Why stainless steel corrodes

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...

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Bibliographic Details
Published in:Nature (London) 2002-02, Vol.415 (6873), p.770-774
Main Authors: Ryan, Mary P, Williams, David E, Chater, Richard J, Hutton, Bernie M, McPhail, David S
Format: Article
Language:English
Subjects:
Applied sciences
Chromium
Corrosion
Corrosion environments
Exact sciences and technology
Humanities and Social Sciences
letter
Manganese
Mass spectroscopy
Metals. Metallurgy
multidisciplinary
Science
Science (multidisciplinary)
Spectrum analysis
Steel
Sulfides
Sulfur
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Summary: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 difficult. 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 particles. Chemical changes in and around sulphide inclusions have been postulated 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.
ISSN:0028-0836
1476-4687
DOI:10.1038/415770a