Virtually all metal parts and many plastic parts have some sort of finish applied to them. These finishes are applied to improve appearance, increase corrosion resistance and enhance wear resistance.
To increase the effectiveness of the finish, no matter what the substrate, coating or application is, the part must be clean prior to coating. Any number of cleaning processes, including aqueous cleaning, vapor degreasing or ultrasonic cleaning, can be used.
However, sometimes just cleaning the part isn’t enough. This is especially true for parts that will be finished with an organic coating such as paint or powder. Prior to the coating film application, most parts need a surface pretreatment. This process is most commonly known as conversion coating.
In general, the purpose of a conversion coating is to promote adhesion, reduce part-coating interactions, improve corrosion resistance and increase blister resistance.
Conversion coatings are typically iron and zinc phosphate and can be applied at a number of points in the pretreatment process (see Table I).
The three major driving forces in the pretreatment industry are quality, cost and the environment. While these aren’t new issues, the pretreatment industry has responded to the needs of finishers in novel ways by creating technology to address each of these requirements.
Although these driving forces may appear to be mutually exclusive, it’s possible to address each of them simultaneously with a complete understanding of the entire manufacturing and finishing process, including paint formulation, application equipment and regulatory impacts.
Cleaning of the substrate is the first step in the pretreatment process and is critical to success in producing a quality coating.
Since the method of cleaning can affect the coating characteristics (coating, weight, crystal structure, etc.), it’s critical to work with your pretreatment supplier for the proper cleaner recommendations.
Proper rinsing is a critical yet often overlooked step in the pretreatment process.
The major function of water rinses is to remove unreacted chemicals from the part surface. This prevents or minimizes drag-in of chemicals from one stage to the next. For effective rinsing, the amount of contamination present in the rinse water should be kept to a minimum.
Using multiple rinse stages that are counterflowed can effectively minimize rinse water use. This allows the process to use a fraction of the amount of water and reduce the amount of effluent produced.
You can also reduce water consumption by optimizing your equipment design with proper racking of parts.
Surface conditioning rinses are used in zinc phosphating to refine crystal morphology and control coating weight. Surface conditioners are colloidal suspensions of a titanium salt that ages with time and loses its effectiveness regardless of use. Typically, the bath is dumped frequently or, in the case of immersion applications, automatically drained. State-of-the-art conditioners are liquid products that allow more consistent application by metering pumps. Make-up water quality can adversely affect the bath life of the conditioning rinse, and it is recommended that soft water be used for make-up.
Conversion Coating Processes
Typical systems used today include both zinc and iron phosphate systems.
Iron phosphate. Iron phosphate systems (more appropriately termed alkali metal phosphates) are used for a range of products requiring a durable finish that are not exposed to severely corrosive environments. These systems can vary from two to six stages, the shortest sequence being a cleaner-coater stage followed by a tapwater rinse if the requirements are low. Parts that are either more difficult to clean or have higher quality requirements will incorporate a separate cleaning stage, appropriate rinse tanks, final seal rinse and a DI rinse. The use of a final seal rinse (chrome or non-chrome) will result in better corrosion performance than the phosphate alone.
Iron phosphates produce an amorphous conversion coating on steel that ranges in color from iridescent blue to gray, depending on operating conditions and product formulation. Mixed metals may be treated with modified formulas that typically contain fluoride.
Iron phosphate processes are much easier to operate than zinc phosphate processes and require fewer process stages, but iron phosphates do not provide the degree of corrosion protection imparted by zinc phosphates.
A new generation of alkali metal phosphate plus non-chrome seal systems has been developed, yielding substantially improved corrosion protection on steel, galvanized steel and aluminum alloys. The alkali metal phosphate chemistry is free of toxic heavy metals and consists of complex mixtures of alkaline-earth acid phosphates with improved mixed accelerator packages, chelators and rust inhibitors. These chemical components are optimized to produce corrosion-resistant conversion coatings in relatively short application times of 30–180 sec in spray or immersion applications.
These chemistries yield uniform amorphous coatings over mixed metal substrates.
They also exhibit pH stability coupled with a long operating bath life and low levels of non-hazardous sludge.
Zinc phosphate. Zinc phosphating processes have been developed to provide exceptional painted part durability in corrosive environments. Typical industries using zinc phosphate processes include automotive and appliance.
Zinc phosphating processes have been developed to handle the increasing demands of industry. This includes the ability to handle mixed metals (steel, zinc-coated steel and aluminum), a decrease in environmental impact, improved performance and ease of operation. These processes operate at lower temperatures, are free of nitrites and nickel, reduce sludge levels and have internally accelerated processes that increase quality, operate easily and eliminate the need for additional accelerators.
There are many additives that can be formulated into a conversion coating. Fluoride is added to optimize the conversion coating on aluminum and/or zinc. Calcium ions are added to zinc baths to produce a microcrystalline phosphate coating or other processing characteristics. Various other metal ions, organic acids, chelating agents and other chemicals are added to modify the overall characteristics of a crystalline conversion coating.
Zinc phosphating systems have progressed from conventional systems containing high levels of zinc and nickel and being accelerated by separate additions of sodium nitrite to the polycrystalline systems used today. Current polycrystalline systems can be either internally or externally accelerated with latest developments being nickel-free. Table II shows the performance of these nickel-free, internally accelerated processes versus a conventional system.
After a metal surface receives a conversion coating, the surface is water rinsed to remove unreacted conversion coating chemicals and a post-treatment is applied.
The post-treatment can provide a two- to ten-fold increase in corrosion resistance and humidity resistance when compared to conversion coatings without final rinses.
Oftentimes, particularly in conjunction with electrocoating, a final DI rinse is required to minimize drag-in of high conductivity water into the electrocoat operation. In these cases, it is imperative to have a reactive final rinse that maintains its properties after DI rinsing.
Post-treatments historically have been based on chromic acid. With effluent guidelines getting more stringent, most finishers have converted to either trivalent chrome or non-chrome post-treatments.
Recent advances in dry-in-place (DIP) polymeric post-treatments have shown excellent results when compared with standard non-chrome/DI rinsed systems.
There are several typical coating evaluations performed on phosphate pretreatment systems.
Visual inspection. It’s desirable for phosphate coatings to be uniform in appearance whenever possible. Variations in color are normal on mixed metal sub-assemblies such as automobiles using various zinc-galvanized steel alloys.
Although color may vary, there should not be any visible shiny spots in the coating. If shiny areas exist, a condition known as inhibition could be present. Inhibition is where the phosphate coating has not formed due to surface contamination.
Mapping is a widely used term today that describes various types of visible patterns that are visually apparent in the coating. Mapping is normally caused by uneven chemical reaction with the metal. This can be due to oils, compounds, sealers or other materials reacting with the metal and forming a permanent stain or bond to the metal surface.
Patterns such as “lace curtaining,” streaking and other mild patterns are many times caused within the phosphate system. These patterns may be caused by drying in drain vestibules, misaligned spray nozzles or other air and solution flow imbalances in the system. In most cases, skilled operators can rapidly correct these patterns by realigning nozzles, adjusting pressures, etc. In some instances, additional wetting harnesses are added to systems to address these problems. Some systems are incapable of correcting certain patterns due to original design flaws versus the capital required for modification. Slight patterns are not normally detrimental to ultimate coating quality.
Coating weight testing. One tool used to rapidly determine proper system operation is to determine the amount of phosphate coating deposited on the surface. The term used to identify this amount is coating weight. The amount of coating should never be misrepresented as an indication of actual coating quality. Coating weight is, however, an excellent indication of the proper chemical balance and physical performance of the phosphate system when used in conjunction with visual inspection.
The normal expression for coating weight in the United States is mg/sq ft of surface (see Table III, left).
A polycrystalline zinc phosphate coating weight of 75 mg/sq ft on steel would most likely indicate some type of chemical or physical imbalance in the phosphate system requiring immediate attention. Coating weights within the range for each substrate are a good indication that no major system imbalances are present. However, further evaluations must be performed (crystal structure, coating composition, paint adhesion, corrosion resistance, etc.) to determine the ultimate coating quality.
Crystal structure. The most consistent, accurate and rapid tool for evaluating coating quality is a microscope. The preferred magnification for prepaint zinc phosphate coatings is 500–1,000 ×. Since the phosphate coating is a combination of crystalline structures chemically deposited on the metal surface, a microscope can be used to determine the size, shape, uniformity and orientation of the crystals present. In many instances foreign debris, residues and metal imperfections can also be detected that would otherwise go unnoticed with the naked eye. It is well-known that the crystal orientation and size can have a dramatic effect on paint film bonding in terms of both ductility and appearance.
Crystal composition analysis. In addition to crystal shape and size, the chemical composition of the crystals may have a primary role in certain durability concerns such as resistance to corrosive salts. These tests are currently performed using electron microscopes, x-ray diffraction, atomic absorption and other sophisticated techniques that render them impractical for on-site quality determinations in a production facility.
Recent innovations in polymeric post-treatments have led to the development of DIP products that can be applied directly over clean substrates with certain types of coating systems, particularly powder and liquid. These systems contain either very low or no phosphates. They also reduce operating costs in the areas of water use, waste treatment, energy costs and labor.
These coatings have been widely accepted for aluminum finishing and have almost completely replaced chrome-based pretreatments in paint applications.
Table IV (left) shows corrosion test results comparing a traditional iron phosphate to a DIP technology.
Tomorrow’s pretreatment users will have many new and exciting improvements open to them. These improvements will not only be in the chemistry and technology of the processes but in the types of services that the pretreatment suppliers will provide.
To contact Henkel Surface Technologies, call 248-583-9300, or visit henkel.com/industrial-12131.html
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