Dealloying and stress corrosion cracking in alloys with only a few percent of noble metal – experiments and atomistic simulations

One model for the propagation of stress corrosion cracking (SCC) in solid solution alloys is film-induced cleavage [1,2]. According to this model, microcracks nucleate in a nanoporous dealloyed metallic layer at the crack tip and do not stop when they reach the ductile substrate, but continue to grow for a few microns before arresting. Recent advances in the measurement of mechanical properties in dealloyed materials have tended to support such a radical suggestion by showing extraordinary behaviour, such as near-theoretical compressive strength of individual ligaments within the nanoporous material [3].

It is important to understand the range of SCC phenomena that might be accounted for by such a model. According to the Galvele school [4] the occurrence of SCC at low contents of more-noble metal is inconsistent with film-induced cleavage, but there is experimental evidence for nanoporous layers at Au contents as low as 5 at% in dealloyed AgAu alloys, and the lower limit of noble-metal content could be as low as 1 at%. Clearly such nanoporous layers contain a large residue of less-noble metal. Regarding Ni as the noble metal in FeCrNi alloys, the possibility of forming a nanoporous layer at Ni contents as low as a few percent would account for chloride- or caustic-induced SCC in austenitic or duplex stainless steels containing 4-8% Ni.

Atomistic simulations of dealloying using a modified version of Erlebacher’s MESOSIM code [5,6] have confirmed that strong nanoporous metallic layers can form at low contents of more-noble metal, and have also detailed the conditions under which such layers disintegrate into dust as assumed by the Galvele group. Comparisons with experiment, using AgAu alloys with low Au contents, are in progress.

1.   K. Sieradzki and R.C. Newman, Philos. Mag. A,  51, 95‑132 (1985).

2.   A. Barnes, N.A. Senior and R.C. Newman, Metall. Trans. A, 40, 58-68 (2009).

3.   J. Weissmüller, R.C. Newman, H. Jin, A.M. Hodge, J. W. Kysar, MRS Bulletin, special issue, 34, 577-586 (2009).

4.   S.A. Serebrinsky and J.R. Galvele, Corros. Sci., 46, 591-612 (2004).

5.   J. Erlebacher, J. Electrochem. Soc., 151, C614-C626 (2004).

6.   D.M. Artymowicz, R.C. Newman and J. Erlebacher, Philos. Mag., 89, 1663-1693 (2009).