Correlation of Laboratory Results with Observations on Long-Term Corrosion of Iron and Copper Alloys
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CORRELATION OF LABORATORY RESULTS WITH OBSERVATIONS ON LONG-TERM CORROSION OF IRON AND COPPER ALLOYS M. B. MCNEIL*, D. W. MOHR* AND B. J. LITTLE** *Naval Coastal Systems Center, Panama City, FL 32407-5000 **Naval Oceanographic And Atmospheric Research Laboratory, Stennis Space Flight Center, MS 39529 ABSTRACT The data base for compounds produced during long-term Compounds are corrosion of iron and copper objects is reviewed. identified that occur in long-term but not in short-term corrosion. Where possible, the formation of these compounds is explained or at least rationalized. An effort is made to discriminate those compounds whose formation depends on microbiological action from those forming abiotically. INTRODUCTION 1 23 In the study of iron and copper objects ' ', the very detailed studies which have characterized the archaeological literature the material available have not always been integrated with all in the corrosion and mineralogical fields. This paper will review corrosion product mineralogy for iron and copper and some of their alloys and point how this integration can be of benefit, particularly in regard to microbiologically influenced corrosion (MIC).4
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will be subdivided into microbiologically induced corrosion and microbiologically intermediated corrosion, depending on the relationship of the microbe to the corroding surface. Microbiologically induced corrosion requires intimate contact of the micro-organisms with the metal surface, creating In this an interfacial chemistry that is conducive to corrosion. type of corrosion, biofilms support large chemical gradientsb which permit the formation of compounds quite unstable in the bulk environment. In microbiologically intermediated corrosion, the precise location of the microbes is irrelevant because microbes only act by changing the bulk properties of the solution or atmosphere. MIC
IRON AND NICKEL ALLOYS Body-centered cubic iron, which is the major metallic component of cast component of wrought iron and forms the ferrite When it iron and carbon steel, is generally rather pure iron. occurs in two-phase alloys, galvanic effects become important.6 In gross two-phase situations such as an iron blade in a copper handle, galvanic effects can be treated by modifications of
Mat. Res. Soc. Symp. Proc. Vol. 185.
1991 Materials Research Society
754
8 7 In two-phase alloys such diagrams and Evans diagrams" stability as octahedrites the detailed transport effects may need to be studied.
Iron objects corroding at rapid rates produce goethite, The details of cc-FeOOH, haematite (Fe 2 0), and magnetite (Fe 30 4 ) . the layers are the subject of continuing study, and the magnetite Dr. Marsh, morphology depends upon the corrosion conditions.9 Dr. Sharland, and their colleagues at the United Kingdom Atomic Energy Research Establishment have studied long-term corrosion of iron extensively,"° and have developed a technique of plotting conditions at a point a fixed distance from the metal surface as a trajectory on the iron Eh-pH diagram as a function of tim
