Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 30 of 331

pretreatment ZINC PHOSPHATING BY JOHN DONOFRIO CHEMETALL PRODUCTS INC., NEW PROVIDENCE, N.J. The phosphating of steel, galvanized steel, zinc, and sometimes aluminum represents an essential stage in many areas of surface finishing. It is employed for the purpose of pretreatment prior to painting, increasing corrosion pro- tection, assisting cold forming, and improving the sliding friction properties of components in sliding contact. Each of these areas will be covered with ap- plication examples. Zinc phosphate is a crystalline conversion coating that is formed on a metal substrate utilizing the chemical reaction between metal ions that have been dis- solved in mineral acids and then diluted with water to form the process solution. Contrary to traditional plating operations where electrical current forces the coating formation, zinc phosphating processes rely on the basic pickling reaction that occurs on the metal substrate when the process solution comes in contact with the metal. The mineral acids that are normally used to dissolve the metal ions are nitric acid and phosphoric acid. Metals, such as zinc, nickel, and manganese, are dissolved depending on the process necessary. Several other metals can be dissolved to create specific char- acteristics. Nickel plays a major role in achieving an acceptable corrosion resis- tance of the coating as well as accelerating the process chemistry. More recent de- velopments have created nickel-free processes that can compete with the nickel- containing processes. Accelerators are added to phosphating processes for specific reasons such as reaction speed, hydrogen elimination, and sludge formation control. Acceler- ators can be used as single materials or they can be mixed to achieve the most effective combination. Several materials can be used including nitrite/nitrate, chlorate, nitroguanidine, hydroxylamine, peroxide, and organic compounds such as sodium nitrobenzene sulfonate (SNBS). Other additives are used such as free and/or complex fluorides when, for ex- ample, hot-dipped galvanized and/or aluminum substrates are treated. Figure 1 shows some phosphate crystal structures created by different processes. REACTION MECHANISMS Pickling Reaction The first reaction that occurs when the zinc phosphate solution comes into con- tact with the metal surface is the pickling reaction, which will dissolve some metal from the surface. On steel, depending on the process solution used, the pick- ling rate is in the range of 1–3 g/m2 of surface area treated; however, it can be much higher for higher coating weights. This pickling reaction is essential for the coating formation since this can be said to be a chemical cleaning of the surface and, as such, will affect the adhesion of the coating to the base metal. During the pickling reaction, for free acid of the solution close to the metal sur- face is consumed because of the dissolution of the metal surface. Metal ions are trans- ferred into the process solution. The type of metal ion depends on the type of sub- 29

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