Letter 19005

Can Stainless Steel Rust ? [UK] 

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Can some grades of Stainless Steel rust ? I think they can, but have no REAL evidence to support this. In short, part of our electro-mechanical system that we make incorporates Bearing Raceways made from EN19 Stainless Steel (I think). In the past, a fine brown/red powder has been observed on the raceway after prolonged run-hours - this powder has been found to contain a high Chromium content, which is why we think it may originate from the Raceways themselves. We have only observed this phenomenon on very rare occasions, and do not think that the problem is "production" related. Can anyone shed some light on this ?

Thank you,

David Harriman
- Basildon, Essex, UK

En19 is not a stainless steel. It is a chrome-moly low alloy steel, perhaps more widely known as AISI 4140. Nominal composition is 0.4% carbon, 1% chromium, 0.2% molybdenum, plus the usual silicon, manganese, etc, contents.

It has no greater claim to corrosion resistance than mild steel has (although it can certainly be heat treated to much higher hardness than mild steel!).

Depending on the environment and the lubrication if any, it may not be at all surprising that it corrodes.

Bill Reynolds    metallurgical services Melbourne, Victoria, Australia

Dear Basildon,

Stainless steels (from diff grades/series) do rust. (I welcome any comment on this statement). Literally, metals found in ore form in nature, which is very noble and resistance to env deteriotion. Steel, a relatively 'active' element extracted from its ore. In terms of thermodynamics, this steel has absorded much energy during the processing, extraction etc from its ore, this energy-intensed element will intend flow/turn to low-energy state to achieve equilibrium with the surrouding. There are quite number of REAL literature or publication out there that explain phenomenon of SS corrosion. Many works have been done on establishing the polarisation curve of SS in acidic, salty env. SS (low grade type) will tend to corrode easily in high-cloride env, we classified this as localised corrosion (may include intergranular, SCC, pitting and crevice corrosion). This corrosion topic alone can be explained in a VOLUME of Metal Handbook (Vol 13), check for it. Very likely to find Cr in yr corrosion products, SS contain Cr as alloy element, only differ by its % compositions, Cr may be ranging from 12% to 27%.

Shall I conclude yr situation is a normal circumstance? say if nothing abnormal happened to the SS production. Some info to share with you. Please comment, tq.

Lee, Chee Hong
- Singapore

I agree with the above submissions. Stainless steel can and does rust, but its rate of rusting is much less than that for low chromium alloys. Stainless steel must contain at least 12% chromium to be classified as truely stainless. Other metals are added to enhance more specific properties of the steel.

Trevor Crichton R&D practical scientist - UK

Just a note for info, some of the best stainless steel will show signs of rust if it was in contact with mild steel befor it was installed, @ hammer marks, rust on the skin from when it was slid across a mild steel work bench and rust from being wire brushed with a normal steel wire brush.

Bill Mashburn
- Hermiston, OR, USA

Have you considered the possibility of fretting corrosion in your application? This can produce a rusted appearance.

John W. Harrison
diagnostics - Tarrytown, NY, USA

As I understand it, there are at least two types of stainless steel - ferritic and austentitic. Ferritic will rust in the right environment, especially if water is present.

Dave Fisher
- Northants, England

What makes stainless steel "stainless" is a chromium oxide passive layer. If this layer is damaged, be it as a result of the manufacturing process or some other means, the exposed metal is able to be attacked and your stainless steel will show signs of rust. The trick is to restore the passive layer as soon after manufacturing as possible, as well as limit the exposure of the material to iron contamination through the manufacturing cycle. There are various means to restore this passive layer, be it through acid treatment or electropolishing. The difficulty is finding the best process.

Tyrone Bagg
- Perth, Western Australia

Rust of s.s generally happen with 304,316,321&347.

FeO is an oxide encapsolated with in other oxides in the s.s and this very oxide has a property of being unstable and therefore it decomposes when the metal is cooled from high temperature ( e.g after welding) according to reaction :4Fe=Fe3O4+Free Fe

The fine particles of free iron in the oxide film of the HAZ then react with the moisture in the air to form a uniform "band" of rust on either side of the weld(similar s.s weld)

Mohsin Alhadad
- Libya

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