What is passivation?
In chemistry, the term passivation means making something chemically passive - or inactive.
Many metals, such as aluminum and titanium, are self-passivating. The atoms react with oxygen to form a strong layer of oxides. If, for example, iron-containing tools are used with these materials, iron residues can remain on the surface and rust. Passivation in this case simply means the chemical removal of free iron from the surface using an acidic solution.
Similarly, free iron on the surface of otherwise stainless or corrosion-resistant steel alloys (stainless steel) can lead to rust. This depends on the type of stainless steel and the environmental conditions. Again, the free iron must be removed from the surface with an acidic solution. In this case, other components such as chrome or nickel remain on the surface, where they form a protective oxide layer for the base material when in contact with the oxygen in the air, thus protecting it from corrosion. This definition of passivation is laid down in the US standard ASTM A-967.
The passive oxide layer of the stainless steel can be damaged by mechanical processing. Passivation should therefore be the last step in the manufacture of stainless steel products. For example, welding involves local heating, which can change the alloy. Welded areas therefore not only have to be pickled, but also passivated. Stainless steel surfaces can also be attacked by chlorides in the area. Bleaching, salts or a maritime environment are particularly challenging for the rust resistance of stainless steel.
Passivation with citric acid has decisive advantages over passivation with nitric acid. Citric acid is less dangerous in terms of occupational safety and is also more effective to use. This also makes an elementary difference at the atomic level: nitric acid attacks the iron content first, but also reduces the proportion of the other alloy components. As a result, the thickness of the chromium oxide layer can only be represented to a limited extent. In addition, when using nitric acid, heavy metals are released into the bath, which have to be disposed of very costly as special waste.
Citric acid, on the other hand, only attacks the free iron, all other components are left in the surface. This safe chemical enables higher temperatures (up to a maximum of 60 ° C) that not only accelerate the passivation process, but can also create a thicker passive layer. In order to uniformly wet the surface, ultrasound can also be used during passivation, which leads to improved cleaning and passivation. A maximum thick and stable passive layer can be formed using citric acid. To do this, it is also necessary to rinse the material thoroughly and allow it to dry sufficiently. In this way, the formation of the passive layer can also penetrate into deeper layers.
The selective removal of free iron by citric acid can lead to problems if the treated stainless steel contains a high proportion of carbon or sulfur. Both elements, like chrome and nickel, are left on the surface during the citric acid treatment. In the case of carbon, this can lead to gray discolouration. With sulfur, deposits can appear that look like rust. These problems can be easily avoided by treating the workpieces with a highly alkaline cleaner before passivation.
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