News Release

Stainless steel corrosion mystery solved by UK researchers

Peer-Reviewed Publication

Imperial College London

From cutlery and cooking pans to the inside of a Formula 1 car engine or a huge chemical process plant, stainless steel is all around us. It’s not meant to corrode, but it can, and when it does the results can be disastrous, whether it’s a hole in your dishwasher or a failed industrial plant.

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.

Why stainless steel fails has long been a mystery, but today with the publication of research in the journal Nature*1, British researchers claim to have solved it.

‘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*2 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 say 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, they suggest.

Stainless steel was first made in Sheffield, UK, in 1913.

The research was supported by the Engineering and Physical Sciences Research Council (EPSRC).

###

For more information please contact:

Dr Mary Ryan
Department of Materials
Imperial College
Tel: +44 (0)20 7594 6755
E-mail: m.p.ryan@ic.ac.uk

Professor David Williams
Department of Chemistry
University College London
Tel: +44 (0)20 7679 7482
Email: d.e.williams@ucl.ac.uk

Tom Miller
Imperial College Press Office
Tel: +44 (0)20 7594 6704
Mob: +44 (0)7803 886248
Email: t.miller@ic.ac.uk

Notes to Editors:

1. The research is published in the scientific journal, Nature (14 February).

Title: Why stainless steel corrodes
Authors: Mary P. Ryan*, David E. Williams#, Richard J. Chater*, Bernie M. Hutton# and David S. McPhail*

* Department of Materials, Imperial College, London
# Department of Chemistry, University College London

2. Named FIB-SIMS - Focussed Ion Beam – Secondary Ion Mass Spectrometry. A relatively new analytical technique based on the FIB microscope. FIB’s are typically found in the semiconductor industry, where they are used to make integrated circuits. Here the authors have used it to achieve nanometre scale local chemical measurements.

3. Established in 1826 as the original University of London, UCL has been consistently rated amongst the top three multi-faculty universities in the UK. As the largest university in London, UCL also boasts the largest medical school and strength in depth in medical and biomedical research. UCL's reputation for excellence in teaching and research continues to grow both nationally and internationally. The latest Research Assessment Exercise (RAE 2001) judged that 80% of UCL research was world class with 40 out of 48 subject areas assessed being rated 5 or 5*.

4. Imperial College of Science, Technology and Medicine is the largest applied science, technology and medicine university institution in the UK. In the December 2001 Research Assessment Exercise, 75 per cent of staff achieved a 5* rating, the highest proportion in any UK university. All departments visited and assessed for their teaching have scored between 21 and 24 points out of 24 or, in the previous system, have been judged excellent. www.ic.ac.uk


Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.