Surface Protection

Surface protection is achieved by chemical/electro-chemical or physical application of coatings or by changing the component surface into a corrosion or wear resistant condition.

Apart from possible decorative effects, the main objective of surface treatment is the achievement of corrosion protection adapted to stress and requirements.

Hydrogen induced brittle failure may occur on high strength steel parts e.g. due to pickling or electroplating. Therefore, safety-relevant, high stressed parts with a tensile strength of Rm ≥ 1 000 MPa should not be electroplated.

In such cases

− dimensioning shall be such that materials with lower strength can be used alternately,

− a material shall be specified that is not susceptible to hydrogen induced brittle failure,

− a material shall be selected that is durable without surface treatment or

− a different coating process shall be selected in agreement with material development.

Examples of such processes are as follows: 

− special processes (ZNS2, ZNS3);

− mechanical plating;

− thermo-chemical treatment;

− organic coatings.

The parts shall always be cleaned with mechanical or alkaline methods. If chemical cleaning is required due to functional reasons only acids with suitable inhibitors shall be used for the preparation/cleaning.

In case of exceptions, which are subject to agreement with material development, measures shall be taken in accordance with DIN EN ISO 4042 and DIN 50969-1 to avoid hydrogen-induced brittle fractures. 

Requirements for high strength parts

For parts with a tensile strength of Rm ≥ 1 000 MPa the surface and heat treatment processes shall comply with state-of-the-art technology for prevention of hydrogen induced brittle failures.

This applies particularly for electroplated coatings. Even with zinc flake coatings, the parts may not be pickled in acids without inhibitors during pretreatment.

The time between pretreatment and coating shall be as short as possible.

Electroplated zinc and zinc alloy coatings are not permitted for parts with a tensile strength of Rm ≥ 1 200 MPa.