Scientists from Imperial College and University College London claim in the journal Nature to have solved the enduring mystery of why stainless steel corrodes.
Unlike rusting, stainless steel corrosion is highly localised and apparently random. Tiny holes called pits can drill through a substantial thickness of steel in a relatively short time. The pits can cause leaks or act as points from which cracks initiate, similar to the type of defect caused by scoring glass before breaking it, and can cause some of the most catastrophic industrial accidents known.
‘Stainlessness’ is created by alloying iron with chromium. As the steel ingot cools after it has been made, tiny sulphur-rich impurity particles, about 10 millionths of a metre in diameter, solidify at a lower temperature than the steel, remaining molten for a time after the metal has solidified.
Using an advanced new microscope the team from Imperial College and University College London found a region around these impurity particles that has significantly less chromium than the rest of the steel. During cooling of the steel the impurity particles ‘suck’ chromium out of the steel around them, creating a tiny nutshell of steel that is not stainless.
Corrosion of this layer, just one 10 millionth of a metre thick, is the virus that triggers the main attack said scientists Dr Mary Ryan of Imperial College and Professor David Williams of University College London.
‘Most of your household appliances contain stainless steel,’ said Dr Ryan of the department of materials at Imperial College.
‘It’s quick to clean and has an attractive shiny appearance – this cleanability also makes it the material of choice for applications requiring sterile surfaces such as surgical instruments or plants for producing pharmaceuticals.
‘Overall it’s used in countless engineering applications and, in general, it has very good resistance and performs well but it is susceptible to this devastating pitting corrosion. Now we’ve worked out the sequence of events that cause it, we know what causes this Achilles heel, and we can use this information to work out how to fix it,’ she said.
The authors suggest that altering the conditions under which it is made could cure the problem without using very expensive low sulphur steels. Another alternative is to use heat treatments after the steel is processed, causing chromium to replenish those sites it has been depleted from, the authors said.