Elektrolytic zinc plating

Introduction

The electrolytic galvanising of parts is mainly intended to protect the parts from corrosion, but it can also be done for decorative reasons.

By means of electrolysis, zinc is applied in relatively thin layers (a few microns) giving the part a shiny appearance. Then the zinc layer is passivated in one of the following colours: blue, yellow, black or olive green. For convenience, people talk about yellow passivation, blue passivation, etc.

Duroc offers as standard the blue and yellow passivation. For specific projects, black and olive green are also available.

In addition to the difference in colour the various passivations give a different corrosion resistance. Thus, the yellow passivation offers a better corrosion resistance than the blue. The flipside of the coin is that the yellow passivation, in contrast to the blue, contains toxic hexavalent chromium. Blue passivation is performed on the basis of trivalent chromium.

Hexavalent chromium

As a result of European directives 2000/53/EG and 2002/95/EGthe use of (hexavalent chromium-based) yellow passivation is restricted in certain sectors. So in order to obtain a high corrosion resistance blue passivated parts can be given an additional top coat (Delta Coll), or the parts can be treated with a silicate nanoparticle reinforced passivation layer (Zink-Top). Delta Coll gives higher corrosion protection than Zink-Top and also regulates the friction coefficient, but it is more expensive than Zink-Top.

Hydrogen embrittlement

Steel with a high tensile strength (>= 1000 N/mm²) can become brittle when taking up hydrogen. During galvanising steel parts come into contact with hydrogen, amongst other things, during the staining process. To achieve zero-risk of hydrogen embrittlement, other surface treatments such as Delta Protektneed to be applied. However, the risk of brittle breakage can also be minimised by giving the galvanised parts a thermal treatment (annealing or tempering).

more.....