of the electrolyte surrounding the electrode or an insulation effect of the film on the electrode surface formed by the reaction products. Resistance polarization may be expressed by the Ohms Law.


Importance of Polarization: Knowing the kind of polarization which is occurring can be very helpful, since it allows an assessment of the determining characteristics of a corroding system. Concentration polarization usually predominates when the concentration of the active species is low; for

example, in dilute acids or in aerated waters where the active component, dissolved oxygen, is only present at very low levels.

If corrosion is controlled by concentration polarization, then any change that increases the diffusion rate of the active species (e.g., oxygen) will also increase the corrosion rate. In such a system, it would therefore be expected that agitating the liquid or stirring it would tend to increase the corrosion rate of the metal.

If a corrosion reaction is activation controlled then stirring or agitation will have no effect on the corrosion rate.

The ohmic drop becomes an extremely important factor when studying corrosion phenomena for which there is a clear separation of the anodic and cathodic corrosion sites, e.g. crevice corrosion.

The ohmic drop is also an important variable in the application of protective methods such as anodic and cathodic protection that force a potential shift of the protected structure by passing a current in the environment.

Activation polarization is usually the controlling factor during corrosion in strong acids.

Different Aspects of Corrosion

Thermodynamic Electrochemical Metallurgical Physical

Thermodynamic Aspects of Corrosion: Thermodynamic considerations allow the determination of whether a reaction can occur spontaneously.

If metal dissolution is unfavorable thermodynamically in a given set of circumstances the job of the corrosion engineer is done.

Free Energy of a Corrosion Reaction: In electrical and electrochemical processes, electrical work is defined as the product of charges moved (Q) times the potential (E) through which it is moved. If this work is done in an electrochemical cell in which the potential difference between its two half-cells is E, and the charge is that of one mole of reaction in which n moles of electrons are transferred, then the electrical work (-w) done by the cell must be nE. In this relationship, the Faraday constant F is required to obtain coulombs from moles of electrons. In an electrochemical cell at equilibrium, no current flows and the energy change occurring in a reaction is expressed in equation:

EnFG

G- Gibbs Free Energy (Joules) E-Emf (volts) n-number of electrons involved in the reaction F-Faraday (96500 C/equivalent)

The larger the value of E for any cell more is the tendency for the overall cell reaction to proceed.

Ecell = Ecathode - Eanode

When a metal M is immersed in an aqueous electrolyte, it acquires a certain potential. If the activity of the metal ions Mn+ in aqueous environment is unity, then the acquired potential is known as Standard Potential The Law of Thermodynamics states that: The direction of a reaction is governed by the requirement of a material to be in a lower or more stable energy state.


Standard Hydrogen Electrode (SHE) The potential of the electrode equals zero if the hydrogen ion activity and the pressure of hydrogen gas

in the atmospheres are both unity. This is the standard hydrogen potential. The half - cell potential for any electrode is equal to the emf of a cell with the standard hydrogen

electrode as the other electrode. The half - cell potential for any electrode expressed on this basis is said to be on the normal hydrogen

scale or on the standard hydrogen scale, sometimes expressed as H or (S.H.E)

EMF Series All metals have been arranged in a series according to their standard potential (0) values. The more positive value corresponds to noble metals and the more negative value corresponds to more

reactive metals (when arranged according to reduction potential) Of the EMF series if two metals make up a cell, the more active metal acts as the anode and the more

noble metal of the two will act as cathode

Pourbaix Diagram [Potential pH Diagrams] Marcel Pourbaix developed potential-pH diagrams

to show the thermodynamic state of most metals in dilute aqueous solutions.

With pH as abscissa and potential as ordinate, these diagrams have curves representing chemical and electrochemical equilibria between metal and aqueous environment.

These diagrams ultimately show the conditions for immunity, corrosion and passivation.

Pourbaix diagrams offer a large volume of thermodynamic information in a very efficient and compact format.

The information in the diagrams can be beneficially used to control corrosion of pure metals in the aqueous environment By altering the pH and potential to the regions of immunity and passivation, corrosion can be

controlled. For example, on increasing the pH of environment in moving to slightly alkaline regions, the corrosion of iron can be controlled

Changing the potential of iron to more negative values eliminate corrosion, this technique is called cathodic protection.

Raising the potentials to more positive values reduces the corrosion by formation of stable films of oxides on the surface of transition metals

These diagrams are purely based on thermodynamic data and do not provide any information on the reaction rates

Pourbaix diagrams deal with pure metals which are not of much interest to the engineers


Electrochemical Aspects of Corrosion: When the oxide-free surface of a metal becomes exposed to the solution, positively charged metal ions tend to pass from the metal into the solution, leaving electrons behind on the metal, i.e.

The accumulation of negative charge on the metal due to the residual electrons leads to an increase in the potential difference between the metal and the solution. This potential difference is called the Electrode Potential. This change in the potential tends to retard the dissolution of metal ions but to encourage the deposition of dissolved metal ions from the solution onto the metal, i.e. the reverse of reaction occurs. Continuation of the dissolution and deposition of metal ions would result in the metal reaching a stable potential such that the rate of dissolution becomes equal to the rate of deposition. This potential is termed as Reversible Potential Er and its value depends on the concentration of dissolved metal ions and the standard reversible potential Eo for unit activity of dissolved metal ions.

The potential of a metal in a solution does not usually reach the reversible potential but remains more positive because electrons can be removed from the metal by alternative reactions. In acid solutions, electrons can react with hydrogen ions, adsorbed on the metal surface from the solution, to produce hydrogen gas.

This reaction permits the continued passage of an equivalent quantity of metal ions into solution, leading to corrosion of the metal. Normally hydrogen escapes from the system, so that the potential remains more negative than the reversible potential and corrosion continues.

In neutral solutions, the concentration of hydrogen ions is too low to allow reaction to proceed at a significant rate, but electrons in the metal can react with oxygen molecules, adsorbed on the metal surface from air dissolved in the solution, to produce hydroxyl ions.

Again, the potential of the metal remains more negative than the reversible potential for reaction.

In Electrochemical terminology, an electrode at which an oxidation reaction occurs is called an anode. The process of oxidation involves a loss of electrons by the reacting species, as occurs in the metal dissolution reaction. Thus an area of a corroding metal where metal dissolution occurs is an anode and metal dissolution is the anodic reaction of corrosion. An electrode at which a reduction reaction occurs is called a cathode. Reduction involves a gain in electrons. The reduction of hydrogen ions and oxygen are thus the cathodic reactions of corrosion and the area of a corroding metal where these reactions occur is a cathode.


Economic Considerations for Corrosion Control

Studies had shown that the annual corrosion costs range from approximately 1 to 5 percent of the GNP of each nation. Several studies separated the total corrosion costs into two parts:

Avoidable Costs: The costs that could be avoided if better corrosion control practices were used Unavoidable Costs: Costs where savings required new and advanced technology.

Estimates of avoidable corrosion costs in these studies have varied widely with a range from 10 to 40 percent of the total cost. Factors that need to be considered in calculating Net Present Value (NPV) include: 1. Initial cost 2. Best estimate of expected life 3. Length of typical shutdown for emergency repair 4. Cost of planned maintenance during scheduled

shutdowns 5. Effect of failure on total plant operation Study identified ten elements of the cost of corrosion: Replacement of equipment or buildings Loss of product Excess capacity Maintenance and repair Redundant equipment Design Corrosion control Technical support Insurance Parts and equipment inventory Economic problems result from damage to pipes, storage tanks, valves, and meters. Damage to pipes is the most prevalent, consisting of leaks and reduced carrying capacity. These pipe corrosion problems often result from tuberculation, which is the production of mounds of rust on the inside of the pipe. These mounds reduce the space in the pipe available to carry water, just as scaling does. In addition, tubercles are usually associated with pits in the pipe wall, which may go all the way through the pipe and cause leaks.


Passivity Passivity is defined as the loss of chemical reactivity. It is the characteristic of a metal exhibited when the metal does not become active in the corrosion reaction.

Cause of Passivity: [Formation of a Protective Layer] Passivity is caused by the buildup of a stable, tenacious layer of metal oxide (30 oA or less thick) on the

surface of the metal. Once the layer, or film, is formed, it acts as a barrier separating the metal surface from the environment.

Numerous metals and their alloys with a high affinity for oxygen have the characteristic of covering themselves with a protective layer of oxide which prevents their corrosion in corrosive environments. These conditions are described as passivity conditions and determine the state of inertia of the metal, which behaves like a noble metal.

Examples: An oxide or nitride film naturally forms due to exposure. However, Rust is not a passive layer. Fe, Cr, Ni, Ti, and alloys containing significant quantities of these elements exhibit passive behavior in

specific environments. E.g. Stainless steel in oxygen in specific environment or iron in 70% nitric acid possesses intrinsic corrosion resistance.

Stainless steel owes its passivity due to the formation of a very protective chromium oxide layer on its surface which makes it resistant to many environments where iron or carbon and low alloy steels may suffer even very severe corrosion.

The reinforcement bars of reinforced concrete are perfectly passivated by the alkalinity of the hydrated cement mixture. The iron in concrete (alkaline solutions) behaves like stainless steel in fresh water.

Aluminium and titanium are resistant to corrosion thanks to their ability to passivate since oxides such as Al2O3, TiO2, and Cr2O3are very effective passive films.

Behavior of Oxide Films in Metals: The surfaces of all metals (except for gold) in air are covered with oxide films. When such a metal is immersed in an aqueous solution, the oxide film tends to dissolve. If the solution is acidic, the oxide film may dissolve completely leaving a bare metal surface, which is said to be in the active state. In near-neutral solutions, the solubility of the oxide will be much lower than in acid solution and the extent of dissolution will tend to be smaller. The underlying metal may then become exposed initially only at localized points where owing to some discontinuity in the metal, e.g. the presence of an inclusion or a grain boundary, the oxide film may be thinner or more prone to dissolution than elsewhere. If the near-neutral solution contains inhibiting anions, this dissolution of the oxide film may be suppressed and the oxide film stabilized to form a passivating oxide film which can effectively prevent the corrosion of the metal, which is then in the passive state.

Overpassivation: It results from the formation of higher oxides of the metal, which dissolve completely to give anions, for example, CrO4

2-, or supply the solution with their own cations, which decompose with the release of oxygen; NiO2 is a typical higher oxide that supplies cations. The oxygen that participates in forming the passivated layer can be supplied by such oxidizing agents as H2O2 or HNO3. Passivation may be facilitated by anions that give sparingly soluble salts or mixed oxides of the metal that is being passivated. However, the most widespread source of passivating oxygen is water, which reacts either chemically or electrochemically with the metal.

Active Passive Behavior of Metals: Transition metal such as Fe, Cr, Ni, Al and Ti demonstrate an activepassive behavior in aqueous solutions such metals are called active-passive metals. They exhibit S-shaped polarization curves. In active materials, it is a straight line with a slight slope.


Active Region: The potential of an electrode in an electrolyte when no current is passed and forward rate of reaction is

equal to the rate of reverse reaction is called as Equilibrium Potential (Eeq or EM/M+ or E)

The current density (i) increases with E i.e. the corrosion rate increases with increasing oxidizing power

until primary passive potential is achieved.

Critical current Density (i critical) the maximum current density observed in active region for metal or

alloys that exhibits an active Passive behavior.

Passive Region: The potential of an electrode where a change from an

active to a passive state occurs is known as Passive Potential (Ep).

A passive film is formed and the current density drops to ipass which is very small but sufficient enough to maintain a protective film on the metal. The corrosion current remains constant with increasing potential so there is no effect on corrosion of the metal.

Transpassive Region: The protective anodic film is damaged and may break

down completely. Flade Potential (EF) the potential at which metal changes from a passive state to an active state. Transpassive Potential (Et) the potential corresponding to the end of passive region which corresponds

to the initial point of anodic evolution of Oxygen. This may correspond either to breakdown (electrolysis) voltage of water or to the pitting potential

Pitting Potential (Epitting) It is the potential at which there is a sudden increase in the current density due to breakdown of passive film on the metal surface in the anodic region.

Stainless Steel Passivation Stainless passivation is the process by which stainless steel will spontaneously form a chemically inactive surface when exposed to air or other oxygen-containing environments. Steels containing more than 11% Chromium are capable of

forming an invisible, inert or passive, self-repairing oxide film on their surface. It is this passive layer that gives stainless steels their corrosion resistance.

If a stainless steel surface is scratched, then more Chromium is exposed which reacts with oxygen allowing the passive layer to reform.

If a particle of carbon steel is embedded in the scratch then the passive layer cannot reform and corrosion will occur when the metal is wetted or exposed to a corrosive environment.

Passivating stainless steel is the removal of exogenous iron or iron compounds from the surface by means of a chemical dissolution, most typically by a treatment with mild oxidant like citric acid passivation solution that will remove the surface contamination but will not significantly affect the stainless steel itself.

Passivation also is accomplished by Electropolishing. Electropolishing is an electrochemical process that is a super passivator of stainless steel and results in a more passive surface.


ENVIRONMENTAL EFFECTS

Effect of Oxidizer Concentration:

For metals that demonstrate active-passive transition, passivity is achieved only if a sufficient quantity of oxidizer is added to the medium. Increasing concentration increases the corrosion rate in active region followed by a rapid decrease that is essentially independent of oxidizer concentration.

When Active-Passive metal is initially passive in corrosive medium then the effect of concentration is negligible. This condition frequently occurs when the metal is immersed in a strong oxidizing medium like Nitric Acid or Ferric Chloride.

The Transpassive region is observed usually in hot nitrating mixtures containing conc. Sulfuric Acid.

Example: 18Cr-8Ni Stainless Steel in H2SO4 & HNO3 shows the typical behavior at elevated temperatures.

Effect of Solution Velocity:

Curve B: For corrosion processes that are controlled by Activation Polarization, Agitation & velocity have no effect on corrosion rate.

Curve A: If the corrosion process is under cathodic diffusion control (concentration polarization), then agitation increases the corrosion rate. This generally occurs when oxidizer is in small amount e.g. dissolved oxygen in acids or water. At a specific velocity the corrosion rate drops and becomes independent of the velocity as the metal is now in passive state. E.g. Stainless steel and titanium.

Curve C: Some metals owe their corrosion resistance in certain mediums due to the formation of massive bulk protective films on their surfaces. E.g. Lead & Steel are protected from attack in in sulfuric acid by insoluble sulfate films. However, if they are exposed to extremely high corrosive velocities then mechanical damage or removal of the films can occur resulting in accelerated attack. This is called Erosion Corrosion. Before mechanical damage the effect of velocity is negligible.

Effect of Temperature: Temperature increases the rate of almost all chemical reactions.

Curve A: It represents an exponential rise in corrosion rate with increasing temperature.

Curve B: It shows initially almost negligible effect on CR at low temperatures followed by a very rapid rise in CR at high temperatures. E.g. 18Cr-8Ni stainless steel in nitric acid shows the typical behavior. Increasing the temperature of HNO3 greatly increases its oxidizing power. At low or moderate temperatures, stainless steel exposed to nitric acid is in the passive state very close to Transpassive region.


An increase in temperature of a corroding system has four main effects: The rate of chemical reaction is increased. The solubility of gases in the solution is decreased. The solubility of some reaction products may change that result in different corrosion products. Viscosity is decreased.

Effect of Oxygen [Oxygen as a Depolarizer] The presence of oxygen in water increases the corrosion rate. This due to the rapid reaction between

oxygen and the polarized layer of atomic hydrogen absorbed on the oxide layer. Therefore, oxygen is known as a Depolarizer.

Oxygen, therefore, has two effects: It removes the polarizing layer of atomic hydrogen, and It can react directly with the metal or metal oxide; thus, the corrosion rate increases.

Having hydrogen in the water helps minimize the amount of corrosion by eliminating the amount of oxygen in the system.

Effect of pH PH also affects the corrosion rate. In the range of pH

4-10, the corrosion rate of iron is relatively independent of the pH of the solution.

At lower pH the film cannot accumulate in the surface of metal. Corrosion takes place at higher rate.

At pH>10 corrosion rate decreases. This is believed to be due to an increase in the rate of the reaction of oxygen with Fe (OH) 2 (hydrated FeO) in the oxide layer to form the more protective Fe2O3 (note that this effect is not observed in deaerated water at high temperatures).

For pH


Uniform Corrosion (General Attack Corrosion) A corrosive attack proceeding evenly over the entire surface area, or a large fraction of the total area. It is relatively uniform reduction of thickness over the surface of a corroding material (decrease in metal

thickness per unit time or a weight loss per unit area per unit time). It is the uniform thinning of a metal without any localized attack, corrosion does not penetrate very

deep inside, and the most familiar example is the rusting of steel in air. It is relatively easy to measure, predict and design against this type of corrosion damage. While uniform corrosion may represent only a small fraction of industrial corrosion failures, the total

tonnage wasted is generally regarded as the highest of all forms.

Mechanism: What causes Uniform Corrosion? The anodic reaction in the corrosion process is always the oxidation reaction: M M+ + e In acidic environments, i.e., pH7, reduction of dissolved oxygen is the predominant cathodic process that causes uniform corrosion.

2H+ + 2e H2 O2 + 2H2O + 4e 4OH

With uniform distribution of cathodic reactants over the entire exposed metal surface, reactions take place in a "uniform" manner and there is no preferential site or location for cathodic or anodic reaction. The cathodes and anodes are located randomly and alternating with time resulting in uniform loss of dimension.

Common Examples: Aqueous corrosion of iron (Fe) in H2SO4 solution: Fe is dissolved (oxidize) at a uniform rate according to the following anodic and cathodic reactions.

Atmospheric Corrosion of Aluminium: It is due to a passive oxide film formation instead of a porous layer. The gray/black-color film may form.

Tarnishing of silver ware. Tarnishing of electrical contacts. Corrosion of underground pipes

(Composite asphalt coated).

Corrosion of offshore drilling platforms. Corrosion of galvanized steel stairways. Corrosion of automobile bodies.


Atmospheric Corrosion of Zinc: Zinc can uniformly corrode forming a White Rust.

The compound Zn4CO3. (OH6) or ZnCO3.Zn (OH) 2 is zinc carbonate or white rust or wet-storage stain (porous).

Atmospheric corrosion of a steel structureRusting It is a common example of uniform corrosion, which is manifested as a brown-color corrosion layer on the exposed steel surface. This layer is a ferric hydroxide compound known as Rust. The compound precipitates as a solid.

Facts about Uniform Corrosion:

Uniform corrosion is the most commonly found form of corrosion. Rust formed on low alloyed steel is a special form of uniform corrosion. Uniform corrosion occurs mainly on very active metals (low Reversible potential). Noble metals (gold, platinum) are immune against uniform corrosion. Chromium, Titanium, Nickel, stainless steel are protected by a passive film and are therefore not

concerned by uniform corrosion. Prevention:

Uniform attack can be controlled or prevented by: Proper Material Selection including Coatings Inhibitors Cathodic Protection


Galvanic Corrosion (Dissimilar Metal or Bimetallic Corrosion) It is an accelerated corrosion of an active metal because of an electrical contact with a more noble metal in a corrosive electrolyte. The contact of two dissimilar metals constitutes a Galvanic Couple. When two electrochemically dissimilar metals are in contact, a conductive path occurs for electrons and ions move from one metal to the other due to which one metal corrodes as its ions are deposited onto the other metal. This is known as Galvanic Action.

In general, the reactions which occur are similar to those that would occur on single, uncoupled metal, but the rate of attack is increased, sometimes dramatically.

Mechanism: The driving force for corrosion is a potential difference b/w electrochemical potential of different materials.

Anode & Cathode: In a bimetallic couple, less noble material being less corrosion resistant become the anode of the corrosion cell & tends to corrode at an accelerated rate while the more noble material act as cathode in the corrosion cell and will undergo relatively reduced or no corrosion. The effect of coupling the two metals together increases the corrosion rate of the anode and reduces or even suppresses corrosion of the cathode.

Galvanic Series Galvanic corrosion potential is a measure of how dissimilar metals will corrode when placed against

each other in a series. Galvanic series is an arrangement of both metals and alloys according to their actual measured

potentials in a particular environment. There would be one galvanic series for each environment. Metals and alloys showing active-passive behavior are listed in both active and passive states.

Factors Influencing Galvanic Corrosion:

1. Electrode Potential: The position of metal in the galvanic series significantly affects the magnitude of galvanic corrosion. Metals close to one another in the list generally do not have a strong effect on one another, but the farther apart are any two metals, the stronger the corroding effect on the higher metal.

2. Distance Effect: Solution conductivity varies inversely with the length of the conduction path. Most corrosion damage is caused by current which cover short paths. Hence, the greatest galvanic damage is likely to be encountered near the junction of the two metals and the severity would be decreased with increased length. If two different metals are far away from each other, there would be no risk of galvanic corrosion, because of very little current flow.

3. Area Effect: The area ratio is very important in consideration of the likelihood of bimetallic corrosion. The larger the cathode compared to anode, more oxygen reduction can occur & hence greater galvanic current & corrosion. Under static conditions, where bimetallic corrosion current is often dependent upon rate of diffusion of dissolved oxygen to the cathode, the amount of bimetallic corrosion is independent of the size of anode & is proportional to the area of cathodic metal surface. This is sometimes known as catchment area principle, & has important implications in designing to minimize the risk of bimetallic corrosion. For a constant area of cathode metal the amount of corrosion of anode metal is constant, but intensity of corrosion is increased as the area of the anodic metal is decreased.

4. The nature of the environment: The severity of galvanic corrosion largely depends upon the type & amount of moisture in the environment. Water containing copper ions, like seawater, are likely to form galvanic cells on a steel surface of the tank. If the water in contact with steel is either acidic or contains salt, the galvanic reaction is accelerated because of the increased ionization of the electrolyte. In marine environments, galvanic corrosion may be accelerated due to increased conductivity of the electrolyte. In cold climates, galvanic corrosion of buried material is reduced because of the increased resistivity of soil. In warm climates, on the other hand, it is the reverse because of the decreased resistivity of the soil.


Methods of Preventing Galvanic Corrosion Methods of preventing or minimizing bimetallic corrosion are based upon following factors:

Breaking the electrical path in the metallic or electrolyte parts of the system. Excluding oxygen from electrolyte Adding inhibitors to electrolyte Sacrificial corrosion

Electrical Insulation: Galvanic corrosion can be prevented by the use of electrical insulation between different metals thus stopping formation of the electrical circuit. Insulation can be achieved by the use of plastic bushes and washers such as nylon or PTFE or rubber gaskets.

Use of Inhibitors: Inhibitors are often injected to reduce corrosion of the carbon steel and this will also minimize corrosion at bimetallic junctions. Galvanic action in systems containing copper can sometimes be controlled by a specific inhibitor such as benzotriazole.

Metallic Coatings: A wide range of paint and taping systems are available for protecting metals; paints can contain inhibitors or sacrificial metals such as zinc. Ideally both members of a bimetallic couple should be painted, but where this is impracticable the cathodic member should be coated in preference; treating only the anodic metal increases the risk of severe localized bimetallic corrosion at any defect in the coating.

Metal coatings can be applied by electroplating, dipping or spraying to give close identity with the second metal; an example is the Aluminium coating of steel in contact with Aluminium.

Alternatively compatibility between two metals can be obtained by coating one of them with a third metal such as zinc or cadmium.

In some cases it is preferable to coat both metals with a third metal. In industrially polluted atmospheres zinc coatings on steel are superior to those of cadmium, while the

latter are preferred where high humidity and condensation occur. In case of microbial corrosion, materials can be protected by coating with resistant synthetic polymers or

inhibited paints, by dosing with biocides or by designing to avoid conditions that favor microbial growth.

Sacrificial Anodic Protection: Galvanic corrosion can be used to advantage in certain cases if an anodic metal is deliberately exposed to corrosion and sacrificed in order to protect part of the structure. Sacrificial anodes tend to reduce the difference of the potentials of the metals involved by polarization. They can be used to protect stainless steel. Since they corrode, sacrificial anodes need periodic inspection and replacement to maintain the protection. In practice magnesium and carbon steel are used for sacrificial anodes. Zinc and Aluminium can also be used but their corrosion products are often not acceptable.


Preventive Measures: [Important Considerations] Select metals, close together, as far as possible, in the galvanic series. The area of the more active metal must be kept smaller than the area of the less active metal. If dissimilar metals are to be used, insulate them. Use inhibitors in aqueous systems whenever applicable and eliminate cathodic depolarizers. Apply coatings with judgment. Do not coat the anodic member of the couple as it would reduce the

anodic area and severe attack would occur at the inevitable defect points in the coating. Therefore, if coating is to be done, coat the more noble of the two metals in the couple which prevents electrons being consumed in a cathodic reaction and likely to be corrosion rate controlling.

Avoid joining materials by threaded joints. Use a transition piece (third metal) in the couple that must have similar properties than the two metals. Use Sacrificial Anodic material, such as zinc or magnesium. For instance, zinc anodes are used in cast

iron water boxes of copper alloy water-cooled heat exchangers. In designing the components, use replaceable parts so that only the corroded parts could be replaced

instead of the whole assembly.

Crevice Corrosion

Crevice corrosion refers to corrosion occurring in confined spaces to which the access of the working fluid from the environment is limited. These spaces are generally called crevices.

Crevice corrosion is a form of localized attack that occurs in zones of restricted flow where a metallic material surface is in contact with a small volume of confined, stagnant liquid.

It is encountered particularly in metals and alloys which owe their resistance to the stability of a passive film, since these films are unstable in the presence of high concentrations of Cl and H+ ions.

Crevice zones may result from the design of the component or from the formation of deposits during service, shutdown, or even fabrication. These deposits may come from suspended solids in the environment, corrosion products, or biological activity. Low-flow areas are prone to the formation of such deposits.

Examples of crevices: Gaps and contact areas between parts, under gaskets or seals, inside cracks and seams, spaces filled with deposits and under sludge piles. (Green corrosion product called Patina is formed)

Examples of Crevice Corrosion: Stainless steels, nickel-base alloys, aluminum alloys, and titanium alloys in aerated chloride environments, particularly in sea or brackish water, but also in environments found in chemical, food, and oil industries. Other cases of crevice corrosion are also known such as the so-called corrosion by differential aeration of carbon steels, which does not require the presence of chloride in the environment. Other examples include crevice corrosion of steels in concentrated nitric acid and inhibited cooling water and of titanium alloys in hot sulfuric environments. The major factors influencing crevice corrosion are: Crevice type: metal-to-metal, metal-to-non-metal Crevice geometry: gap size, depth, surface roughness Material: alloy composition (e.g. Cr, Mo), structure Environment: pH, temperature, halide ions, oxygen

Basic Mechanism: The specificity of the crevice zones arises as a result of: 1. Limited mass transport by diffusion & convection between the inside of crevice & the bulk environment

due to the local accumulation of corrosion products, and secondly due to 2. The difference in solution resistivity between the inside and the outside of the crevice.

This results in the buildup of a galvanic cell between crevice and external surfaces.


Explaining the Mechanism: Stage 1: Corrosion occurs as normal both inside and outside the crevice (uniformly over the entire surface): The overall reaction involves dissolution of metal and reduction of oxygen to hydroxide ion.

Anodic reaction: MMn+ + ne, Cathodic reaction: O2+2H2O+4e 4OH.

The positively charged metallic ions are electrostatically counterbalanced by OH ions. Stage 2: At this stage, the cathodic reaction inside the crevice consumed most of the oxygen available. So oxygen within the crevice is depleted because of the restricted convection due to which the oxygen reduction ceases in the area. However, the overall rate of oxygen reduction remains the same since crevice area is very small.

Stage 3: Dissolution of metal continuous, therefore Cl ions diffuse into the crevice to maintain a minimum potential energy resulting in formation of a Metal chloride. Hydrolysis of metal chloride lowers the pH.

MCln + nH2O M (OH) n + nHCl

Stage 4: More Mn+ ions attack more Cl ions leads to lower pH inside the crevice, metal dissolution accelerates and more Mn+ ions will be produced that will lower the pH.

As the corrosion within the crevice increases the rate of oxygen reduction on the adjacent surface also increases. This cathodically protects the external surface. Thus during crevice corrosion the attack is localized within the shaded area while the remaining surface suffers little or no corrosion.

Crevice corrosion is initiated by a difference in concentration of some chemical constituents, usually oxygen, which set up an electrochemical concentration cell (differential aeration cell in the case of oxygen). Outside of the crevice (the cathode), the oxygen content and the pH are higher - but chlorides are lower.

Chlorides concentrate inside the crevice (the anode), worsening the situation. Ferrous ions form ferric chloride and attack the stainless steel rapidly. The pH and the oxygen content are lower in the crevice than in the bulk water solution, just as they are inside a pit. The pH inside the crevice may be as low as 2 in a neutral solution. Once a crevice has formed, the propagation mechanism for crevice corrosion is the same as for pitting corrosion.


Factors Affecting Crevice Corrosion

Combating Crevice Corrosion: Use Welded butt joints instead of riveted or bolted joints. Sound welds and complete penetration are

necessary to avoid porosity and crevices on the inside. Close crevices in existing lap joints by continuous welding, caulking or soldering. Design vessels for complete drainage; avoid sharp corners and stagnant areas. Complete drainage

facilities, washing & cleaning tends to prevent solids from settling on the bottom of the vessel. Inspect equipment and remove deposits frequently. Remove solids in suspension early in the process. Provide uniform environments as in the case of backfilling a pipeline trench. Use solid non-absorbent gaskets such as Teflon, wherever possible. Weld instead of rolling in tubes in tube sheets.


Pitting Corrosion Pitting Corrosion is the localized corrosion of a metal surface confined to a point or small area that takes

the form of cavities. It is one of the most damaging forms of corrosion. Pitting corrosion occurs at passivated metal surfaces during the access of aggressive anions. Many metals and their alloys are subject to this type of corrosion (e.g., iron, nickel, copper, aluminium,

steels), whereas chromium is one of the few exceptions that resists pitting in aggressive environments. The restriction of the dissolution to pits within a large passivated metal surface, which may serve as a

large cathode for the reduction of oxidants such as dissolved oxygen, leads to fast perforation of the metal, which weakens the construction and thus causes large economic losses and safety problems.

The main reasons for localized corrosion failures are: Insufficient passive film stability (for stainless steel, insufficient chromium content cannot prevent

chemical dissolution at the oxide-solution interface) Design errors: the presence of areas with stagnant solution will accelerate the generation of local

aggressive conditions Inadequate surface preparation: the presence of deep scratches or defects during surface preparation

can act as pit initiation sites.

Steps involved in Pitting: Pitting at passivated metal surfaces is a complex process with a sequence of steps which are as follows: 1. Breakdown of passivity 2. Early stages of pit growth 3. Late stages of pit growth 4. Re-passivation phenomena

Pit initiation [Breakdown of Passivity] The first stage of the pitting process called pit initiation is related to the breakdown of the protecting passive oxide. Different mechanisms have been proposed for the various passive materials depending on the thickness of their oxides and also on the chemical stability at the oxide-solution interface.

1) The penetration mechanism: the Cl- ions (or other halides anions) are integrated instead of OH- and migrate through the passive film inducing hole formation at the metal-oxide interface. It is assumed that cation vacancies migrate from the oxide electrolyte to the metal-oxide interface. The related voids lead to stresses within the passive film and its final breakdown. The inward diffusion or migration of cation vacancies is affected by the incorporation of Cl ions at the oxide-electrolyte interface. This causes a critical concentration for breakdown at the pitting potential. This type of initiation process is typical of corrosion resistant material with very thin thermodynamically stable oxides like Chromium.

2) The adsorption mechanism: the Cl- ions (or other halides anions) adsorbs locally on the surface replacing OH- and decreases the stability of the oxide surface. The passive layer dissolves faster until the metal surface is exposed. This process is typical of less corrosion resistant materials such as stainless steels where the passive oxide is in a dynamic formation-dissolution state.

3) The oxide cracking mechanism: in this case, the surface oxides (hydroxides) film cracks open due to stresses (internal or applied). This initiation mode is rarer and implies as prerequisite a poorly passivating surface (thicker oxide) or the presence of a thick anodized layer for example. Magnesium in the alkaline domain would be such an example of a material able to passivate but forming a thick oxide layer that can crack in presence of chlorides.


The pitting potential is defined by the threshold potential at which the measured current rapidly increases because of the growth of a stable localized attack. Pit formation can however already occur at potentials lower than the pitting potential.

Pit growth: Once a metastable pit reached stabilized growth, in-depth propagation will occur. Pit usually grows in the direction of gravity. Most pits develop and grow downwards from horizontal surface.

The question is now why a localized attack can be so detrimental. The answer becomes clear when considering the electrochemical reactions involved.

Reaction steps during pit growth processes: Dissolved metallic ions are hydrolyzed. As a result, large amount of protons (H3O+) are formed and the

pH will drop quickly in the pit. Mn+ + H2O Mn (OH) + H3O

+ The electro-neutrality in the local pit solution volume has to be achieved at any time so that fast Cl-

migration in the pit will occur producing locally more aggressive HCl solutions. Cl + H3O

+ HCl + H2O The diffusion of O2 is more difficult in the occluded pit area resulting in local O2 depletion. When an active-passive transition is present for the passive material, a decrease of the pH will result in

an increase of the critical current density (icrit); the re-passivation process is therefore even more hindered.

The combination of all these phenomena induces an autocatalytic process for the localized attack where the dissolution rate is accelerating during a first phase.

If the pH drops below a value of 4, it is also necessary to take into account an additional cathodic reaction taking place only in the pit, the hydrogen reduction (H-type corrosion process). This cathodic reaction is charge transfer controlled so that it can induce very high dissolution rate in the pits and acidification will accelerate the pit growth rate.

2H+ + 2e H2

Setting up of a Galvanic Cell: For a passive metal, an active passive element (micro-galvanic coupling) is furthermore created and maintained during the pit growth phase: Active anode (pit) Passive cathode (intact passive layer around the pit) Concerning the galvanic coupling conditions, the area ratio is catastrophic with a small anode surrounded by a very large cathode. If the electrolyte conductivity is good and the passive film supports oxygen reduction, then the pits will grow very fast!


Effect of Halides: Halides very effectively attack passivating thin oxide layers, leading to an intense localized dissolution of

the metal surface. Chlorides cause the most serious problems due to their presence in many environments such as

seawater and salt on roads, in food, and in the chemical industry. Hypo-chlorides (HOCl) are difficult to handle because of their strong pitting tendencies. Halides of non-oxidizing metals such as NaCl and CaCl2 causes pitting but, to much lesser degree. Cupric, Ferric and mercuric halides are extremely aggressive. They dont even require oxygen to

promote attack because cations are cathodically reduced as follows: Cu 3+ + 3e Cu

Effect of Velocity on Pitting: Pitting is usually associated with stagnant conditions such as a liquid in a tank or liquid trapped in a low pipe system. Increase in velocity often decreases pitting attack. E.g. Stainless steel pump would give good service handling of sea water if it were to run continuously but would start pitting if shutdown for an extended period. Effect of Metal Composition on Pitting: Stainless steel alloys are most susceptible to pitting than any other group of metals & alloys. E.g. presence of Cr, Ni, Mo, N increases pitting while Si, S, Se, C decreases pitting.

Evaluating Pitting Damage:

Pitting factor is the ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss.

Conventional weight loss (%) methods cant be used to quantify the corrosion rate in case of pitting because usually the associated metal loss is small and doesnt indicate the depth and penetration of pit.

Measurement of pit depths is complicated by the fact that there is a statistical variation in depth of pits on an exposed specimen.

Average Pit depth is a poor way of estimating the damage since the deepest pit causes the fracture. Measurement of maximum pit depth would be a more reliable way of expressing pitting corrosion. Prevention:

No. of Pits

Pit Depth

Max. Pit Depth


Stress Corrosion Cracking (SCC)

Stress corrosion cracking (SCC) is a catastrophic cracking failure of a susceptible material in a particular environment. This phenomenon can produce cracking at stress levels well below a materials rated tensile strength. The crack or fracture will appear brittle, with no localized yielding, plastic deformation or elongation. Rather than a single crack, a whole network of fine, feathery, branched cracks will form. Pitting is frequently seen and will often serve as a stress concentrator to initiate cracking. One or more cracks will grow from the pit, eventually leading to the failure. SCC is the initiation and slow growth of cracks under the simultaneous influence of tensile stresses and aggressive environment.

Stresses to consider include: Operational (Applied)

Thermally induced (temperature gradients, differential thermal expansion and contraction)

Induced by build-up of (voluminous) corrosion products

Assembly - Poor fit up (tolerancing problems) - Torqueing - Press and shrink fits - Fastener interference - Joining (see below)

Residual (due to manufacturing processes)

- Joining (welding, brazing, soldering) - Casting - Machining - Forming and shaping - Cutting and shearing - Surface treatment (plating, mechanical cleaning, etc.) - Heat treatment (e.g. quenching, phase changes)

Mechanisms: SCC is the result of a combination of three factors:

1. A susceptible material, 2. Exposure to a corrosive environment, and 3. Tensile stresses above a threshold.

Cracking Morphology: [Type of crack propagation] Intergranular: Attack at the grain boundaries under tensile stress. Transgranular: Attack through (with in) the grains.


There is no unified mechanism for stress corrosion cracking in the literature. Various models have been proposed which include the following:

Adsorption model: the specific species adsorb and interact with strained bonds at the crack tip which causes in a reduction of the bond strength or in thermodynamic term, the reduction in surface energy resulting in a brittle fracture.

Film rupture model: The protective film present at the crack tip is ruptured by continued plastic deformation and thus exposing the metal to environment at a very small area (crack tip) setting up an active-passive cell. The small area behaves as anode and rest of the walls of the crack behaves as cathode. The cathode to anode area ratio is very high and a localized type of attack takes place.

Pre-existing active path model: Pre-existing path such as grain boundaries where intermetallics and compounds are formed. Localized preferential corrosion (dissolution) at the crack tip, along a susceptible path, with the bulk of the material remaining in a more passive state. At the crack tip, adsorbed or species diffused in the metal weaken the metallic binding forces.

Embrittlement model: Hydrogen embrittlement is a major mechanism of SCC for steels and other alloys such as titanium. Hydrogen atoms diffuse to the crack tip and embrittle the metal.

Mechanism Type Process Pictorial View Hydrogen Embrittlement

Dissolved H-atoms dilate lattice & Weaken atomic bonds.

Adsorption Induced Cleavage Adsorption of A weakens crack tip

bonds.

Surface Mobility Atoms migrate out of the crack tip.

Film Rupture Crack grows by anodic dissolution at

crack tip where film is ruptured.

Film Induced Cleavage First brittle crack initiates in brittle

film and propagates in ductile crack tip metal.

Localized Surface Plasticity Brittle crack propagates into crack tip

metal weakened by anodic dissolution resulting from film rupture.


Controlling Factors in SCC Crack: Corrosion film characteristics

(i.e., passivation) and local anodic attack (i.e., de-passivation) serve as controlling factors in SCC crack initiation and growth.

Localized corrosion can promote SCC making exposure geometry and specimen design important factors.

Mechanical straining or electrochemical inducements such as crevices or controlled potential are utilized to overcome the problems and uncertainties of SCC initiation.

HYDROGEN EMBRITTLEMENT: [Hydrogen Induced Cracking (HIC), Stepwise Cracking (SWC) or Blister Cracking] Hydrogen embrittlement occurs when atomic hydrogen diffuses into the metal structure and interacts within the metal to form a brittle material. This brittle material then fails by brittle cracking due to the internal stresses created by the entrapped hydrogen molecules. Mechanism: Hydrogen dissolves in all metals to a moderate extent. Being in smaller in size it fits in between the metal atoms in the crystals of the metal. Consequently it can diffuse much more rapidly than larger atoms. Hydrogen tends to be attracted to regions of high triaxial tensile stress where the metal structure is dilated. Thus, it is drawn to the regions ahead of cracks or notches that are under stress. The dissolved hydrogen then assists in the fracture of the metal, possibly by making cleavage easier or possibly by assisting in the development of intense local plastic deformation. These effects lead to embrittlement of the metal.


Sulfide Stress Cracking (SSC): Sulfide stress cracking (Wet H2S cracking) is a special case of hydrogen embrittlement that occurs when H2S dissociates, in the presence of water, into hydrogen and sulfide ions. Hydrogen absorption is clearly responsible for SCC of high-strength steels in aqueous environments, notably in the presence of H2S, where the fraction of discharged hydrogen that is absorbed by the metal can approach 100% owing to poisoning of the recombination reaction (H + H H2) by sulfur adsorption.

Sulfide stress cracking (SSC) has special importance in gas and oil industry, as the materials being processed there (natural gas and crude oil) often contain considerable amount of hydrogen sulfide. SSCC is a low temperature (temperature where water is liquid) effect of H2S in an aqueous environment and Sulfidation is the term used for the high temperature (>450F) sulphur corrosion.

Chloride Stress Corrosion Cracking (CSC): Chloride stress corrosion involves selective attack of the metal along grain boundaries. It is the most widely encountered form of SCC. In the formation of the steel, a chromium-rich carbide precipitates at the grain boundaries leaving these areas low in protective chromium, and thereby, susceptible to attack. Stainless steels are covered with a protective oxide film. The chloride ions rupture the film at weak spots setting up a galvanic cell which produces accelerated attack at the sites, which when combined with tensile stresses produces cracking. The minimum stress level required for chloride SCC for 300 series stainless is extremely low and most commonly used fabrication techniques (welding, bending, rolling, etc.) will produce residual stress exceeding the minimum value. This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and the use of low carbon steels.

Caustic Embrittlement: Caustic embrittlement is generally encountered in boilers or steam systems where sodium or calcium scales form on heated surfaces [Occurs in zircalloy and ferrous alloys]. The susceptibility of a welded steel structure to SCC is a well-defined function of sodium hydroxide concentration and temperature. The cracking is associated with the presence of sodium hydroxide. However, the metal away from the cracks is ductile. The cracking is predominantly intercrystalline while in some cases they may be transcrystalline. The concentrated basic conditions are caused by boiling (basic solution concentrates in crevices). The caustic material plates out of solution. When it is remoistened it creates a region with a highly caustic environment. Strict pH control is the only solution. Nickel cladding has been used successfully to prevent caustic embrittlement.


Corrosion Fatigue Cracking [Low Amplitude Cyclic (or Ripple) Loading] It is a Special case of Stress Corrosion Cracking (SCC) Applied stress is not constant (not static) but experiences cyclic

variations aggravated SCC attack Results in Transgranular crack propagation The cyclic loading can induce constant de-passivation of the

material. At the induced micro notches, additional gliding and accelerated

corrosion is induced. The cracks are generally transgranular in nature, with little

tendency for branching. However, a few small secondary cracks may appear in the vicinity of the main crack.

Factors Affecting CF The corrosion fatigue crack initiation and growth of a metal/alloy, depends on the following factors: Amplitude of cyclic load or stress intensity factor Ratio of stress intensity factor Frequency of fluctuating load Corrosiveness of environment, Geometry of crack tip or stress concentration. Corrosion Fatigue (CF) Control All most all the techniques used in SCC control can be used for fatigue crack initiation and growth. There is an old rule of thumb about fatigue tests which says that 10% of damage occurs in 90% of time and 90% of the damage in the last 10% of the time. This means that more importance should be given in controlling initiation than the propagation of corrosion fatigue. The following consideration should be taken into account to reduce the possibilities of fatigue failures: 1. Materials choice: Strong alloys exhibit poor resistance to C.F. 2. Design aspects: Stress concentrations such as flaws, grooves, pits notch etc. are potent sites for CF

initiation 3. Cold working: Shot peerning or grit blasting provide compressional stresses and control C.F. 4. Nitriding: Certain special steels containing elements like Cr and Mo possess affinity towards nitrogen.

When they are heated in ammonia, the surface is converted into nitrides and the product is voluminous than the base alloy surface. This provides compressive stress and raises fatigue strength of the material.


Intergranular Corrosion It is a localized attack at and adjacent to grain boundaries with relatively little or no corrosion of the grains. Penetration of the corroding medium follows grain boundaries and destroys the bond between grains. This action can seriously affect the strength of an alloy. When severe, this corrosion causes a loss of strength and ductility which is out of proportion to the amount of metal actually destroyed by corrosion. It may eventually result in intergranular - stress corrosion - cracking (IGSCC) along the metal's grain boundaries.

Mechanism: Intergranular corrosion occurs when grain boundaries and matrix (inner part of grains) have different

electrochemical behavior. It is either due to the presence of impurities in the boundaries, or due to local enrichment or depletion

of one or more alloying elements. This situation can happen in corrosion-resistant alloys, when the grain boundaries are depleted (grain

boundary depletion) of the corrosion-inhibiting elements such as chromium by some mechanism. For a given materials, grain boundaries or areas near grain boundaries are less noble (less corrosion

resistant) compared to the inner part of grains because of a differing composition and formation of carbides. As a result, the corrosive attack is localized at these less noble areas.

Damage: The observed damages are not only the attack and dissolution of grain boundary areas but also result in severe falling out of entire grains inducing macroscopic damages. The degradation processes are: 1. Electrochemical material removal evolving at relatively small rate with local increased corrosion at

grain boundaries. 2. Removal of undermined grains is the most dangerous aspect regarding fast damages.

Weld Decay [Sensitization] It is the most well-known form of intergranular corrosion caused by inadequate heat-treatment or welding procedures which result in the precipitation of intergranular compounds (e.g., austenitic stainless steels).

Sensitization refers to the precipitation of carbides at grain boundaries in a stainless steel or alloy, causing the steel or alloy to be susceptible to intergranular corrosion or stress corrosion cracking (Weld Decay).

In nickel alloys and austenitic stainless steels, where chromium is added for corrosion resistance, the mechanism involved is precipitation of chromium carbide at the grain boundaries when cooling from welding temperatures, resulting in the formation of chromium-depleted zones adjacent to the grain boundaries. These zones are highly subject to corrosion attack and also act as local galvanic couples, causing galvanic corrosion.

The combination of chromium and carbon in the stainless steel provide the necessary ingredients to form chromium carbide; of atomic composition Cr23C6. The other necessary ingredient is thermal treatment. The

carbide is stable between the ranges of 950 and 1450oF, but requires some time to grow due to the nucleation and growth required for second phases. The nucleation site of the chromium carbide is the austenite grain boundaries. The second phase therefore grows on the grain boundaries. The distribution of the chromium in the steel then becomes important. Chromium can only diffuse by substitutional diffusion in the matrix which is slow or along grain boundaries which is faster but limits the location of the chromium for the carbides to regions near the grain boundaries. The carbon diffuses by interstitial diffusion, which is much faster and easier than substitutional or again along the boundaries.

1. Vacancy controlled also Substitutional a vacancy or missing atom on its lattice site is needed for the atom to jump into. As one atom jumps into a vacancy it leaves a vacancy.

2. Interstitial diffusion involves small atoms diffusing in a matrix of large atoms. For metallic atoms, the H, C, N, O, B atoms are the ones small enough to fit in the spaces between the large metallic atoms.


3. Grain boundary diffusion uses defect structure of grain boundary so much faster than vacancy or substitutional diffusion rates

Knifeline Attack (KLA) KLA is a specific intergranular corrosion phenomenon of stabilized austenitic stainless steels. The attacked areas have been found to contain precipitated chromium carbides; the stabilizing elements niobium (columbium) or titanium failed "to do their job" in tying up the carbon in the alloy. KLA is similar to weld decay in that they both result from intergranular corrosion and both are associated with welding. The two major differences are that:

1. KLA occurs in presumably stabilized steels; 2. KLA occurs in a very narrow band in the parent metal immediately adjacent to the weld, whereas weld

decay develops at a greater distance from the weld.

Mechanism: The mechanism for KLA is based on the high-temperature solubility of niobium (columbium) carbides in stainless steel. Niobium and niobium carbides dissolve in the metal when it is heated to a very high temperature and they remain in solution when cooled rapidly from this temperature. The niobium stays in solution when the metal is subsequently heated into the chromium carbide precipitation range, niobium carbide does not form, and the metal behaves (sensitizes) as though it were austenitic steel without niobium stabilizer.

Remedy: The remedy for avoiding KLA is to heat the completed structure (after welding) to around 1065C. Chromium carbide dissolves and niobium carbide forms - which is the condition desired for resistance to KLA and weld decay. The rate of cooling after the 1065C treatment is not important.

Exfoliation A unique form of intergranular corrosion is observed in aluminum alloys. Intergranular attack proceeds laterally through plate and sheet in planes parallel to the surface (direction of cold work). The corrosion products force the metal away from the body of the materials and give rise to a layered or leaf-like appearance ("exfoliation") in un-recrystallized alloys.


Example: In Al-Mg alloys, the magnesium tends to segregate or preferentially collect at the grain boundaries. As magnesium is a very active element, then the grain boundaries become active and dissolve. The grain surface area that is not being attacked becomes cathodic with a resulting large cathode to small anode ratio which increases the corrosion rate for the grain boundary regions.

Measures to avoid intergranular corrosion:

From a materials perspective: Heat treatment at higher temperature (1100C) followed by Solution quenching or Quench annealing

(Rapid cooling from the solution welding temperature). Decrease of the carbon content of steel (not always possible because it decreases the machinability of

the steel). However, low carbon content makes carbide formation impossible. Lowering the carbon to below 0.03% is referred to as Extra Low Carbon (ELC).

Use Stabilized steels by the addition of Titanium, Niobium, and Tantalum known as Stabilizers (have higher affinity for carbide formation than Chromium). Alloying with titanium (Ti = 5 * C concentration) leads to formation of (non-interfering) titanium carbides, and the chromium content remains unchanged through the whole grain boundary.

From a design and construction perspective: Controlling the temperature flow during welding, Heat affected zones away from the weld areas are often the location of IGC failure. This problem is by far the most acute as it is not easy to avoid the intermediate temperature domain during welding. It is currently the major still remaining cause of IGC failure for steel.

Selective Leaching (De-alloying or De-Metalification or Parting or Selective Corrosion)

Selective leaching is the preferential dissolution of one element from an alloy. It is also known as de-alloying as it results in the breaking of an alloy. It may occur uniformly or locally, often without a measurable change in dimension. The less noble metal is removed from the alloy by microscopic-scale galvanic corrosion mechanism. The most susceptible alloys are the ones containing metals with high distance between each other in the

galvanic series, e.g. copper and zinc in brass. The elements most typically undergoing selective removal are Zn, Al, Fe, Co, Cr etc.

What causes de-alloying? Different metals and alloys have different electrochemical potentials (or corrosion potentials) in the same electrolyte. Alloys contain a number of different alloying elements that exhibit different corrosion potentials. The potential difference between the alloying elements is the driving force for the preferential attack on the more "active" element in the alloy.

1. Dezincification It is the selective removal of zinc from zinc-containing alloys (Brass). The zinc is separated by dissolution from copper & can be observed by naked eyes because alloy changes color from yellow to red.

Mechanism: Dissolution of Brass Zn ions stay in solution Cu plates back on.

Zn is quite reactive, whereas Cu is nobler. Zn can corrode slowly in pure water by cathodic ion reduction into H2 and hydroxide ions. Therefore, de-zincification can proceed even in the absence of O2. The analysis shows 90-95% Cu with some present as copper oxide. The amount of CuO is related to the oxygen content in the environment.


Types of Dezincification Mechanisms: 1. Uniform (Layer) Type 2. Localized (Plug) Type It seems to favor the high brasses (i.e. High Zn content) and acidic environments.

It seems to occur in the low brasses (low Zn content) and neutral, alkaline or slightly acidic environment (high in salt).

Uniform-layer dezincification leaches zinc from a broad area of the surface.

Plug-type dezincification is localized within surrounding surfaces mostly unaffected by corrosion.

This type of dezincification uniformly reduces the wall thickness of the valve or fitting.

This type of dezincification penetrates deeply into the sidewalls of valves and fittings.

A complex set of conditions must be present for dezincification to occur, and the occurrence is often related to region of the country. E.g. Slow H2O flow or stagnant H2O especially when O2 & CO2 is high in H2O makes a material more susceptible to uniform corrosion. Slightly acidic water that is low in salt & at room temperature also facilitates selective leaching.

Common failures associated with plug-type attack include penetration through the sidewalls that causes water seepage or loss of mechanical strength in threaded sections to the point of fracture.

2. Graphitization (Graphitic corrosion) It is the deterioration of gray cast iron in which the metallic constituents are selectively leached or

converted to corrosion products leaving the graphite intact. Graphitic corrosion should not be confused with another term graphitization, which is used to describe

the formation of graphite in iron or steel, usually from decomposition of iron carbide at elevated temperatures.

Mechanism: The gray cast iron show the effect of selective leaching out of iron in mild corrosive environments. During Graphitic Corrosion, the porous graphite network that makes up to 4-5% of the total mass of the

alloy is penetrated by the insoluble corrosion products. As a result cast Iron retains its shape and appearance but this weakens its internal structure.

The corrosion process is a Galvanic Action between C and Fe with Carbon acting as the least noble so anode while Iron acting as the most noble so act as a cathode.

The surface layer of the iron becomes like graphite and it can be easily cut with a knife. Iron is dissolved and leaves behind an interlocking nobler graphite network and a porous mass of voids and complex iron oxides. This graphitized cast iron loses its strength and other metallic properties, but unchanged in the shape.

How to prevent de-alloying? De-alloying, selective leaching and graphitic corrosion can be prevented through the following methods: Select metals/alloys that are more resistant to de-alloying. For example, inhibited brass (Red brass

15% Zn) is more resistant to dezincification than alpha brass; ductile iron is more resistant to graphitic corrosion than gray cast iron.

Control the environment to minimize the selective leaching. Use sacrificial anode cathodic protection or impressed current cathodic protection

Decarburization: It is the selective loss of carbon from the surface layer of a carbon-containing alloy due to reaction with one or more chemical substances in a medium that contacts the surface.

Denickelification: It is the selective leaching of nickel from Ni based alloys. E.g. Cu-Ni alloys.

Decobaltization: It is the selective leaching of cobalt from Co based alloys. E.g. Stellite


EROSION CORROSION It is a degradation of material surface due to mechanical action, often by impinging liquid, abrasion by

slurry, particles suspended in fast flowing liquid or gas, bubbles or droplets, cavitation, etc. Erosion corrosion is acceleration in the rate of corrosion attack in metal due to the relative motion of a

corrosive fluid and a metal surface. The increased turbulence caused by pitting on the internal surfaces of a tube can result in rapidly increasing erosion rates and eventually a leak.

Erosion corrosion occurs due to the velocity of the fluids around the metal components. The removal of the ionic species promotes the rapid dissolution of the anode material. In addition the removal of protective films by the flowing solution can decrease the protection they offer.

Erosion-corrosion is most prevalent in soft alloys (i.e. copper, aluminum and lead alloys). Alloys which form a surface film in a corrosive environment commonly show a limiting velocity above which corrosion rapidly accelerates. With the exception of cavitation, flow induced corrosion problems are generally termed erosion-corrosion, encompassing flow enhanced dissolution and impingement attack. The fluid can be aqueous or gaseous, single or multiphase. There are several mechanisms described by the conjoint action of flow and corrosion that result in flow-influenced corrosion:

Mass transport-control: Mass transport-controlled corrosion implies that the rate of corrosion is dependent on the convective mass transfer processes at the metal/fluid interface. When steel is exposed to oxygenated water, the initial corrosion rate will be closely related to the convective flux of dissolved oxygen towards the surface, and later by the oxygen diffusion through the iron oxide layer. Corrosion by mass transport will often be streamlined and smooth.

Phase transport-control: Phase transport-controlled corrosion suggests that the wetting of the metal surface by a corrosive phase is flow dependent. This may occur because one liquid phase separates from another or because a second phase forms from a liquid. An example of the second mechanism is the formation of discrete bubbles or a vapor phase from boiler water in horizontal or inclined tubes in high heat-flux areas under low flow conditions. The corroded sites will frequently display rough, irregular surfaces and be coated with or contain thick, porous corrosion deposits.

Erosion-corrosion may be enhanced by particles (solids or gas bubbles) and impacted by multi-phase flows. The morphology of surfaces affected by erosion-corrosion may be in the form of shallow pits or horseshoes or other local phenomena related to the flow direction.

Effect of Velocity [Flow Induced Mechanical Removal & Shearing] Velocity strongly influences mechanics of corrosion. It exhibits mechanical wear effect at higher values

and particularly when solution contains solids in suspension. At first effect of Velocity may be negligible but when critical velocity is reached then the attack increases at a rapid rate.

Erosion-corrosion is associated with a flow-induced mechanical removal of the protective surface film that results in a subsequent corrosion rate increase via either electrochemical or chemical processes.

It is often accepted that a critical fluid velocity must be exceeded for a given material. The increase in fluid velocity usually increases the erosion corrosion rate.

The mechanical damage by the impacting fluid imposes disruptive shear stresses or pressure variations on the material surface and/or the protective surface film.

Example: Velocity may increase attack on steel by increasing supply of O2, CO2, H2S in contact with metal surface or It may decrease diffusion (i.e. transfer) of ions by reducing thickness of stagnant film at the surface. If the passive film is removed from stainless steel then it will corrode uniformly. Other cases exist for example stainless steel in fuming nitric where the erosion corrosion rate decreases due to removal of nitrous acid at increased velocity. When this component remains at low or static conditions it promotes attack of the steel.


Jet o f Fluid Impacts Surface When Bubble Contacts

Turbulent Flow [Inlet Corrosion] Turbulent flow leads to a phenomenon called inlet corrosion. It results in greater agitation of liquid at metal surface than in case of laminar flow. The most frequently occurring example of this type of failure occurs at inlet ends of tubing in condenser and similarly in Shell end tube heat exchanger. The extra turbulence at an inlet or change in direction will produce turbulent flow. The material loss is greatest nearest the inlet as the flow and agitation is highest leading to enhanced erosive reaction. Impeller and propeller are typical components operating under turbulent conditions.

Impingement [Localized Erosion Corrosion] Impingement attack is related to the cavitation damage, and has been defined as localized erosion-

corrosion caused by turbulence or impinging flow. Entrained air bubbles tend to accelerate this action, as do suspended solids. This type of corrosion occurs in pumps, valves, orifices, on `heat-exchanger tubes, and at elbows and

tees in pipelines. Impingement corrosion usually produces a pattern of localized attack with directional features. The pits

or grooves tend to be undercut on the side away from the source of flow, in the same way that a sandy river bank at a bend in the river is undercut by the oncoming water.

When a liquid is flowing over a surface (e.g. in a pipe), there is usually a critical velocity below which impingement does not occur and above which it increases rapidly.

In practice, impingement and cavitation may occur together, and the resulting damage can be the result of both. Impingement may damage a protective oxide film and cause corrosion, or it may mechanically wear away the surface film to produce a deep groove.

Impact Erosion Corrosion [Micro Machining Operation] When particulate matter is added to the flow an additional surface damage mechanism is additive to the

erosion process. The particles act as small machine tools and remove the surface material in a micro-machining operation.

The angle of impact that is most deleterious is 30 to 45o for metals and 90o for ceramics. Elbows and bends are particularly prone to this type of damage.

Cavitation Cavitation occurs when a fluid's operational pressure drops below its vapor pressure causing gas

pockets and bubbles to form and collapse. Cavitation erosion is due to either the bubbles in a fluid flow or droplets in an air stream impacting a

surface at high velocity. In this case the bubble collapses on to the surface of the material and causes a small portion of material

to be removed from the surface. The corrosive environment will attack the highly anodic material exposed by the cavitation damage.

Cavitation sometimes is considered a special case of erosion-corrosion and is caused by the formation and collapse of vapor bubbles in a liquid near a metal surface. Cavitation removes protective surface scales by the implosion of gas bubbles in a fluid.

The subsequent corrosion attack is the result of hydro-mechanical effects from liquids in regions of low pressure where flow velocity changes, disruptions, or alterations in flow direction have occurred.

Cavitation damage often appears as a collection of closely spaced, sharp-edged pits or craters on the surface.

Micro-machined crater during impact erosion corrosion

Machined Lip


Effect of Environment: pH: Variation in pH can result in abnormalities in behavior. For example, one pH may induce passivation and another may not. This is the case for iron, which is very resistant to erosion at pH of 6 and 10 but poor at other pH conditions as the films former are non-passive.

Oxygen: If oxygen promotes a stable film formation then it will be advantageous. One case is titanium in which increasing oxygen promotes a stable film formation. For brass and copper, brass formed a gray CuO film while copper formed a CuCl2film which was not stable under identical environmental conditions.

Prevention 1. Choose erosion resistant metals: Choose materials that are strengthened by solid solution rather than by second phase strengthening. These materials generally do better in erosion. In addition the removal of second phases often decreases the corrosion rate of the material as no galvanic contacts are available. 2. Design: Try not to provide enhanced erosion conditions. For example, Try to avoid sharp elbows when a particulate flow is present. Thicken sections so more erosion is possible before refitting is necessary. Make component that is going to suffer erosion corrosion easily replaceable or repairable. Design a portion that can be employed while the other pipe is being repaired or redundant design. It is generally desirable to reduce the fluid velocity and promote laminar flow; increased pipe diameters

are useful in this context. Rough surfaces are generally undesirable. Designs creating turbulence, flow restrictions and obstructions are undesirable. Abrupt changes in flow direction should be avoided. Tank inlet pipes should be directed away from the tank walls, towards the center. Welded and flanged pipe sections should always be carefully aligned. Impingement plates of baffles designed to bear the brunt of the damage should be easily replaceable. 3. Change environment: Slow flow by wider diameters. Use filters to remove particles. Change impellor speed to reduce cavitation. 4. Electrochemical Protection: Use impressed currents to reduce anodic effects on erosion. De-aeration and corrosion inhibitors are additional measures that can be taken. Cathodic protection and the application of protective coatings may also reduce the rate of attack. Offshore Well Systems: In offshore well systems, the process industry in which components come into contact with sand-bearing liquids, this is an important problem. Materials selection plays an important role in minimizing erosion corrosion damage. Caution is in order when predicting erosion corrosion behavior on the basis of hardness. High hardness in a material does not necessarily guarantee a high degree of resistance

corrosion engineering notes (p.e-409)

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