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Each class of metals has its own unique surface characteristics that will affect the performance of an applied powder finish. These characteristics include the not only the solid properties of the base metal itself but also the many surface attributes that are determined by the chemical composition and processes used in the manufacture of the metal.
These surface attributes consist primarily of a mixture of the oxides, hydrates, and salts of the metallic elements which make up the metal composition and very little of the metal itself.
In addition to these natural constituents, a number of contaminants are present. For example, on steel these would include surface carbons, oils, lubricants, metal fines, non-metallic surface inclusions, rolled-in scale, large surface carbides, dirt, corrosion products, and by-products of bacterial action, mildews and other fungal deposits. Zinc surfaces will have oils, lubricants, corrosion products, metal fines, dust, dirt, and other extraneous soils. Aluminum will be similar to zinc but also include heavy deposits of aluminum oxide. The aluminum oxide is not considered a classic contaminant in itself but it may interfere with the removal of other contaminants.
There is no universal part preparation method that can be used to deal with all of the different metals and their respective surface contaminants and be effective in all cases. An understanding of the different metals, their typical surface characteristics and the different treatment methods is very important to the powder coater.
Steel Steel is an alloy of iron and carbon with small amounts of other elements. The steel manufacturing process can be varied to produce steels with different properties and gauges. Heavy gauges of steel are typically hot-rolled steel. Hot-rolling is the first step in the development of steel slab. It produces a surface that carries a layer of mill scale that is developed after the steel is rolled, during the cooling process. The scale is relatively adherent to the metal surface beneath it but it is often cracked and loosened during the manufacturing process. Application of powder over HRS surfaces that have not had the scale removed is very likely to result in adhesion failure. Complete and reliable removal of mill scale requires acid pickling or mechanical cleaning. Welded areas on HRS may cause further problems with adhesion failure and application. The welding process leaves a rough surface with dried-on compounds, oils and stains that can interfere with application and adhesion. Welded areas should be mechanically cleaned by wire brush or blasting to remove contamination that will resist chemical cleaning.
Cold rolled steel (CRS) is a further reduction in the process that produces steel in thinner gauges (0.005 to 0.080 inches or 0.0127 to 0.20 centimeters thick) and has a finer microstructure than hot-rolled steel. It has the same basic elements as the HRS but not the mill scale and heavy carbon smut. CRS sheets will normally have light oil on the surface to act as a rust inhibitor but it is relatively clean and free of oxides. Chemical cleaning can be a very satisfactory way to prepare CRS for powder coating. Some products, such as automotive panels and wheels, are manufactured from high strength-low alloy steels (HSLA). HSLA steels have small amounts of alloying elements included in their composition to provide better strength-to weight ratios. HSLA will normally respond well to the same treatment methods as other carbon steels and oxidation can be removed by grit blasting. Blasting to a near-white or white metal clean surface may produce some ”shadowing,'' due to the nature of the oxides on the surface. Typically, this is not a problem and coating adhesion is good over these surfaces. Some HSLA materials contain silicone, which can accumulate as oxides on the surface and interfere with subsequent cleaning and conversion coating processes.
Stainless Steel Stainless steel (SS) is relatively free of the iron hydrates that are a common component on the surface of regular grades of carbon steel. SS should be cleaned to remove the surface oils and dirt from manufacturing and handling. Many stainless steel products do not have a protective coating because they are not sensitive to ordinary atmospheric conditions.
The passive oxide layer on stainless steel is relatively inert to alkaline cleaners and other chemical products that are commonly used on carbon steel. Normal cleaning will remove lose soils but it will not create a surface that is receptive to iron phosphating. Acid etchants or mechanical abrasion processes are sometimes used to remove the oxide layer and create a slightly roughed surface that is better for adhesion of the coating. These processes help promote adhesion but they do not provide any additional resistance to moisture penetration. Galvanized Steel Zinc coating of steel to produce galvanized steel can be produced by hot-dipping the steel into a molten bath of zinc or by electrolytic application in an ionic zinc solution. Galvanized materials are used to provide an additional layer of corrosion protection. The performance properties of the galvanized product produced by hot-dip or electrolytic process are not much different. However, the surface chemical properties do have some significant differences. Hot-dipped galvanized steel has a spangled appearance, which is determined by the specific chemical composition of the molten zinc bath and the cooling process used for solidification of the coating. The coating has a layered structure that includes a thin layer of alloyed iron-zinc coating adjacent to the steel interface with a layer of zinc coating proper on the outer surface. Various elemental impurities or additives may tend to segregate to the grain boundaries of the spangles because of their limited solubility in the solidifying matrix, while other elements, such as aluminum, tend to diffuse into the entire surface of the zinc coating.
Hot-dipped galvanized coatings can be used to produce galvannealed coatings by sustaining the alloying reaction. The coating is applied at a predetermined thickness and the steel and coating are held at temperatures where the diffusion of iron is very rapid. Diffusion continues until complete alloying has occurred. Galvannealed coatings have a matte gray color and low gloss compared to the bright silvery look of the non-alloyed zinc coatings. Galvannealed coatings can provide better adhesion for organic coatings without a phosphate treatment than the free zinc coatings.
Zinc coatings applied to steel electrolytically in zinc ion solutions can provide the same corrosion protection and hot-dipped zinc coatings but they are very different in composition and structure. Electrogalvanized steel is comparatively free of the minor impurities that are common to the hot-dipped zinc coatings.
In the electrogalvanizing process, there is no need for the metallic element additives that are used to control the behavior of the hot-dipped bath and the spangle size and pattern of the coating. Electrolytic zinc coatings are relatively uniform in composition without the thermally induced diffusions of iron that produce alloy layers in the hot-dipped process. Coating deposition occurs from sulfate or chloride saline solutions so it is possible to have some minor inclusions of these salts in voids of the coating if the surface is improperly rinsed but good control of the rinse process will normally eliminate this problem. Also, since there is no recrystallation from the molten state, there is no variation of the spangle.
Aluminum Pure aluminum (99.5% Al) has low density, high ductility and low strength. Aluminum can be alloyed to produce metals with many of the desirable characteristics of the pure metal and added properties from the alloy for strength. Aluminum is commonly alloyed with one or more of the elements of copper, manganese, magnesium, silicon, nickel, tin, and zinc as major constituents and chromium, iron, nickel, silicon, and titanium as minor constituents or normal impurities. Since some alloys may have less corrosion resistance than the pure metal, they are sometimes clad with pure aluminum or another alloy with better corrosion resistance. The various alloys may respond differently to cleaning and treating.
Aluminum alloys are classified into two general types;
These two types are pre-determined by their elemental compositions and how these compositions react to mechanical stress and temperature. Cold working or heat treating of aluminum alloys will develop a more homogenous surface texture and distribution of the various metal elements than the original wrought aluminum. Heat treating of aluminum alloys can affect the chemical responses of their surfaces and the receptivity to cleaning and chemical treatment. For example, a manganese alloy will tend to collect manganese oxides on the surface, in addition to the normal aluminum oxides. These cold worked or heat treated aluminum alloys will generally have better corrosion resistance than the softer and more heterogeneous wrought material. The raw ingot will have larger, more segregated inclusions while on the treated alloy these particulate intermetallics will be more uniform and less likely to develop corrosion cells. Manganese, lead, zinc, and titanium have less effect on the corrosion resistance of aluminum alloys than magnesium, iron, silicon, and copper. Different chemical surface characteristics of the alloy will react differently to certain types of exposure. For example, magnesium will retard corrosion if exposed to saline chloride but promote corrosion when exposed to alkalinity. Aluminum alloys are identified by a series of numbers. Different series of alloys will react differently to chemical treatment. The 1000 series products have the least amount of alloying impurities. These products are readily treated by chemical process and have excellent corrosion resistance. The 2000 series uses copper as the major alloying element to add strength to the metal. They may not always respond to chemical treatment. Since each of the different series will have somewhat different properties, it is wise to know what basic elements are used in the raw material and how they affect pretreatment.
Powder Coaters Manual
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June 14, 2012