Material AMPTIAC CORROSION A NATURAL BUT CONTROLLABLE PROCESS

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1 24 Gretchen A. Jacobson, Managing Editor Materials Performance NACE International Houston, TX CORROSION A NATURAL BUT CONTROLLABLE PROCESS INTRODUCTION Corrosion is a naturally occurring phenomenon commonly defined as the deterioration of a substance (usually a metal) or its properties because of a reaction with its environment. [1] Like other natural hazards such as earthquakes or severe weather disturbances, corrosion can cause dangerous and expensive damage to everything from vehicles, home appliances, and water and wastewater systems, to pipelines, bridges, and public buildings. Unlike many natural disasters however, there are time-proven methods to prevent and control corrosion that can reduce or eliminate its impact on public safety, the economy, and the environment. With its many forms, causes, and associated prevention methods, corrosion obviously is highly complex and requires extensive expertise and significant resources to control. The 2001 US Federal Highway Administration-funded cost of corrosion study, Corrosion Costs and Preventive Strategies in the United States, [2] which was initiated by NACE International and conducted by CC Technologies, Inc., determined the annual direct cost of corrosion to be a staggering $276 billion or 3.1% of the gross domestic product. This finding was based on an analysis of direct costs by industry sector. When based on the cost of corrosion prevention and control, the study found that $121 billion is attributed to corrosion control methods, services, research and development, and education and training. Corrosion is so prevalent and takes so many forms that its occurrence and associated costs never will be completely eliminated; however, the study estimates that 25 to 30% of annual corrosion costs could be saved if optimum corrosion management practices were employed. The science of corrosion prevention and control is highly complex, exacerbated by the fact that corrosion takes many different forms and is affected by numerous outside factors. Corrosion professionals must understand the effects of environmental conditions such as soil resistivity, humidity, and exposure to salt water on various types of materials; the type of product to be processed, handled, or transported; required lifetime of the structure or component; proximity to corrosion-causing phenomena such as stray current from rail systems; appropriate mitigation methods; and other considerations before determining the specific corrosion problem and specifying an effective solution. The first step in effective corrosion control, however, is to have a thorough knowledge of the various forms of corrosion, the mechanisms involved, how to detect them, and how and why they occur. [3] BASIC FORMS OF CORROSION There are 10 primary forms of corrosion, but it is rare that a corroding structure or component will suffer from only one. The combination of metals used in a system and the wide range of environments encountered often cause more than one type of attack. Even a single alloy can suffer corrosion from more than one form, depending on its exposure to different environments at different points within a system. All forms of corrosion, with the exception of some types of hightemperature corrosion, occur through the action of the electrochemical cell (see Figure 1). The elements that are common to all corrosion cells are an anode where oxidation and metal loss occur, a cathode where reduction Corrosion Occurs Here A N O D E Electron Flow Current [+] from Anode Metal [Corrosion] Figure 1. The Electrochemical Cell. + Ion Flow Electrolyte Voltage Difference No Corrosion Occurs Here and protective effects occur, metallic and electrolytic paths between the anode and cathode through which electronic and ionic current flows, and an electrical potential difference that drives the cell. The driving potential may be the result of differences between the characteristics of dissimilar metals or surface conditions, and the environment, including chemical concentrations. There are specific mechanisms that cause each type of attack, different ways of measuring and predicting them, and various methods that can be used to control corrosion in each of its forms. - C A T H O D E Current [-] from Cathodic Reduction The Quarterly, Volume 7, Number 4 39

2 General Attack Corrosion Also referred to as general corrosion or uniform corrosion, general attack corrosion proceeds more or less uniformly over an exposed surface without appreciable localization (see Figure 2). This leads to relatively uniform thinning on sheet and plate materials and general thinning on one side or the other (or both) for pipe and tubing. It is recognized by a roughening of the surface and usually by the presence of corrosion products. The mechanism of the attack typically is an electrochemical process that takes place at the surface of the material. Differences in composition or orientation between small areas on the metal surface creates localized anodes and cathodes that facilitate the corrosion process. Most often caused by misapplying materials in corrosive environments, general corrosion often can be tolerated because the effect of metal loss is relatively easy to assess and allowances can be made in the initial design. Protective coatings are particularly effective in controlling uniform corrosion. Cathodic protection (CP) an electrochemical technique used for corrosion control (see Methods of Corrosion Control later in this article) can be used in underground or immersion situations. Figure 2. General Corrosion of a Steel Storage Tank. Localized Corrosion Unlike general attack corrosion, localized corrosion occurs at discrete sites on a metal surface. Types of localized corrosion include pitting, crevice, and filiform corrosion. Figure 3. Pitting Corrosion Inside of a Pipe. Pitting Pitting is a deep, narrow attack that can cause rapid penetration of the substrate (metal) wall thickness. It is characterized by corrosive attack in a localized region surrounded by noncorroded surfaces or surfaces that are attacked to a much lesser extent (see Figure 3). Pits initiate at defects in a protective or passive film. The corrosion is caused by the potential difference between the anodic area inside the pit which often contains acidic, hydrolyzed salts and the surrounding cathodic area. Pitting corrosion is controlled by using more pitresistant materials, protective coatings, and/or CP, or by modifying the environment (i.e., by introducing deaeration, chemical corrosion inhibitors, etc.). Crevice Corrosion Crevice corrosion occurs at localized sites where free access to the surrounding environment is restricted, such as crevices where materials meet either metal-to-metal or metal-to-nonmetal. Crevices also can form under deposits of debris or corrosion products. Also called concentration cell corrosion, crevice corrosion is caused by two basic mechanisms: oxygen concentration cell corrosion and metal ion concentration cell corrosion. In the first mechanism, the difference in oxygen concentration between the areas inside and outside the crevice causes a potential difference between these areas. The area within the crevice becomes anodic with respect to the outside area, where the high oxygen content drives the cathodic reaction and causes corrosion to occur deep within the crevice. In metal ion concentration cell corrosion, the difference in potential between the inside and outside of the crevice is caused by a difference in metal ion concentration. When this mechanism occurs, corrosion usually is concentrated at the entrance of the crevice. Crevice corrosion control is complicated by the difficulty of reaching the environment within the crevice. The principal options for corrosion control in this instance are appropriate materials selection, improved design (by eliminating as many crevices as possible), the use of coatings to seal crevices, and CP. Filiform Corrosion Filiform corrosion is a special form of oxygen cell corrosion occurring beneath organic or metallic coatings on materials. This form is recognizable by the appearance of a fine network of random threads of corrosion product beneath the coating material (see Figure 4). Filiform corrosion is associated with mild surface contamination of solid particles deposited from the atmosphere or residue on the metal surface after processing. When exposed to humid conditions (relative humidity greater than 60%), these surfaces often will suffer filiform corrosion. Corrosion proceeds because of the potential difference between the head of the advancing filament, which becomes anodic, with a low ph and lack of oxygen compared to the cathodic area immediately behind the head. This type of corrosion, particularly on painted surfaces, can be prevented by proper cleaning and preparation of the metallic surface and then applying a protective coating to a clean, dry surface. Figure 4. Example of Filiform Corrosion. 40 The Quarterly, Volume 7, Number 4

3 A DVANCED M A TERIALS AND P ROCESSES T ECHNOLOGY Table 1. Galvanic Series for Metals in Seawater. Active (More Negative) End Magnesium Zinc Aluminum Alloys Carbon Steel Cast Iron 13% Cr (Type 410 SS (Active) 18-8 (Type 304) SS (Active) Naval Brass Yellow Brass Copper Copper-Nickel Alloy 13% Cr (Type 410) SS (Passive) Titanium 18-8 (Type 304) SS (Passive) Graphite Gold Noble (More Positive) End Platinum Source: NACE International Basic Corrosion Course Handbook, p. 2:17. Galvanic Corrosion Galvanic corrosion occurs because of the potential differences between different metals when they are in electrical contact and exposed to an electrolyte. It also can occur between a metal and an electrically conductive nonmetal such as graphite. The rate of attack of one metal or alloy usually is accelerated; in contrast, the corrosion rate of the other usually is decreased. Galvanic corrosion often is pronounced where the different materials are immediately adjacent to one another. The mechanism is a classic electrochemical cell electrons flow through a metallic path from sites where anodic reactions are occurring to sites where the electrons cause cathodic reactions to occur. At the same time, there is a migration of ions (charged particles) in the electrolyte. The positive ions are charged metal particles dissolved at the anode; loss of these ions causes the corrosion of the anode. The negative ions come from the cathodic reactions. Thus, the anode corrodes and the cathode does not in fact, the cathode is protected against corrosion. There also is a voltage, or potential, difference between the anode and cathode (Figure 1). The cell is driven by the potential differences between metals or electrically conductive nonmetals when exposed to an electrolyte. A galvanic series can be used to determine the likely interactions between adjoining metals. Table 1 shows an example of a galvanic series for metals in seawater. When two metals are connected together, the more active one becomes the anode (corroding) and the less active one the cathode. Galvanic corrosion can be controlled with materials selection, barrier coatings, CP, modification of the environment, electrical isolation (the connection between dissimilar metals is insulated to break the electrical Figure 5. Micrograph of SCC Fracture Surface. continuity), and design. When designing the system, unfavorable area ratios should be avoided by using metal combinations in which the more active metal or alloy surface is relatively large. Rivets, bolts, and other fasteners should be of a more noble metal than the material to be fastened. Environmental Cracking Unlike many other forms of corrosion where corrosion occurs over long periods of time, environmental cracking can occur very rapidly. Because it is unanticipated, it can be catastrophic. Environmental cracking is the brittle failure of an otherwise ductile material caused by the combined action of corrosion and tensile stress. It can be identified by tight cracks that are at right angles to the direction of maximum tensile stress. Types of environmental cracking include stress corrosion cracking (SCC), hydrogen-induced cracking (HIC), liquid metal embrittlement (LME), and corrosion fatigue (CF). There are many ways to control the various forms of environmental cracking, including materials selection, modification of the environment, protective coatings, CP, reduction in residual surface stress, and by changing the design to lower tensile stress levels. Stress Corrosion Cracking An anodic process, SCC occurs in metals exposed in an environment where no damage would result if tensile stresses were reduced or absent. Examples of media that promote SCC of specific alloys include strongly alkaline solutions with carbon steel (CS), chlorides with stainless steel (SS), and ammonia (NH 3 ) with copper alloys. Usually there is incubation period during which cracking initiates on a microscopic level, followed by propagation (see Figures 5 and 6). Typically there is little metal loss or general corrosion associated with SCC. Hydrogen-Induced Cracking HIC results in the brittle failure of otherwise ductile materials when exposed to an environment where hydrogen can enter the metal. It is caused by the combined action of tensile stress and hydrogen. A cathodic phenomenon, HIC occurs when the normal evolution of hydrogen at cathodic sites is inhibited and the atomic hydrogen in the cathodic reaction enters the metal. Higherstrength alloys (those with a tensile strength of 1,034 MPa or greater) are more susceptible to HIC than lower-strength alloys. Sulfide stress cracking (SSC) is a specific form of HIC wherein the presence of sulfides suppresses the evolution of hydrogen. This is a common problem in sour service processes or conditions involving wet hydrogen sulfide (H 2 S) such as in oil fields. Figure 6. SCC Induced Failure of a Tube. The Quarterly, Volume 7, Number 4 41

4 Liquid Metal Embrittlement LME is defined as the decrease in strength or ductility of a metal or alloy as a result of contact with a liquid metal. A normally ductile material under tensile stress while in contact with a liquid metal may exhibit brittle fracture at low stress levels. Unlike fracture by SCC, LME is not time dependent cracking can begin immediately upon application of stress. Embrittling agents cause different reactions depending on the alloy; for example, SS are quite resistant to degradation when contacted by liquid metal whereas mild CS and copper-based alloys can become severely embrittled. Corrosion Fatigue Corrosion fatigue (CF) is caused by the combined action of a cyclic tensile stress and a corrosive environment (see Figure 7). It is characterized by a premature failure of a cyclically loaded part. The petroleum industry encounters CF problems in the production of oil the exposure of drill pipes and sucker rods to brines and sour crudes causes costly failures and loss of production. Flow-Assisted Corrosion Flow-assisted corrosion is caused by the combined action of corrosion and fluid flow. This includes erosion-corrosion, impingement, and cavitation. Erosion-Corrosion This form of corrosion can occur in flowing liquids or gases with or without abrasive particles (see Figure 8). The velocity of the flow is sufficient to remove weakly adhering corrosion products from the surface or by damaging the protective oxide film, reducing the protective effect of each, and also pit or otherwise remove substrate. Mechanical erosion is caused by hard particles impacting the surface, causing craters in the metal. If erosion-corrosion can be identified and there is no evidence of particle impingement, a possible solution is to reduce the flow rate or remove flow-disturbing surface discontinuities. Impingement Impingement is caused by turbulence or impinging flow (directed at roughly right angles to the materials). Entrained air bubbles tend to accelerate this action, as do suspended solids. This type of corrosion occurs in pumps, valves, and orifices; on heat exchanger tubes; and at elbows and tees in pipelines. It usually produces a pattern of localized attack with directional features. When a liquid is flowing over a surface, there usually is a critical velocity below which impingement does not occur and above which it rapidly increases. Impingement first received attention because of the poor behavior of some copper alloys in seawater. Cavitation This mechanical damage process is caused by collapsing bubbles in a flowing liquid, usually forming deep aligned pits in areas of turbulent flow. Cavitation occurs when protective films are removed from a metal surface by high pressures generated by the collapse of gas or vapor bubbles. In general, higher-strength alloys are more resistant to this type of corrosion than lower-strength alloys. When cavitation damage is caused primarily by corrosion following the removal of protective films, the corrosion portion of the damage may predominate. Under extreme cavitation conditions, the cavitation itself is capable of removing the metal directly and corrosion effects are insignificant. Figure 8. Failure of an Impeller Due to Erosion Corrosion. With all forms of flow-assisted corrosion, proper materials selection for a particular environment is crucial to preventing damage. Other methods include modification of the environment, protective coatings, CP, and controlling flow velocity and patterns through design. Figure 7. Example of a Fracture Surface Resulting From Corrosion Fatigue. Intergranular Corrosion Intergranular corrosion is the preferential attack at, or adjacent to, the grain boundaries of a metal. Almost all engineering metals are composed of individual crystals, or grains, that meet at areas of relative 42 The Quarterly, Volume 7, Number 4

5 A DVANCED M A TERIALS AND P ROCESSES T ECHNOLOGY impurity and misalignment. Intergranular corrosion occurs when the grain boundries or areas directly adjacent to them are anodic to the surrounding grain materials. This can happen because of differences in impurity levels or strain energy of the misalignment of atoms in the grain boundaries. In some cases, individual grains are loosened and lost from the material. In other cases, the localized loss of grain boundary material causes localized attack similar in appearance to cracking. Intergranular corrosion can be controlled using proper material selection, design, fabrication, and weld procedures; modification of the environment; and heat treatment to dissolve the undesirable constituents at the grain boundaries. Dealloying Dealloying is a corrosion process in which one constituent of an alloy is removed preferentially, leaving an altered residual structure. Many alloys consist of mixtures of elements (i.e., zinc and copper are alloyed to produce brass), where one element can be anodic with respect to the other element(s) and can selectively corrode by galvanic action. This phenomenon is commonly detectable as a color change or drastic change in mechanical strength. For example, brasses will turn from yellow to red and cast irons will turn from silvery gray to dark gray. Various laboratory techniques, such as cross-sectioning the part in question, can provide evidence of color changes. Metallographic examination at high magnification and x-ray spectroscopy also can provide positive identification. It can be controlled through materials selection, modification of the environment, protective coatings, CP, and design (e.g., controlling temperature to minimize hot-wall effects in heat exchangers). Fretting Corrosion Fretting corrosion is defined as metal deterioration caused by repetitive slip at the interface between two surfaces in contact that were not intended to move in that fashion. The motion between surfaces either removes protective films or, combined with the abrasive action of corrosive products, mechanically removes material from surfaces in relative motion. For fretting to occur, the interface must be under load and the motion must be sufficient for the surfaces to strike or rub together. Results of fretting include metal loss in the area of contact; production of oxide and metal debris; galling, seizing, fatiguing, or cracking of the metal; loss of dimensional tolerances; loosening of bolted or riveted parts; and destruction of bearing surfaces. Techniques for controlling this type of corrosion include materials selection, designing to avoid motion between surfaces, and the use of lubricants such as molybdenum disulfide. High-Temperature Corrosion Direct chemical reactions, rather than the reactions of the electrochemical cell, are responsible for the deterioration of metals by high-temperature corrosion. The actual temperature at which corrosion occurs depends upon the material and the environment, but corrosion usually starts when the temperature is within approximately 30 to 40% of the alloy s melting point. High-temperature corrosion usually is associated with the formation of thick oxide or sulfide scales, with reactions that cause internal swelling of the metal. It is dependent on reactions that include oxygen effects, sulfidation, carburization, decarburization (hydrogen effects), halide effects, and molten-phase formation. Methods to control this type of corrosion are largely confined to materials selection and design, although limited modification of the environment can be achieved and protective coatings can be effective. METHODS OF CORROSION CONTROL As is true of the various forms of corrosion, there are many different methods of corrosion prevention and control. Each offers its own complexities and purposes. In general, the approach to control most corrosion is to understand the corrosion mechanism involved and remove one or more of the elements of the corrosion cell; for example, by electrically separating the anode and cathode from each other or from the electrolytic environment by reducing the driving potential. The most commonly used corrosion control methods include materials selection and design using corrosion-resistant alloys, plastics, and polymers; organic and metallic protective coatings; CP; and corrosion inhibitors. All of these methods are appropriate for controlling corrosion in certain situations and not for others. They often are used together to solve a particular corrosion problem (for example, protective coatings and CP are a common and effective combination). A complete description of all the variations of corrosion control methods and systems is beyond the scope of this article. Brief descriptions of the most common methods follow. Materials Selection & Design There is no one material resistant to all corrosive situations but materials selection is critical to preventing many types of failures. When selecting a material, the required characteristics need to be defined in advance. If no material has every characteristic that a specific project requires, a corrosion control system will be needed or the service conditions must be adjusted to meet the characteristics of the candidate material. Factors that influence materials selection are listed in Table 2. Appropriate system design also is highly important for effective corrosion control. Design includes the consideration of many factors, such as materials selection, process and construction parameters, geometry for drainage, avoidance, or electrical separation of dissimilar metals, avoiding or sealing of crevices, corrosion allowance, operating lifetime, and maintenance and inspection requirements. Protective Coatings Putting a barrier between a corrosive environment and the material to be protected is a fundamental method of corrosion control. There are many organic and metallic coating systems to choose from, and they are available in various combinations. Coating system selection is similar to materials selection in that many factors need to be considered as seen in Table 3. Common coating application methods include brush or roller, spray, and dipping. In addition to proper coating selection and application The Quarterly, Volume 7, Number 4 43

6 Table 2. Factors Affecting Materials Selection. Corrosion resistance in the environment Availability of design and test data Mechanical properties Cost Availability Maintainability Compatibility with other system components Life expectancy Reliability Appearance methods, substrate preparation is critical to the success of the coating. The majority of coating failures are caused either completely or partially by faulty surface preparation, [3] such as leaving contaminants on the surface or having an inappropriate surface morphology. Cathodic and Anodic Protection CP is an electrochemical technique used on facilities like pipelines, underground storage tanks, and offshore structures that makes the structure to be protected a cathode relative to an external anode that discharges protective current to all exposed surfaces. The source of the protective current may be an active (impressed current) or passive (sacrificial) system of galvanic anodes (usually magnesium, aluminum, or zinc) (see Figure 9). CP is widely used in several environments, including water and soil. It often is used in combination with coatings that reduce the exposed surface area to receive protective current. Anodic protection has more limited, but important, applications in chemical environments. It is achieved by maintaining an activepassive metal or alloy in the passive region by an externally applied anodic current. Corrosion Inhibitors Corrosion inhibitors are substances that, when added to an environment, decrease the rate of attack. Inhibitors are commonly added in small amounts to acids, cooling waters, steam, and other fluids, either continuously or intermittently. They generally control corrosion by forming thin films that modify the environment at the metal surface. Some retard corrosion by adsorption to form a thin, invisible film only a few molecules thick. Others form bulky precipitates that coat the metal and NACE International The Corrosion Society Founded in Houston, Texas, in 1943 by 11 pipeline corrosion engineers, NACE International now has approximately 15,000 members worldwide and is involved in every industry and area of corrosion prevention and control. With the mission of providing education and communicating information to protect people, assets, and the environment from the effects of corrosion, NACE offers a wide variety of activities, services, and benefits to the corrosion control community: More than 1,500 NACE members participate in the activities of more than 250 technical committees to write and publish reports and standards, and exchange technical information. The NACE Annual Conference and Exhibition, held each spring, attracts approximately 6,000 attendees from around the world for a week of technical symposia, meetings, training opportunities, and exhibits of the latest products and services for corrosion control. Other conferences and symposia, including the popular Pipeline Integrity Management Symposium series, are held on a recurring basis. NACE education and training courses, including the renowned Coating Inspector Program, cover a variety of corrosion control technologies and levels of certification and are held all over the world. To date, more than 10,000 people have been certified or professionally recognized by NACE. NACE members receive the monthly journal Materials Performance and many subscribe to CORROSION, the journal of corrosion science and technology. NACE offers the world s largest selection of books on corrosion control, along with software and other products. NACE Public Affairs is working with industry, academia, and government to increase the visibility of the Society and the critical importance of corrosion prevention and control. For more information about NACE, visit 44 The Quarterly, Volume 7, Number 4

7 A DVANCED M A TERIALS AND P ROCESSES T ECHNOLOGY Power Source Offset Current Provides Electrons Table 3. Coating System Selection Factors. Types of exposure Operating conditions Substrate Ambient conditions during application Environmental regulations Cost Application during operation or shutdown Time constraints New construction or maintenance Shop or field application Design/fabrication considerations A N O D E Cations [+] from Anode A. Impressed Current CP Electron Flow + Ion Flow - Cations [ ] from Cathodic Electrons Protected Structure [Cathode] protect if from attack. A third mechanism consists of causing the metal to corrode in such a way that a combination of adsorption and corrosion product forms a passive layer. CORROSION CONTROL OF THE FUTURE Society will continue to face critical challenges in corrosion prevention and control, where aging equipment and infrastructure, new product formulations, environmental requirements, and strict budgets require corrosion control programs that are designed by highly skilled professionals for specific situations. The fields of corrosion science and engineering are of utmost importance to develop the experience and tools necessary to successfully reduce the incidence, problems, and expense caused by corrosion. By following appropriate strategies and obtaining sufficient resources for corrosion programs, the best engineering practices can be achieved. The payoff includes increased public safety, reliable performance, maximized asset life, environmental protection, and more cost-effective operations in the long run. Galvanic Anode B. Galvanic Anode CP Cation [+] from Anode Metal Electron Flow Figure 9. Cathodic Protection Methods. + Ion Flow - Cations [ ] from Cathodic Electrons Protected Structure [Cathode] REFERENCES [1] Corrosion Basics An Introduction, L.S. Van Delinder, ed. (Houston, TX: NACE, 1984) [2] G.H. Koch, M.P.H. Brongers, N.G. Thompson, Y.P. Virmani, J.H. Payer, Corrosion Costs and Preventive Strategies in the United States (Washington D.C.: FHWA, 2001) [3] NACE International Basic Corrosion Course Handbook (Houston, TX: NACE, 2000) Ms. Gretchen A. Jacobson has been in the business of magazine writing, editing, and production for more than 20 years. She began her career at Petersen Publishing Company in Los Angeles, California, where she worked for several specialty automotive and outdoor enthusiast magazines. She was production manager of a group of lifestyle, business, and arts magazines in California before moving to Houston, Texas, in She was the science writer and editor for a National Science Foundation-funded Science and Technology Center on parallel computation research that was based at Rice University with affiliated universities and government laboratories around the country. She has been the managing editor of NACE International s membership magazine, Materials Performance, since The Quarterly, Volume 7, Number 4 45