During the last ten years, the percentage of of zinc coated steel used on PSA's cars has considerably increased: 15 percent ten years ago to about 70 or 80 percent today. Zinc coated steel sheets are the basis of our protection against corrosion. All these steel sheets are coated with at least 7.5 microns of zinc on one or both sides of the sheet. This policy is based on studies of natural corrosion in Europe later confirmed by natural studies on the Asian continent. Both are in good agreement with actual corrosion resistance on customers cars. Within PSA, we only use metallic coatings and only pure zinc coatings for car body panels and structures; that is to say hot dip galvanized and pure zinc electrocoated products. No alloyed metallic coatings such as Zn-Ni or Zn-Fe are used. The main reason for this is the high cost of Zn-Ni coatings. Furthermore this coating is also brittle and sensitive to corrosion (pitting and surface aspect corrosion). The advantage generally stated for galvannealed products is its weldability. However, three major disadvantages exist: - these coatings are sensitive to powdering in stamping conditions, - they are susceptible to cratering in painting conditions, - the aptitude of these coating to meet our specifications concerning the formation of red corrosion products has not been validated. For all these reasons, we decided to use pure zinc coatings with a modification of our welding process. To optimize the quality of PSA's cars, the characteristics of the coatings have been specifically defined. For exterior panels, we use double-sided electrocoated products and the zinc thickness depends on the position of each part on the vehicle. In order to respect our corrosion specifications, it is absolutely necessary to have electrocoated products with minimal thicknesses of 7.5 or 10 microns by side. Electrocoated products could be replaced by hot dip galvanized products if the same aspect after painting is reached. For car structures, we used 10 microns on one or both sides of the sheet, depending on the vehicle's component. This minimum thickness has to be respected on parts exposed to corrosion agents order to guarantee our specifications in term of corrosion resistance. We use either electrogalvanized or hot dip galvanized sheet steel depending on the price and on the possible problems of welding. In the first case, sufficient electric power is needed and/or enough electrolytic cells on electrocoating lines to obtain these thicknesses. Concerning hot dip galvanized products, there are two main aspects to our specifications which determine the coating quality: - the control of coating thickness, - the control of the chemistry of the coating. In our specifications, the thicknesses must be between 10 and 13 microns on each side of he sheet. This value is absolutely necessary in order to guarantee the constant quality of the welded points without changing the parameters of the welding machine. In terms of chemistry, four elements have to be very well controlled: Fe, Al and Sb or Pb. The control of the percentage of these elements within precise values is necessary to guarantee weldability (iron), to facilitate the phosphatation (aluminium), to avoid pollution of the phosphatation bath (Pb or Sb) and to avoid intergranular corrosion of the coating in tropical conditions (Pb or Sb).
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