Letter 34000

What is the nature of the bond between chromate and zinc  

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Whwn zinc chromate plating is applied to steel, the zinc is electrically deposited, and then the part is dipped in a chromate solution. What is the nature of the bond between the chromate and the zinc?

Dave Butka
molded rubber parts - Bridgeport, CT, USA

Hi,

The film that is formed on the freshly plated zinc surface is a chemically formed one when the zinc metal dissolves in the chromate passivation solution.The Zinc chromate is a complex gel structure with zinc oxide and chromium oxide with water molecules.The hexavalent chromium present in the film is a corrosion inhibitor especially under high humid conditions.The film gets stabilised after 24 hrs of ageing.Until then the film is very soft and is susceptible to mechanical damages.The different colors that appear are due to the light incident on the bright zinc surface through a transparent chromate film of varying thickness.It is also called rainbow passivation or iridescent yellow passivation.The film loses its corrosion-resistant property when it is subjected to heating above 60C due to the loss of water molecules.Hope this information helps you.

Subramanian Ramajayam
- Bangalore, Karnataka, India

Hi Subramanian,

Are you suggesting that if any zinc plated+passivated parts stored around 60C (with the high humidity this may even less than 60C ??), the corrosion-resistant property of the plated part will disappear and the part will start to corrode quickly?

We have some white/red corrosion problem with our yellow zinc plated & passivated nutserts inserted to 316L SS parts shipped form Malaysia where the parts are stored in a container at certain period of time (2 days to 1 week) after assembled. I assume that the temp inside the container may reach to 50C to 60C easily in summer time and that together with the high humidity may trigger the parts to corrode prematurely on the way back to Australia as per your theory..

Any comment(s) ?

Alex Kultur
- Brisbane, Qld, Aust

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