Letter 8014

HOURS OF CORROSION RESISTANCE 

+

Dear Sir,

We would like to know the number of hours the following metals/finish will withstand salt spray test/natural corrosion.

1. SS 403
2. SS 304
3. PRECOATED METAL SHEETS
4. POWDER COATED CRCA SHEETS
5. NI-CR PLATED CASTINGS (12 MICRONS)
6. ZINC PLATED (12 MICRONS) ON CRCA (YELLOW & BLUE PASSIVATION)

Thanks

KARTHIK SOMANATHAN
- Bangalore, India

Why not just run the salt spray tests, Karthik? As the card layers say: a peek is worth a thousand finesses.

Most of our readers are not interested in doing an abstract academic exercise. You probably have some real-world questions, and if you ask them, I think you'll get more satisfactory responses.

Note that a powder coated sheet will have much better salt spray resistance than a zinc plated one, yet highway guard rails are usually zinc plated or galvanized. 12 microns is a fairly heavy thickness for zinc plating, but very light for nickel-chrome plating. Salt spray is not a useful test for nickel-chrome except as a go/no-go porosity test. I believe that a given grade of stainless steel will either remain rust-free or rust immediately in a given environment--I don't think it's a matter of how long it retains rust resistance before giving out. The point being that if anyone gives you the numbers you seek, chances are it will be largely useless information as long as it's an abstract question. Make the question real and you're likely to get useful answers. Good luck!

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Are you looking for the first sign of red rust? There are many ways to measure how long something lasts in salt spray. Some look at white corrosion instead of red rust, and some look for 5% instead of the first sign of corrosion.

Tim Neveau Rochester Hills, Michigan

Sir,

The subject of corrosion is somewhat complex.What is good for one situation may not suit the other. There is aestheitic and functional values to be preserved in some cases. In some of them one may be more important than the other. You may kindly see the galvanic series in the web site, www.powers.com/CarbideBit/2.3.htm [Ed. note 11/27/06: that page is apparently no longer functional].

Subramanian Ramajayam     plating chemist Bangalore, Karnataka, India

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