Abstract
Immersion tests of 0.004-0.20% C steels containing less than 0.002% P and 0.18-0.20% C steels alloyed with 0.002-0.031% P were carried out at 460-540°C in molten zinc. Vacuum melted specimens were used to avoid the influence of other elements, for example, silicon. After dipped in liquid zinc for a given time, specimens were pickled in 10%HCl and weighed to measure iron loss. Addition of carbon up to 0.20% lowered the height of the maximum peak at 500°C, which appears generally in the curve expressing the relationship between iron loss and temperature. The reaction rate at 500°C followed a linear law for 0.004% C steel, but was almost parabolic for 0.20% C steel. The δ1P layer of the former was completely broken up at grain boundaries, which is responsible for the linear attack. On the contrary that of the latter was stable and possessed protective property. According to scanning electron micrographic examinations the δ1P layer formed on 0.20% C steel at 460°C is hardly destroyed on etching in a nital solution. Carbon was found to be distributed in alloy layers as mixed carbide, Fe3ZnCx. In view of the fact that the break-up is caused by both the chemical reaction with molten zinc and the shear stresses existing at grain-boundaries, it can be concluded from above mentioned observations that precipitated carbides improve the grain-boundary strength to prevent fracture. Since a continuous ζ layer protects δ1P layer, carbon has no effect at 460°C. At 500°C due to the absence of ζ layer δ1P layer reacts with molten zinc. 0.004% C steel is attacked violently, whereas 0.20% C steel shows good corrosion resistance. Phosphorus added to 0.18-0.20% C steels accelerated corrosion at 500°C. In other words a high concentration of phosphorus, which makes grain-boundaries fragile, cancels the effect of carbon.
