[Rufus]

Hello - I'm new here and a complete novice at chemistry.  (I did NOT do well in the high school or college chemistry courses I had to take.)

Anyway, I'm considering the use of a product containing phosphoric acid on a rusty part for a car I'm restoring.  The advantage of doing is to arrest the rust without needing to disassemble the part from a stainless steel ring - which cannot be accomplished without destroying the originality of the part. 

Rust will be converted to iron phosphate.  Since the iron phosphate will be in direct contact with the stainless steel, I'm wondering what the possibility is of galvanic action between the two?

Any help would be greatly appreciated!

Bob....

[Arkcon]

No, compounds do not cause a galvanic reaction.  Compounds may corrode a metal, or attack it chemically, but the term galvanic action applies to less reactive pure metals in contact with another metal, in an environment where there's another, corrosive agent ...

Anyway, many people use phosphoric acids to clean rust off of iron.  The phosphoric acid reacts with the clean iron to form an iron phosphate coating -- which is widely held to help protect the iron from future rusting.  I don't know if you really get an good, uniform, iron phosphate plating with naval jelly or the like, usually such a coating is put on by hot dipping a cleaned part in an alkaline medium, or by electroplating.  But at any rate, a little bit of iron phosphate may form, and it is a good porous coating that holds primer and paint better than a sanded clean iron part.

[Rufus]

Thanks Arkcon!

See, I told you I don't know anything about chemistry!!

Good...galvanic reaction is not a factor, but you've reminded me I'll have to worry about surface coverage since the 2 materials (what's now rusty iron and the stainless steel) are already in contact...sandwiched together.  I'm planning to use 'Ospho' rather than naval jelly.  Hopefully by being much thinner than naval jelly, it's coverage will be better.  Or is there another product or method that would creep into small crevises better...??

Does it make sense that priming or painting won't serve any purpose, since the ospho should penetrate small crevises better than primer or paint could?  Since the part is hidden, costmetics are unimportant to me.

Regards,
Bob S.

[Arkcon]

I think carefully descaled iron should be coated with something, to prevent it from rusting in moist air.  I don't believe descaling with phosphoric acid really gives it a uniform, durable, iron phosphate coating -- only a hot alkaline dip or electroplating does that.  Removing rust from crevices doesn't change their future corrosion.

I don't know if you get a galvanic reaction between stainless steel and iron.  Generally, you do get a galvanic reaction between a less reactive metal and iron in contact.  But stainless, that's a tough one to figure out -- is it less reactive because of a surface coating, or some internal stopping of corrosion by grains of other metals?  'Cause I've heard both explanations for stainless steel. 

Some other source may know more.  If it's typical to weld a stainless steel part to an iron part in automobile manufacture, they may have studies this phenomena thoroughly.  If you identify the two parts by name on the correct forum, as in -- "Corrosion likelihood: stainless steel fuel tank welded to cast iron strut" or some such foo.