Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 25 of 331

pretreatment IRON PHOSPHATING BY BRAD GRUSS PRETREATMENT & PROCESS INC., ASHBY, MINN. Phosphate coatings are produced on ferrous and nonferrous metal surfaces and are composed of tiny crystals of iron, zinc, or manganese phosphates. These in- organic coatings produced on metal surfaces retard corrosion and promote bet- ter paint bonding. Phosphate coatings are produced after precleaning or are formed in a combination bath known as a cleaner-phosphate. Phosphate coat- ings are generally used by the metal finishing industry for the following rea- sons: (1) to provide a base for bonding organic finishes such as paints, lacquers, plastics, rubber, adhesives, etc.; (2) to provide a base of oils, waxes, and rust pre- ventives to reduce metal corrosion; (3) to provide a base for lubricant on bearing surfaces to reduce friction; and (4) to aid in drawing and forming of metals. The primary use of phosphate coatings, however, is for the bonding of paint. Coatings produced on metal are not only stable and chemically inert toward or- ganic finishes, but they are also absorptive and bind organic finishes to the met- al. The most important reason to use a phosphate coating is to prevent or retard the spread of corrosion under paint, including the areas near a ruptured film. Phosphate coatings consist of crystalline salts of the metal being treated and/or crystalline salts of metal ions added to the phosphating solution. When the metal comes in contact with the phosphating solution, pickling occurs, which results in a reduction of acid concentration at the liquid-metal interface. At this point iron is dissolved, hydrogen is evolved, and the phosphate coating is deposited. Should the solution contain additional metal ions, such as zinc or manganese, phosphate coatings of these ions are also deposited. Accelerators, such as nitrite, nitrate, chlorate, peroxide or special organic chem- icals, may be added to increase the rate of coating deposition. IRON PHOSPHATING Iron phosphating is the workhorse of the general finishing industry because of the quality delivered with regards to the newer coatings, its ease of control, non- sludging characteristics, and economics. Iron phosphating may be facilitated through a high-pressure, dry-steam process.1 By using dry steam for cleaning and phosphating metal surfaces for painting and powder coating, substantial sav- ings can be achieved. Iron phosphate coatings are usually derived from solutions that contain very little iron. They are produced on ferrous metals through the combined use of acid phosphate salts, free phosphoric acid, plus accelerators. For nonferrous metals, 24

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