Corrosion properties of galvanized auto steel sheet with breakages of zinc coating in NaCl containing solution and mud

The corrosion properties of the galvanized steel sheets for autos with various broken areas of their zinc coating were evaluated and compared by immersion tests in a mud of Yingtan soil containing 3.5% NaCl and a solution with the same concentration of NaCl. The results show that corrosion weight-loss rate of the sheet with intact zinc coating was higher than that of the substrate; the corrosion rate increased with the broken area increasing, and was higher in the mud than in the solution. In the two cases, the variations of corrosion rate with time were also different. The analysis of EIS diagrams show that the electric charge transfer resistances of the specimens with broken zinc coating were much lower than that of the steel substrate, so the corrosion of zinc coating played a major important role in the weight loss. Higher oxygen concentration and lower pH value in the mud were two key factors to give rise to the corrosion behavior of specimens with broken zinc coating in the mud different from that in the NaCl solution.