Initially, one would probably say that not much has changed in the area of automotive pretreatment for a number of years. Globally, most automotive assembly plants have used immersion processes with tri-cationic (Zn-Ni-Mn) technologies for several years. However, upon further consideration, there have been several significant changes in the conventional automotive pretreatment process, with more changes to come. When examining the reasons for change, it is apparent that there are two major drivers: 1) Environmental regulation; and 2) Substrate selection
Regulation of heavy metals has been a concern in the pretreatment process for several years. These regulations were initially driven from Europe, resulting in a push toward nickel-free technologies. Industry attempts to qualify a commercial nickel-free technology, without compromising overall adhesion and corrosion performance, fell short of the original objective. However, the research and development work completed toward this objective resulted in commercialization of tri-cationic pretreatment technologies with much lower metal levels. In the United States, the Metal Products and Machinery (MP&M) initiative was published in the Federal Register in January 2001. This initiative sought to reduce effluent of chromium, nickel, manganese and zinc up to 92% from current standards by the year 2005. Although this initiative has since been revised, it does serve as an indication of future technology direction.
Chrome containing post rinses have largely been replaced by chrome-free rinses or the elimination of post rinses altogether. The initiative for this comes from the European End of Life Vehicle Directive. This regulation assigns European automakers full responsibility for the recycling of their vehicles at the end of its usefulness or “life.”
Demands to limit the use of water and energy reduction initiatives have created a drive toward low-temperature and ambient processing as well. The reduction of water consumption and subsequent cost reductions also have the added benefit of lessening the burden on down stream processes such as wastewater treatment systems. Reductions in processing temperatures have significant impact on the use and expense of gas and electricity (cost) historically needed to heat the large processing baths.
Without question, the selection of substrates used in the manufacturing of automobiles is an area under continual evaluation by all automakers. Changes in substrate selection can significantly influence the pretreatment chemistry to be used. Substrate selection is important for several reasons, including corrosion performance, weight impact on corporate average fuel economy (CAFE) standards, design flexibility as well as overall cost consideration.
Over the past few years the automotive industry has widely adopted the use of galvanized or galvanealled substrates on exterior panels to extend corrosion protection. With the ever-increasing demands to lower the weight of vehicles to meet CAFE standards, the use of aluminum offers significant reductions in the overall weight of the vehicle.
With U.S. manufacturers, the use of cold-rolled steel is limited to low corrosion areas such as automobile roofs. The increased use of aluminum and multiple substrate combinations offer several challenges to the traditional pretreatment process. Technologies that have traditionally been effective in treating the cold rolled steel and galvanized substrates often need to be modified to accommodate the various alloys of aluminum, depending on the volume of aluminum that is being processed. There is a pretreatment chemistry that can accommodate high aluminum loading while maintaining excellent pretreatment quality on CRS and zinc coated substrates processed through a common pretreatment bath.
Another trend that is affecting the pretreatment process is the expanding use of mill-applied coatings. These coatings range from dry-film lubricants and pre-phosphates to weldable primers. While dry-film lubricants and pre-phosphates are used primarily for improvements in stamping difficult to draw parts, weldable primers potentially can change the footprint of a conventional paint shop, through the elimination of processes such as hem-flange sealants and cavity wax operations.