The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Corrosion resistance of Bronze Dichromate vs. Decorative Chrome Plating

Hello! I am an engineering student currently on a COOP (internship) term. I was wondering how does the Corrosion resistance of Bronze Dichromate fare against Decorative chrome plating. Both are on leaded steel (SAE 12L14).

I know the Dichromate is cheaper and is yellow/gold colour, but are there other reasons why I might choose one over the other? (ie: adhesion?)

Joey Lee
Engineering COOP Student - Vancouver, BC, Canada
May 18, 2011

Hi, Joey. First, we need to clarify what it is that we are talking about so that we do not mislead the readers.

When we speak of bronze dichromate, there is no bronze metal involved; rather, we are referring to the color of the finish: vaguely the color of bronze. Secondly, we need to note that dichromate cannot be applied directly to steel; rather the steel must first be electroplated with something that the dichromate solution can react with. Most commonly this would be zinc electroplating, although it could be a zinc alloy, or cadmium plating.

When we speak of decorative chrome plating, we are actually speaking of a rather heavy layer of nickel plating followed by a very thin flash of chrome.

Now for the comparison: decorative chrome plating is decorative; zinc plating plus dichromate is considered an engineering finish rather than a decorative one. The chrome plating is hard and abrasion resistant, the zinc plus dichromate is not. But perhaps the single biggest difference is that the zinc plating plus dichromate is a sacrificial plating which offers cathodic protection to the substrate whereas the chrome plating is a barrier layer plating which, once compromised by a scratch, scrape, or porosity actually accelerates the rusting of the underlying steel. If the chrome plating is of very high quality and is not compromised to allow exposure of the underlying steel, I think it will significantly outlast the dichromate.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 2011