Materials Performance

JUN 2019

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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76 JUNE 2019 W W W.MATERIALSPERFORMANCE.COM We're busy updating our extensive library of NACE Press books so you can benefit from the convenience of an electronic version! Look for this symbol on your favorite title and take advantage of pure portability! Select titles now available! www.nace.org/store e BOOK T he corrosion behavior of materi- als under conditions of service should be an important consid- eration during the design of a system. However, although it is relatively straightforward to obtain quantita- tive information on physical and mechanical properties such as tensile strength, yield strength, impact values, fatigue limit, effect of temperature on properties, and so forth, truly representative corrosion data are much more difficult to obtain. It is not always possible to anticipate the actual environment in which a metal will operate. Even if initial conditions were known completely, often there is no assur- ance that operating temperatures, pres- sures, or even chemical compositions will remain constant over the expected life span of equipment. Under these circumstances, one is tempted to ask, "Why bother?" Experience has shown, however, that even when impossible circumstances appar- ently are present, substantial cost savings are possible by intelligent use of corrosion data and good design practices. Most published data have limited useful- ness because the character of attack is often omitted. Another factor to keep in mind is that published data usually are obtained under carefully controlled laboratory condi- tions using high-purity, reagent-grade mate- rials. is is very different than plant condi- tions using commercial-grade materials, which are less pure. Minor elements or im- purities can play an important role in the stability of metals and alloys when they are exposed to service conditions. A classic example is the catastrophic influence on water-handling aluminum equipment even when only a minute amount of copper ions are present in the water. If laboratory tests indicate that a given chemical is seriously corrosive to a particu- lar material of construction, a decision can usually be made to eliminate this material from further consideration. Favorable labo- ratory or field test data, on the other hand, are not positive assurances of good perfor- mance of a material service, unless the data were developed under precisely the same circumstances that will be encountered in use. Even two different petroleum refineries belonging to the same oil company and us- ing the same basic processes find differences in performance of materials traceable to differences in local conditions; for example, different source of crude, different mixture of crudes, a different pretreatment because of different end products (lube oil, asphalt, fuel oil, etc.). In a soda ash plant, for example, consid- eration was being given to the use of alumi- num heat exchanger tubes and shells for the ammonia (NH 3 ) recovery condensers. is is a standard aluminum application in many plants of this type. Sheet specimens of alu- minum alloys were carefully mounted in the existing cast iron shells using brackets coated with electrical insulating material to avoid dissimilar metal attack. After six months of exposure, examina- tion of the specimens showed no discern- able attack. erefore, because this evidence seemed to confirm experience in other soda ash plants, a decision was made to build these larger, water-cooled condensers of aluminum alloys. One week after the plant went into ser- vice with the new condensers, it was neces- sary to shut the plant down because the new condensers had failed. Although the walls of the condensers were unaffected by corro- sion, the tubes were completely perforated from the condensate side (exterior). Cautionary Use of Test Data The evaluation tests using aluminum coupons had not taken into account the in- fluence of heat transfer on corrosion behav- ior. Actually, the specimens accurately simu- lated only the sidewall conditions. In this particular plant, the ratio of carbon dioxide (CO 2 ) to NH 3 in the moist gaseous stream was lower than was customary in other soda ash plants; consequently, the inhibiting ef- fect of CO 2 was incomplete. is illustrates the importance of local situations that can complicate the task of material selection. This article is adapted from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 557-558. Understanding the basic principles and causes of corrosion CORROSION BASICS

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