Like the name suggests, stainless steel does not corrode or rust like carbon steel does. However, stainless steel is not 100{19cc6dfde2095820d75378ef5ea2a7429af84c4e3caa3f1811d59515c3bb9677} stain proof and in certain situations can be subject to staining, especially in coastal regions. What separates stainless steel from carbon steel is the amount of chromium that is present in the steel and that consequently forms a protective layer or passive film of chromium oxide. This film prevents corrosion by blocking oxygen diffusion to the steel surface and prevents corrosion from spreading into the metal’s internal structure. To prevent stainless steel from staining, a basic maintenance routine is recommended. We recommend cleaning stainless steel as often as you would your windows. Cleaning can be performed with clean water or a detergent. For stubborn marks we recommend contacting your supplier for expert advice. A basic maintenance schedule will help ensure that your stainless steel has a long, stain free lifespan. The most common type of stainless steel staining is known as tea staining. Tea staining is the brownish discolouration of the stainless steel surface. It does not affect the structural integrity of the steel and is entirely aesthetic. Tea staining occurs most commonly within about five kilometres of the ocean and becomes progressively worse the closer the stainless steel is to the shore. However wind exposure, pollution levels, local sheltering and high temperatures are known to create environments where tea staining has occurred up 20 kilometres from the ocean shore. In total there are 6 factors that play a part in tea staining, namely:
- Presence of corrosive substances – The presence of sea salt on the surface of the stainless steel is one of the major factors that cause tea staining.
- Atmospheric conditions – A combination of atmospheric conditions with high humidity and a high temperature creates optimum conditions for the occurrence of tea staining.
- Surface orientation and design – Poor drainage promotes tea staining whether it is because the surface is near horizontal or has a texture that traps contaminants.
- Surface roughness – Deep grooves or metal folds on a surface are more susceptible to staining because they can trap salts (chlorides). When the surface dries the salts become concentrated, making the conditions more aggressive.
- Surface characteristics – To achieve the best corrosion performance of a stainless steel, the surface should be clean, free of contamination such as carbon steel or manganese sulphide inclusions, and have a continuous passive layer. Neglecting the surface of stainless steel increases the chances of tea staining.
- Appropriate grade – There are several hundred grades of stainless steel with different chemical composition but only about 10 in common use. All owe their corrosion resistance to the thin chromium oxide film on the surface, although other additions such as molybdenum and nitrogen can improve the corrosion resistance, especially in corrosive environments.