Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 27 of 331

THREE-STAGE SYSTEM 1. Cleaning: The first stage is a multipurpose stage where cleaning or oil removal must take place (see Table I). Displacement of soils is produced through spray impingement and wetting afforded by the detergent packages built into acidic solutions. A fine balance of the surfactants is necessary to adequately remove these normally alkaline sensitive soils. Upon completion of soil removal, the acidic solution dissolves a minute layer of the metal substrate. A slight pH rise takes place at the substrate–solution interface causing an insoluble reaction and producing the iron phosphate coating. Typical iron phosphate coatings range from 20 to 40 mg phosphate/ft2 on steel substrates. The wide range of soil types and soil loads distributed unevenly across metal surfaces places severe demand on the cleaner portion of cleaner phosphate chemistries. 2. Rinsing: The continuous overflowing rinse stage is designed to flush non- adherent soils and phosphate solution from all parts. 3. Seal rinsing: This final step in a three-stage spray pretreatment removes any trace chemical residue, prevents flash rusting, and seals the pores of the iron phosphate coating. This three-stage system has several benefits. It provides for good multimetal preparation and cleaning of controlled soils, and establishes dry-film adhesion characteristics. The system, however, also has some negatives. There is no alka- line cleaning stage and the phosphate accelerator choices exhibit limited abili- ty. Furthermore, control is difficult because of the narrow window available for it and finally phosphate shows a lack of uniformity. Nonetheless, cleaner coalers or cleaner–phosphate systems are the most prevalent spray systems for organic finishing. Cleaning is critical to ultimate success in the three-stage system. Acidic de- tergent systems are not as effective as alkaline cleaning products. This requires the manufacturer to have very tight control over incoming soil types and soil loads. The window for successful cleaning is much narrower, requiring full knowledge of incoming soils and very tight control over washer maintenance and overall cleaning efficiency. The type of iron phosphate and how the coating weight is accelerated or pro- duced is limited in the currently available chemistries. This sometimes limits the phosphate coating and the ultimate corrosion resistance offered! Three-stage iron phosphate systems can offer high-quality results when the pre- treatment chemicals are chosen for their ability to clean. FIVE-STAGE APPLICATION METHODS Iron phosphate systems can utilize five or more stages, where additional stages may include additional cleaning, rinsing, and DI rinse stages (see Table II). Five- stage systems are best suited for delivering high-quality phosphate development and long-term coating life. Six-stage systems most often include a final deionized water mist rinse. 1. Cleaning: Cleaning, typically alkaline cleaning, produces a metal surface free of organic and inorganic reactive soils. Alkaline cleaners incorporate deter- gents and surfactants to wet the soil, alkaline builders to degrade, emulsify, and saponify organics, and water conditioners to soften and control contaminants. Cleaning chemistries generated with alkaline systems offer a much wider range when considering the difficulty of removal and heavier loads of soil. 26

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