finishing.com -- The Home Page of the Finishing Industry
23+ years of serious education, promoting Aloha,
& the most fun you can have in metal finishing smiley
    no popups, no spam
HomeFAQsSuggested
Books
Help
Wanteds
Advertise
on this site
FORUM
current topics
topic 60766

Trivalent chromate on bare steel

May 22, 2018

Alodine 1201

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

virgs laroza
- batangas city, Philippines


May 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 [link by ed. to spec at TechStreet] or ISO 9227 [link by ed. to spec at TechStreet].

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, but trivalent chromate coatings are not used directly on steel. They are used on aluminum and on certain electroplatings such as zinc plating ... 8-20 microns is probably the thickness of the zinc plating. Even 72 hours would be a very minimal SST test. Please clarify the situation. Thanks.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



September 24, 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 2018

A. Hi Mark. Unfortunately that won't work. You can dip into water soluble lacquer or oil as a very moderate preservative, and it might be possible to iron phosphate the interior of the tubing before dipping into it.

Depending on what the item and the tubing are, it might be possible to hot dip galvanize it instead, but this can't be done on very light tubing and may be cost prohibitive.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


September 25, 2018

thumbs up sign Hi Ted, thank your response; it has been very helpful, very much appreciate your time and knowledge! Mark

Mark Liu
- Orange County, California



This public forum has 60,000 threads. If you have a question in mind which seems off topic to this thread, you might prefer to Search the Site

ADD a Comment to THIS thread START a NEW threadView CURRENT TOPICS



Disclaimer: It's not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations may be deliberately harmful.

  If you need a product/service, please check these Directories:

JobshopsCapital Equip. & Install'nChemicals & Consumables Consult'g, Train'g, SoftwareEnvironmental ComplianceTesting Svcs. & Devices


©1995-2018 finishing.com, Inc., Pine Beach, NJ   -   About finishing.com   -  Privacy Policy
How Google uses data when you visit this site.