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Corrosion is when a material breaks down because of its environment. In general, stainless steel is much more resistant to corrosion than carbon or alloy steels.
Stainless steel is an iron alloy and is made up of 10.5% chromium as a minimum. This chromium causes a very thin layer of oxide to build up on the surface of the metal which is often referred to as the ‘passive layer’ and gives stainless steel its characteristic shiny finish.
A passive layer like this helps to prevent corrosion of the surface of the metal and increasing the amount of chromium in stainless steel subsequently increases its ability to resist corrosion. Different stainless steel alloys can be formed by adding elements such as Nickel and Molybdenum, the result of which is the metal having even more useful properties such as increased formability and a higher ability to resist corrosion.
In ‘normal’ atmospheres or aqueous environments stainless steel will not corrode, hence why domestic sinks, cutlery, work surfaces and saucepans made of stainless steel are so widely used. However, it is important to remember that this material is ‘stain-less’ and not ‘stain-free’ and therefore corrosion can occur in some situations.
Here at Special Piping Materials, we pride ourselves on supplying products made from some of the most high-performing materials available – stainless steel, duplex, super duplex, 6% Moly and Nickel Alloy. Different materials are naturally suited for different purposes and environments and one of the factors that needs to be considered is the probability of corrosion. Our expert teams around the globe work with our clients to ensure that they choose the materials that best suit their requirements. Avoiding corrosion is a very important consideration.
Carry on reading to find out more about what causes corrosion and the different types of corrosion that can impact stainless steel. It’s a big subject so we will follow this up at a later date with how to prevent corrosion and what corrosion resistant coatings are.
What can cause corrosion in stainless steels?
Many different factors can cause corrosion of stainless steels.
At its most basic description, corrosion is a chemical reaction that affects the integrity of a metal. This type of chemical reaction can be caused when the metal comes into contact with an electrolyte like water, oxygen, grime, or other metal.
Once the chemical reaction has taken place, the metal loses electrons and therefore becomes weaker. Once the metal is weaker it is then vulnerable to the effects of further potential chemical reactions that can produce things such as rust, cracks, and holes in the material.
Corrosion can also be self-perpetuating meaning that it is hard to stop once it starts. If corrosion reaches a certain point, then it could cause the metal itself to become unsteady and it could collapse.
There are ways to prevent and avoid corrosion though – many of which Special Piping Materials is able to help with – and we will cover these in a later blog.
Different types of stainless steel corrosion
Most people see rust as the only form of corrosion but there are many different types:
- Uniform corrosion:
Uniform corrosion is thought of as the most common type or corrosion that can affect stainless steel and other metals. It is described as an ‘even’ spread of corrosion across the surface of a material.
Interestingly, although it can cover relatively large areas of a metal’s surface, it is also considered to be one of the most ‘benign’ types of corrosion. This is because it is possible to evaluate the impact it is having on the material’s performance as it can be easily tested.
- Pitting corrosion:
Pitting corrosion can be hard to predict, identify and distinguish meaning it is often thought of as one of the most destructive types of corrosion.
It is a very localised form of corrosion in which a local anodic or cathodic point creates a small pitted area of corrosion. Once this pit takes hold it can ‘grow’ so a small hole can quickly form a cavity that can be a variety of different sizes and shapes. Corrosive pits most commonly ‘travel’ downwards and can be particularly dangerous because if left unchecked they can lead to a failure in the structure of a metal even if a relatively small area has been affected.
- Crevice corrosion:
Crevice corrosion is a localised form of corrosion that results from a microenvironment where there are different concentrations of ions between two areas of a metal.
This type of corrosion occurs in places such as bolts, washers and gaskets which are small areas with little circulation that allow corrosive agents to enter. The lack of circulation causes a diminishing amount of oxygen so re-passivation cannot occur. Subsequently, the pH balance of the crevice is affected, and an imbalance is caused between that area and the external surface. This, in turn, causes higher rates of corrosion which can be further exacerbated by low temperatures. A way to avoid this type of corrosion is to utilise proper joint design that reduces the chance of a corrosive crevice.
- Intergranular corrosion:
Intergranular corrosion can be caused by impurities that takes place at the boundaries between the grains that form during the solidification of an alloy. If intergranular corrosion occurs, then it can seriously affect the mechanical properties of the metal even while the majority of the metal remains intact.
- Stress corrosion cracking (SCC):
Stress corrosion cracking (SCC) is caused when a metal is exposed to a particularly corrosive environment, usually at higher temperatures. If the metal experiences external stress such as a tensile load or the metal expands and contracts quickly (due to rapid temperature changes) then stress corrosion can occur. Another circumstance in which it can occur is due to residual stress caused by the manufacturing process such as cold forming, welding, machining, grinding, etc.
Stress corrosion can be hard to spot because the majority of the surface remains intact but fine cracks emerge in the microstructure of the metal that then spread.
Stress corrosion can be avoided if the correct materials are chosen for the environment, they need to perform in.
- Galvanic corrosion:
If two electrochemically dissimilar metals come into contact when they are immersed in a corrosive or conductive solution, then an electron flow is produced between them. The least corrosive-resistant metal is usually affected more, with the less resistant metal becoming anodic and the more resistant metal becoming cathodic. This type of corrosion is called galvanic or two-metal, corrosion.
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