Building a Better Powder Coating System: Pretreatment Chemicals

It has been said by some that the quality of metal finishing is like a threelegged stool. The legs represent pretreatment, substrate and paint.

The first leg of quality starts with the cleaner. Proper cleaner selection for the substrate involved and processing system ensures a surface that is ready to be treated. A spray or immersion treatment system that processes only steel is best suited to strong alkaline and hydroxide cleaners. They are very effective at removing organic soils such as rust preventatives and mill oils. Immersion and spray cleaner chemistries vary, based on surfactants and additives that are related to the temperature of the system and the need to control oil emulsification or separation. Splitting out oil in an immersion system can lead to redepositing it on the surface. Soft metals, such as aluminum, brass and galvanized (zinc coated) steel can be greatly affected by the choice of cleaners. Cleaners for these metals tend to be lower in sodium or potassium hydroxides, buffered with silicate or run at a lower pH value. In addition to the cleaners mentioned, another category of products used are acidic cleaners. Acidic cleaners have their pros and cons as well, but they can be beneficial for problematic steel. We will touch on that in a little bit. If the cleaner is too aggressive, smutting of the aluminum or over-etching of the zinc layer on the steel can occur. This can result in a poor treatment and poor paint adhesion. If the cleaner is too mild or buffered, it may not remove oxides that form readily on both steel and soft metals. This situation can also lead to reduced paint adhesion and poorer salt spray results. The choice of cleaners becomes more critical when the system runs mixed metal loads. To ensure good cleaning, your chemical supplier should conduct a complete evaluation of the process, substrates and soils involved in the system. 

Now that the parts are clean, they are ready to be treated. The treatment is intended to increase paint adhesion and improve corrosion resistance. Iron phosphate conversion coatings have been around for well over one hundred years. They are currently the most prevalent treatments used in conjunction with powder paints and can provide outstanding quality for the finished goods. Depending on the processing system, iron phosphate can be a stand-alone treatment, after cleaning, or in stage three, of a four- five- or six- stage washer. Iron phosphate cleaner-coaters are used in stage one of a three-stage washer. They combine both the cleaning and conversion functions of the washer. Cleaner-coaters allow for a smaller footprint when production space is limited, while also eliminating alkaline and caustic cleaner solution entrapment. Entrapped cleaner solution has a tendency to boil out and splash on the part surface in the dry-off oven or the paint cure oven itself. This boil out always results in a paint film defect. The defects can range from craters in the film to massive adhesion loss. A second type of phosphate used as a pre-paint treatment is zinc phosphates. Incorporating zinc into the phosphate conversion coating improves corrosion resistance, although it comes with a significant increase in operating costs, washer equipment, and chemistries required to operate the bath and process control. Poor process control can actually result in reduced corrosion resistance and salt spray results. Treatment processes fall into two broad categories: phosphate coatings and oxide coating. The newer transition metal oxide coatings for steel have been around for about fifteen years now. These coatings provide an array of benefits to the paint finisher, ranging from reduced operating temperatures to sludge elimination. This all results in cost saving for the user. In addition to cost savings, the transition metal coatings (TMC) can provide a synergistic effect on quality with powder coatings. These treatments have replaced zinc phosphate on galvanized steel with a direct to metal powder and can increase corrosion resistance threefold on steel. If your powder operation has not evaluated a TMC you should talk to your chemical supplier. 

The second leg of quality is the substrate. Steel has a long history in the United States. It was the foundation of financial empires, but steel production has changed over the years. Currently, US steel production ranks fourth, with China being the largest producer/provider in the world. Quality issues arising from the steel are generally related to residual surface carbon or oxide formation. Residual surface carbon has a direct detrimental effect on salt spray testing results. Surface oxide formation from lubricant dry-down or batch annealing of the steel affects the quality of the conversion coating. This also results in reduced corrosion resistance. When trying to get the best out of the “discount steel” that purchasing brings in whenever they can, you need to go back to the first step. Depending on the quality of the steel, the right choice of cleaner will help. As an acid pickle will remove laser scale (iron oxide film), a mild acidic cleaner can help remove surface oxides and activate the metal. This results in a more adherent conversion coating and better paint adhesion. Proper cleaner formulation is needed to maximize soil penetration, lifting and removal so as to minimize residual surface carbon. Setting maximum surface carbon level as part of a steel quality specification, and sticking to it will also help.