Stainless steel is an alloy of iron that is composed of at least 10.5% chromium. This chromium allows a very thin oxide layer to build upon the metal surface, which is also referred to as the ‘passive layer’ and gives its distinctive shiny finish to stainless steel.
A passive coating like this helps avoid corrosion of the metal surface and consequently improves its ability to resist corrosion by increasing the amount of chromium in stainless steel. By combining elements like nickel and molybdenum, different stainless-steel alloys can be developed, resulting in the metal having even more useful properties such as improved formability and greater corrosion resistance capability.
Stainless steel products manufactured by steel pipe fabricators will not corrode in ‘natural’ conditions or aqueous environments, hence why domestic cutlery, sinks, work surfaces and stainless-steel saucepans are so commonly used. It is important to note, however, that this material is ‘stainless’ and not ‘stain-free’ and therefore, in certain cases, corrosion will occur.
What could cause stainless steel corrosion?
Stainless steel corrosion can be caused by several different causes.
Corrosion, at its most simple description, is a chemical reaction that affects a metal’s integrity. If the metal comes into contact with an electrolyte, such as water, oxygen, grime, or other metal, this form of chemical reaction may be induced.
The metal loses electrons after the chemical reaction has taken place and hence becomes weaker. It is then vulnerable to the effects of more future chemical reactions that can create things such as corrosion, cracks, and holes in the material until the metal is weaker.
Corrosion may also be self-perpetuating, meaning that once it begins, it is difficult to stop. It could cause the metal itself to become brittle once corrosion reaches a certain stage, and it could collapse.
Different forms of corrosion in stainless steel
Rust is the only form of corrosion most people see, but there are several different types:
- Corrosion uniform
The most common type of corrosion that can affect stainless steel and other metals is known to be uniform corrosion. It is known as an ‘even’ spread of corrosion across a material’s surface.
Interestingly, it is also known to be one of the most ‘benign’ forms of corrosion, although it can cover relatively wide areas of the surface of a metal. This is because the effect it has on the performance of the material can be measured, since it can be easily checked.
It can be difficult to anticipate, recognize and differentiate pitting corrosion, meaning it is often considered to be one of the most damaging forms of corrosion.
It is a very localized type of corrosion in which a tiny pitted corrosion area is formed by a local anodic or cathodic point. Once this pit holds, it can ‘build’ so that a small hole can easily form a cavity that can be of various shapes and sizes. Corrosive pits most often ‘travel’ downwards and can be especially dangerous since if unchecked, even if a comparatively small area has been impacted, they can lead to a breakdown in the structure of a metal.
- Corrosion of a crevice
Crevice corrosion is a localized type of corrosion that results from a microenvironment in which two areas of a metal have different concentrations of ions.
In the places likewashers, bolts and gaskets with little circulation that let acidic agents to enter, this form of corrosion happens. A declining amount of oxygen is caused by the lack of circulation, so that re-passivation does not occur. The pH balance of the crevice is subsequently affected, and an imbalance between that region and the external surface is caused. This, in fact, triggers higher corrosion rates that can be aggravated further by low temperatures. The use of proper joint design that decreases the risk of a corrosive crevice is a way to prevent this form of corrosion.
- Corrosion in galvanic form
If, when immersed in a corrosive or conductive solution, two electrochemically dissimilar metals come into contact, an electron flow is formed between them. With the less resistant metal being anodic, the least corrosive-resistant metal is generally more affected. This form of corrosion is called galvanic corrosion, or corrosion of two metals.
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