Getting Your Pretreatment Process Ready for a Changeover to Thin Film Coating
Tomasz Slezak from Chemetall says thin-film technology enhances corrosion protection and paint adhesion, while providing a cost-efficient, eco-friendly process.
Q: I am considering a change to thin-film coating. How can I ensure that a process is ready for a successful transition?
A: Metal pretreatment plays an important role in the manufacturing process. Iron and zinc phosphates have been long-time predominant methods for metal preparation prior to paint application. Specialty chemical suppliers have followed the green technology trend by developing and continuously improving thin-film coatings. Whether you are calling it thin film, advanced pretreatment, transition metal coating, nanotechnology or zirconium coating, this technology enhances corrosion protection and paint adhesion, while providing a cost-efficient eco-friendly process.
Still considered by many to be new, this coating has been around for more than two decades. Performance levels have matched and exceeded quality test results for parts treated with iron and zinc phosphate. Many of these products are approved and specified as the pretreatment process of choice by many heavy equipment, agricultural and automotive manufacturers. With increased regulations on phosphate discharge and the evolving trend promoting green technology, metal fabricators and finishers are looking at transition metal coatings as a pretreatment option. Zirconium coatings are not a drop-in replacement for conventional conversion coatings, such as iron and zinc phosphates. A thorough system audit and review of application steps should prepare you for a successful transition.
Proper cleanout and complete descale of the washer are required before the new system startup. Any leftover residue will react with new chemistry and could degrade the new process performance. Descale should include spray risers and nozzles. This is a great time to complete maintenance inside the washer and make any changes to optimize the system. These include reviewing nozzle size and alignment; fixing leaks found during descale; checking filter screens; and installing exit halo risers and counter flows. Installing filter systems, oil removal or auxiliary equipment will increase the bath lifetime.
Advanced pretreatment doesn’t require as much contact time and higher temperatures as conventional conversion coating; therefore, it is possible to move the thin-film application to a smaller tank. Convert existing iron or zinc phosphate stage into a rinse and charge subsequent tank with an advanced pretreatment chemical. Review the layout with chemical supplier and equipment manufacturer.
Chemical compatibility should always be verified when a new product is introduced to the process. Tanks, pumps, filters and all other auxiliary equipment should be checked and replaced with compatible materials. Check the chemical compatibility of all gaskets and seals. Thin-film chemicals can be used in stainless steel, mild steel or plastic washers. Most applications do require stainless steel equipment. If stainless steel is required, verify that all of the construction materials (including welds, screens and valves) are 304 or higher grade stainless steel.
Water quality is very important in the advanced pretreatment application. Makeup water should not exceed conductivity of 100 micro Siemens per centimeter (µS/CM) and contain less than 25 ppm of calcium, magnesium, chloride, sulfate and sodium. Higher levels can increase chemical consumption and degrade performance. Incoming water sample should be analyzed, and results reviewed with the chemical supplier. Reverse osmosis (RO) or deionized (DI) water systems are required in most applications.
Excellent pre- and post-rinses need to be maintained with the zirconium coating process. Maximum conductivity of single-stage rinse prior to advanced pretreatment should not exceed 100 (µS/CM). In the dual-rinse stage, maximum recommended ranges are 200 µS/CM in the first rinse stage, and 100 µS/CM in the second. Exit halo riser (with direct RO or DI water feed overflowing to the rinse stage) would ensure the purest quality water flow over the part. Makeup water fed directly from the exit halo will assist with maintaining lower conductivity. Rinse tanks should be plumbed with continuous counter flow to previous rinse stage (a typical overflow rate of 2 to 10 GPM).
Check the parts for excessive solution dragout or pooling. Solution dragout can contaminate subsequent stages. Pooling can lead to paint defects and delamination. Review parts racking or possibly add drain holes to reduce the amount of solution carried through the system.
Thorough cleaning is an essential part of pretreatment, especially with thin-film technology. All soils (organic and inorganic) need to be removed prior to the coating and paint application. Alkaline cleaners are a good choice for removing organic soils, such as oils and greases. As a quick check, look for water-break-free surface after the rinse stage following the cleaning cycle. Consider adding an acid or neutral pickle step to your pretreatment process to replace mechanical removal of laser and weld scale.
Following proper application procedures is vital in manual spray applications. To ensure proper cleaning, work within specified process parameters. Keep in mind that the time required to completely clean the entire surface will depend on soil loading. Work with your chemical supplier to train operators and establish a “Best Practice Guide” for manual wand applications. Apply thin-film coating at low pressure, completely flooding the entire surface of the part. Misting nozzles or high-pressure systems are not recommended. Do not allow more than two to three minutes between cleaning and rinsing. Treated parts should be painted immediately.
There are many thin-film products available on the market. Your chemical supplier can recommend products that will meet your quality requirements or provide you with a list of products that have already been approved by OEMs. Any recommendation should be validated by an additional test with production substrates. Remember that good test results will not only depend on pretreatment but also on the quality of the paint and its application.
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