Pickling of stainless steel

1. Characteristics and properties

After appropriate chemical treatment, stainless steels are non-rusting, have a high surface hardness, can be produced as spring steels, are mechanically very resilient and have an optically attractive appearance. Throughout Europe, so-called “stainless steels” are standardized in EN 10088/Part 1-3. According to this standard, steels must have a chromium content of at least 10.5% and a carbon content of less than 1.2%. The main negative effects on the stainless properties of stainless steels are scale layers resulting from heat treatment, tarnish from welding and grinding, residues of welding spatter, deposits of metal oxides, extraneous rust from normal steel parts which can be transferred by contact, and abrasion of the steel tool parts which is transferred during the mechanical processing of the stainless steel parts.

The pickling of stainless steels differs considerably from the pickling of normal steels, as it is supposed to lead to the removal of unwanted foreign substances, but without attacking the actual surface of the stainless steel. In particular, the surface of the stainless steel consisting of chromates must not be destroyed, as it is primarily responsible for the stainless properties. For this reason, chloride-based acids must not be used. By combining several acids, the stainless steel can be completely pickled without irreparably destroying the chromate layer. During this process, the surface is additionally passivated against contact corrosion.

2. Benefits and areas of application

The areas of application for pickling stainless steel are very diverse. Basically, it is suitable for all stainless steel workpieces which are subject to thermal treatment, which show an attack of rust spots or which have been soldered or welded in any form. The same applies to stainless steel workpieces intended for decorative applications, as pickling and passivation improve the resistance to hand perspiration by touch.

3. Basic material

Stainless steels are divided into 4 subgroups. Stainless austenitic steels, stainless ferritic steels, stainless martensitic steels and stainless austenitic ferritic steels. The pickling acids and processing parameters are matched to the respective stainless steel group. In addition to chromium and nickel, vanadium, molybdenum, aluminium, titanium, tantalum, niobium and tungsten are the most common alloying partners found in stainless steels. However, the rust resistance of these steels does not depend solely on their composition. Among other things, a smooth, clean, metallic surface is important for the rust-resistant behaviour of these steels.

4. Required delivery condition

In order to achieve an optically and technically excellent result by pickling stainless steel, it is necessary to process workpieces that have a surface quality that permits this. The workpieces must be free of dirt, oils and greases.  Silicone-containing, alkali-/alkaline-earth-containing oils as well as high-pressure additives must be avoided at all costs.

5. Pre-treatment

Before pickling, the workpieces must be thoroughly degreased and cleaned. There must be no oil or grease on the surface, as this would not allow pickling in the areas wetted with oil and would therefore not achieve a perfect result.