Odd acid effect cleaning steel, any chemist answers?

[J Tiers]

I have cleaned a lot of steel with phosphoric. Mild steel, hardened steel, HSS, you name it. Probably have cleaned several hundred pieces, including hardened cutters, ordinary steel, tools, etc. (I have tried electrolytic, uniformly got bad results, de-rusted, but often pitted and always covered with black gook).

Every time with phosphoric, I have either suceeded in cleaning all or most of the corrosion. I have never had any sort of attack on the steel.

The last time, I had some hardened steel pins with some rust , so I did the usual.

This time, the steel was actually eaten severely, with a network of "martian canals" eaten into the surface. Easily felt with a fingernail, easily seen, probably 0.005+ deep. Pins were totally ruined, they aren't even scrap now, because they were hardened, and would need drawn to even re-work.

Has never happened before. I have left steel in for a day, and had NO progression of any sort of attack on the steel.

I know the rust wasn't at fault. The rust was all on one side, but the "canals" were all over the pieces.

All the pins are known to be of similar/same composition, and in any case, they were not touching each other. I always avoid that for exactly this reason.

Any chemical reason for this?

Best I can figure is local electrolytic action if the steel composition and hardness was not uniform. It may not have been.

I think the pins may have been case hardened, and the "canals" are in a pattern very similar to the mottling on those "color case hardened" tools. That suggests a variation in composition, and resulting difference in local potential.

[wierdscience]

Differential density as the result of the quench,the parts probibly had stress micro-cracks in the surface that were not visable until the acid made the point.

[J Tiers]

Maybe, but phosphoric just does NOT eat steel normally. I have left parts in for days with zero problems. It had to have help from electrolytic action in dissolving the steel.

Would the density difference (I don't understand the mechanism for that) cause a local difference in electrolytic potential?

I'm thinking maybe diffrences in the case, case hardening can't be totally even.



[This message has been edited by J Tiers (edited 07-18-2005).]

[jburstein]

Well doesn't case hardening usually involve a difference in chemistry? Meaning either higher carbon or sometimes nitrogen content?

That would provide an explanation, although I don't know that phosphoric acid would necessarily attack either of those. Just a thought.

-Justin

[JRouche]

Maybe they were corroded that way and the acid just revealed what the rust had accomplished. Did you save any pins from the drink to test? I also use Phos. acid alot and have had good results. JRouche

[Paul Alciatore]

What were the pins in contact with? Basket or whatever they rested on?

Paul A.

[WJHartson]

I agree with wierdscience that there was some surface problems and the acid found the cracks and cleaned them out resulting in the "martian canals" as you call them.

Joe

[Steve Stube]

On two occasions I can remember I obtained a strange outcome using Metal Conditioner (phosphoric acid). One was a railroad track spike I wanted to clean lots of old rust from to use it in some metal art. I let it set for days in a bottle of Metal Conditioner. When I took it out it looked like it had been made using the Damascus (sp)method of laminating with deep etching at the layer boundaries. The second case was a pair of machinist Vee Blocks I got pretty cheap at a flea market because they were rusted. Again submerged in a bottle of Metal Conditioner for perhaps a month or two, not sure but definitely weeks. Result was similar to the railroad spike example but because years had separated these two events, I forgot the earlier lesson. The Vee Blocks were shot. They had actually grown (gotten bigger and greatly distorted) separating at boundaries.
I have no answers to offer only these results/experiences. If I learned anything from this it is that I may not get what I was after with extended soak times.

[tonydacrow]

This is just a theory, but here goes.

As steel oxidizes, those external areas affected will become poorer in iron and richer in carbon, chromium, manganese, and other alloying elements less inclined to oxidize. In addition, the resulting iron oxide will physically occupy more space than the iron from which it is formed.

This would create a push-pull effect in which the depletion of the iron and the increased size of the resulting oxide could cause the corrosion to migrate deeply into the metal by “wedging†as opposed to remaining on the surface. I.e., the iron poor regions would “pull†iron atoms from the interior, iron-rich areas causing them to migrate toward the corroding area as the iron-oxide molecules push the corrosion toward the interior. (This mechanisim seems likely to work only over extended periods of time due to the required migration of iron atoms.)

Since the edges of the “tunneling†would be composed of elements less apt to oxidize than the iron, very definite boundary layers occur along the iron depletion routes and look like "martian canals".

. . .

Or, . . . it could be Pixies…

[Mike Gibson]

I'll vote for Pixies

[J Tiers]

Must be the pixies, because the attacked areas were shiny and fine before, there was just one line of rust.

I am putting it down to case hardening, with differing penetration in different areas. Must have made small regional cells.

Phosphoric wouldn't have etched cracks any faster than regular steel areas. It just does NOT attack steel as a rule.

Must have been assisted by electrolytic action.