Metal Finishing Guide Book

2012-2013

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tralized, and produce an integral crystal growth on the metal surface. The aluminum surface is therefore converted to a finely crystalline phosphate film with acceptable texture for paint bonding. Crystalline phosphate films may be iridescent to gray. Coating weights range from 10 to 50 mg/ft2 for iron phosphates and 100 to 300 mg/ft2 for zinc phosphates. Properly applied, this group of phosphates provides good corrosion protection. Iron and zinc phosphates find widespread use in mixed steel and aluminum product lines. They are popular because of low operational costs and mild environmental toxicity. Bath life is, however, very limited due to low tolerance for aluminum ion accumulation. Application is by immersion at 125 to 140�� for 1 to 4 minutes, or spraying at 125 to 160��F for 30 seconds to 2 minutes. Product selection should be restricted to moderate field service environments. CHROMATE PHOSPHATES Chromate-phosphate coatings enjoy a privileged position in aluminum prepaint treatment. They have a historic significance as being the first pretreatment specifically developed for aluminum in 1945. Since then, these products have performed remarkably well for the architectural metal and beverage can industries. Demand for cans,however, is on the decline. Recently introduced high-performance topcoats are more forgiving toward nonchromate prepaint treatments. Chromate-phosphate coatings are applied by spray or immersion.Immersion times range from 30 seconds to 3 minutes at 110 to 130��F, whereas spraying is done at 15 to 45 seconds at 95 to 130��F. These baths produce crystalline or amorphous coatings of 15 to 1,000 mg/ft2. The film is iridescent to grayish green. Thickness can be as high as 0.1 to 0.4 mil. A typical air-dried coating is given as 50% to 55% chromic phosphate, 17% to 23% aluminum phosphate, 22% to 23% water, and a trace of fluorides. Performance properties of chromate-phosphate films are generally very close to chromic acid anodizing films and those of chromate-oxide films to be discussed later. Adhesion and corrosion protection increase with coating weight up to a point, then fall off. The best range is between100 and 200 mg/ft2. Because of its excellent qualities, the American Architectural Manufacturers Association (AAMA) has designated the chromate-phosphate process as a standard prepaint treatment. It also meets U.S. military specifications. Generally, this process is recommended for severe and long-term service conditions. CHROMATE-OXIDE COATINGS Chromate-oxide films are more versatile and widely applied than the chromatephosphate treatments. They comprise the bulk of treatments for the coil stock and transportation industries. In applications where anodizing is not feasible, for example, where parts are too long or assembled with dissimilar metals, chromate treatments of this type have been used in place of anodizing. Typically, a chromate-oxide bath consists of three principal constituents: acid chromates, etchants and accelerators or complexing agents. Application may be by spray, immersion, or brush at 70 to 110��F for 15 to 45 seconds. The aluminum surface is converted to an iridescent golden yellow color. The film is tightly adherent, amorphous, and mixed with metallic oxide products. Film thicknesses range between 0.005 and 0.04 mil. Coating weights are from 15 to 100 mg/ft2. A freshly formed film can be leached to a nearly 137

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