Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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descent to gray. Coating weights range from 10 to 50 mg/ft2 and 100 to 300 mg/ft2 for zinc phosphates. Properly applied, this group of phosphates provides good corrosion protec- tion. Iron and zinc phosphates find widespread use in mixed steel and aluminum product lines. They are popular because of low operational costs and mild en- vironmental 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 specif- ically developed for aluminum in 1945. Since then, these products have per- formed remarkably well for the architectural metal and beverage can industries. Demand for cans,however, is on the decline. Recently introduced high-perfor- mance 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 giv- en as 50% to 55% chromic phosphate, 17% to 23% aluminum phosphate, 22% to for iron phosphates www.metalfinishing.com/advertisers 47

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