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Iron sulphate versus manganese phosphate (blackening) information needed

2006

Dear Finishers:

I was planning a manganese phosphate (parkerized) coating for some machined parts. I have a large tank that I use.

To clean the metal beforehand I soaked it in sulfuric acid. I left them in too long, and to my surprise they came out black/grey, similar to parkerized. I have since learned that I inadvertently made an iron sulphate coating.

My question is, why had I never heard of this before? The finish was incredibly easy-- no heat, and only one ingredient-- and to my eye, seems comparable to manganese phosphate in color and durability.

Is there any reason to use manganese phosphate instead? Why is iron sulphate not more widely used?

I appreciate any replies.

Isisdore Hazwell
- Fairfield, Virginia

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First of two simultaneous responses -- 2006

Good afternoon:

Are you sure that you formed iron sulphate? Both the ferrous and ferric forms of this compound are soluble in water; my guess is that if you formed any in the pickling tank, it would have been removed in the rinse tank. Maybe you formed an adherent pickling smut instead?

Ferrous and ferric sulphates have health hazards associated with them that would probably make them less desirable than phosphate coating. You might want to google "ferrous sulfate ⇦ this on eBay or Amazon [affil links] msds" and "ferric sulphate msds".

Also, you might want to consider the reason you're phosphating. Is it strictly for color? Or are you looking for an adherent surface with enough porosity to act as a carrier for oil, wax, etc.; or as a base for paint?

Steve Bizub
- St Louis, Missouri

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Second of two simultaneous responses --

Perhaps, overpickling dissolved the ferritic matrix and left a "black smudge of carbonaceous, insoluble material...particularly undesirable when close inspection must follow." -- The Making, Shaping and Treating of Steel [on Amazon affil links], (9th Edn., p. 803, (1971)).

Iron sulphate is a useful coating only in concentrated sulfuric acid, which allows carbon steel to be used for 96-98% sulfuric acid storage tanks and low velocity piping (< 3 ft/sec). Unfortunately, the iron sulphate is only weakly adherent to the steel surface and, in more dilute solutions, dissolves and promotes corrosion.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2006