In fig. 5.5, it can be observed that, starting from the melting temperature of the zinc towards increasing values of the bath temperature, there is an interval (area highlighted with blue), in which the galvanizing can be successfully obtained. A thoughtful choice generally leads to choosing operating conditions between 440 and 460 ° C, which allows variations in the temperature inside the bath, without significant effects on the growth of the layer. Under different conditions, there may be “anomalies” in the composition and crystal structure.
However, technological uses of high temperature baths are also possible. In this case, 550 ° C is reached. The formation of the ζ (zeta) layer no longer takes place and, therefore, the coating is composed of a mixture of δ (delta) phase crystals and zinc. It is difficult to obtain coatings with a thickness greater than 100μm at such temperatures.
In general galvanizing, the immersion time of the pieces in the usual conditions is generally included in the interval between 1.5 and 5 minutes, depending on the more or less linear shape of the articles, and the thickness of the sections with which they are assembled. It is in fact in these first minutes that the greatest increase in thickness occurs. Particularly complex elements may however require the immersion to be extended beyond 10 minutes.
In fact, the thickness of the steel plays a decisive role in determining the residence time of the product inside the galvanizing bath, as already stated in the previous chapter. The thicker profiles require a longer time to uniform their temperature to that of the bath and during extraction they keep warmer and this positively affects the kinetics of formation of the layer.
The roughness of the surfaces can also significantly influence the thickness of the coating, due to dragging effects and the increase in the specific surface of the steel exposed to the action of zinc. In some cases, the effect is more evident, as occurs for very rough pieces because they have been sandblasted with particularly angular means or with pieces that originally have very corroded surfaces before pickling.
Reactivity of steels: influences of the substrate composition
The differences in the composition of the steel, due to the technological addition of metals or other elements in addition to iron and carbon in the alloy, lead to a greater or lesser increase in thickness or, as is commonly said, a greater or lesser reactivity.