Why does stainless steel pit

Stainless steel is well-known for being resistant to corrosion from things that make plain steel and iron and other materials rust away. Under certain circumstances, stainless steel alloys can become corroded, showing signs of rust or other problems. One of the first things to know about stainless steel is that there are countless formulations of it on the market. What separates one of these stainless steel alloys from a plain steel alloy aside from having different component metals in different ratios is that protective oxide layer of film.

As long as this oxide layer remains unhindered, the steel will retain a shiny surface appearance. So, what causes stainless steel to form this oxide layer? The answer lies in the specific elements used in most forms of stainless steel: Iron, manganese, silicon, carbon, and chromium. Of these elements, chromium has the highest impact on the rust resistance of stainless steel, meaning chromium-rich stainless steel alloys such as most austenitic stainless steels tend to have the highest overall resistance to corrosion.

For example, grade stainless steel has molybdenum which grade stainless steel lacks. Because of this, grade stainless steel is more resistant to chlorides. There are a number of reasons why a piece of stainless steel might start to develop rust. However, because there are hundreds of different alloys of stainless steel, what might cause one stainless steel alloy to corrode might not affect another one.

Many types of stainless steel alloys will suffer extreme pitting corrosion when exposed to environments that are rich in chlorides such as salt. For example, grade stainless steel, when used in naval applications, may start to suffer pitting as a result of contact with seawater which is rich in salt or salt-enriched sea breezes. Alternatively, a specialized coating can be applied to the steel to prevent direct contact with chlorides in the environment. One basic mistake that some manufacturers may make when creating a custom steel wire or sheet metal form is that they may weld two dissimilar metals together —whether by accident or by design.

Why is this a problem? 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 l ow-carbon stainless steel or uniform heating and rapid cooling of the steel. 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 L and L , and are able to realize on demand both standard or customized shapes in other materials like duplex steels or high alloy steels.

Skip to content. Different kinds of corrosion on stainless steel. The most common types are: Pitting Corrosion on stainless steel The passive layer on stainless steel can be attacked by certain chemical species. Crevice Corrosion on stainless steel Stainless steel requires a supply of oxygen to make sure that the passive layer can form on the surface.

General Corrosion on stainless steel Normally stainless steel does not corrode uniformly like ordinary carbon and alloy steels. Galvanic corrosion on stainless steel If two dissimilar metals are in contact with each other and with an electrolyte e. 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.

Intergranular Attack on stainless steel Intergranular Attack is also a quite rare form of corrosion. 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. Tungsten, although not commonly used, is about half as effective on a weight percent basis as molybdenum in improving corrosion resistance.

The PRE relationship for austenitic and duplex stainless steels is usually given as follows. The relative resistance to localized corrosion for different alloys can be quantified by determining the critical temperature required for initiating attack.

When a higher critical temperature is necessary to initiate attack of a stainless steel alloy, it is more resistant to localized chloride attack. These critical temperatures relate to standard laboratory environments and are not readily transferable to practical operating environments but they provide an indication of relative performance.

Figures 2 and 3 also illustrate that crevice corrosion can occur when the remaining fully exposed surface of the stainless steel is not exhibiting pitting corrosion. This makes designing to avoid or seal crevices an important factor in achieving the desired level of performance. Sealing or eliminating tight joints during design can often avoid crevice corrosion.

Pitting and crevice corrosion can be avoided by either choosing a more corrosion resistant stainless steel or by changing the service environment so that it is less aggressive. For example, lower service temperatures or reduced levels of chloride can substantially reduce the aggressiveness of the environment.

In immersed applications or where standing water will be present, welding is the preferred solution. In true water shedding applications, like roofs and wall panels, flexible inert washers or construction sealants can often be used effectively but they should not be considered for water pooling or immersion.