Paint Damage to Galvanized Steel and Polyurethane

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Q: The Phosphoric acid is 85% concentrate - it is diluted 20:1 with water and then applied with a power hose.  The applied area is rinsed with water following application (though this was not done that well in this case).  The polyurethane is the painted finish of the steel - the whole point of this phosphoric wash is to remove corrosion from the steel work which has accumulated due to grinding above.  This is supposed to be a cleaning process - and for removing the rust it has worked - however this damage to paint work and galvanized steel is concerning.  I would appreciate any advice you can give.

A:  I too am puzzled.  I can understand that there might be some slight surface corrosion of the galvanized steel, especially if the operators do not thoroughly rinse these surfaces after using the acid.  It is possible that some of the residual (unreacted) acid might have continued to react with the zinc.  In general, I would expect the galvanizing to show some dulling after it comes into contact with phosphoric acid, because a zinc phosphate, or perhaps even an iron phosphate, (if iron is present) will form on the metal surface. Is this, in fact, what you are observing?

But what puzzles me more is the damage to the polyurethane.  Generally, these coatings stand up well to acids and chemicals.  Can you clarify what you mean by "damage"?  Are you seeing staining from the phosphoric acid that can be wiped away, or is the coating being eaten away?  Without seeing the coating I would assume that what you are observing is simply a surface condition, unless you tell me otherwise.  Can you wipe the surfaces clean with a Scotchbright pad, or similar? Is it like the water staining you might see on a motor car finish?

Based solely on what I think might be happening, I would advise you to make sure that the operators thoroughly rinse the surfaces immediately after they have completed the rust removal process.  If you can mask off the galvanized and painted surfaces, all the better.

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