It is a common belief that all stainless steel is rustproof. Our service experts keep getting asked whether that is true. Not all stainless steel is rustproof. Even stainless steel can corrode under certain conditions. You can find out why this is so in today’s blog post.
What types of stainless steel are there?
The term stainless steel is divided into alloyed, unalloyed, and stainless steel. In this context, stainless steel is often called just stainless steel.
To answer the question when “stainless steel” can rust, it must first be clarified why ordinary steel rusts. Unalloyed steels or unalloyed and low-alloy stainless steels primarily consist of the chemical element iron. In the atmosphere, the iron of the unprotected steel in connection with a moist, oxygen-containing environment can form iron oxide (rust).
The oxygen also reacts on the surface of the stainless steel. Since, by definition, stainless steels must have a chromium content of at least 10.5%, the oxygen reacts preferentially with the chromium. A firmly adhering, dense chromium oxide layer forms instead of an iron oxide layer. This chromium oxide layer is called a passive layer in technology. It is a few atomic layers thick and not visible. Small mechanical damage to the layer is repaired immediately and self-healing in the presence of oxygen.
The most popular colloquial stainless steels are V2A and V4A. The V2A is also available as 18-8 or 18-10 in the household items, such as z. B. pots and cutlery knew. V4A has a high level of corrosion resistance and, for this reason, is often used in swimming pools and the chemical industry.
When can stainless steel rust?
The passive layer is responsible for protection against corrosion and the aesthetics of the products. For this to persist, it is imperative to clean the stainless steel regularly. A protective layer can only be formed and maintained on a clean surface.
Contamination by environmental influences, such as B. sulfur dioxide, soot particles, but also through other compounds (road salt, table salt, aqueous liquids containing chloride, such as drinking water) can lead to permanent damage to the protective layer. If such damage is present, a corrosion attack can occur, and the material rusts. The occurrence and progression of rust are highly dependent on the choice of material. As a simple rule, the higher the alloy of the stainless material, the higher the corrosion resistance.
Brown rust deposits on stainless steel are rarely corrosion products of the stainless steel. In most cases, these come from particles of unalloyed steel that have reached the surface and form rust (extraneous rust) there.
The following practical examples provide a further explanation:
Example 1: You have a stainless steel railing outside of your house. There is an intersection nearby. When the vehicle brakes, brake dust with rusting material is generated, which lands on the stainless steel railing via the air, in the presence of humidity, these microscopic particles can corrode on the stainless steel surface. One speaks here of flash rust. If you clean these “stains” in time, the corrosion resistance is retained.
Example 2: You have a stainless steel sink in your kitchen. If you put a paper clip in the wet sink in the evening, it will rust the next morning. Remove the clip, and you will see a slight discoloration on the sink that matches the contour of the paper clip. The remnants of the rust can be traced back to the clamp. It can be easily removed with a household cloth.
In the case of corrosion, when two different metals come into contact, it is often referred to as “contact corrosion.” An increased corrosive attack always accompanies this on the “less noble” metal of the pairing. In the example mentioned, it would be the paper clip. Does stainless steel come with other common metals such as B. Iron or aluminum in contact, this is the nobler partner, and there is no risk of corrosion as long as the passive layer is present.
It all depends on the right tool.
Therefore, when processing stainless steel, make sure that you use the right tools. Please do not use tools that were used to machine normal steel. Otherwise, stainless steel could be contaminated. Our tip: Use separate tools for stainless steel and steel.
How can you remove rust from stainless steel?
How stainless products are cleaned is highly dependent on the degree of soiling. If the surface is slightly discolored, it is usually sufficient to use a commercially available sponge, water and dishwashing detergent. After the cleaning process, rinse with clean water and dry the cleaned surface.
If the stains are stubborn, you can work with mild scouring milk and sponge. Rinse with water and dry afterward. In principle, cleaning agents containing chlorine should be avoided in the private sector.
If pronounced damage can be seen, abrasives must be used to remove material. In industrial applications, stainless steels are often pickled in acidic pickling baths.