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Letters to Nature

Nature 415, 770-774 (14 February 2002) | doi:10.1038/415770a; Received 5 November 2001; Accepted 19 December 2001

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Why stainless steel corrodes

Mary P. Ryan1, David E. Williams2, Richard J. Chater1, Bernie M. Hutton2 & David S. McPhail1

  1. Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BP, UK
  2. Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK

Correspondence to: Mary P. Ryan1 Correspondence and requests for materials should be addressed to M.P.R. (e-mail: Email: m.p.ryan@ic.ac.uk).

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Stainless steels are used in countless diverse applications for their corrosion resistance. Although they have extremely good general resistance, they are nevertheless susceptible to pitting corrosion. This localized dissolution of an oxide-covered metal in specific aggressive environments is one of the most common and catastrophic causes of failure of metallic structures. The pitting process has been described as random, sporadic and stochastic and the prediction of the time and location of events remains extremely difficult1. Many contested models of pitting corrosion exist, but one undisputed aspect is that manganese sulphide inclusions play a critical role. Indeed, the vast majority of pitting events are found to occur at, or adjacent to, such second-phase particles2, 3. Chemical changes in and around sulphide inclusions have been postulated4 as a mechanism for pit initiation but such variations have never been measured. Here we use nanometre-scale secondary ion mass spectroscopy to demonstrate a significant reduction in the Cr:Fe ratio of the steel matrix around MnS particles. These chromium-depleted zones are susceptible to high-rate dissolution that 'triggers' pitting. The implications of these results are that materials processing conditions control the likelihood of corrosion failures, and these data provide a basis for optimizing such conditions.

  1. Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BP, UK
  2. Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK

Correspondence to: Mary P. Ryan1 Correspondence and requests for materials should be addressed to M.P.R. (e-mail: Email: m.p.ryan@ic.ac.uk).