Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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such as aluminum or zinc, a microetched surface is produced in place of iron phos- phate coating formation. Thorough understanding of available water for processing will help to min- imize sludge formation in tanks and prolong tank life. Water should be ana- lyzed for its hardness and dissolved solids in micromhos. For extremely hard waters, select alkaline cleaners or phosphate compounds with hard water stabi- lizers. It will be especially important for phosphate compounds to have a hard water stabilizer system built in to minimize sludge formation as well as frequent operating pH adjustment to obtain quality phosphate coating formation. Operating pH will vary widely with type of phosphate compounds and some will favor a pH range of 3.5 to 5.0, whereas others will favor a range of 4.8 to 6.0. It is more economical to use pH adjustment acid concentrate than to use phos- phate compound. pH will tend to rise in operation in most instances. For combination cleaner–iron phosphates, the cleaning issue, or cleaning ability of the chemical formulation is critical. Quality phosphating will not take place until the surface is sufficiently void of all organic soils. In many instances, operators and management stress coating weights and salt spray requirements for three-stage systems, but their concern should be more focused on control of soils, the cleaning ability offered, and maintenance of the system. Mixed metal lines, or systems treating nonferrous metals through an iron phosphate system, are generally faced with a major trade-off. It is very difficult to maximize salt spray performance on both ferrous substrates and nonferrous substrates running through the same line at the same time. Serious evaluation and pretesting should be done prior to the installation of a multimetal line. When high corrosion protection is required, serious consideration should be given for two pretreatment lines, especially when the finisher desires to maximize performance testing with mixed metals operations. The cleaning of nonferrous materials, such as aluminum, zinc, galvanneal, and galvanized or terne plated steel, require special consideration, especially with cast materials. Many die lubes proposed to be water soluble are in fact very difficult to clean. If the metal finisher does not have his or her own in-house die casting operation, then it is advised to have either a great relationship with the casting vendor or a thorough understanding of the soils used and the process utilized. Precleaners and iron phosphates for mixed metal lines are typically modified slightly to clean and deoxidize. Work with your chemical ven- dor to choose the right materials that offer sound cleaning, deoxidizing, and microetching in the process, without being overly aggressive. Adhesion fail- ures occur when either the substrate is insufficiently cleaned or microetched and when there is excessive etch. 25

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