Comparing Blasting to Conversion Coatings

Q. We run a batch powder coating system and we coat a variety of different parts that are made of different metals. A lot of the parts that we get for coating are made of steel. In some cases there are a lot of new parts to coat and sometimes we take on an old part that needs to be recoated. We often use a pressure blast system to remove dirt, oil and old paint from parts before we coat them.

Sometimes we will get a request for a part that needs exceptional corrosion resistance and we are wondering what the best options are to achieve good rust resistance. Is a blast-only surface good enough, or should we do a chemical treatment?

Chemical treatment is foreign to us so we do not know what options are available, how much the materials and equipment cost and what is involved in using them. Iron phosphate is what most people ask for, but we have never used it. How complicated is it to apply a phosphate coating? Can it be done manually? Does it really help prevent corrosion? —A.A.

A. A blast-only surface can be a very clean surface that provides excellent mechanical adhesion. Since good adhesion is critical for good corrosion resistance, the blast surface can be effective in some cases. However, there are a lot of factors to consider. A blast system can use spherical shot or sharp-edged grit. A spherical media will be very effective at removal of inorganic contamination such a scale and oxidation.

However, it does not do much to remove organic soils like oil, and it may even imbed oily materials in the surface of the steel. A grit media will cut at the surface and do a better job of removing organic soils.  But there are some limitations. Mainly, the media must be kept clean. The media must be turned over fast enough to avoid accumulation of oily soils.

With no conversion coating (phosphate for example) on the surface, the metal is highly reactive and it does not add any protection from oxidation. One of the better ways to enhance corrosion resistance on blasted steel is application of a primer layer. Electrocoating and autophoretic coating are two dip-coating processes that provide excellent corrosion resistance as a primer layer. They can provide good edge coverage and also get deep into recesses that are difficult to coat with a spray process. A layer of powder coating primer is also an excellent way to enhance corrosion resistance. It provides more film thickness and an excellent moisture barrier. A powder primer is also nice because you can do it with the equipment you already have.

If you do look at conversion coatings, you will find that they can create a surface condition that helps promote good adhesion and retard the spread of corrosion. Iron phosphate coatings can be applied by dip or a spray system. It is somewhat more complicated than a manual blast operation, because you must control the chemical solutions. You also need to invest in tanks or a spray wand. It works very well on light gauge parts that need to be cleaned and degreased and may not hold up to the pressure of a blast operation.

Iron phosphate helps promote coating adhesion but does very little to inhibit corrosion. Some non-chrome seal rinse products can help to improve the corrosion resistance but iron phosphate is more effective at improving adhesion and not necessarily great for corrosion resistance.

For extreme corrosion resistance from a chemical product, you will need zinc phosphate or chrome, processes that would be much more costly and complicated, not to mention the environmental concerns associated with these materials.

If the powder is applied properly with sufficient thickness and complete coverage, the adhesion over the blasted surface is very good and the corrosion protection from the powder film is very good. It can be improved substantially with the addition of a primer layer and the primer can be a zinc-containing powder.

If a customer insists you add corrosion protection with a conversion coating, I suggest that you look for a source that provides zinc phosphating as a service and have them clean and phosphate it for you. Bring it back to your shop and coat it as soon as possible so that it does not have a chance to pick up any moisture or contamination.