corrosion prevention 1-4
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corrosion preventionTRANSCRIPT
Corrosion Protection
Ahmad Ivan Karayan, ST, MEng
Department of Metallurgy and Materias EngineeringUniversity of Indonesia
CATHODIC PROTECTION ANODIC PROTECTION MATERIAL SELECTION
ALTERATION OF ENVIRONMENT (INHIBITOR)
PROPER DESIGN COATINGS & WRAPPING
Cathodic Protection
• Sacrificial Anode (Galvanic Cathodic Protection)
• Impressent Current
Voltage (IR) Drops• The difference between the the real potential and the displayed potential• Methods suggested for determining IR drop involve one or more of the
following:
ON and OFF Potential
• ON Potential is the potential adjusted from the T/R when T/R is on regardless IR Drop, so CP criteria -850 mv shall be measured by OFF Potential
• OFF Potential Potential measured when T/R is off
Galvanic Cathodic Protection
• Alloys of magnesium, zinc, and aluminum have been developed to enable the anode to remain active and extend the life of the anode. The pure forms of the metals are often not suitable as anodes because they undergo too much “self corrosion” in the environment and do not stay active. Galvanic anode alloys include: magnesium, zinc, aluminium
Galvanic Anode Efficiency
• The efficiency of a galvanic anode depends on the alloy of the anode and the environment in which it is installed.
• The consumption of any metal is directly proportional to the amount of current discharged from its surface.
• For galvanic anodes, part of this current discharge is due to the cathodic protection current provided to the structure and part is caused by local corrosion cells on its surface.
• Anode efficiency is the ratio of metal consumed producing useful cathodic protection current to the total metal consumed.
• For magnesium, the anode efficiency is generally less than 50%, while zinc has an efficiency of 90%.
Applications of Galvanic Anode System
The following are among the conditions where galvanic anodes are used:
• When a relatively small amount of current is required. • Usually lower resistivity electrolytes.• For local cathodic protection to provide current to a specific area on
a structure. Some pipeline operators install galvanic anodes at each location where a leak is repaired rather than installing a complete cathodic protection system. Such practices may be encountered on bare metal or very poorly coated systems where complete cathodic protection may not be feasible because of cost.
• When additional current is needed at problem areas. Some structures with overall impressed current cathodic protection systems may have isolated points where additional current in relatively small amounts is needed. These requirements can be met with galvanic anodes.
• Poorly coated buried valves • Interiors of water storage tanks • Shorted casings that cannot be cleared (to improve potentials in the
surrounding area) • Underground storage tanks • Isolated sections where the coating has been badly damaged • Areas where electrical shielding impairs effective current distribution from
remotely located impressed current systems • In cases of cathodic interference, if the conditions are suitable, galvanic
anodes can be used at the discharge point to return interfering current. • To provide protection to structures located near many other underground
metallic structures where conditions make it difficult to install impressed current systems without creating stray current interference problems. Galvanic anodes can be an economical choice for a cathodic protection current source under such conditions.
• Galvanic anodes find extensive use in protecting the interior surface of heat exchanger water boxes and other vessels. They are also used within oil heater-treater vessels, depending on the quality of the interior lining and the fluid chemistry and temperature.
• On offshore structures, large galvanic anodes may be used to protect the underwater components.
Anode Backfill• Zinc and magnesium anodes used in cathodic protection pplications
in soil are sometimes supplied prepackaged with a prepared backfill material in a cloth or cardboard container.
• The special backfill prevents direct soil contact to reduce localized corrosion of the anode, prevents passivation of the anode caused by reactions with soil salts, provides a low-resistivity environment around the anode, and expands when wet to fill the hole and eliminate air voids.
• The most common backfill material contains 75% hydrated gypsum, 20% bentonite clay, and 5% sodium sulfate.
• Zinc anodes can also be packaged in a backfill consisting of 50% hydrated gypsum and 50% bentonite clay.
• Since zinc anodes are normally installed in low-resistivity soil, it is not necessary to add sodium sulfate to lower resistivity.
Wiring and Connections• Galvanic anodes can be attached to the structure either directly by
welding or bolting integral straps to the structure, e.g., hull mounted anodes and bracelet anodes, or by connecting a wire between the anode and structure.
• If a wire is used, the manufacturer attaches it to the anode. The wire is attached to the structure using a mechanical connection, thermite weld, or other suitable method. The thermite weld is the preferred method since it provides the most reliable connection. The wire should be coated with a dielectric insulation and the connections should be coated.
Component of Impressed Current Cathodic Protection
• The components of an impressed current cathodic protection system are anodes, anode backfill, a power supply (rectifier), structure, wiring, and connections.
• The anodes used in impressed current CP systems are different from those used in galvanic systems.
• Impressed current anodes are manufactured from materials that are consumed at low rates. Impressed current CP systems generally operate at higher current and driving voltage levels than galvanic anode CP systems.
Applications of Impressed Current
Typical uses of impressed current are: • For large current requirements, particularly for bare or poorly coated
structures • In all electrolyte resistivities • As an economical way of protecting structures having dissipated galvanic
anodes • To overcome stray current or cathodic interference problems • For protection of large heat exchanger water boxes, oil heater-treaters, and
other vessels • For interiors of water storage tanks • For exterior bottoms (both primary and secondary) of aboveground storage • tanks • For underground storage tanks • For underwater components of offshore structures • For foundation piles and sheet piling, both underground and in the water.
Anodes for Impressed Current
• Graphite Graphite anodes are used in soils, flowing seawater, and mud. They are practically immune to chlorine attack. Graphite anodes are usually impregnated with a sealer to prevent mechanical failure from gas evolution in pores. Graphite is also brittle. Consumption rates are 0.45 kg/A-y (1 lb/A-y) in seawater, 0.9 kg/A-y (2 lb/A-y) in carbon backfill, and 1.36 kg/A-y (3 lb/A-y) in mud. Graphite anodes are usually available as cylindrical anodes.
Anodes for Impressed Current
• Conductive Polymer Carbon is used as filler in polymer materials having a copper core for use as an impressed current anode. This type of anode looks like an insulated wire but the covering is conductive. (Note: this type of anode wire must not be used where dielectrically insulated wire is needed.) This material has a maximum rating of 51 mA/m (16 mA/ft) of material.
Carbon has also been used as conductive filler in water- or solvent- based coatings for application as an anode to protect reinforced concrete structures.
Anodes for Impressed Current
• High-Silicon Chromium-Bearing Cast Iron
High-silicon cast iron (HSCI) is a chemically resistant alloy containing silicon, chromium, and iron. HSCI anodes are commonly used in fresh water, seawater, or underground applications. HSCI is very brittle and forms a SiO2 film on the surface in underground applications that can increase the resistance of the anode in dry environments. The consumption rate of HSCI ranges from 0.25 to 1 kg/A-y (0.55 to 2.20 lb/A-y).
Anodes for Impressed Current
• Lead
Lead-silver alloys have been used in seawater applications. The lead under anodic current develops a PbO2 film that is conductive and prevents deterioration of the lead. The consumption rate of lead-silver alloys is on the order of 0.09 kg/A-y (0.2 lb/A-y).
Extruded lead with platinum pins has also been used in seawater applications. The purpose of the platinum pins is to promote the ormation of the PbO2 film.
Anodes for Impressed Current
• Mixed-Metal Oxide
Mixed-metal oxide (MMO)anodes, also called dimensionally stable anodes, consist of rare earth oxides baked onto a titanium substrate. These anodes were developed for the electrolytic production of chlorine and hypochlorites, but are now used for cathodic protection applications. The consumption rate is on the order of 1 mg/A-y. This anode material is typically available in rod, wire, tubular, or mesh form.
Anodes for Impressed Current• Platinum
Platinum is used as an anode material when either metallurgically clad or plated onto either a titanium or niobium substrate. Titanium and niobium form stable oxide layers when made anodic. These layers are stable up to 12 V in the case of titanium and 90 V in the case of niobium. The consumption rate of platinized anodes is on the order of 6 to10 mg/A-y. Platinized anodes are available in wire or mesh form. Platinized anodes are subject to rapid deterioration if the breakdown voltage is exceeded or if the environmental conditions surrounding the anode become acidic. Other deleterious factors include the presence of low-frequency AC ripple, current reversal, biofouling, scales, and the presence of certain organic materials.
Platinized anodes are most suitable in fresh-or salt-water applications rather than in underground applications.
Anodes for Impressed Current
• Scrap Metal
Scrap iron or steel can be used as an anode material. In situations where the current requirements are low, scrap iron is readily available, and the anode can be readily replaced, scrap iron might be an economical choice. The consumption rate of iron is 6.8 to 9.1 kg/A-y (15 to 20 lb/A-y). The relatively rapid dissolution rate and difficulty in maintaining the integrity of the connection between the power supply and anode are disadvantages of this material.
Anodes for Impressed Current
• Metallized Titanium
• Thermal Sprayed Zinc and Aluminum Alloys
• Magnetite
• Aluminum
Anode Backfill
Carbon is used as a backfill material around impressed current anodes for underground CP applications. The purpose of the backfill material is to:
• Reduce the resistivity of the environment surrounding the anode to increase the amount of current the anode can discharge
• Extend the anode surface area, thus increasing the amount of current the anode can discharge; and
• Reduce consumption of the anode since the carbon becomes the part of the anode consumed before the anode itself.
Anode Backfill
• Carbon backfill for cathodic protection purposes is available as calcined petroleum or metallurgical coke, each being the product of its respective industry. Non-calcined carbon is also available, but is not suitable for CP use since it can have too high an electrical resistance.
Power Supply (T/R)
• Unlike galvanic anode systems where the natural potential difference between the anode and cathode provides the driving force for current, an impressed current CP system must be supplied with power from an external source.
Wiring and ConnectionsIn impressed current CP systems, all wiring and connections must be made to totally isolate the metal from the electrolyte. Unlike a galvanic anode system where exposed wire and connections are protected by the anode, any exposed metal in an impressed current CP system is part of the anode. Thus, exposed metal will corrode rapidly. Only cable having approved cathodic protection dielectric insulation can be used. Types of insulation found on CP cables include:
• High-Molecular-Weight Polyethylene (HMWPE) commonly used for direct burial cathodic protection installations for both anode and structure wiring.
• Halar/Polyethylene Layered Insulation.• Kynar/Modified Polyolefin.
Anode Configurations
Configurations is chosen based on calculation
Remote anodeDistributed anode
Stray Current
• Stray currents are currents flowing in the electrolyte from external sources. Any metallic structure, for example a pipe line, buried in soil represents a low resistance current path and is therefore fundamentally vulnerable to the effects of stray currents
Stray Current
• Stray current tends to enter a buried structure in a certain location and leave it in another. It is where the current leaves the structure that severe corrosion expected.
• Overprotection might also occur at a location where the high current density of stray current enter a structure.
• There are a number of source of undesirable stray currents, including foreign cathodic protection installations, dc transit systems such as electrified railways, subway systems, and streetcars, welding operations, and electrical power transmission systems.
Direct Stray Current
• Direct stray currents come from foreign cathodic protection systems, transit systems, and dc high voltage transmission line.
• Direct stray current can cause :
1. Anodic interference
2. Cathodic interference
3. Combined interference
Anodic interference• It is found in relatively close proximity to a buried anode.• At location close to anode the pipeline will pick up current. This
current will be discharged at a distance farther away from the anode.
Anodic interference
• In the current pickup region, the potential of the pipeline will shift in the negative region. It receives a boost of cathodic protection current locally. This local current boost will not necessarily be beneficial, because a state of overprotection could be created. Excess of alkaline species generated can be harmful to aluminum and lead alloys
Cathodic Interference
• Cathodic interference is produced in relatively close proximity to a polarized cathode.
• Current will flow away from the structure in region in close proximity to the cathode. The potential will shift in the positive direction where the current leaves this structure, and this area presents the highest corrosion damage risk.
• Current will flow onto the structurew over a large area at farther distance from the cathode.
• Current pickup occurs close to an anode, and current discharge occurs close to a cathodically polarized structure.
• The degree of damage of the combined stray current effects is greater than in the case of anodic or cathodic interference acting alone.
• The damage in both the current pickup (overprotection effects) and discharge regions (corrosion) will be greater.
Combined Anodic and Cathodic Interference
Controlling Stray Current Corrosion
In implementing countermeasures against stray current effects, the nature of stray currents has to be considered. For mitigating dc interference, the following fundamental steps can be taken:
• Removal of the stray current source or reduction in its output current.
• Use of electrical bonding• Cathodic shielding• Use of sacrificial anodes• Application of coatings to current pickup areas
Corrosion control of metal structure by impressed anodic current.
Interface potential of the structure is increased into passive corrosion domain.
Protective film is formed on the surface
of metal structure which decrease the corrosion rate down to its passive current.
Can be applied for active-passive metals/alloys only.
Anodic protection can decrease corrosion rate substantially.
Acid concentration, M
NaCl, M Cor. Rate μm/y (Unprotected)
Cor. Rate μm/y (Protected)
0.5 10-5 360 0.64
0.5 10-3 74 1.1
0.5 10-1 81 5.1
5 10-5 49000 0.41
5 10-3 29000 1.0
5 10-1 2000 5.3
Anodic protection of 304SS exposed to an aerated H2SO4 at 300C at 0.500 vs. SCE
Metals which can be passivated and de-activated
• The metals which can be passivated by oxidation and activated by reduction are those which have a higher oxide less soluble than a lower oxide and will thus each corrosion domain forms an angle.
• The lower the apex of this angle in the diagram (such as titanium, chromium and tin etc.), the easier it will be to passivate the metal by oxidation and it will be difficult to reactivate the passivated metals by reduction.
Anodic protection parameters :
(can be obtained from anodic polarization measurement)
Range of potential in which metal is in passivation state (protection range)
Critical current density Flade potential
Optimum potential for anodic protection is midway in the passive region
Flade potential (EF)
In which EFO : Flade potential at pH = 0
n : a constant (between 1 and 2) depends of metal composition and environment conditions
• Metals having EF < equilibrium potential of hydrogen evolution reaction (HER) can be passivated by non oxidizing acid (i.e. titanium)
• Increasing temperature will reduce the protection potential range and increase the critical current density and therefore anodic protection will be more difficult to be applied.
pH059,0nEE OFF
57
Parameters that should be considered for anodic protection
design (Flade potential is not included in the figure)
Influences of temperature and chloride concentration on anodic polarization curve of stainless steels
(schematic figure)
• For metals exposed in aggressive ions containing - environment
• Interface potential of metal should be :
Eprot>Elogam>Eflade
• Basically : Eflade is equal or slightly lower than Epp.
Schematic figure of potential range for anodic protection of a stainless steel which is susceptible to pitting corrosion in an
environment containing aggressive [email protected]
• Increasing of chloride ions concentration results in a significant decrease of protection potential range.
• Consequently, in aggressive ions containing-environment anodic protection is applied only for metals which have relatively high protection potential and high pitting potential.
• Increasing temperature leading to a decrease of [email protected]
Schematic figure of anodic protection system for protecting inner surface of
storage tank
CATHODES FOR ANODIC PROTECTION
• Should be permanent and can be used as current collector without any significant degradation.
• Having large surface area in order to suppress cathodic overpotential.
• Low cost. Platinum clad brass can be used for anodic
protection cathodes because this cathode has low overpotential and its degradation rate is very low, however it is very expensive.
Comparison of anodic and cathodic protection :
Anodic protection
Cathodic protection
Applicability Active-passive metals only
All metals
Corrosives Weak to aggressive
Weak to moderate
Relative investment cost
High Low
Relative operation cost
Very low Mediums to high
Equipment Potentiostat + cathode/s
Sacrificial anodes or DC power supply + ICCP anode/s
Throwing power
Very high Low to high
Significant of applied current
Often a direct measure of protected corrosion rate
Complex
Does not indicate corrosion rate
Operating conditions
Can be accurately and rapidly determined by electrochemical measurement
Must usually be determined by empirical testing
Typical applications of anodic protection
• Anodic protection has been applied to protect storage tanks, reactors, heat exchangers and transportation vessels for corrosive solutions.
• Heat exchangers (tubes, spirals and plates types) including their anodic protection systems can be easily to purchase in the market.
• i.e. AISI 316 SS HE is used to handle 96-98% sulfuric acid solution at 1100C. Anodic protection decreases corrosion rate of the stainless steel, initially from 5mm/year down to 0.025mm/year and therefore less contaminated sulfuric acid can be obtained.
DATA
Effect of chromium content on critical current density and Flade potential of iron
exposed in 10% sulfuric acid.
Effects of nickel and chromium contents on critical current density passivation potential in 1N and 10 N H2SO4 containing
0.5 N K2SO4
Requirement of critical protection current densities for several austenitic stainless steels (18-20 Cr , 8-12
Ni) exposed in different electrolytes
Protection current density : current density required to maintain passivity
Effect of sulfuric acid concentration at 240C on the corrosion rate and critical current density of stainless steel
Effect of stirring of electrolyte on the corrosion rate and requirement of current density to maintain passivity on a
stainless steel at 270C
Anodic Protection Using a Galvanic Cathode A cylindrical tank of 304 stainless steel for
storing deaerated sulfuric acid (pH=0) is found to corrode rapidly. To provide anodic protection, a galvanic cathode of platinum will be installed. The tank has a diameter of 5 m and the depth of acid is 5 m.
a. Draw a labeled sketch of the polarization diagram for the tank and calculate the passivation potential versus SHE.
b. What is the area of platinum required to ensure stable passivity?
c. What will the corrosion potential be when the tank achieves passivity?
Data:• 304 stainless steel:• Ecor = -0.44 V vs SCE• icor = 10-3 A/cm2
• Tafel slope anodic = 0.07 V/decade• icrit = 1.4 x 10-2 A/cm2 • ipas = 4 x 10-7 A/cm2
• H+ reduction on platinum• i0 = 10-3 A/cm2
• Tafel slope cathodic = 0.03 V/decade
• SCE = +0.2416 V vs.SHE