Zum Einfluß von Si und P auf das Verzinkungsverhalten Von Baustählen

The influence of Si and P contents in steels on the galvanizing behavior The galvanizing behavior of 40 structural steels in current use was investigated in relation to the complex influence of the Si/P content under conditions that are usual in the hot dip galvanizing industry (440/450/460 °C,5/10/15 min immersion time). The effect of P on the increase of coating thickness on steels with 0,01 to 0,40 % Si begins to be perceptible at > 0,020 % P. The influence of P increases with decreasing Si content of the steels and decreasing temperature of the melt. In steels with ≤ 0,12 % Si, an increased P content causes a shift of the thickness maxima in the temperature range from 440 to 450 °C, such as is otherwise typical of steels of 0,12 % to 0,28 % Si with < 0,020 % P. Zinc coatings on steels with the critical P/Si content (0 to 0,20 % Si/> 0,020 % P) as a rule are more unstable. With increased immersion time at the temperature stated above, floating away of the ξ phase into the zinc melt may occur. This could also be observed with Sandelin steels in the most critical Si range (0,07 to 0,10 %) with < 0,020 % P, with an immersion time of 15 min. It is possible to reduce the thickness of zinc coatings by adding small amounts of Al to the zinc melt (< 0,03 %). This effect of Al at a concentration that is below what is required for the known inhibitory action by the formation of an thin Fe₂Al₅-or Al-containing δ₁ film on the steel surface is attributed to the instability of the ξ phase, a proportion of which floats away into the zinc melt.