Coating and Inhibitors

Introduction to Corrosion MonitoringWhat is Corrosion Monitoring? The field of corrosion measurement, control, and prevention covers a very broad spectrum of technical activities. Within the sphere of corrosion control and prevention, there are technical options such as cathodic and anodic protection, materials selection, chemical dosing and the application of internal and external coatings. Corrosion measurement employs a variety of techniques to determine how corrosive the environment is and at what rate metal loss is being experienced. Corrosion measurement is the quantitative method by which the effectiveness of corrosion control and prevention techniques can be evaluated and provides the feedback to enable corrosion control and prevention methods to be optimized.

Classification of corrosion protection methods

Active corrosion protectionPassive corrosion protectionPermanent corrosion protectionTemporary corrosion protection

Protective Metallic Coatings

Metallic coatings provide a layer that changes the surface properties of the workpiece to those of the metal being applied. The workpiece becomes a composite material exhibiting properties generally not achievable by either material if used alone. The coatings provide a durable, corrosion resistant layer, and the core material provides the load bearing capability. The deposition of metal coatings, such as chromium, nickel, copper, and cadmium, is usually achieved by wet chemical processes that have inherent pollution control problems. (corrosion costs study)Alternative metal deposition methods have replaced some of the wet processes and may play a greater role in metal coating in the future. Metallic coatings are deposited by electroplating, electroless plating, spraying, hot dipping, chemical vapor deposition and ion vapor deposition. Some important coatings are cadmium, chromium, nickel, aluminum and zinc.Plating and surface treatment processes are typically batch operations, in which metal objects are dipped into and then removed from baths containing various reagents to achieve the desired surface condition. The processes involve moving the object being coated through a series of baths designed to produce the desired end product. These processes can be manual or highly automated operations, depending on the level of sophistication and modernization of the facility and the application.

Organic Protective CoatingMany paints, coatings and high performance organic coatings have been developed as a need to protect equipment from environmental damage. Of prime importance in the development of protective coatings was the petroleum industry that produced most of the basic ingredients from which most synthetic resins were developed.

Metal CladdingThe corrosion resistance of a substrate can be improved by metallurgically bonding to the susceptible core alloy a surface layer of a metal or an alloy with good corrosion resistance. The cladding is selected not only to have good corrosion resistance but also to be anodic to the core alloy by about 80 to 100 mV. Thus if the cladding becomes damaged by scratches, or if the core alloy is exposed at drilled fastener holes, the cladding will provide cathodic protection by corroding sacrificially.Cladding is prevalently applied at the mill stage by the manufacturers of sheet, plate or tubing. Cladding by pressing, rolling or extrusion can produce a coating in which the thickness and distribution can be controlled over wide ranges and the coatings produced free of porosity. Although there is almost no practical limits to the thickness of coatings that can be produced by cladding, the application of the process is limited to simple shaped articles that do not require much subsequent mechanical deformation.Aluminum CladdingAmong the principal uses are aluminum cladding in the aircraft industry, lead and cadmium sheathing for cables, lead-sheathed sheets for architectural applications and composite extruded tubes for heat exchangers. The thickness of the cladding is usually between 2% and 5% of the total sheet or plate thickness, and since the cladding is usually a softer and lower strength alloy, the presence of the cladding can lower the fatigue strength and abrasion resistance of the product. In the case of thick plate where substantial amounts of material may be removed from one side by machining so that the cladding becomes a larger fraction of the total thickness, the decrease in strength of the product may be substantial.A clad finish being soft in nature is subject to damage during manufacturing and while in service. Caution must be exercised while polishing or cleaning, since it is sensitive to harsh chemicals and abrasive materials.Cladding and Weld OverlayingCompared to carbon and alloy steels, all corrosion resistant alloys are expensive. In many cases, corrosion resistance is required only on the surface of the material and carbon or alloy steel can be clad with a more corrosion resistant alloy. Cladding can save up to 80% of the cost of using solid alloy. Cladding of carbon or low alloy steel can be accomplished in several ways including roll bonding, explosive bonding, weld overlaying and wallpapering. Clad materials are widely used in the chemical process, offshore oil production, oil refining and electric power generation industries. The use of clad steel is not new. Corrosion resistant alloy clad steel has been available for over 40 years. Almost any corrosion resistant stainless steel or nickel alloy can be bonded to steel. The steel can be clad on both sides or on one side only. The hot roll bonding process is used to produce over 90% of clad plate products.

Vapor Deposition (VD)Vapor deposition refers to any process in which materials in a vapor state are condensed through condensation, chemical reaction, or conversion to form a solid material. These processes are used to form coatings to alter the mechanical, electrical, thermal, optical, corrosion resistance, and wear properties of the substrates. They are also used to form free-standing bodies, films, and fibers and to infiltrate fabric to form composite materials. Vapor deposition processes usually take place within a vacuum chamber. There are two categories of vapor deposition processes: Physical vapor deposition (PVD) Chemical vapor deposition (CVD)

Electroless platingElectroless nickel (EN) plating is a chemical reduction process which depends upon the catalytic reduction process of nickel ions in an aqueous solution (containing a chemical reducing agent) and the subsequent deposition of nickel metal without the use of electrical energy. Due to its exceptional corrosion resistance and high hardness, the process finds wide application on items such as valves, pump parts etc., to enhance the life of components exposed to severe conditions of service ,particularly in the oil field and marine sector. With correct pretreatment sequence and accurate process control , good adhesion and excellent service performance can be obtained from EN deposited on a multitude of metallic and non-metallic substra6tes.

Electroless PlatingDue to its unique properties of excellent corrosion resistance, combined with a high wear resistance and uniformity of coating, EN finds extensive applications in a number of fields. Some of the prominent areas of application are :-- Oil & Gas: Valve components, such as Balls, Gates, Plugs etc. And other components such as pumps, pipe fittings, packers, barrels etc. Chemical Processing: Heat Exchangers, Filter Units, pump housing and impellers, mixing blades etc. Plastics: Molds and dies for injecting and low and blow molding of plastics components, extruders, machine parts rollers etc. Textile: Printing cylinders, machine parts, spinneret's, threaded guides etc. Automotive: Shock Absorbers, heat sinks, gears, cylinders, brake pistons etc. Aviation & Aerospace: Satellite and rocket components, rams pistons, valve components etc. Food & pharmaceutical: Capsule machinery dies, chocolates molds, food processing machinery components etc.

Corrosion Inhibitors

Corrosion InhibitorsThe use of chemical inhibitors to decrease the rate of corrosion processes is quite varied. In the oil production and processing industries, inhibitors have always been considered to be the first line of defense against corrosion. A great number of scientific studies have been devoted to the subject of corrosion inhibitors. However, most of what is known has grown from trial and error experiments, both in the laboratories and in the field.Rules, equations or theories to guide inhibitor development or use are very limited. A synergism, or cooperation, is often present between different inhibitors and the environment being controlled, and mixtures are the usual choice in commercial formulations.

Classification of inhibitors

Inhibitors are chemicals that react with a metallic surface, or the environment this surface is exposed to, giving the surface a certain level of protection. Inhibitors often work by adsorbing themselves on the metallic surface, protecting the metallic surface by forming a film. Inhibitors are normally distributed from a solution or dispersion. Some are included in a protective coating formulation. Inhibitors slow corrosion processes by either:Increasing the anodic or cathodic polarization behavior (Tafel slopes);Reducing the movement or diffusion of ions to the metallic surface;Increasing the electrical resistance of the metallic surface. The scientific and technical corrosion literature has descriptions and lists of numerous chemical compounds that exhibit inhibitive properties. Of these, only very few are actually used in practice. This is partly due to the fact that the desirable properties of an inhibitor usually extend beyond those simply related to metal protection. Considerations of cost, toxicity, availability and environmental friendliness are of considerable importance.

Inhibitors have been classified differently by various authors. Some authors, for example, prefer to group inhibitors by their chemical functionality. However, by far the most popular organization scheme consists in regrouping corrosion inhibitors in a functionality scheme as follows:Passivating inhibitorsCathodic inhibitorsOrganic inhibitorsPrecipitation inhibitorsVolatile corrosion Inhibitors

Passivating InhibitorsPassivating inhibitors cause a large anodic shift of the corrosion potential, forcing the metallic surface into the passivation range. There are two types of passivating inhibitors:Oxidizing anions, such as chromate, nitrite and nitrate, that can passivate steel in the absence of oxygen Non oxidizing ions such as phosphate, tungstate and molybdate that require the presence of oxygen to passivate steel.

These inhibitors are the most effective and consequently the most widely used. Chromate based inhibitors are the least expensive inhibitors and were used until recently in a variety of applications, e.g. recirculation-cooling systems of internal combustion engines, rectifiers, refrigeration units, and cooling towers. Sodium chromate, typically in concentrations of 0.04-0.1% was used for these applications.At higher temperatures or in freshwater with chloride concentrations above 10 ppm higher concentrations are required. If necessary, sodium hydroxide is added to adjust the pH to a range of 7.5-9.5. If the concentration of chromate falls below a concentration of 0.016% corrosion will be accelerated. Therefore it is essential that periodic colorimetric analysis be conducted to prevent this from occurring.

In general, passivation inhibitors can actually cause pitting and accelerate corrosion when concentrations fall below minimum limits. For this reason it is essential that monitoring of the inhibitor concentration be performed.

Cathodic inhibitors

Cathodic inhibitors either slow the cathodic reaction itself or selectively precipitate on cathodic areas to increase the surface impedance and limit the diffusion of reducible species to these areas. Cathodic inhibitors can provide inhibition by three different mechanisms as:Cathodic poisonsCathodic precipitatesOxygen scavenger.

Some cathodic inhibitors, such as compounds of arsenic and antimony, work by making the recombination and discharge of hydrogen more difficult. Other cathodic inhibitors, ions such as calcium, zinc or magnesium, may be precipitated as oxides to form a protective layer on the metal.

Oxygen scavengers help to inhibit corrosion by preventing the cathodic depolarization caused by oxygen. The most commonly used oxygen scavenger at ambient temperature is probably sodium sulfite (Na2SO3).

Organic Inhibitors

Both anodic and cathodic effects are sometimes observed in the presence of organic inhibitors but, as a general rule, organic inhibitors affect the entire surface of a corroding metal when present in sufficient concentration. Organic inhibitors usually designated as 'film-forming', protect the metal by forming a hydrophobic film on the metal surface.The effectiveness of these inhibitors depends on the chemical composition, their molecular structure, and their affinities for the metal surface. Because film formation is an adsorption process, the temperature and pressure in the system are important factors.Organic inhibitors will be adsorbed according to the ionic charge of the inhibitor and the charge on the surface. Cationic inhibitors, such as amines, or anionic inhibitors, such as sulfonates, will be adsorbed preferentially depending on whether the metal is charged negatively or positively. The strength of the adsorption bond is the dominant factor for soluble organic inhibitors.For any specific inhibitor in any given medium there is an optimal concentration. For example, a concentration of 0.05% sodium benzoate, or 0.2% sodium cinnamate, is effective in water with a pH of 7.5 and containing either 17 ppm sodium chloride or 0.5% by weight of ethyl octanol.

The corrosion due to ethylene glycol cooling water systems can be controlled by the use of ethanolamine as an inhibitor.

Precipitation Inhibitors

Precipitation inducing inhibitors are film forming compounds that have a general action over the metal surface, blocking both anodic and cathodic sites indirectly. Precipitation inhibitors are compounds that cause the formation of precipitates on the surface of the metal, thereby providing a protective film. Hard water that is high in calcium and magnesium is less corrosive than soft water because of the tendency of the salts in the hard water to precipitate on the surface of the metal and form a protective film.

The most common inhibitors of this category are the silicates and the phosphates. Sodium silicate, for example, is used in many domestic water softeners to prevent the occurrence of rust water. In aerated hot water systems, sodium silicate protects steel, copper and brass. However, protection is not always reliable and depends heavily on pH and a saturation index that depends on water composition and temperature. Phosphates also require oxygen for effective inhibition. Silicates and phosphates do not afford the degree of protection provided by chromates and nitrites, however, they are very useful in situations where non-toxic additives are required.

Volatile Corrosion Inhibitors

Volatile Corrosion Inhibitors (VCI), also called Vapor Phase Inhibitors (VPI), are compounds transported in a closed environment to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds, such as morpholine or hydrazine, are transported with steam to prevent corrosion in condenser tubes by neutralizing acidic carbon dioxide or by shifting surface pH towards less acidic and corrosive values. In closed vapor spaces, such as shipping containers, volatile solids such as salts of dicyclohexylamine, cyclohexylamine and hexamethylene-amine are used.On contact with the metal surface, the vapor of these salts condenses and is hydrolyzed by any moisture to liberate protective ions. It is desirable, for an efficient VCI, to provide inhibition rapidly while lasting for long periods. Both qualities depend on the volatility of these compounds, fast action wanting high volatility while enduring protection requires low volatility.

Electrochemical Tests to Monitoring inhibitors effectivenessWhile all laboratory corrosion tests require accelerating corrosion processes, only electrochemical tests can directly amplify the impact of corrosion processes. The main reasons why this is possible is that all electrochemical tests use some fundamental model of the electrode kinetics associated with corrosion processes to quantify corrosion rates. The amplification of the electrical signals generated during these tests has permitted very precise and sensitive measurements to be carried out. Potentiodynamic polarization methodsLinear polarization resistance (LPR)Electrochemical impedance spectroscopy (EIS)

Attributes of a Corrosion EngineerKnowledge of corrosion Knowledge of corrosion resistant characteristics of materials Knowledge of corrosive characteristics of chemicals Information on physical and mechanical properties of materials Information on availability and cost Information on fabrication techniques Knowledge of special requirement of what is being produced Proficiency in planning, executing, and interpreting test programs Ability to get along with others Common sense

Corrosion EngineerWork closely with the engineering staff in working out new designs or modifying existing ones to reduce the opportunity for corrosion. Close contact with the maintenance engineers. Work with the production department. Collaborate with accounting department in establishing the actual cost of corrosion. Work closely with the purchasing department. Assist the sales department by helping discover any deficiencies in products. Keep management informed of his need and his accomplishments so that he may be provided with the support he will require to be effective.