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The Theory and Practice of Corrosion and Its Control in Industry? Norman Hackerman Departments of Chemistry, Rice University, Houston, Texas 77005,and the University of Texas, Austin, Texas 78712 Received September 1, 1986 While the forces which go to cause corrosion are important in accessing the possibilities of corrosion, the dynamics of the system are equally important. In this same vein, protection from corrosion either chemically or electrically requires not only knowing that protection is possible but also that it can be delivered to the solid-fluid interface where it is required. It is necessary to state first that this paper concerns corrosion of metals mainly since the word has been used more broadly to include all materials and even nonmaterial things like the soul. It is then useful to state that all metals are inherently unstable in air, in water, and especially in both. While other factors make the corrosion process undesirable, the cost alone makes it a serious societal problem. A few years ago that cost was estimated to be $126 billion per year just in the United States.' It was noted that this amounted to 4.2% of that year's GNP or to $556 per each US. resident. It could also be shown to be between a third and a half of the US.Department of Defense budget. It is interesting that in 1949 Uhligh2 estimated the cost to be $5.37 billion or about 2.1% of that year's GNP. These cannot be firm numbers for the same reason as that which induces managers to ignore corrosion until forced to deal with it. That reason is that treating corrosion does not produce income, so its absence (by good design, good treatment, or good fortune) is not seen on the books. An example from F. L. LaQue3 concerned a manager whose plant handled 10%H 8 0 4and who maintained he had no corrosion problem. The gap was bridged when it was learned that he replaced his pumps every 6 weeks and considered that to be a normal cost. It is instructive to recognize that the range of effects of corrosion reach from merely aggravating to catastrophic. Thus the overriding question of safety, environmental impact, and health hazards is clearly pertinent. For example, the potential critical problem of impairment of the industrial world's potable water supply because of leaks in buried tanks and pipelines stems from corrosion of such vessels over time. This is not isolated on large industrial sites but applies to all gasoline service stations and even to buried domestic fuel tanks. This is a case of the effect on the system itself being of considerably less i m p ~ r t . ~ Somewhat more technical is the need to know that there are numerous operative forces, with chemical forces being the common one. To that requisite force must be added the possible effects of light (radiation), heat, mechanical, flow, and bacterial influences. In particular, the variations of these within the system as well as their changes with time are very influential in advancing the overall effect from aggravating toward catastrophic. The most widespread instances of corrosion are not necessarily the most destructive. These involve the common system of metals in very dilute to dilute aqueous solutions of electrolyte open to air. All domestic, commercial, and industrial systems in which metal is exposed Presented a t the Symposium on "Corrosion", 191st National Meeting of the American Chemical Society, New York, NY, April 13-18, 1986.
to air and humidity are included. More complex problems stem from more severe conditions, e.g., temperature, stress, radiation, erosion, and so on. Other complexities arise from the less ordinary chemistry of nonaqueous systems-both conducting and nonconducting-and the less ordinary property, for example, of volatile solids. Examples of such systems constitute a hall of horrors amonst those experienced in corrosion. One is the catastrophic loss of metal in boilers fired by fuel oil containing sometimes even only traces of vanadium. At elevated temperature an oxide of vanadium and iron sublimes, causing rapid metal loss. Another example is that of rapid destruction, in times past, of steel field tanks holding a solution of ammonia, ammonium nitrate, and water. Stresses due to thermal changes on passage of the sun overhead exposed bare metal to this ammonia-base liquid and appropriate chemistry occurred, leading to soluble iron-ammonium complexes, which are stable in the absence of solvent water. This brings us to the point of stating that corrosion is not necessarily a single effect. If the force acting is mainly chemical, the effect should be mainly uniform, i.e., wall thinning, product contamination, and downstream damage such as catalyst poisioning, plugging, and metal redeposition (e.g., Cu on Fe). A more localized and potentially structurally more damaging outcome arises from chemical forces, plus differentials in temperature, flow rate, or concentration, e.g., of oxygen or electrolyte, etc. Here, one encounters pitting, roughening, selective action (dezincification, decarburization), cavitation, and so on. This leads to contamination, interference with flow, and most importantly loss of containment and/or structural strength. A third form of corrosion involves catastrophic losses such as those that occur with stress corrosion cracking. This requires both stress and chemical force acting together. In this case, the fracture need not be preceded by much metal loss overall. Another form of such an effect already noted involves steel boiler tubes a t furnace temperature when vanadium-containing fuel is used. The vanadium oxide destroys the integrity of the oxide cover with subsequent rapid loss of metal and collapse of tube. As noted above also, a steel field tank containing a liquid fertilizer (60 mol % NH4N03plus 20 mol % NH, is 20 mol % HzO) collapsed. The reason is that this is not a water-base system but an ammonia-base system, and soluble iron-ammonia complexes form but are not hy(1)Editors reply to: Fontana, M. G. Mater. Perform. 1983,22(6),64. ( 2 ) Uhlig, H. H. Chem. Eng. News 1949,97, 2764. ( 3 ) LaQue, F. L., private communication. ( 4 ) See, for example: Houston Chronicle; June 25, 1986,Section 1 p 3.
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Corrosion and Its Control drolyzed. Thus it forms from the base metal when thermal changes due to the solar transit produce fissures in the covering oxide. The metal is removed a t a great rate, as high as 4 in. of penetration per year (cf. to a "high" corrosion rate of 0.1 in. per year). The range of corrosion is so large and the variety is so great that even in a simple system there is no panacea. Nonetheless, corrosion is treatable, but only by prescription for most problems. Further, one must heed the caveat that once solved not likely forever solved. In other words, the problem may change with time and treatment. There is another important caveat; namely, the conditions as normally measured are not necessarily those a t the reaction site, i.e., a t the metal surface, within the crevice space, or inside the pit. It helps to know the system pH, oxygen concentration, or composition generally, but it is much more critical to understanding the problem and its solution to know these parameters a t the initiating reaction site. This holds also for temperature, especially its flux, and for the nature and velocity of flow. This is a basic surface chemistry problem. In addition, it is self-evident that the actual aggressor be identified. For example, the conversion of Ni to aqueous Ni2+may take place slowly enough in aqueous solution, but the presence of ammoniacal salts aggravates the problem markedly. In fact, the rate of soluble complex ion formation is likely to be limited only by local ion concentration buildup. This leads naturally into the important proposition that many corrosion problems are transport limited; so flow rate, flow pattern, and changes in these frequently contain the solution to corrosion control. This segment has emphasized the need to know the system, the real conditions (generally as estimated from the measured conditions), and the likely changes in these. I t also helps to have information on such things as viscosity, surface tension, ohmic resistance, and various nonequilibrium electrode potentials. In connection with the last, some actual measured corrosion current would do much better. Finally, it behooves the treater to recognize and estimate the effect of the corrosion process, its treatment, and time on the efficiency and products of the system. This requires forethought as well as effective m~nitoring.~ In somewhat more scientific language, the corrosion reaction requires a t least one redox process, which takes place either by direct electron transfer or by the same transfer separated by some distance via a conduction path, Le., electrochemically. In aqueous systems with one or another oxidizer available, the reaction will proceed with -AG. Thus kinetics govern; so transport, precipitation site, complexing propensity, thermal differences, concentration differences, localized stress, and even such apparently unlikely things as radiation and bacteria all play a part. For the quite common electrochemical corrosion any potential difference across a conducting path has an influence. Reprising the matter of corrosion control in somewhat more technical terms requires the reminder that each individual system is likely to be singular and requires individual assessment. This is particularly true of the effect of the treatment on other aspects of the system. There are some half-dozen basic treatment methods. By far the best is good design. That is, design to minimize temperature and composition differences as well as flow (5) Turner, M.; King,R. The Chemical Engineer 1986 (February),17.
Langmuir, Vol. 3, No. 6, 1987 923 problems. Corrosion, cavitation, and similar effects of flowing streams add to the metal loss burden both directly, as is obvious, and indirectly by maintaining bare metal surfaces. Minimizing stress and crevices such as those provided by layers of metal is also helpful. Nonetheless, it must be kept in mind that the principal reason for designing the system is for a certain product, and design to minimize corrosion must be consistent with the main purpose of the process. Given the best design possible, there are treatments to handle such corrosion as appears. For instance, it might be possible to change some condition(s) by eliminating or reducing the availability of moisture or air or by changing the heat flux or flow rate-up or down. Still, one must remember to check the effect of such changes on system productivity. A little more complex approach is to treat the environment chemically to reduce its agressiveness. An example is that of simply adjusting pH appropriately or removing an inadvertent complexing agent. The same possibility exists on the metal side, namely, reduction of its reactivity. This is the reason for many alloys, although it is still not possible to specify corrision-resistant alloy compositions from first principles (from experience yes, but from basic solid-state theory, not yet). Another way to control corrosion is to separate the reacting phases. Indeed, that may be the intrinsic way alloys function-by forming a solid barrier toward the solution or vapor phase. It is possible to form conversion coating or to interdict with an applied coating. When the latter is done in situ it falls under the heading of corrosion inhibition. More generally, corrosion inhibition involves adding chemicals to the metal's environment. Earlier it has been noted that the aggressiveness of the environment could be altered, e.g., by decreasing its acidity. This may simply result in decreasing the effect of the solution on barrier compounds. As stated just above, the added chemical may provide a conversion coating, again a barrier. Some chemical inhibitors function by adsorbing on the solid surface-perhaps on the metal, perhaps on the solid metal compounds-and retarding the corrosion reaction in some fundamental way. That is, this may involve actual alteration of metal reactivity, or it may be simply a very thin film barrier. There are also redox inhibitors which are expected to alter the electrochemical reaction in a suitable manner. This leads to the last general procedure for corrosion control. This is the application of potential or current to the interface, in order to reduce the corrosion current. Here we must be certain of suitable current distribution and path. Crevices play an important role in impairing the effectiveness of the method. More generally, highresistance paths can be deterimental. In addition to the method noted above, it is evident that any two or more can be used in some combination. Thus, while it is clear that the theory of corrosion reactions and their conttrol is vital and useful it is equally clear that the practice of the control of corrosion processes requires judgement and experience. In conclusion, several propositions deserve a restatement: The theory is that of the chemistry or electrochemistry of the system. In practice, modern instrumentation provides valuable insight but is still only useful with corroboration. Laboratory conditions usually do not equate to site conditions: real vs. test surface, real vs. test heat flux, and real vs. test flow, etc. Corrosion is sequential, so interposing a slowdown in some step of the
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sequence lowers the rate. Corrosion changes with time, therefore its control may require change with time. Electrochemical protection is not better than the conducting path. Corrosion inhibitors can be no better than the transport of the “active* particle to the corrosion site. Coatings can be no better than the quality of surface preparation permits. Corrosion control must not interfere
with the system’s product. Corrosion control must not pollute or create safety hazards.
Acknowledgment. I am pleased to acknowledge support Of this work by The Robert A. Welch Foundation of Houston, TXRegistry No. Steel, 12597-69-2.
Breakdown of Passivity and Localized Corrosion Processed H.Bohni Institute of Materials, Chemistry, and Corrosion, Swiss Federal Institute of Technology, CH-8093 Zurich, Switzerland Received July 10, 1986 Breakdown of passivity and localized corrosion in the presence of C1- or other halides is usually restricted to certain potential ranges. At low potentiah the local activation is followed by a rapid repamivation process forming a new passive film. At higher potentials repassivation is supressed and stable pit growth may occur. The present understanding of the initiation process as well as the growth kinetics indicates that the transition from instable to stable pit growth may not be generalized for all metals but rather has to be considered individually for each system. In the case of rapid mass transport conditions between pit and bulk electrolyte (e.g., aluminum), usually well-defined critical potential values for stable pit growth (pitting potentials) are observed. However, when the exchange of the electrolyte is slow and often less reproducible the transition from instable to stable pit growth occurs within a whole potential range. The localized active dissolution may then be extended to very low potentials as in the case of crevice corrosion of stainless steels. Therefore, sufficient knowledge of the mechanism of localized corrosion is necessary to predict the corrosion behavior of passive metals correctly. 1. Introduction Localized corrosion processes such as pitting and crevice corrosion as well as intergranular attack are major practical problems affecting the performance of passive metals. The often unpredictable occurrence may cause serious difficulties when using materials such as stainless steels, nickel and nickel-base alloys, or other generally corrosion-resistant materials. Furthermore, in the case of stress corrosion cracking and corrosion fatigue the crack initiation quite often starts a t sites where localized corrosion processes occur. Despite the fact that during the past years large efforts were made for a better understanding of the processes involved in localized corrosion, the mechanisms are not yet sufficiently well understood, and important knowledge is still lacking, especially for the nucleation stage of pitting corrosion. Nevertheless, valuable contributions with respect to the growth and the stability of this type of corrosion have been made, which are not only of scientific but also of great practical importance. Pitting corrosion on passive metals in the presence of chloride ions as well as other halides usually occurs above a critical potential range. Therefore the susceptibility of passive metals to pitting corrosion is often studied by electrochemical methods. Most commonly, current-potential curves are measured either stepwise or by applying a constant potential sweep rate and recording the resulting current. Typical current-potential curves with a sudden current increase within the passive potential range are usually obtained as shown in Figure 1. The following + Presented at the symposium on “Corrosion”, 191st National Meeting of the American Chemical Society, New York, NY, April 13-18, 1986.
values are often determined and used to characterize metals and alloys with respect to pitting as well as crevice corrosion: (1)The critical current density characterizing the active/passive transition, (2) the pitting potential ep where pits start to grow, and (3) (after reversal of the potential sweep direction) the repassivation potential trep below which already growing pits are repassivated and the growth is stopped (growth-limiting potential). Obviously, one or several of these parameters can be measured as a function of the alloy as well as the electrolyte composition, as shown, for instance, by Horvath and Uhlig,’ who were able to demonstrate the beneficial effect of chromium and molybdenum in stainless steels (Figure 2). Still other studies2have shown the detrimental influence of manganese in various stainless steels (Figure 3). The pitting potential is significantly displaced in the less noble direction when the manganese content is increased. Without discrediting the practical value of such investigations, it has to be pointed out that the pitting as well as the repassivating potentials do not only depend on the type of metal and the environment but unfortunately often also on the applied method. Therefore, the numerous pitting potentials published in the literature are mainly used to compare alloys and environments with respect to their pitting susceptibility or pitting-promoting tendency rather than to predict true limiting potentials (Figure 4). 2. Nucleation and Growth Processes A valuable improvement of the present state of knowledge is already achieved if localized corrosion pro(1) Horvath, J.; Uhlig, H. H.
J. Electrochem. SOC.1968, 115, 791. (2) Degerbeck, J.; Wold, E. Werkst. Korros. 1974, 25, 172.
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