NACE Basic Corrosion Course
National Association of Corrosion Engineers, Houston, Texas, 1970.
This NACE Basic Corrosion Course has been prepared under the direction of the National Association of Corrosion Engineers to help provide a broader program for corrosion education for its membership and the industries which have corrosion problems. (reference)
The preparation of this course has been assigned to a special committee composed of former NACE Presidents-C. G. Munger, Amercoat Corporation; W. H. Burton, Allied Chemical Corporation; E. C. Greco, Consultant; and Aaron Wachter, Consultant. This committee has served for two years under the Chairmanship of Professor Anton deS. Brasunas of the University of Missouri-Rolla. They secured the services of numerous NACE members who contributed time and effort in preparing this seventeen-chapter text on corrosion which is intended to be used by individuals throughout the world as a correspondence course. For that reason, the writing and preparation was so patterned as to maximize the clarity of the discussion and minimize the need for any additional instruction.
This text is liberally illustrated to help clarify various situations and the language which was used throughout the text has been made as nonmathematical and nontechnical as possible to discuss this very technical subject in a relatively easy-to-understand manner. As corrosion technology develops and as the need for additional, new, and more detailed information becomes necessary, NACE will then consider the preparation of a more advanced text, as well as preparing up-to-date and supplementary data for this basic course.
NACE gratefully acknowledges the assistance of the various authors and editorial committee who helped make this course possible and we trust that their arduous efforts will be amply rewarded by the realization that this course will be of considerable benefit to the nation, to numerous industries, and to individuals all over the world who will take advantage of the opportunity afforded them by this educational effort. (back)
Anton deS. Brasunas, Editor, NACE Basic Corrosion Course October, 1970
Chapter 1 - The Scope and Language of Corrosion
By Anton deS. Brasunas and Norman E. Hamner
When a study of corrosion is undertaken, it may be natural to think that corrosion is a simple single reaction and that when understood, it can be turned off like a spigot.
If cost and availability were not factors, we could select the very best materials and come close to doing just that. But let us dismiss consideration of materials like gold or platinum and think in terms of practical substances that we can afford to use in our homes, industries, automobiles, etc.
Practical materials like iron and steel, aluminum and copper alloys, plastics, ceramics, wood, refractory metals, stainless steels and many other modern alloys and superalloys, all have certain advantages as well as disadvantages. A selection of one of these alloys or classes of alloy can be a "best choice" for a certain application. Learning when to choose which, comes with experience and knowledge. This is a part of what we hope you can learn from this course. (back)
Chapter 2 - Introduction to Corrosion
By F. L. LaQue
Ordinarily, people become engaged in controlling or preventing corrosion by appointment rather than as the final step in a process of formal education having this as its original goal. This course is designed to be helpful to that segment of such a group entering this field without the benefit of any extensive training in the basic sciences related to corrosion, but who may be called upon from time to time to take at least the first steps in anticipating, diagnosing and otherwise dealing with corrosion problems, either on their own or in collaboration with others.
While corrosion processes form an interesting basis for scientific studies which are frequently undertaken as exercises in chemistry, and particularly electrochemistry, by far the greatest interest in, and concern for corrosion stems from its practical effects and how they may be avoided. Various estimates have been made of the annual economic loss resulting from corrosion. There is no general agreement as to just what should be included in calculating this loss, for example, should we include the coating on tin cans which would not be needed if the contents were not corrosive to steel. It is, therefore, fruitless to argue about the figure that should be used. However, there is ample evidence that annual losses attributable to corrosion in North America amount to several billions of dollars and, depending on what is included in the estimate, could well surpass the $10 billion
Chapter 3 - Corrosion-Related Chemistry and Electrochemistry
By N. D. Greene
Corrosion as a Chemical Reaction
Although at first sight the corrosion of metals may appear to be a rather complex process, the general reaction can be readily understood by considering elementary chemical principles. Almost all of these have been discussed in your high school chemistry courses and therefore their application to the phenomenon of corrosion should be relatively simple. In this chapter, the principles of chemistry and chemical reactions important to the understanding of corrosion are discussed. Since space limitations prevent a complete review of basic chemistry, it is suggested that if necessary, a high school chemistry text or review book be utilized for additional study purposes. (back)
Chapter 4 - Atmospheric Corrosion
By Kenneth G. Compton
Corrosion in the atmosphere usually occupies a minor position in the writings of scientists and corrosion engineers dealing with the basic subject. The corrosion phenomena in chemical plants, underground structures and, to a lesser degree, in sea water or at elevated temperatures seem to offer more glamour and to be more spectacular. Actually, most of the destructive damage to equipment and structures caused by corrosion occurs in the atmosphere. The large segment of the vast paint industry concerned with the manufacture and application of its products for the protection of metals and the large scale operations of the galvanizing industry attest to the -importance of controlling atmospheric corrosion.
Most of the broad forms of corrosion occur in the atmosphere and some appear to be largely restricted to it. Since the corroding metal is not bathed in large quantities of electrolyte, most atmospheric corrosion operates in highly localized corrosion cells. Calculation of the electrode potentials on the basis of ion concentration, the determination of polarization characteristics and other electrochemical operations are not possible as contrasted to the situation in liquid immersion types of corrosion. Yet, all of the electrochemical factors significant in corrosion processes operate in the atmosphere, so their comprehension is vital to an understanding of its operation. (back)
Chapter 5 - Principles of Cathodic Protection
by A. W. Peabody
The purpose of this chapter is to introduce the student to cathodic protection, one of the widely used "tools" for the control of electrochemical corrosion throughout industry. After a brief description of the meaning of cathodic protection and how it works, the balance of the chapter will consist of a discussion of the practical aspects of cathodic protection systems as well as various factors that may influence cathodic protection designs.
As a first step, it is necessary to agree on a definition for "cathodic protection" so that a basis will be established on which can be built a better knowledge of the manner in which it functions and of its practical use.
A cathode is the electrode where reduction (and practically no corrosion) occurs. Prior to applying cathodic protection, most corroding structures will have both cathodic areas and anodic areas (those areas where corrosion is occurring, see glossary). It follows, then, that if all anodic areas can be converted to cathodic areas, the entire structure will become a cathode and corrosion will be eliminated.
The second step is to show how application of direct current electricity to a corroding metallic structure can cause it to become a cathode throughout its area. To begin with, direct current electricity is associated with the corrosion process on a buried or submerged metallic structure. (back)
Chapter 6 - Corrosion by Soils
By Marshall E. Parker, P.E.
As was mentioned in previous chapters, various metals have varying tendencies to corrode as illustrated by their relative positions on the Galvanic or Emf Series. Since this chapter will center largely around iron, it is well to note that iron is intermediate in these lists. Furthermore, it has been shown that pure iron, wrought iron, or mild steel behave just about the same in underground situations.
It has been observed that iron pipe buried in bone dry soil suffers little or no corrosion. However, because of rain, natural springs, rivers, etc. soils are rarely dry, especially beneath the surface, so we must consider real cases. Even if we considered the simple case of moisture from natural rain, we would soon be dealing with the complex solutions similar to well water, river water, etc. All soils contain a variety of mineral matter, some of which is soluble to a greater or lesser degree and we must deal with these "solutions".
Had there been no minerals to dissolve in rainwater, corrosion by soils would be almost nil. Iron in pure water does not corrode, but when there is oxygen dissolved in it the situation is very different. Furthermore, when other substances dissolve in it the situation becomes even more complex. An explanation of corrosion behavior under a variety of conditions is not easy to give, but one important factor is the electrical conductivity of the medium which surrounds the metal being corroded. Pure water is a very poor electrical conductor but as substances dissolve in it, particularly those which ionize, conductivity rises sharply. As it will be seen later, the relationship between conductivity and corrosion is a very logical one. (back)
Chapter 7 - Corrosion by Water and Steam
By Warren E. Berry
A major use of water in industry is the transfer of heat and the production of steam. There is extensive use of cooling water in almost every manufacturing process, in commercial air conditioning, and even a fair percentage of domestic air conditioning. Fossil and nuclear-fueled steam plants and attendant steam generation dominate the power generating field.
Water is corrosive to most metals. Pure water without dissolved gases (as O2, CO2, SO2) in it does not cause undue corrosion attack of most metals and alloys at temperatures up to the boiling point of water. Even at temperatures of about 850 F (450 C) almost all the common structural metals, except the light metals aluminum and magnesium, possess adequate corrosion resistance to highpurity water and steam.
Naturally-occurring or man-made contaminants make water corrosive. The most significant of these contaminants is oxygen from the air that is dissolved in the water. As described in Chapter 3, oxygen is a cathodic depolarizer that reacts with and removes reaction products from the cathode during electrochemical corrosion, thereby permitting the attack to continue.
Other contaminants that are often found in water and which contribute to corrosion include chloride salts (usually sodium chloride-common table salt) from the sea, wells, or industrial sources; sulfides from wells, mining wastes, or sewage; and carbon dioxide from combustion-products in the air or certain water treatment practices. Of course, many other man-made contaminants can be found in local areas where industries discharge their wastes into streams.
This chapter will discuss the corrosion problems associated with natural and treated waters at temperatures ranging from room temperature up to 1000 to 1100 F (540 to 600 C); the means of preventing or slowing down corrosion; and the best choice of materials for each environment. (back)
Chapter 8 - Localized Corrosion
By Hugh P. Godard
Localized corrosion can be defined as selective removal of metal by corrosion at small special areas or zones on a metal surface in contact with a liquid environment. Note that this discussion is limited to corrosion in liquids, whereas pitting and other forms of localized corrosion can also occur in other environments. It usually occurs under conditions where the largest part of the original surface either is not attacked or is attacked to a much smaller degree than at the local sites.
The most common type of localized corrosion is pitting, in which small volumes of metal are removed by corrosion from certain areas on the surface to produce craters or pits. Pitting corrosion may occur on a metal surface in a stagnant or slow moving liquid. It also may be caused by crevice corrosion, poultice corrosion, deposition corrosion, cavitation, impingement and fretting corrosion.
Another common type is intergranular corrosion (sometimes called "intercrystalline corrosion"). In this form, a small volume of metal is preferentially removed along paths that follow the grain boundaries to produce what might appear to be fissures or cracks. The same kind of subsurface fissures can be produced by transgranular corrosion (sometimes called "transcrystalline corrosion"). In this, a small volume of metal is removed in preferential paths that proceed across or through the grains. This occurs only under certain conditions and with certain alloys.
Intergranular and transgranular corrosion sometimes are accelerated by tensile stress. In extreme cases, the cracks proceed entirely through the metal, causing rupture or perforation. This condition is known as "stress corrosion cracking", a subject that will be dealt with in Chapter 10. Intergranular and transgranular subsurface cracks also can be produced by hydrogen. Caustic embrittlement and corrosion fatigue are two other mechanisms of metal deterioration which form fissures at or beneath the surface.
In a completely different type of corrosion which may become localized one of the metals in an alloy may be selectively leached out without producing visible pits or cracks, and without changing the dimensions of the metal. At a casual glance the metal may appear to be intact. Under a microscope it can be seen to be porous. The mechanical properties of the alloy are greatly reduced by the selective attack. The most common example of this type is dezincification of brass in which the zinc is selectively dissolved out of the alloy. Another case is "graphitic corrosion" of cast iron, in which the iron is selectively dissolved or leached away leaving a porous mass apparently intact but in reality consisting largely of graphite. (back)
Chapter 9 - Fundamentals of Inhibitors
By Norman Hackerman and E. S. Snavely
Definition of Corrosion Inhibitor
An inhibitor is a substance which retards or slows down a chemical reaction. Thus, a corrosion inhibitor is a substance which, when added to an environment, decreases the rate of attack by the environment on a metal. Corrosion inhibitors are commonly added in small amounts to acids, cooling waters, steam and other environments, either continuously or intermittently to prevent serious corrosion.
It would be awkward to include mechanisms of inhibition in the definition of a corrosion inhibitor because inhibition is accomplished by one or more of several mechanisms. Some inhibitors retard corrosion by adsorption to form an invisibly thin film only a few molecules thick; others form visible bulky precipitates which coat the metal and protect it from attack. Another common mechanism consists of causing the metal to corrode in such a way that a combination of adsorption and corrosion product forms a passive layer. We also include in the definition those substances which, when added to an environment, retard corrosion but do not interact directly with the metal surface. This type of inhibitor causes conditions in the environment to be more favorable for the formation of protective precipitates or it removes an aggressive constituent from the environment.
The use of corrosion inhibitors has grown to be one of the foremost methods of combating corrosion. To use them effectively, the corrosion engineer must, first of all, be able to identify those problems which can be solved by the use of corrosion inhibitors. Second, he must consider the economics involved, viz., whether or not the loss due to corrosion exceeds the cost of inhibitor and maintenance and operation of the attendant injection system.
Third, he must consider the compatibility of inhibitors with his process to avoid adverse effects such as foaming, decreases in catalytic activity, degradation of another material, loss of heat transfer, etc. Finally, he must apply the inhibitor under conditions which produce maximum effect. This chapter on inhibitor fundamentals was written with the above tasks in mind. Corrosion inhibitors are discussed from four points of view:
Chapter 10 - Stress Corrosion
By Hugh L. Logan
Mechanical forces, that is, tensile or compressive forces, will have little if any effect on the overall corrosion of metals as measured for example in mils per year corrosion penetration. However, a combination of tensile stresses and a corrosive environment is one of the most important causes of failures of metal structures. This type of attack is properly known as "stress corrosion cracking." It is defined as the spontaneous failure of metals as the result of the combined action of a corrosive environment and tensile stresses, either applied or residual. In brass it has been called "season cracking" and in low-carbon steels, "caustic embrittlement." It is a fairly common cause of failure in most common alloys and has even been reported as a cause of the cracking of a gold ring found buried in the soil.
Stress corrosion cracking was first extensively studied in small arms brass cartridge cases. Explosions in riveted steam boilers were believed to be triggered by stress corrosion cracking called caustic embrittlement because of caustic deposits found adjacent to the cracks. Construction of steel boilers by welding rather than riveting has reduced but by no means eliminated this type of failure. The development of numerous stress corrosion cracks in welded steel structures used in producing gas (by coal distillation) was reported in England. Recently in the United States, the failure of a large interstate natural gas pipeline was attributed to stress corrosion cracking.
Leaks that develop in stainless steel heat exchangers and other stainless steel equipment used in the petrochemical industries are believed to be due generally to stress corrosion cracking. Even titanium alloys, considered to be highly inert to general corrosion, will fail by stress corrosion cracking when stressed and in contact with sea salt above 550 F (290 C) and under some conditions in contact with dilute chlorides, wood alcohol or some other materials at room temperature.
The foregoing paragraphs may suggest, and it is fortunately true, that only a limited number of corrodents will produce stress corrosion cracking in a given material. Table 10-1 gives a brief list of corrodents that are known to have produced stress corrosion cracking in the more common alloys. (back)
Chapter 11 - Metallurgical Factors Affecting Corrosion
By R. F. Hochman
Since metals are the principal material suffering corrosive deterioration, it is important to develop a background in the principles of metallurgy to fully understand corrosion.
General Characteristics of Metals
Nearly all metals and alloys exhibit a crystalline structure. The atoms which make up a crystal exist in an orderly three dimensional array. There are solid materials, principally glass, that exist in an amorphous state. However, only crystals have the unique condition in which atoms are geometrically and uniformly arranged in all three dimensions. The unit cell is the smallest portion of the crystal structure which contains all talc, geometric characteristics of the crystal. It can be considered the smallest building block of the crystal. The crystals, or grains, of a metal are made up of these unit cells repeated in a three dimensional array.
The crystalline nature of metals is not readily obvious because the metal surface usually conforms to the shape in which it has been cast or formed. Therefore the crystalline nature of metals is difficult to understand since the usual concept of a crystal is a geometrically shaped object. In some rare instances, this crystallinity can be observed naturally, i.e., brass door knobs are normally bright and shiny, however, after a time, the corrosive perspiration from hands etches the crystalline features of the alloy on the surface. Metal crystals may be precipitated in the cold zones of liquid metal systems due to mass transfer phenomena. Controlled etching with selected electrolytes will normally show the granular characteristics of metals and alloys. (back)
Chapter 12 - Corrosion at High Temperatures
By John J. Moran
The behavior of materials at elevated temperatures is becoming of increasing technological importance, yet, it is a problem man has had to face and solve from the very beginnings of his existence. Understanding the behavior of metals at elevated temperatures and especially their corrosion behavior has only comparatively recently become an object of scientific investigation. Techniques for studying reactions at high temperatures had to be developed. It is obviously difficult to observe the actual reaction between gases and metals at high temperatures and watch the reaction products build up. It is easier to measure the change in weight after some interval or even to measure the weight change continuously during the test. But these techniques are fairly recent developments. Formerly, one could only observe the appearance of the scale after the test was concluded and examine it carefully after it had cooled.
How it had changed, how it actually had appeared at any given temperature was the subject of much speculation but little accurate evidence was available. And, as has subsequently become clear, the appearance of a scale after cooling to room temperature may be different from the same scale at a high temperature. Many scales which are continuous and thus protective at a given temperature will flake off or spall upon cooling and hence create a misleading impression of their effectiveness during continuous service.
It is not surprising, therefore, that the first quantitative approach to oxidation behavior was made in the early nineteen twenties with the postulation of the parabolic rate theory of oxidation by Tammann and, independently, by Pilling and Bedworth,2 nor that a more formal treatment of the problem would be delayed another decade, until the middle nineteen thirties, when Wagner' presented his theory of oxidation. Modifications and alternative theories continue to appear and presumably will continue to appear for a considerable time, because there are still many gaps in the theory; particularly in its application to practical problems and service experience involving complex systems.
In the following discussion we shall consider first the behavior of pure metals when exposed to oxidizing conditions at elevated temperatures and secondly the modifying effect of alloying additions upon the performance of the base metals. At least qualitatively the correlation with theory will be shown. In this fashion it is hoped that an appreciation of the important factors controlling oxidation behavior will be gained so that later when more complex alloys exposed to more complicated corrosive environments than air or oxygen are considered, it will be evident what characteristics of the alloy and the environment are likely to affect behavior. Finally the principles developed from the study of high temperature corrosion by air or oxygen will be applied to the behavior of alloys exposed to other high temperature corrosive gaseous media. Some effort will be devoted also to the effect of nongaseous contamination on high temperature corrosion behavior and to the problems of fused salt and liquid metal corrosion. (back)
Chapter 13 - Alloy Behavior at High Temperatures
Anton deS. Brasunas
Factors that affect corrosion under a variety of conditions and the possible means commonly employed for its control have been discussed at length in earlier chapters. Considerable attention has been given to the corrosion of various alloys in water, chemical solutions and in underground or concrete structures. Relatively little attention is ordinarily given to corrosion by gases, especially very hot gases.
In Chapter Four, the subject of corrosion by the atmosphere was discussed and in Chapter Twelve, some of the principles of high temperature corrosion were presented. In this chapter, we will take a more detailed look at the behavior of engineering alloys at elevated temperatures in several different commonly-encountered environments, namely, air and flue gas atmospheres as well as in low pressure gases, vacuum and molten metals and salts. Brief attention will be given also to mechanical properties at high temperatures.
Although the temperatures above approximately two or three hundred degrees F are sometimes considered "high temperature", this chapter will be concerned primarily with temperatures in the "red hot range" primarily from 1200 degrees Fahrenheit and up (above 650 C).
A major portion of this chapter will be devoted to high temperature corrosion of heat resistant alloys and the effects of alloying elements, but some attention also will be given to the effects of high temperatures on mechanical properties and structural instability. (back)
Chapter 14 - Coatings for Corrosion Protection
By N.E. Hamner
The concept of placing a protective barrier between materials and their environment is so ancient that its origin is lost in the mist of history. As can be expected with a concept so old, its materials, methods and qualifications are numerous and diverse. Furthermore, some. uses of a barrier, while originally satisfactory, are now obsolete or obsolescent in the light of new discoveries about the properties of matter and because the merits of individual components of barrier systems are better understood now.
There are three main kinds or compositions of barriers: Inert or essentially inert, inhibitive and sacrificial. Various combinations of these types are found in coatings systems designed to use some or all of the several protective advantages provided. It must be remembered, however, there is no such thing as a "perfect" coating in a practical sense so none of these types or any combination can be expected to give perfect protection. The properties of materials being what they are, none are so inherently stable that they will permanently resist attacks by the environment.
Thus, practical coatings are a compromise between the maximum protection that can be extracted from a system and how much is available to pay for them. With respect to economics it probably would be as expensive to achieve a "perfect" coating as it is to make anything else perfect. As the effective life of a coating system increases, its cost usually increases also. From another aspect, coatings protect by one or more of the following mechanisms:
The effectiveness of a coating system is directly related to the degree to which it effectively interposes itself between the environment and the substrate or reduces attack by the environment. This concept applies equally well to both cathodic and anodic (electrical) protection, both of which in an absolute sense are coatings in that they interpose barriers between the environment and the substrate.' In a similar way, such methods as peening2 to produce compressively stressed surface material resistant to attack also are coatings in an electrochemical sense.(back)
Chapter 15 - Corrosion Testing
By Frank L. McGeary and Bernard W. Lifka
The Value of Corrosion Testing
The corrosion of metals is governed by fundamental laws. As we understand these laws better, we become better able to predict the performance of a particular metal, how it reacts under a given set of conditions and how its performance can be improved. At present, however, it is necessary to develop experimentally most of the information we need about corrosion of metals.
Metals are used under countless and changing conditions so that the unexpected is not uncommon. It is because of this limited predictability of metal performance that corrosion tests are so important.
Properly conducted, corrosion tests can mean the savings of millions of dollars. They are the means by which we can avoid using a metal under unsuitable conditions or of using a more expensive material than is required. Corrosion tests also help in the development of new alloys that perform more inexpensively, efficiently, longer, or more safely than the alloys now in use. Also, quality control corrosion tests are a means of ensuring that the alloys we make and purchase have the capabilities expected of them.
Scope of this Chapter
Because of the large number of metals, alloys and applications, it is impossible to cover all phases of corrosion testing in this chapter. The intent, therefore, is to cover the fundamentals of testing commonly used for metals and to develop guidelines for proper test methods. Because this is an elementary corrosion course, emphasis is given to those responsibilities frequently assigned to a laboratory technician.
Corrosion testing programs can be simple ones which are completed in a few minutes or hours, or they can be complex and require the combined work of a number of investigators over a period of years.
This discussion on corrosion testing is intended for high school graduate technicians as well as engineers who may be new to the field. The trained technician is vital to a corrosion testing program. Engineers and scientists who devise and supervise the programs are liable to promotion and transfer. Consequently, the well trained technician becomes a means for maintaining the "continuum of quality" through a long testing program.
Types of Corrosion Tests
Corrosion tests are in two broad categories:
Chapter 16 - Materials for Corrosive Environments
By M.G. Fontana and J.H. Peacock
A large variety of materials, ranging from platinum to concrete, is used by the engineer to construct bridges, automobiles, process plant equipment, pipelines, power plants, etc. The corrosion engineer is primarily interested in the chemical properties (corrosion resistance) of materials, but he must have knowledge of mechanical, physical, and other properties to assure desired performance. The properties of engineering materials depend upon their physical structure and basic chemical composition.
These properties are related to behavior under load or stress in tension, compression, or shear. Properties are determined by engineering tests under appropriate conditions. Commonly determined mechanical properties are tensile strength, yield point, elastic limit, creep strength, stress rupture, fatigue, elongation (ductility), impact strength (toughness and brittleness), hardness and modulus of elasticity (ratio of stress to elastic strainrigidity). Strain may be elastic (present only during stressing) or plastic (permanent) deformation. These properties are helpful in determining whether or not a part can be produced in the desired shape and also resist the mechanical forces anticipated.
The corrosion engineer is often required to consider one or more properties in addition to corrosion resistance and strength when selecting a material. These include density or specific gravity (needed to calculate corrosion rates); fluidity or castability; formability; thermal, electrical, optical, acoustical, magnetic properties; and resistance to atomic radiation. For example, a particular part must be castable into an intricate shape, possess good heat-transfer characteristics, and not be degraded by atomic radiation. In another case, the equipment must be a good insulator, reflect heat and have low unit weight. Incidentally, radiation sometimes enhances properties of a material, e.g. the strength of polyethylene can be increased by controlled radiation.
Cost is not a property of a material, but it may be the overriding factor in selection of a material for engineering use, based on economic considerations and therefore should always be kept in mind. (back)
Chapter 17 - Analysis and Correction of Corrosion Failures
By Ellis D. Verink, Jr.
This final chapter of the NACE Basic Corrosion Course integrates information presented in earlier chapters and shows how these data can be used to analyze and improve conditions that will minimize corrosion failures and improve the useful life of many materials that must be exposed to a "hostile environment."
What Constitutes Failure?
The dictionary defines failure as "a falling short, a deficiency or lack, an inability to perform, ". For the corrosion engineer, the term "failure" is defined in terms of how well a material fulfills all aspects of the functional requirements of the application for which it was selected.
It is not enough, however, merely to provide functional capability. The best choice is the material which fulfills the required function most economically, taking into account initial cost, maintenance costs, reliability, return on invested capital, product quality, need for inhibitors, product degradation, product loss, unscheduled shutdowns, etc. A few examples will illustrate the consequences of failures in materials selection.
The, Naval Stores industry, located mainly in the southeastern and south-central parts of the United States, has an interesting materials selection problem. The raw material for this industry is pine gum collected from long-leaf yellow or slash pine trees, or extracted from pine stumps. Typical products of this industry include rosin, turpentine, dipentene, etc. These products are not especially corrosive to mild steel although the cleansing action ofturpentine and some of the processing solvents can make steel more vulnerable to atmospheric attack.
The real problem is catalytic degradation of the product from contact with metals. Rosin is priced on the basis of its paleness in color. The paler it is,, the higher the price it commands. If small metal cups of copper, steel, aluminum and stainless steel were placed on a table and each was filled with molten rosin, in a matter of minutes. the rosin in the copper and the mild steel cups would turn black whereas the rosin in the aluminum and the stainless steel cups would remain pale amber in color. As a consequence, the materials used for construction of . process equipment, storage and shipment of Naval Stores are usually either aluminum or stainless steel alloys despite their .initial, cost premium over steel.
The textile and paper industries also are concerned with color of their finished products. Since the corrosion, products of~ many metals are highly colored, care must be, exercised in selection of materials to be used in contact with the finished products. Mills of this type generally are highly automated so accidental product contamination may, go, unnoticed until the finished product reaches a customer's plant.
For example, undesirable rust spots were discovered on paper stock being processed at a paper specialities plant. Careful investigation revealed that a steel electrical conduit passed above the paper machine in the paper mill. Corrosion of the steel in the humid paper mill environment permitted iron corrosion products to fall and contaminate the paper.
Edible products also must be protected from degradation. Vegetable oils tend to become rancid in contact with some materials. Copper base alloys have significantly undesirable effects on edible oils. Accordingly, such products normally are handled in nickel, stainless steels, plastics, or aluminum vessels.
Corrosion of certain metals introduces toxic reaction products into process streams. So, for this reason, in the concentration of sap in the maple sugar making process, use of lead equipment or lead-base solder for fabrication of steel equipment is forbidden. Similarly, lead alloys are forbidden in the processing of edible gelatin.
Excessive Maintenance Cost
The cost of maintaining plant and equipment is an operating expense which directly reduces profit. Therefore, any reduction in maintenance expense would appear desirable from a profit standpoint. When taxes are taken into consideration, however, an economic decision must be made as to whether it is better from an overall profitability standpoint to maintain and protect a lower price material (as an expense) or invest in higher cost capital equipment.
Because company managements will appraise recommendations of corrosion engineers in the light of overall profitability, a working knowledge of engineering economy by corrosion engineers is strongly urged. Such knowledge will help define just what is "excessive maintenance cost." A typical example of a decision based on maintenance costs is given. In soda-ash plants, exposure to sodium chloride, calcium chloride, high humidity, etc., produces an environment spectacularly corrosive to unprotected steel.
It is almost useless to try to maintain protective coatings on steel grating-type stair treads because pedestrian traffic damages the coatings, permitting corrosion to undermine them and expose additional metal to attack. As a consequence because of their higher degree of corrosion resistance, aluminum stair treads are commonly employed in soda-ash plants. Care must be exercised, however, to avoid galvanic action between the aluminum tread and steel structural members.
It is good practice to plan periodic shutdowns of process equipment for inspection and maintenance purposes. This permits orderly repair and reconditioning without disruption of operations or inconvenience to customers. Unfortunately, emergencies sometimes arise requiring unscheduled shutdowns.
Consider the example of a large central station steam-power plant which was forced to shut down a unit because of excessive leakage of condenser tubes. Power plants are committed to supply power on a continuous basis at a fixed price to a large cross-section of domestic and industrial customers. Loss of the use of a major power unit forces the power company to purchase power from other power companies in order to supply their customers. They usually must pay a premium price of the order of $10-15,000 per day or even more for this "back-up" power.
The unscheduled shutdown of a chemical plant or refinery may involve disruption of activities not only at the plant where failure occurs, but also may interrupt operations of several other plants which depend on the first plant for their supply of raw materials. As a consequence, it sometimes is considered necessary to provide large storage capacity for certain products as a hedge against disruption of operations during an unscheduled shut down.