a newbie looking for info

[Andy330hp]

Hi,
I'm a recent college grad working at a new job, and one of the projects I am working on involves a number anti-corrosive (I assume) coatings. For example, we have part that is cast Al that is to be "zinc plate with yellow chromate conversion".
I have 2 questions
First, I don't want to waste peoples time explaining the basics, I don't think that is what this site is for, so could anyone just suggest some good reading on the selection of coatings for different applications. I don't necessarily need to get into the detailed chemistry, I am more interested in the functional aspects.

Second and more specific, from what I've read so far, aluminum can be converted directly. So, why is the zinc plate necessary?

 

[sascha]

Hi Andy,

Conversion coatings directly applied to aluminium are normaly used prior to painting. They give some corrosion resistance but are mainly used for good paint adhesion.
Zinc or even zinc-alloy deposits on top of aluminium will give a far better corrosion resistance.
If you need any further information contact me or a chemical supplier like ENTHONE in Woking. They are specialised in surface treatment.
Hope this helps.

 

[kenvlach]

I agree somewhat with sascha.
This supplemental zinc plating prior to chromating is pretty much only ever done for castings, not wrought aluminum.
Aluminum casting have much poorer corrosion resistance than most wrought alloys. This is due to heterogeneous microstructure (lots of Si and intermetallic grains to set up galvanic corrosion cells) and porosity. Also, the chemfilm doesn't 'take' very well on the Si phase.

Yellow chromate on aluminum per MIL-C-5541E, Class 1A, has a 168 hour salt spray requirement, but only wrought alloy (Al 2024, 6061 and 7075) panels need be tested.
Yellow chromate on electroplated zinc (on steel) per ASTM B633, Type II, has a only a 96 hour salt spray requirement.