Why stainless steel rust

Stainless steel is the most durable of metals. Its mechanical properties enable its structures to remain extremely resistant to rust. Nevertheless, corrosion can’t be precluded. But there are ways to minimize the risk of corrosion.

Stainless steel’s fine layer of chromium oxide has natural techniques to self-repair when breached or broken. However, if the damage is too extensive corrosion can occur. There are various types of corrosion to be aware of.

The most common types are:

Pitting Corrosion on stainless steel

The passive layer on stainless steel can be attacked by certain chemical species. Chloride ion is the most common of these and is found in everyday materials such as salt and bleach. Harsh pitting corrosion is a localized damage that eats pits into stainless steel. In addition to chloride ion, can be caused by elevated temperatures for extended amounts of time or lack of oxygen to the surface.

You can avoid pitting corrosion by ensuring that stainless steel does not come into prolonged contact with harmful chemicals or by choosing a grade of steel which is more resistant to attack.

Crevice Corrosion on stainless steel

Stainless steel requires a supply of oxygen to make sure that the passive layer can form on the surface. In very tight crevices it is not always possible for the oxygen to gain access to the stainless steel surface thereby causing it to be vulnerable to attack. Crevice corrosion is avoided by sealing crevices with a flexible sealant or by using a more corrosion resistant grade.

General Corrosion on stainless steel

Normally stainless steel does not corrode uniformly like ordinary carbon and alloy steels. However, with some chemicals, mainly acids, the passive layer may be attacked uniformly depending on concentration and temperature and the metal loss is distributed over the entire surface of the steel. Hydrochloric and sulphuric acids at some concentrations are particularly aggressive towards stainless steel. General corrosion can be quite destructive and happen to the entire surface at once.

Galvanic corrosion on stainless steel

If two dissimilar metals are in contact with each other and with an electrolyte (e.g. water or another solution), it is possible for a galvanic cell to be set up and to accelerate corrosion of one of the metals. It can be avoided by separating the metals with a non-metallic insulator such as rubber.

Less common types of corrosion are:

Stress Corrosion Cracking (SCC) on stainless steel

Stress Corrision Cracking is a relatively rare form of corrosion, which requires a specific combination of tensile stress, temperature and corrosive species, often the chloride ion, for it to occur. SCC can occur typically in hot water tanks or swimming pools. But if it does happen, it can be rapid, breaking down the mechanical properties of steel in days rather than months or years. Another form known as sulphide stress corrosion cracking (SSCC) is associated with hydrogen sulphide in oil and gas exploration and production

Intergranular Attack on stainless steel

Intergranular Attack is also a quite rare form of corrosion. If the carbon level in the steel is too high, chromium can combine with carbon to form chromium carbide. This occurs at temperatures between about 450-850 C. This process is also called sensitisation and typically occurs during welding. The chromium available to form the passive layer is effectively reduced and corrosion can occur. This corrosive harm happens between the grains and can be avoided by using a low-carbon stainless steel or uniform heating and rapid cooling of the steel.

The choice of the right structural stainless steel sections

Montanstahl does supply structural stainless steel sections for all different kinds of applications where corrosion is an issue. We carry as stock items the common grades 304L and 316L, and are able to realize on demand both standard or customized shapes in other materials like duplex steels or high alloy steels.

Reba Scientific thanks Montanstahl of Switzerland for permission to publish this article