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Zinc Plating & Passivation Of Mild Steel Sheet Work

Zinc plating and passivation is traditionally used to provide “sacrificial” corrosion protection on mild steel and is usually deposited on commercial sheet metal work at thicknesses varying from 5μm up to 25μm depending upon the levels of corrosion protection required for the finished component application. Sheet metal components which require zinc plate coatings are normally surface passivated by submersing the steel work in a chromate solution to increase the resistance of the zinc to atmospheric corrosion.

With the right pre-treatment and preparation zinc can be deposited on almost any metal but the most common being mild steel.

Zinc is deposited from one of these three types of plating solution;

 

  • Alkaline Con Cyanide
  • Cyanide
  • Acid Chloride

 

The deposits gained from each of these solutions have different properties. Differences include; their appearance, coating thicknesses, successful coverage of deep recesses and various mechanical properties such as the ductility of the final subsequent deposit.

When zinc plating, the plated components are usually passivated to maximize the protection offered by the zinc on the steel by immersing the sheet metal parts in a chromate solution. There are various classes of chromate coatings that can have different levels of corrosion protection in addition to their different aesthetic appearances. The appearance of the finish cannot be guaranteed from batch to batch due to the batch usage. With the introduction of the ELV and RoHS directives; passivates contains hexavalent (Cr VI) that cannot be applied to components that are going to be used in automotive and electrical/electronic industries. Current chromate solutions that are being commercially used are:

 

  • Black passivate – Contains Cr VI not ELV/RoSH complaint
  • Thick film trivalent - Contains Cr III ELV/RoSH complaint, in this class of passivate is developed to approximate the corrosion resistance of the colour/Yellow passivate as an ELV/RoSH complaint alternative.
  • Clear/Blue - Contains Cr III ELV/RoSH complaint

 

We are often asked to have mild steel sheet metal components zinc plated and clear, yellow, silver or black passivated is where the protection against atmospheric corrosion is more important than the final cosmetic appearance. The colour of zinc plating surface treatments can vary from batch to batch and matching components can be a problem. If a consistent finish or colour is required then powder coating or wet spraying may be a more suitable option.

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