| Abstract: | The phosphatizing quality is very sensitive to variations in the chemical composition of the surface. By annealing the steel sheets in a H2/N2-mixture with a dew point less than - 20 °C, an enrichment of strong oxide-forming elements (e.g. Mn, Si, V) occurs. The most important of these elements for the phosphatizing treatment is manganese. After annealing, the surface composition of the steel sheets was determined by ESCA analysis and the coverage of the enriched oxides was calculated. Zinc phosphatizing was performed in two different solutions, one with pH = 1.9 and the other with pH = 2.9. The nucleation rate was evaluated from SEM pictures. After phosphatizing, the samples were painted and exposed in laboratory test equipment to a SO2-polluted atmosphere (1 ppm) with a relative humidity of 90%. The weight increase gave a value of the corrosion rate. This investigation shows that both the nucleation rate of phosphate crystals and corrosion resistance are affected by the manganese enrichment, especially if the phosphatizing solution has a high pH (pH = 2.9). In this case, both the number of phosphate crystals and the corrosion protection are enhanced by the enrichment. At the lower pH value (pH = 1.9), no increased corrosion protection is obtained by the annealing treatment. The particle-size distribution of manganese oxide, which can be evaluated by consecutive ion etching and ESCA analysis of the surface, is also important to phosphatability. A large number of small particles uniformly distributed on the surface is the most favourable situation. |
