Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 305 of 707

you pay attention to the pretreatment and pH of the bath. The normal operat- ing pH range of this bath is 3.6–4.0. A pH that is lower than the operating range may cause early corrosion in salt spray testing as the bath becomes more aggressive. A combination of lower pH and slightly elevated temperature makes it extremely aggressive and supports a shorter time cycle. Studies were performed by a licensee of NAVAIR's trivalent chromium con- version coating that examined test results of various aluminum alloys treated with different process cycles. It included various times, temperatures, and concen- trations of more than one detergent. Special attention was given to the etching nature of some of the detergents. The objective was to achieve a clean surface with- out any modification or powdery film. After cleaning, the surface was activated by various chemical methods that were chosen based on the alloy. Also, impor- tant factors included concentration and cycle time in activation bath. Rinsing was also given special attention. Tap water or deionized water was selected as required. Quality of rinse water, such as the levels of chlorides and total dissolved solids, were observed. The trivalent chromium pretreatment bath was operated at ambient temperatures as well as at elevated temperatures for evalu- ation purposes. Testing of different alloys included different concentrations of trivalent chromate in the bath. It also included different time cycles. The results were based on the performance in a neutral salt spray chamber. We evaluated different test matrices and found that certain alloys required a specific overall treatment. Alloys, which had not been so difficult to process, offered us excellent results. Salt spray hours in neutral salt spray ranged from 168–800 hours. When a specific process cycle was followed, 2024 alloy also performed well in salt spray corrosion testing, with a range from 168 to more than 1,000 hours. ENHANCED PERFORMANCE ADDITIVE One of the most daunting aspects of trivalent conversion coatings as a drop in replacement for hexavalent chromium chemistries is the requirement of rigid pre- treatment parameters in order to achieve maximum corrosion performance. Across the board, job shops and formulators alike go to great lengths to produce consistent results that pass requirements such as MIL-DTL-5541 and other corrosion specifications. Application facilities not well equipped to perform experiments for pretreat- ment optimization are finding themselves making small beaker size batches of chemistry with the intention of scaling up to their production line. This is good practice when tight controls are met with proper standards, but the process can be convoluted for some and more work than what a typical job shop is used to when integrating a new product into their line. The standard practice in industry is to purchase a material, follow the operating parameters, and, voilá, it works. Trivalent chromium conversion coatings are not, in general, that straightforward. The nature of the technology poses the burden of tailoring the facility's resources and equipment constraints to the product, which requires a slightly more sophisticated level of understanding. This may come as a shock to those transitioning to trivalent chromium chemistry and can often be discouraging. There have been successful attempts in creating a more robust process to allow leeway in pretreatment parameters. Research institutions and private industry groups are looking for additives to put in trivalent chromium conver- sion coating baths. Anyone with experience using trivalent conversion coatings 304

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