Does Stainless steel oxidise?
In short, yes, but at a rate much, much slower than other forms of steel, hence its long durability and superior performance. A study in a highly corrosive coastal environment showed that Grade 316 Stainless had 9,000 times the life of carbon steel - so Stainless definitely lives up to its name!
Stainless steel corrosion can occur due to a handful of factors but can be prevented against and in many cases, treated. Keep reading to learn how to get the longest life out of your Stainless, and what we can provide to help!
Why is Stainless steel "Stainless"?
Stainless steel resists corrosion better than many other metals because of a very thin, colourless passive layer that forms spontaneously on the surface.
This is thanks mainly to chromium content. Stainless steel contains a minimum of 10.5% chromium, which can increase in different alloys. The most common grades are Austenitic grades (e.g. Grade 316), which comprise over 70% of total Stainless steel production & contain a minimum of 16% chromium, yielding strong protection against oxidation (rust). Unlike iron, which develops rust (iron oxide) when exposed to oxygen, the chromium in Stainless steel develops an outer-layer film that is highly resistant to further oxidation and protects the underlying metal.
Stainless steel can thus look virtually brand new after years of elemental exposure or use.
So why does Stainless steel oxidise?
All Stainless steel contains iron in varying ratios. When the passive layer of Stainless steel is compromised, this exposes the iron to the oxygen in the environment and allows it to oxidise, resulting in rust.
The addition of salts (chlorides), harsh chemicals, heat, an electric current, or even a combination of these will typically speed up how quickly rust forms.
If the Stainless steel's surface is contaminated with carbon steel, this can also cause rust to form. This is seen in practice through poor handling and fabrication practices.
How does the passive layer corrode, resulting in rust?
The corrosion of Stainless steel can be categorised into 7 different main forms:
1. Abrasion
One of the biggest threats to Stainless steel work surfaces, equipment, and other areas which might see regular interaction.
This makes it a common culprit for rust in residential, food service, and medical settings. It can also occur during fabrication as metals are cut, sanded, or otherwise worked. When you bang other items into Stainless steel or run hard items across the surface, you risk penetrating the passive layer. Additionally, abrading the surface (particularly with finer surface finishes like No 4 and higher) through scratches decreases corrosion resistance as at a micro level.
2. General corrosion (or uniform corrosion)
General corrosion is the uniform loss of metal over entire surface. Stainless steels tend not to suffer from general corrosion unless exposed to highly oxidising environments such as certain acids.
3. Galvanic corrosion (or bimetallic corrosion)
Galvanic corrosion occurs when two dissimilar metals are immersed in a conductive solution and are electrically connected. One metal (the cathode) is protected, whilst the other (the anode) is corroded. The rate of attack on the anode is accelerated, compared to the rate when the metal is uncoupled.
For example, if aluminium and carbon steel are connected and immersed in seawater, the aluminium will corrode more quickly, whilst the steel will receive protection.
4. Intergranular corrosion
Exposure to temperatures between 550C and 850C (i.e. during welding) or prolonged periods, can lead to an issue known as weld decay, or sensitisation.
At these specific temperatures, chromium carbides form at the boundaries of grains, thus reducing the available chromium to resist corrosion in adjacent areas.
5. Pitting corrosion
Pitting is localised corrosion resulting in cavities or holes and occurs when Stainless steel is exposed to environments containing chlorides (including coastal and marine environments).
While Stainless steel offers good acid resistance, there are certain grades that fare better when used with strong acids and high saline environments.
6. Crevice corrosion
Crevice corrosion is localised at the crevice between two joining surfaces and is formed between two metals or between metal and non-metal.
Concentrations of chemicals can increase quickly in tiny spaces leading to corrosion that spreads fast with little or no warning. When possible, always ensure that submerged parts see a steady movement of any chemicals and allow for plenty of air circulation in dry areas.
7. Stress corrosion cracking
Tensile stresses in combination with corrosive environmental conditions can lead to cracking in some grades of Stainless steel, particularly Grade 304 and Grade 316. This is similar in nature to intergranular corrosion.
How do I prevent Stainless steel corrosion?
Follow these tips to get the most out of your Stainless steel!
1. Get the right Stainless steel for the job.
Make sure that the grade of Stainless steel is suitable for the use and environment it will be subjected to. Our friendly team can help with your questions and enquiries.
2. Ensure your full design is corrosion-proof.
Proper planning in the design stage of will reduce the potential for surface damage. For example, in a piping system, drainage holes for water should be used when possible, and cavities and crevices limited. The design should also encourage air to circulate freely throughout the application.
Where harsh chemicals are present, be sure to allow tolerance windows. There will often be areas of high and low concentration. Always consult with qualified engineers to conduct risk assessments and design with tolerances above what you plan to encounter during typical operations.
Where galvanic corrosion might occur due to dissimilar metals in contact, eliminating contact through non-conductive materials (e.g. Teflon washers) can be necessary.
3. Avoid contamination in the fabrication process.
During the fabrication stage, it is important to prevent Stainless steel from contacting iron or ordinary steel. This requires surveying the surrounding worktables, tools, storage units, steel turning rolls, and chains.
Any carbon steel dust particles settling onto the Stainless steel during fabrication can contaminate its surface, increasing the potential for rust formation. Furthermore, cleaning and grinding tools that have been used with carbon or low alloy steel must be kept separate from Stainless steels.
4. Maintain your Stainless properly.
In most cases, warm water and a mild detergent are enough to perform Stainless steel cleaning. For tougher cases, there are Stainless steel specific cleaners that will not compromise the passive layer, which can be found in our Shop.
For cleaning which requires physical contact, be sure to use soft cloths or iron-free scouring pads. Ensure abrasive materials have not had previous contact with steel materials.
5. Bolster the passive layer.
Passivation treatments use specific chemicals to promote rapid healing or growth of your Stainless steel’s passive layer. While the passive layer does naturally form under ambient conditions, the application of passivation treatments can result in a more durable protective film.
Pickling treatments use specific acids to help counteract the effects of welding or high temperatures on the surface passive layer. They’re ideal for removing mill scale, oxides & reducing impurities or removing stains. These are generally required after welding in particular to ensure optimal corrosion resistance.
A rhyme for your time
If you don't want your Stainless steel to rust, these tips and tricks are an absolute must!
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