Black Oxide versus Black Phosphate

"Phosphating of Metals"
by Werner Rausch

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A. David - Yes, black oxide can be used as a rust retardant, but it is not as effective as black phosphate. The black zinc phosphate acts sacrificially to protect the steel, whereas the black oxide will rely on the after-finish for corrosion protection. The decision should be based on how much protection you need - a good oil on black oxide can pass a 96 hour salt spray test.

A. Black oxide is an exceptionally thin process which, as the name implies, is an iron oxide film. It is formed by putting iron parts into a very hot and very strong solution of caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] with nitrates and other ingredients.

Black phosphate, as I understand it and depending on who is selling it, is either a color variation on a standard phosphatized coating achieved with either a pretreatment or post-dip dyeing, or it is a very heavy less porous phosphate.

You didn't say what your interest was, so I would be general and say that the black oxide is probably cheaper, less corrosion resistant, and prettier.

A. AS I HAVE PRACTICALLY HANDLED THE PROCESSES OF CHEMICAL BLACKENING OF FERROUS SUBSTRATES AND PHOSPHATING WITH BLACK FINISH, I CAN SAY THAT PHOSPHATING WILL BE CHEAPER AND MORE CORROSION RESISTANT THAN BLACK OXIDE WHICH LOOKS ATTRACTIVE. THE CHEMICAL BLACKENING USING ALKALINE SOLUTION IS A DIFFICULT PROCESS WITH CLOSE OPERATING RANGE OF 136-140 ° C. IN PHOSPHATING MANGANESE PHOSPHATING WILL LOOK JET BLACK WITH MATTE FINISH BUT THE PROCESS NEEDS TO BE CONTROLLED VERY CAREFULLY. IF ZINC/IRON PHOSPHATING IS CHOSEN BASED ON THE FORMULATION OF THE PROPRIETARY CHEMICAL WITH NICKEL OR CALCIUM THE ATTAINMENT OF BLACK FINISH IS ENSURED. THIS PROCESS WILL BE THE CHEAPEST AND ESPECIALLY FOR SMALL COMPONENTS LIKE FASTENERS WHICH CAN BE HANDLED EASILY BY BARRELS. THERE IS YET ANOTHER COLD BLACKENING PROCESS WHEREIN THE FERROUS MATERIAL IS DIPPED IN A SOLUTION CONTAINING COPPER WHICH GETS COATED WITH COPPER BY DISPLACEMENT REACTION AND IMMEDIATELY BLACKENED BY SUITABLE ACETATE/SILVER CONTAINING PROPRIETARY SOLUTIONS. BUT THIS IS ONLY DECORATIVE AND VERY THIN COATING WHICH DOES NOT CONTRIBUTE TO THE CORROSION RESISTANCE OF THE SUBSTRATE METAL..

Thank you for the fine exposition on this, Ramaswami. After thinking about it further, I'll concede that phosphating may be less expensive than black oxiding, even with the cost of proprietary chemicals whereas black oxide is usually done with commodity chemicals … but your hands-on experience trumps my book knowledge in any case :-)

A. Hi, Ming. No, "blackening/bluing" is the same thing as black oxide. I don't think that phosphate coating does significantly "prevent intrusion of salt", but it can help minimize galvanic hot spots, provide a "key" for paint, and minimize undercutting at scribe marks and filiform corrosion. Phosphating is a base for paint or sort of a oil-retaining dry film lubricant rather than a final finish.

A. Hi, Shahid.

"Phosphating" is a pretty broad subject, involving cleaning the steel, then putting it into one of many different phosphatizing solutions, including iron phosphate, zinc phosphate, or manganese phosphate. Of these, manganese phosphate is the one usually used for internal engine parts. The coating holds oil well, and is somewhat softer than steel, so engine parts are often treated with it as a "break-in" agent.

Regards,

A. No. Since iron, zinc or manganese coatings contain phosphates, along with in some cases calcium, they are non-conductive.

A. Dear Ricardo,

Nope, phosphatizing is not a galvanic barrier. You need a dielectric, an isolating agent like paints or lacquers, to avoid galvanic corrosion of magnesium.

I hope you can solve this issue! Regards,

A. Hi Mustanser. What is the composition of, and the parameters for, the manganese phosphating that you were using which yielded a gray to dark grey color? What grade of steel are these fasteners that you are treating? In addition to processes that may be inherently darker, there are also dyeing processes to obtain black coatings, and perhaps that's what you are seeking?

Regards,

? Hi Rupinder. Sorry, I am not familiar with the phrase "Auto Blackening". Please avoid posting abstract questions; rather, please tell us your situation and what motivates the question -- that way we may be able to figure out from the context how to help you rather than wasting your time and everyone else's. Thanks.

Regards,

"Electrolytic and Chemical Conversion Coatings"
by Biestek & Weber

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A. Hi Christian. Biestek & Weber .
claims that 100 hours is possible with phosphate & oil, and suggests that non-drying vegetable-based oil applied in heavy thicknesses is the most effective for corrosion resistance.

But guessing what will be the most helpful paragraph in a 400-page book is difficult :-)
Please start the dialog by telling us what your processing sequence is, what things you have been trying, and what results you have been getting. Presumably you can't use stoving varnish on the fasteners, but must use oil? Presumably you will reject the idea of a hot chrome based passivation rinse before the oil based on environmental considerations? Presumably the process line exists and there are limitations regarding what changes are possible? Are you, together with the process vendor, exercising close regulation over the process control parameters? Thanks.

Regards,

A. Hi Christian,

It is possible, with a heavy coat of zinc or manganese phosphate, hex chrome sealer and a heavy water displacement oil.

We make phosphate coating to screws to meet 72-120 hours of SST, with good results. Hex chrome replacement in this case is not too easy, but there are some non-chromic sealers you could try.

Best of luck!