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Maximum operating temperature of zinc plating with chromate conversion


An ongoing discussion from 1998 through 2016 . . .

(1998)

Q. We are using a threaded steel fitting finished with zinc plating & chromate treated to a bronze color. The fitting is exposed to conducted heat during an aluminum brazing process, but is not directly exposed to the flame heaters.

After brazing, the fitting and tube are water quenched to cool & remove flux. The plating on the fitting has darkened and is flaking off.

My question is, What is the maximum operating temperature of zinc plating with chromate conversion treatment?

Grant Repar
climate control - Belleville, Ontario, Canada


(1998)

A. The chromate starts degrading at about 140 °F because it is a very thin gel and is being dehydrated. The high temp of brazing nearby and then quenching is a very severe test of plating. If the plating is on the aluminum, even with an excellent preparation and zincating, I would expect a high failure rate of the plate. The chromate will turn a very dark color, approaching black, in the 600 °F range. If at all possible, let the part at least partially cool before you quench. My preference would be to totally cool and then remove the flux with hot water, assuming that you are using a soluble flux.

James Watts
- Navarre, Florida


(1998)

A. Dear Grant ,

Whilst I agree with everything that Jim Watts says , I think you have another problem about to rear its ugly old head; correct me if I am wrong, but you are braising an aluminum fitting to a steel part that has been Zinc plated -- there is a real corrosion cell looking for somewhere to get wet!

Regards,

John Tenison-Woods
John Tenison - Woods
- Victoria Australia


(1998)

A. With regard to the potential for galvanic corrosion, John has a good point. Now, since the subject has come up though: whether you will have a problem depends not just on how wet the weldment gets, and how active the aluminum is in comparison to the zinc, but (a factor that is crucial but seldom considered) the area of the two parts. A tiny bit of more noble material mounted to an expanse of more active metal will usually have small effect, whereas a tiny bit of active metal mounted to an expanse of more noble material can fail catastrophically in the blink of an eye. I suspect that Grant is already aware of this and that the assemblies perform well.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(1998)

GRANT,

CHROMATES WILL BLOW OFF BETWEEN 140 °F AND 170 °F

ZINC PLATE WILL MELT ABOVE 375 °C

ZINC PLATE WILL BLISTER WITH 2 OF THE FOLLOWING:

1 - THE TYPE OF PLATING SOLUTION (I.E., CYANIDE/CHLORIDE/ALKALINE)

2 - ZINC PLATE THICKNESSES APPROACHING 1.0 MILS ARE PRETTY WELL GUARANTEED TO BLISTER.

COMMENTS:

WE PLATE GM TRUCK TAILGATE HINGES AND WENT THROUGH HELL!

TO GET THE PLATE TO STICK BECAUSE THEY GET WELDED THEN PAINTED FOR FINAL ASSEMBLY. THE BEST PROCESS WE DISCOVERED WAS, AND THIS IS IN A BARREL APPLICATION, IS A CYANIDE SOLUTION WITH NO ORGANICS i.e., BRIGHTENER - 0.2 TO 0.4 MILS YELLOW CHROMATE

REGARDS
RAY

ray delorey
industrial processing - cambridge, ontario


sidebar (2000)

Q. Sir,

Could you please provide me some material on "zinc deposition by theoretical and experimental methods".

Thank you

Yours faithfully,
sandesh

SANDESH PRABHU
- mysore,karnataka, India


(2000)

Hi cousin Sandesh,

Sorry, this site isn't a homework hotline smiley. Maybe scholar.google.com can find you some good stuff.

Or please feel free to ask specific questions and we'll try to answer them. Best of luck.

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



December 16, 2014

Q. I have a metal insert part which is Zn plated with chrome passivation in plastic fuel tank. while manufacturing fuel tank with rotomoulding temperature goes up to nearly 300 °C. as metal insert is integrated part of fuel tank Zn plating is getting damaged because of High temperature.

Can anyone tell me how much temperature Zinc plating with chrome passivation can withstand?
Any other surface treatment which can withstand up to 300 °C?

raj gupta
- chennai, tamilnadu, India


December 2014

A. Hi Raj. You're way too high for zinc plating. I suspect that nickel plating would be fine; but remember that nickel is only a barrier layer plating and, unlike zinc which offers cathodic protection, it must be thick enough to be free of porosity and pinholes.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



January 2, 2016

Q. I recently moved to this area, and purchased a mild steel barrel to burn yard waste (oh the trees/leaves). I modified the barrel significantly to reduce the amount of time needed attending an open flame for mere waste disposal. I've had fairly good success with it. I'd like to provide that efficiency to others, but shipping costs on a barrel are as much as 8 times the actual cost of the barrel itself (not to mention the modification work on top of that), and getting them used is a whole grab bag of what hazardous waste did I get and can I deal with it? These factors would make it unlikely to market successfully. So...

I am designing a more portable burner using mild steel. Rather than tapping dozens of threads, I'm looking at the viability of steel rivet nuts (aka nutserts or rivnuts). The majority of these are "yellow zinc" coated, so I'm fairly concerned that there may be heat related issues if I use them. I know for a fact that I can produce bright yellow flame (edging towards white but still definitely yellow, just not the dull yellow/orange/red of the average campfire) and a fairly intense orange in the steel from just yard waste. Charcoal might get a bit hotter or at least be more consistently hot, but I've not tested it yet. I have no thermometer, so I can't say for certain what the temperature is, sorry. Still, it lives outside exposed to the weather, has long periods of disuse, and occasional bursts of fairly high heat. The longest burn period I've had so far is about 7 hours, but it fully burned (very little ash remained after the burn) enough wood and leaves to have maintained a good campfire for 3 days and nights.

Can you say if these "yellow zinc" coated steel rivet nuts are just as likely to perform as well over time under these conditions or would I truly benefit from just getting the uncoated steel rivet nuts? (The applications mentioned previously seemed more like production processes and I'm just not certain how to factor them in this situation. I think it's probable I'll loose the coating in one way or another, but will it matter? erm... can't tell. I expect the silly thing to rust, but don't want it to have accelerated corrosion at the insert points from an un-advised choice.)

I don't know yet if I will be making more than one of these. I know it will work, and work well, but it still may be more expensive than enough folk will pay, regardless of quality/efficiency. I am trying to consider other purposes and varied construction that might make it more marketable, but... it's still in design/testing, so I just don't know.(Just trying to meet the editorial standards request.)

Thank you for your time and assistance.

John Jordan
- Hope Mills, North Carolina, USA


January 2016

A. Hi John. The best coating for the general case of exposures like this, if the cost of coating application and shipping cost were not a factor, would probably be ceramic coating (flame spraying with ceramic frits at very high temperatures). I mention this because you talk about other potential applications for your general approach, and they may not be as cost sensitive.

"Yellow zinc" means zinc electroplating followed by chromate conversion coating (although these days it's probably trivalent chromium rather than actual hexavalent chromate). The yellow will not survive at all, and the life of the zinc is questionable because the temperature is questionable. The exposure to chrome or zinc from the burning will be minimal, and I am personally not chemophobic, but I guarantee that some of your customers will be -- so if I were you I'd go the bother of obtaining the plain steel hardware. Good luck.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


January 2, 2016

I don't see any "thank you" messages posted, so I truly don't expect to see this one, nevertheless --

Thank you, Ted. Your response was not only much faster than I expected but it very nicely answered my question (with reasons, so ++ for that), and it also pointed me in a direction for future learning and product enhancements. Personally, I appreciate that greatly.

I tend to cobble knowledge to produce something useful, either more cost effectively, or for application in ways that don't seem to have been previously considered. Exposure to things is often the limiting factor, so mention of the ceramics allows me to consider... future things, if not for this application, perhaps another.

Thank you for your endeavor with the website, and again for your specific assistance to me.

With warmest esteem!
John

John Jordan [returning]
- Hope Mills, North Carolina, USA


January 2016

And thank you for the kind words, John. Happy New Year!

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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