We’ve all heard this myth: Stainless steel won’t rust. The word “stainless” is in the name so surely it shouldn’t. Or will it? We believe that stainless steel would be more aptly named “harder to stain steel.” Stainless steel is unique in that it’s strengthened with built-in corrosion resistance, but it’s not foolproof. While it may not corrode in the same manner nor at the same speed as conventional steels, under certain conditions it will begin to lose its integrity.
How is stainless steel constructed?
Stainless steel is an alloy of Iron with a minimum of 10.5% of the mineral Chromium, which is one of the most common elements in the earth’s crust and in seawater. Chromium produces a thin layer of oxide on the surface of steel, known commonly as the passive layer. This is unlike iron oxide, which you often see as flaky rust, the chromium oxide cleanly clings to the steel, acting as a protective barrier.
The higher the Chromium, the stronger the resistance to corrosion. Most stainless steels used in buildings and heavy-duty structures are austenitic (as opposed to ferritic, martensitic and duplex stainless steels) and contain a maximum of 0.15% carbon and 16% chromium, resulting in an incredibly strong resistance
Factors that cause stainless steel to rust
Two of the main causes of rust in stainless steel are environment and steel grade. Environmental factors include exposure to damaging chemicals, saline, grease, moisture or heat for prolonged periods of time. Of course, impurities in the steel can play a part in the corrosion, which is why it’s important to check the grading for the application.
There are different six different forms of stainless steel corrosion, some of which are preventable and others may only be noticed when the application fails. The six forms of corrosion are:
- General corrosion – this is the most preventable type of corrosion and happens when it comes into contact with acid-based material, such as household cleaners. It presents as a uniform loss over the entire surface and steels with a pH value less than one are more prone to this kind of corrosion
- Pitting corrosion – this is generally localised in a particular area resulting in cavities or holes and is caused by exposure to environments containing chlorides
- Crevice corrosion – Found in the crevices between two joining surfaces – usually a metal and non-metal – where the exposure to oxygen might be low. This can be overcome with a higher resistant stainless steel
- Stress corrosion – Usually occurs when tensile stressed in combination with corrosive environmental conditions lead to cracking. It’s quite rare, but mostly occurs in applications such as hot water tanks and swimming pools. Another form known as sulphide stress corrosion cracking (SSCC) is associated with hydrogen sulphide in oil and gas exploration and production.
- Intergranular corrosion – This type of stainless steel corrosion is the rarest. It is where boundaries of crystallites are more likely to corrode than inside surfaces. If the carbon level in the steel is too high, chromium can combine with carbon to form chromium carbide. The steel would have to heated for temperatures of 450 – 850 degrees Celsius for this to occur.
- Galvanic/bimetallic corrosion – This is an electrochemical process that could also be called a battle of the species. It’s where one metal corrodes preferentially compared to another in the presence of electrolyte. It can typically be avoided by separating the metals with a non-metallic insulator, such as rubber.
Preventing rust in steel should ideally be undertaken throughout all stages of the steel lifecycle. Consideration in the design and fabrication phase, as well as regular maintenance will limit the likelihood of rust appearing and limit existing rust.
At ShapeCUT we have real working knowledge of what steel type works the best no matter what type of job it’s being used for. Talk to the team at ShapeCUT today to find out more.