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.
Understanding the basic principles and causes of corrosion Corrosion Basics Cautionary Use of Test Data T
he corrosion be- havior of materials under conditions of service should be an important consider-
ation during the design of a system. Although it is relative- ly straightforward to obtain quantitative information on physical and mechanical prop- erties such as tensile strength, yield strength, impact values, fatigue limit, effect of tem- perature on properties, and so forth, truly representative
corrosion data are often much UWZM LQNÅK]T\ \W WJ\IQV ?PQTM
reliable atlases characterize corrosion resistance of many materials, including metals and alloys, this information typically addresses concentra- tions of a single chemical in a
uniform environment. It is not always possible to anticipate
the more complex environments in which a metal will actually operate. Even if initial conditions were known completely, often there is no assurance that operating temperatures, pressures, or even chemical compositions will remain constant over the expected life span of equipment. Experience has shown, however, that even when unpredictable circumstances
72 MATERIALS PERFORMANCE April 2012
apparently are present, substantial cost savings are possible by intelligent use of corrosion data and good design practices. Most published data have limited
usefulness 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 conditions using high-purity, reagent-grade materials. This is very different than plant conditions using commercial-grade materials, which are less pure. Minor elements or impurities can play an important role in the stability of metals and alloys when they are exposed to service conditions. A classic M`IUXTM Q[ \PM KI\I[\ZWXPQK QVÆ]MVKM WV water-handling aluminum equipment even when only a minute amount of copper ions is present in the water. If laboratory tests indicate that a
given chemical is seriously corrosive to a particular material of construction, a decision can usually be made to eliminate this material from further consideration. .I^WZIJTM TIJWZI\WZa WZ ÅMTL \M[\ LI\I on the other hand, are not positive assurances of good performance of a material in service, unless the data were developed under precisely the same circumstances that will be encountered. Even conditions as seemingly similar I[ \_W LQNNMZMV\ XM\ZWTM]U ZMÅVMZQM[ belonging to the same oil company IVL ][QVO \PM [IUM JI[QK XZWKM[[M[ ÅVL differences in performance of materials traceable to differences in local conditions; for example, different source of crude, different mixtures of crudes, or different pretreatment because of different end
products (lube oil, asphalt, fuel oil, etc.). In another example, the use of aluminum heat exchanger tubes and
shells was considered for the ammonia (NH3
) recovery condensers in a soda-
ash plant. This is a standard aluminum application in many plants of this type. To verify suitability, sheet specimens of aluminum 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, exam-
ina tion of the specimens showed no discernable attack. Because this evidence [MMUML \W KWVÅZU M`XMZQMVKM QV W\PMZ soda-ash plants, it was decided to build these larger, water-cooled condensers of aluminum alloys. One week after being placed in
service, the plant had to be shut down because the new condensers had failed. Although the walls of the condensers were unaffected by corrosion, the tubes were completely perforated from the condensate side (exterior). The evaluation tests using aluminum
coupons had not taken into account the QVÆ]ence of heat transfer on corrosion behavior. Unfortunately, the specimens had accurately simulated only the side- wall conditions. Further, in this particular plant, the ratio of carbon dioxide (CO2 NH3
) to
in the moist gaseous stream was low- er than was customary in other soda-ash plants; consequently, the inhibiting effect of CO2
was incomplete. This illustrates
the importance of local situations that can complicate the task of material selection.
This article is adapted by MP Editorial Advisory Board Member Norm Moriber from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 559-561.
NACE International, Vol. 51, No. 4