Chloride Ion Activity and Susceptibility of Al Alloys 7075-T6 and 5083-H131 to Stress Corrosion Cracking
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INTRODUCTION
ALUMINUM is a relatively active metal that spontaneously reacts with oxygen or water to form aluminum oxides, hydroxides, hydrated oxides, or mixtures of these phases.[1–3] The normally excellent corrosion resistance of aluminum alloys is the result of a thin impervious film of these products forming on the surface. This film isolates the underlying metal from the environment making it nonreactive, or passive, in environments where the metal is thermodynamically unstable.[4,5] As with most passivated metals, corrosion problems arise if this passivating film has difficulty protecting the metal because of chemical or mechanical forces. In particular, localized forms of corrosive attack, such as pitting, crevice corrosion, and stress corrosion cracking (SCC), may occur when halide ions are present in the environment.[6] For aluminum alloys, the chloride ion, which is commonly found in ambient environments, is one of the more aggressive of these ions.[4,5,7] The fact that these ions appear to cause corrosion implies that they increase the thermodynamic forces driving corrosion reactions. However, these ions are the most reduced state for this element and cannot promote oxidation of metals by R.E. RICKER, Metallurgist, is with the Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. E.U. LEE, C. LEI, and B. PREGGER Materials Engineers, R. TAYLOR, Materials Engineering Technician, and E. LIPNICKAS, Chemical & Materials Engineer, are with the Materials Engineering Division, Naval Air Warfare Center Aircraft Division, Patuxent River, MD 20670. Contact e-mail: [email protected] Manuscript submitted March 26, 2012. Article published online November 10, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
accepting electrons. Because aluminum contains sufficient energy to reduce hydrogen ions from water molecules, the oxidizers primarily responsible for causing corrosion of aluminum alloys in aqueous environments are dissolved oxygen and water.[1,8] If the concentration of the chloride ion has no influence on (1) the chemical potential of the elements in the alloy, (2) the chemical potential of the elements in the corrosion products, and (3) the activity of the oxidizers in the solution, then it has no direct influence on the free energy driving corrosion reactions. However, the literature clearly shows that the chloride ion alters the rate and morphology of attack.[7,9,10] Therefore, these ions interact with corrosion processes in a manner that enables the existing corrosion driving forces to exert their influence in a more detrimental manner. The responsible interaction may be as simple as altering the activated complex, adsorbing in a manner that interferes with passivation, or altering hydrolysis equilibria. In all these cases, the influence of the chloride should be related to its activity in a manner that indicates the stoichiometry of this ion in the responsible process. Quantifying the effect of the chloride ion on SCC over a wide range of concentrations wil
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