Q: We powder coat in-house and have a five-stage washer, of which one stage is a phosphoric acid bath. Do we need to have iron phosphate to get good adhesion? Is there an alternative process for iron phosphating?
A: There are a number of alternatives to a five-stage iron phosphate. What you choose as your cleaning/pretreatment process largely depends on a few issues:
- The type of metal(s) that you coat
- The condition of the metal substrates (i.e., dirt, oils, oxides, etc.)
- The specification and expectation of coating performance (i.e., adhesion and corrosion resistance)
The first step in preparing a substrate for powder coating is to clean any foreign materials and compounds from the surface. This can be accomplished by mechanical means (e.g., abrasion, blasting, or tumbling), chemical means (e.g., alkalis, solvents, or soaps) or physical means (e.g., flame, plasma, or corona).
Mechanical means can work very well if the substrate is relatively clean to begin with and if you use an epoxy-based powder. As you probably know, epoxies are great for hardness, corrosion, and chemical resistance but do not fare well in outdoor environments (due to UV degradation). However, epoxy or hybrid (epoxy-polyester) powders can provide very good indoor performance with only a clean, abraded, or blasted surface.
If the coating requirement involves any resistance to outdoor elements you need, at the minimum, to clean thoroughly, rinse, and apply a chemical conversion compound to establish a bond between the powder coating and the metal substrate. This will provide a minimum of outdoor durability for resistance to corrosion and attack by moisture.
If the coating will find service in a more demanding environment (i.e., coastal areas, northern states where salt is used on roads, marine, etc.), then a minimum of a five-stage iron phosphate or better yet a multi-stage zinc phosphate pretreatment system is best. These have a long history of performance and are offered by many chemical companies.
If you wish to avoid using phosphate-type pretreatment systems due to handling or regulatory issues, then you can consider a few other options. Recently, non-phosphate systems based on zirconium, titanates, and/or organo-silanes have replaced iron and zinc phosphate systems. These new systems require close process control and usually have to be tailored to the substrates and substrate conditions at the finishing line. All the major chemical companies have versions of these.
Another option is to consider using a one-step pretreatment. One company in particular, Carpenter Chemicals, has a technology that was developed in Italy and has demonstrated success as a single-stage pretreatment. This technology is known as Plaforization™ and involves an organo-phosphate (not phosphoric acid) that is claimed to provide cleaning and metal pretreatment in one step.
Regarding physical cleaning methods such as plasma, flame, and corona, these are most commonly used to clean and activate plastic surfaces rather than metal substrates. Plasma has an intriguing performance profile but is restricted to relatively small substrates or low volume when used as a pretreatment process.