Zinc Electroplating - Question About Passivation

[chez311]

I was doing some research into zinc plating, and I came across zinc plating types V and VI "colorless/colored passivate" as specified by ASTM B633 which I assume have non-chromium conversion coatings. After reading the specification and doing some research it seems that these categories were introduced to provide a category for alternatives to hexavalent chromate conversion coatings. I have seen some people specify trivalent chromate coatings under type V and VI however that does not make sense to me as a) nowhere does it say type II and III chromate coatings are hexavalent only and b) it specifically notes under section 1.8 that types V and VI are "non-chromate passivate treatments". Where does trivalent chromate belong?

In addition I cannot seem to find much if any information on coatings that fall into this category of type V or VI - is there any widely accepted (or at least somewhat common) conversion coatings that satisfy this requirement/fall into this category? I know the term "passivate" can be somewhat imprecise and can also refer to a process that renders the surface passive through the removal of surface impurities and/or encouraging creation of the oxide layer however I can't really find anything for such a process on a zinc coating/electroplate.

Thanks in advance for your help.

 

[SwinnyGG]

To start off, B633 is not the most clear specification out there.. but.

1) As far as I am aware, all type I coatings are hex chromium.

2) As far as I am aware, all other types may be hex or tri chromium depending on the specific plating house doing the job

3) Types V and VI cover passivation coatings- this is different than that base coating and represents an additional step. So a part could be coated in trivalent chromate, and then passivated, which puts it in V or VI

The spec just isn't that clear. If you need trivalent chromate for ROHS compliance, specify ROHS compliance on your drawings as part of the coating spec.

 

[chez311]

I would tend to agree, some of the language is a little confusing. What alternate specification would you recommend or have you seen commonly used?

When you refer to type I coatings, type I is defined as being "as plated without supplementary treatments" - do you mean type II or III? If so I don't see anywhere defining them as such... If I were specifying it I would probably do as you said and notate Type II or III RoHS compliant/hexavalent chromium free.

So are you saying that a zinc plated part that is given a trivalent chromate coating which would typically put it in the type II or III category could be additionally passivated and would therefore be a type V or VI? In that case what process would that entail? I have not come up with anything about passivation of chromate coatings - typically chromates are regarded as a passivation coating in and of themselves. This is additionally confusing as I noted the standard specifically qualifies type V and VI coatings as non-chromate in section 1.8

 

[SwinnyGG]

Well the critical thing to remember here is that the standard doesn't directly specify materials- it specifies standards of performance.

Read 6.7- the passivation is another film applied over the base zinc coating. The base zinc can be chromate, the passivation layer is not.

My statement that 'As far as I am aware, all type I coatings are hex chromium.' is not based on any direction from the standard- it's just based on what the market makes available to mee ttype I.

I really dislike this standard- if your goal is to gain an in-depth knowledge of zinc plating and alternatives, in my opinion you'll learn way more by talking to a skilled plating house that you trust, than you will by reading standards like this one that don't really tell you much.

 

[chez311]

In that case I am wondering, do you know of any such passivate coatings or processes (other than those already described as either utilizing chromium or phosphate) that could be applied to either a bare zinc plate, zinc chromate, or zinc phosphate coating? I could not find any in my search. My initial assumption was that this could be something similar to stainless steel passivation, but I have as of yet come up empty.

That is good advice about contacting a plating house, however I currently specify the coating requirements to our tier 1 suppliers which is handled through their supply chain so that will be something I will have to seek out. Thanks for the tip.

 

[Compositepro]

There is good information here:

http://www.infinitechfinishing.com/articles/article7.htm

 

[SwinnyGG]

In that case I am wondering, do you know of any such passivate coatings or processes (other than those already described as either utilizing chromium or phosphate) that could be applied to either a bare zinc plate, zinc chromate, or zinc phosphate coating?

I am not aware of the specific chemistry of the sealers used for passivation per 6.7- I have experience with a few products produced with type VI coatings, but the steps used were from a specific supplier and were proprietary; we purchased their services based on ROHS compliance and performance of the parts.

My initial assumption was that this could be something similar to stainless steel passivation, but I have as of yet come up empty.

'Passivation' is a broad chemical term that just means reducing the surface energy of a part so it is harder for corrosion to initiate. In the materials world, 99% of the time when people say 'passivate' they are talking about a stainless steel part- where it is thoroughly cleaned and then acid dipped, so that the oxide barrier is uniform and complete, and free of contamination.

Zinc passivation is not necessarily the exact same process (it very well could be- I'm not an expert and have only purchased treated parts, not dug into the process) but the chemical result is the same, and the reason for improved corrosion resistance is the same.