Stainless steel is subject to corrosion if the carbon steel (free iron – Fe) is not protected by a layer of chromium oxide (Cr203). This happens naturally when the chromium makes contact with the oxygen (O2) in the air.
The problem is when this oxide barrier is scratched or damaged and the newly exposed carbon steel cannot re-form. If no chromium, or not enough, is available to cover the damaged area then the corrosion can start and spread into the sublayers.
The key is to create a chromium-rich surface by removing as much iron as possible and at the same time leave as much chromium as possible behind. This drastically increases the likelihood that an oxide will form before corrosion can take hold.
One of the ways to do this involves a process called Chemical Passivation. The process of passivation has two-steps. The first step, like we mentioned, is to remove any free iron or other contaminants that are on the surface. The second step is to use an oxidizer to supercharge the conversion of chromium on the surface into an instant, strong and uniform oxide layer.
The various methods of passivation
1. Nitric acid – In the past, nitric acid was the most commonly used chemical method to passivate stainless steel, and in some cases it is still a preferred chemical. Nitric acid is a strong mineral acid and works very effectively to dissolve the iron and other contaminants that are on the surface. More importantly nitric acid self-oxidizes the chromium, so a two-step process can be combined, thus saving time. The problem with nitric acid is that it is difficult to handle and to dispose of after use. This process is hazardous and unfriendly to the environment.
2. Citric acid – Citric acid is safer to use than nitric acid because it’s biodegradable and produces fewer to no environmental issues. Citric acid is commonly used in food processing and domestic cleaning supplies. However, it is not an oxidizer, so it cannot oxidize the chromium automatically. For the oxidation to take place a 2nd process is necessary to create the protective properties of the chromium oxide. This can be completed with common air, however, for the best results the surface is sometimes subjected to a peroxide solution to speed and improve the conversion process.
3. Electropolishing – Electropolishing is not true passivation but we are discussing it here because, although the process is different, the results are similar. With electropolishing the iron is removed from the surface by immersing the part in an electrolyte solution and then applying a positive charge under controlled time and temperature. This process removes the iron and leaves behind the oxidized chromium.
One important difference is in the surface finish. During the electrolysis process the high points of the surface get removed first, making the surface much smoother. Smoother surfaces not only look better but have improved anti-corrosion properties because less surface area is exposed to the degrading effects of the environment.
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The Federal Group USA