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Porous diecasting after chromate conversion coating



(-----) 2004

Hi there,

Problem encountered: Surface of zinc alloy die-casted part very porous after chromate conversion coating (clear chromate).

Question: Does this got to do anything with the chromate process or has it got to do with casting process? Anyone has similar experience before? Any ideas are welcome.

Thanks in advance for help.

ML Michelle
die-caster - Singapore
^


2004

A peek is worth a thousand finesses, Michelle. What did the part look like before chromating? It could be the casting, it could be aggressive mechanical treatment, or it could be an over-active cleaning solution in the chromate processing. My guess is mechanical treatment.

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


2004

Dear Mooney,

Thanks for response.

First of all, most basic question: Is roughness = porous?

I do not have any equipment to check the porous level of die-cast surface before chromating. The part did not undergo any mechanical treatment except clear chromate finishing. If this is due to die-casting, is there anyway I can reduce the surface porous through any chemical processes? I checked some parts from other supplier (different die-caster and plater), surface is not as porous compare to ours after clear chromating but our parts is heavier, so more dense.

Assuming my die-cast part surface is good, how does an overactive solution causes the porous? I am sorry that I have to ask from layperson point of view.

Again, thanks for being patient with me.

ML Michelle
- Singapore
^


2004

No, roughness and porosity are not the same thing. Porosity refers to small holes in the coating or the substrate that exposes undesirable materials. In the case of a diecasting, the outside skin is hard and smooth; below the skin, the material is porous. Break any scrap diecasting to see what the bulk of the cross section looks like.

In general if the skin is broken, it is not possible to plate or chemically treat the diecasting satisfactorily.

Inspection of diecastings takes a little experience, but porous castings or castings with a "cold shut" will not take to plating or chromating properly.

The castings are presumably cleaned in a caustic cleaning solution before chromating, and if this solution is overly aggressive it will etch the parts, making them rough and porous.

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

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