Unknown Bluish Oxide

Question:

Some of our end users form our high-strength material and then powder coat for corrosion and appearance sake. I know little about the powder coating industry, however, in the production of the high-strength steel by continuous annealing, a thin, tightly adherent “bluish” oxide is formed on the annealed strip during cooling. It is being proposed that we enter into a post treatment with approximately 10% phosphoric acid to remove the blue coloration. Preliminary testing indicates that an iron phosphate surface compound is formed in place of the oxide film. Is this iron phosphate detrimental to the powder coating process or can it act as a pretreatment for ultimate powder coating processes. If it were not desirable, then we would need to use an inhibitor in the process. Any comments on this subject would be appreciated. T. W.

Answer:

My first reaction is that iron phosphate is a good thing for powder adhesion and corrosion improvement. In fact, it is the most common pretreatment used before powder coating on ferrous substrates. However, I am not sure if the iron phosphate that is developed in your process possesses the same properties that iron phosphate applied in a normal powder coating pretreatment system has. For instance, is the iron phosphate tightly adhered to the metal surface without powdering? Is the crystalline structure good enough for powder coating? Is the phosphate deposited evenly? Is the coating weight sufficient to provide proper adhesion and corrosion resistance? Not being a chemist or metallurgist, I do not feel comfortable giving you a green light without more information.

Pickling is used to remove oxides and other inorganic soils before powder coating on ferrous substrates that are welded or heat treated beforehand. Furthermore, pickling is performed with phosphoric acid (and sometimes citric acid), as you are using. However, pickling is typically followed by applying iron phosphate, and possibly a sealer, onto the surface before drying and powder coating. This leads me to think that the iron phosphate developed during your pickling process is insufficient for most powder coating operations. However, that does not mean that you can’t ship your product to your customer after pickling “as is.” Just don’t mention the iron phosphate on the surface. They should then process the steel as they normally do and will obtain better results than before you removed the oxides. They may even be applying an iron phosphate to the surface as part of their normal powder coating operation. However, I think if you tell them that your surface now has some iron phosphate on it instead of the oxide, it will give them the wrong impression and they may try to eliminate their pretreatment process, causing disastrous results.