CORROSION PROTECTION

[GrzegorzD]

Hello,
I'm looking for information about clear chromate and yellow dischromate coating. Full text about corrosion protection attached:
CORROSION PROTECTION: ZINC PLATED 0.0003 MIN THICKNESS PLUS A CLEAR CHROMATE OR YELLOW DISCHROMATE DIP WHICH MUST WITHSTAND 96 HOURS UNDER ASTM B-117 SALT SPRAY CONDITIONS WITHOUT ANY CORROSION

Any help is gratefully received.

Greg

 

[mshimko]

I have nothing specific to provide, other than wonder what your application is, because in my fields use of chromates are strongly discouraged or even outright prohibited.

 

[TVP]

In order to obtain better corrosion resistance, zinc plated parts are usually chromate coated as well. This involves immersing the zinc plated parts into a bath containing either hexavalent chromium (water + sodium dichromate + other additives) or trivalent chromium (water + chromium trichloride + other additives). Depending on the time, temperature, concentration, etc., the thickness of the resultant chromate film will be ~ 30-300 nm.

Historically, hexavalent chromate films have been used most often. The color of the hexavalent chromate starts out clear, with a blue tint (often called blue bright). As thicker films are deposited, the color changes to an iridescent yellow/gold/bronze, and then to an opaque olive green or brown. The thicker the chromate, the better the corrosion resistance, as the chromate delays the onset of white rust (corrosion of the zinc layer).

The following articles provide a great deal of information about chromate coatings:

http://www.pfonline.com/articles/pfd0020.html

http://www.pfonline.com/articles/pfd0323.html

http://www.pfonline.com/articles/020102.html

 

[kenvlach]

This coating can be specified as ASTM B633 Fe/Zn 8*, Type II (yellow).

*I presume thickness units are inches, so for 0.0003" use 8 microns of electroplated zinc (presumably on steel or iron).
Type II indicates colored chromate giving a corrosion resistance of 96 hrs (to white 'rust'). Best to specify yellow chromate.

Also, you must specify whether corrosion testing is to be done; it can be on actual part if reasonable geometry or on a test coupon coated at the same time. Note: ASTM B117 gives the test equipment & salt spray; B633 gives the applicable length of time.
Testing is often just done on the first lot; saves $ & time.

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/B633.htm?L+mystore+hiws2054

 

[kenvlach]

Colorless chromate is Type III in ASTM B633, but it may not meet the 96 hour salt spray requirement -- only 12 hrs minimum if I remember correctly (need to find my ASTM 2.05 Book).

 

[CoryPad]

Chromates are conversion coatings for metallic surfaces like Zn, Al, and Mg. They improve the corrosion resistance of the Zn plating. These coatings are very common, so you could review industry specifications like ASTM B 633 and B 201 or ISO 2081 and 3613 and 4520, or contact companies that plate parts or supply plating materials. Since platers and materials suppliers are usually local, you would need to tell us where you are located for more specific information.

Regards,

Cory

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

 

[GrzegorzD]

Thanks a lot averyone!