It is a widespread assumption that all stainless steels are non-rusting. Our service experts at materials4me are regularly asked whether this is true? In fact, not every stainless steel is non-rusting. Even non-rusting stainless steel can corrode under certain conditions. Learn why this is the case in today’s blog post.
What types of stainless steel are there?
In principle, the concept of stainless steel can be divided into alloyed, unalloyed and non-rusting stainless steels. In this context, non-rusting steels are often simply just called stainless steel.
To answer the question of when “non-rusting” stainless steel can rust, we must first explain why typical steel rusts. Unalloyed steels as well as unalloyed and low-alloy stainless steels, consist primarily of the chemical element iron. In the atmosphere, the iron of the unprotected steel can form iron oxide (rust) in combination with a damp, oxygen-containing environment.
The oxygen also reacts on the surface of the non-rusting stainless steel. Because non-rusting stainless steels by definition must have a chromium content of at least 10.5%, the oxygen preferably reacts with the chromium. It forms a firmly adhering, dense chromium oxide layer instead of an iron oxide layer. This chromium oxide layer is called the passive layer. It is just a few atoms thick and is not visible. Small areas of mechanical damage to the layer are immediately repaired in a self-healing process in the presence of oxygen.
The generally best-known non-rusting stainless steels are V2A and V4A. V2A is also known as 18-8 or 18-10 in the area of household appliances, such as pots and cutlery. V4A has a high corrosion-resistance and is often used for this reason in swimming pools and the chemicals industry.
When can non-rusting stainless steel rust?
The passive layer is responsible for the protection against corrosion and for the aesthetic properties of the products. To ensure that this is maintained, it is absolutely essential to regularly clean the non-rusting stainless steels. A protective layer can only form and be retained on a clean surface.
Contamination due to environmental influences, such as sulphur dioxides, rust particles and other compounds (road salt, cooking salt, aqueous liquids containing chloride, such as drinking water) can result in permanent damage to the protective layer. When such damage occurs, it can lead to corrosion and rusting of the material. The occurrence and progression of rust depends significantly on the selected material. As a simple rule, the higher the amount of alloying of the non-rusting material, the higher the corrosion resistance.
Brown rust coats on non-rusting steel are rarely corrosion products from the non-rusting steel. In most cases, these original from particles of unalloyed steel that land on the surface and form rust there (called extraneous rust).
Practical examples for further explanation:
Example 1: On the outside of your house, you have a stainless steel railing of non-rusting stainless steel. There is an intersection nearby. When the vehicles brake, this produces brake dust with rusting material that travels through the air and lands on the stainless steel railing. Upon contact with air moisture, these microscopically small particles can corrode on the non-rusting surface of the stainless steel. This is referred to as film rust. If these “spots” are cleaned in time, the corrosion resistance is retained.
Example 2: In your kitchen, you have a sink of non-rusting stainless steel. If you leave a paper clip lying in the still damp sink basin in the evening, it will be rusted by the next morning. When you remove the paper clip, you will see a slight discolouration on the sink in the shape of the paper clip. This rust residue can be attributed to the paper clip. It can be removed easily with typical household cleaning cloths.
In the case of corrosion upon contact between two different metals, this is often called “contact corrosion”. This is always associated with an intensified corrosion of the “less stainless” metal of the pair. In the example above, this would be the paper clip. If non-rusting stainless steel comes into contact with other widely used metals, such as iron or aluminium, it is the more stainless partner and is therefore not at risk of corrosion as long as the passive layer is present.
The right tool matters
For this reason, pay attention to using the right tool when working with non-rusting stainless steels. Please do not use tools that were used for the working of normal steel. Otherwise the non-rusting stainless steel could be contaminated. Our tip: Use separate tools for stainless steel and normal steel.
How can rust be removed from non-rusting stainless steel?
The type of cleaning for non-rusting products depends significantly on the degree of contamination. If there is a slight surface discolouration, it is generally sufficient to use a typical sponge, water and a washing-up liquid. After cleaning, rinse with clean water and dry the cleaned surface.
If the spots are stubborn, a mild scouring cream and a sponge can be used. Here as well, rinse with water and dry afterwards. In principle, cleaning agents containing chlorine should be avoided in private use.
If pronounced damage is apparent, abrasives must be used to remove material. In industrial applications, stainless steels are often pickled in pickling baths containing acid.
Various stainless steel materials at materials4me
Do you need a specific stainless steel material for your next project? We offer V2A or the higher-grade V4A non-rusting stainless steel for special applications. We invite you to take a closer look at our thyssenkrupp online shop.
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