Metal finishing Q&As since 1989
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Chromate on bare steel
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Q. Sir we are going to salt spray test for Trivalent Chromate 8-20 microns, our bare metal is steel, we manufacture parts of weighing scale. I would like to know the standard criteria of SST, is 24 hrs, 48 hrs, 72 hours.
Thank you.
virgs of Philippines
- batangas city, Philippines
May 22, 2018
TUTORIAL FOR NEWBIES:
Steel can be protected from corrosion with organic coatings like paint or powder coating, or with inorganic coatings including zinc plating (an electro-chemical process) or galvanizing (dipping into molten zinc).
Coatings are sometimes subjected to an accelerated corrosion test, most commonly the neutral salt spray test per ASTM B117 or ISO 9227.
Although the zinc coating protects the steel from rusting ("red rust"), the zinc may also corrode ("white rust"). To deter that white rust, zinc plating is almost always 'chromate conversion coated' and hot dip galvanizing is often 'chromate quenched'.
Until about the year 2000 most chromate conversion coatings used hexavalent chromium because it was more effective than trivalent chromium. But in response to European Reduction of Hazardous Substances pressures, most chromates today are trivalent, and development efforts have made them approximately equal to hexavalent chromates in corrosion resistance.
This chromate conversion coating is done not only on zinc plated surfaces but on cadmium plating, on aluminum, and a few other metals.
Something which is running between the lines on this thread is the question of whether chromate conversion coating can be done on bare steel, and the answer is no, it doesn't convert steel and isn't useful.
A. Hi Virgs. I am not sure if I am misunderstanding you, or you are misunderstanding something, or if were just suffering a bit of a language difficulty, but trivalent chromate coatings are not used directly on steel. They are used on aluminum and on certain electroplatings such as zinc plating. your reference to 8-20 microns is probably the thickness of the zinc plating on the steel, not the chromate.
72 hours would be a fairly minimal SST test; 192 hours would be about the minimum for a quality coating. Please clarify the situation. Thanks.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 2018
Q. Hi All, I also have a similar question. Will Trivalent coating form a layer bond to bare carbon steel? We zinc plate the steel tube, but the zinc plating layer only appears about 1-2 inches at end of inner wall, rest of them are still bare metal. Inner wall got rusted during transit. I wonder if trivalent would form a protection layer?
Mark Liu- Orange County, California
September 24, 2018
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A. Hi Mark. Unfortunately that won't work. You can dip into water soluble lacquer or oil as a very moderate preservative, but it might be possible to iron phosphate the interior of the tubing before the lacquer or oil.
Depending on what the item and the wall thickness of the tubing, it might be possible to hot dip galvanize it instead, although this may be cost prohibitive.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Hi Ted, thank your response; it has been very helpful, very much appreciate your time and knowledge! Mark
- Orange County, California
Q. Do you have a recommended CONVERSION COAT product to be used in carbon steel substrate?
Note: I have one in use but is missing in the market.
Regards
- Minneapolis, Minnesota
March 14, 2023
A. Hi Jose. Long time since your last visit -- welcome back!
I might be misunderstanding you. Are you saying that you apply a chromate conversion coat to bare steel?
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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