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Acid zinc barrel inconsistent blue chromate

November 13, 2008

Hi guys

We are a small general production rack and barrel zinc plater in New Zealand.

Recently we have been having problems maintaining a good blue trivalent chromate colour in our automated acid zinc barrel line,especially when plating screws.
One or two barrels can be bright blue and the next 10 might have little or no chromate colour at all! Does this sound familiar to anyone?
Some general barrel chromating tips and information would also be helpful. It seems a lot of platers have quality issues regarding chromate conversions.

Any advice would be greatly appreciated.

Many thanks,

Michell Jagersma
Bright zinc plater - Auckland, New Zealand

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My first presumption would be that you have a problem with drag in and/or pH maintenance. When you loose the blue color, that does not necessarily mean that you are not getting a conversion coating. Many trivalent blue bright chromates begin to lose the blue color when the pH rises (typically above 2.2). This often is because the chromate film is actually thicker and the refracted light makes the coating appear clear or slightly yellow. Another reason may be that the loads have a larger surface area than you are replenishing for and you are exhausting the chemistry quickly (or dragging out more/dragging in more water for the same net result).

One way to control concentration and pH is by using an automatic feed pump hooked to a pH controller.

As I usually tell people "ask your supplier".

Also, blue coloration can be a factor of drying heat. Inadequate heat may cause less blue. It may be a factor of larger load sizes when doing screws.

Really, what you need to do is to try to eliminate each variable.

Gene Packman
process supplier - Great Neck, New York
November 17, 2008

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Thanks For your input Gene.
Raised ph within barrel at chromate station due to drag in. sounds a good place to start.
we do have a .3% nitric pre-dip. however the conversion tank pH is always on the rise, indicating chromate consumption and or drag in. Typical blue pH 2.1-2.3

regards Michell,

Michell Jagersma
- Auckland, New Zealand
November 18, 2008

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Michell,

I would suggest using 0.5% by volume of nitric acid as a predip and I would check it twice a shift. The simplest way would be to check the pH of a freshly made solution at the percentage you wish to use (at these low percentages you will be able to use pH) and then check the tank twice a shift to maintain.

Most manufacturers of trivalent chromates (clear/blue bright) prefer that the pH adjustment to lower the pH be performed by the addition of product not acid. The reason (I am a supplier) is that the chromate is a product of a number of chemicals some of which may have been reacted in the manufacture of the product. Therefore, the addition of a single acid, even if you "know" that the product contains this acid, can disrupt the formulation.

Gene

Gene Packman
process supplier - Great Neck, New York
November 22, 2008

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Try to increase nitric acid bright dip to 0.5% and use better chromate with wider operating pH parameter. (1.8 to 3.0) are available now.

P. Gurumoorthi
electroplating process chemicals - Chennai, Tamilnadu, India
November 25, 2008