Zinc Plating 2

[dblcrona]

We have several of our parts Zinc plated. We have been finding that many of them have a powdery coating on them. Screw threads on these parts get packed full of this powder. Is this caused by the Zinc plating process or could this be caused by something else after they are plated. Our plater says that we must be getting the parts wet which is causing the problem. Any input is appreciated.

 

[TVP]

One possibility is that the chromate conversion coating that is applied over the zinc plating has been damaged/degraded, resulting in corrosion of the zinc plating. This corrosion is often called white rust due to its white, powdery appearance. Are the parts made from a high-strength steel that requires quenching and tempering to achieve the final properties? If so, check into manufacturing sequence from plating to embrittlement relief to chromate coating. Parts with a hardness greater than 32 HRC require embrittlement relief (usually baking at 200 C for 4 hours) and this must take place prior to chromate coating because elevated temperature causes the chromate to degrade, hence corrosion resistance suffers. Another possibility is mechanical damage, scratches, etc.

 

[dblcrona]

After doing some research on White Rust, I have come to the conclusion that my parts must be gathering condensation at some point during the process after plating. When they come back from the plater they are in open bins stacked on a skid and shrink wrapped. If they gather any condensation, the shrink wrap will prevent air from getting to the parts so they can dry. I am going to have our plater start to band the bins together and eliminate the shrink wrap.

Can anyone tell me if there are any chemicals that can be added to the plating process that will help to prevent white rust?

 

[CoryPad]

Chromate conversion coatings, as mentioned by TVP.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[FinishingTalkdotcom]

There are some things the plater can do to enhance corrosion resistance. Zinc dust, hydrogen peroxide, and carbon treat the zinc solution periodically. This removes metallic and organic contamination that can contribute to premature corrosion. Also, the plater should check the bottom of the chromate tank for dropped parts. Dissolved iron from steel parts will reduce corrosion resistance. The chromate should be dumped and recharged when high contamination levels are detected.

 

[dblcrona]

My plater has recommended that we try a yellow Chromate Conversion coating instead of the clear Chromate Conversion coating that we are currently using. Does anyone have any experience with yellow vs. clear and know how much more corrosion resistant the yesslow is?

 

[CoryPad]

For traditional hexavalent chromium versions of conversion coatings, you get more corrosion resistance as the coating becomes thicker/darker. It goes from clear (sometimes called blue) to yellow to olive to black. In a standard salt spray test like ASTM B 117, the hours elapsed until appearance of red rust goes from ~ 96 for clear to ~ 152 for yellow. There is poor correlation between this test and real world applications, but it is the standard that has been (mis)used for almost 100 years.

Hexavalent chromium has become a restricted substance for many countries/industries. Because of this, you may want to consider trivalent conversion coatings. There are no yellow versions of this, but the thick film trivalent passivates have shown comparable perform to yellow hex chromate coatings.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[kenvlach]

Zinc coatings are ordered by specification, Zn thickness & Type of supplemental coating.
See ASTM B633-07, 'Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel,'

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/B633.htm?L+mystore+qlks3416

A colorless chromate (Type III) should provide 12 hours resistance to the ASTM B117 salt spray, and a yellow (Type II) should provide 96 hours, before the appearance of white rust.

After this supplemental coating has been depleted, the thickness of the zinc provides the number of years before red rust according to the environment. See Appendix to B633.

I have some doubt as to whether the plater properly reactivated the surface after HE bake-out prior to the colorless chromating. If not, effectively no chromate. And, are you testing for the colorless chromate (indirectly via salt spray or chemical analysis?). Ask for the paperwork (shop travelers initialed at each process step, QC signature verifying drying, lab reports for salt spray testing, solution analysis records, etc.). I.e., a supplier qualification survey.

 

[dblcrona]

kenvlach,

I have some doubts about our plater as well. If I were to send some samples of our plated parts to another plating vendor, would they be able to tell if it was done incorrectly?

Ken

 

[kenvlach]

I wouldn't really trust one plater's opinion about a competitor.
Use an independent lab to either
a) test for the presence of colorless (clear) chromate per ASTM B201 (an easy test to do yourself although requires lead acetate solution) or
b) salt spray test per ASTM B117 to see if meets the 12 hours w/o white rust for clear chromated zinc per ASTM B633.

 

[FinishingTalkdotcom]

There are many clear chromates on the market, some offering much more corrosion protection than others. For example, we have a clear chromate that will pass 48 hours in salt spray to red rust, and another that will pass 120 hours. We offer zinc plating with clear/blue, yellow, and black chromates - all hexavalent free. If you would like an additional source for zinc plating, please contact me at smoore@scovill.com., even if it is only for comparison purposes. I will be glad to help you with any of your plating needs.

 

[kenvlach]

Interesting table, of coatings for fasteners ranked from best (1000 hours salt spray before red rust) to worst (< 6 hours). Also, given is a column for hours to 'white corrosion products' for zinc & zinc alloy coatings.

http://www.agrati.com/uk/gamma/speciali/index30.htm