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Old 08-18-2003, 09:23 PM
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more on phosphoric acid

here was something posted on the hobart welders forum I thought you may find of interest:

regarding hitting bare or rusted metal with phosphoric acid, and then immediately top coating WITHOUT neutralizing the P/A and WITHOUT priming the bare metal:

Dupont Metal Prep is a Phosphoric acid based product.
I have the actual formulation someplace, but since the BIG computer crash last week, finding a lot of the information I have is somewhat difficult at the moment.

Prime -v- No prime involves a lot of variables, starting with the conditions you're working under. Any primer is nothing more than an adhesion system that allows paint to stick to the metal. Most paints will not in fact stick to bare metal, so primers are used.
When you run a phosphoric acid process on clean new metal, the acid micro etches the surface of the metal, and gives you better primer adhesion. Unfortunately, when you neutralize the acid, unless you are working in absolute perfect humidity conditions, and can immediately dry the surface, oxidation of the microetched surface will begin, and you'll see rust in a few hours.

NO prime ~ in my world, absolutely clean rust free steel is a rarity by the time I get to the paint stage, so the phosphating process comes into play.
Phosphoric acid solution converts Iron Oxide to Iron Phosphate, witch is a layer chemicly bonded to the steel. The IP layer being chemicly bonded has a far superior bond to primer placed onto etched steel, as it is part of the steel rather than a layer stuck to the steel. The IP layer is also far less permiable for oxygen to react with the steel.
The IP layer is also an excellent bonding surface for most enamels, so priming is unnecessary. All that is necessary is to smooth the IP layer to the quality of the topcoat finish desired.
Over the course of 30 years using this system on equipment I've found far less topcoat damage occurrs with the IP topcoat system than occurrs with the etch prime topcoat system. This may be because of the chemicle bond between IP and the steel, and superior adhesion between that layer and topcoat, or it may be because less layers are a stronger coating system.
Phosphating has been around for a long timein various forms, and is proving to be superior to painting in many applications. Here in the NorthEast bridge steel is now being phosphated rather than painted.

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