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Why steel isn't so stainless


Everything including the kitchen sink: Stainless steel is everywhere.

Stainless steel isn't always quite as rust-free as it could be — and now a team of British scientists have worked out why.

Dr Mary Ryan of the Imperial College of Science Technology and Medicine and colleagues report in this week's Nature that they have established the cause of "pitting", where tiny pockets of rust burrow into stainless steel making it weak.

The researchers have found how an unavoidable contaminant in the steel-making process, sulphides, can lead to corrosion in stainless steel.

"Its very nice work," commented Dr Graham Sussex, Technical Officer for the Australian Stainless Steel Development Association. "It is something that has been postulated on a lot."

Stainless steel is an alloy with at least 12 per cent chromium, which creates a very thin layer of inert chromium oxide on the surface.

"It is this inert layer on the surface that stops stainless steel from corroding," explained Dr Sussex.

Nevertheless, stainless steel does corrode in wet, salty environments like those near the sea.

Previously it was thought that the sulphide impurities dissolved in the wet chloride ions to form corrosive hydrogen sulphide, writes Roger Newman of the University of Manchester Institute of Science and Technology in an News and Views article accompanying the article by Ryan et al.

However the new research has found that the sulphur, which has a lower melting temperature than steel, stays molten for a short time after the steel has solidified.

It is during this time the impurity particles "suck" chromium out of the surrounding steel, creating an area of weakness in the steel which is called a "pit initiation point".

"Once you get a pit going it's a stable environment that is much more aggressive on a micro scale so you wind up with a little hole in the stainless steel which will just keep growing," said Dr Sussex. "That's why stainless steel fails."

Pitting can penetrate several millimetres of steel in a few weeks or months and can lead to catastrophic failures.

Dr Ryan and team used secondary ion mass spectrometry, a technique usually used in microelectronics, to observe the behaviour of sulphur in steel.

The steel they used had a chromium level of 17-18 per cent, but in the areas around the sulphur impurities this was down to 12-14 per cent, leaving those areas vulnerable to corrosion.

Sulphur impurities in stainless steel have their uses. While they are unwelcome in marine steel, they are necessary when steel is used to make objects as they help "weaken" the steel enough to enable it to be cut.

In fact some steel has sulphur deliberately added to it so it is very easy to machine, explained Dr Sussex. It was this type of steel that was used in the new research because it made it easier to find examples of pitting.

"If they had taken a piece of sheet steel they would have been hunting forever," he said.