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    Factors influencing the growth of the coating

    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.

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    Formation reaction and characteristics of the coating

    Morphology of the coating
    The products due to the effect of hot dip galvanizing is the result of the diffusion of zinc at the bath temperature, through the most superficial layer of the steel.

    The term reaction to indicate the set of processes that lead to the formation of the coating is now universally accepted. It is not a chemical reaction, but a sort of metallurgical reaction, a physical process.
    At the surface of the steel there is an exchange between the two phases which gives rise to the formation of layers of alloys with different compositions of the two metals iron and zinc. For this reason, the zinc coating is “welded” on the surface of the steel, with obvious benefits compared to other anticorrosive treatments that involve overlapping of metals (such as electroplating or metallization processes) or organic coatings (liquid or powder paints ).

    The iron / zinc alloys developed during immersion in the galvanizing bath are well characterized and recognizable by their composition and crystalline structure. Each of them, in fact, corresponds to one of the homogeneous phases foreseen by the iron-zinc state diagram (ie of “solubility”). Their succession shows an increasing zinc content towards the outside.

    In a typical galvanizing coating, starting from the steel substrate, the γ (gamma) layer with a thickness of about 1μm, in which zinc is present for about 70% (the percentage of iron varies between 26.8 and 31.1 %).

    The subsequent δ (delta) layer contains an amount of iron of the order of 10%.
    In the following ζ (zeta) layer, 7% iron is present.

    In the microscope photos the crystals of layer di oriented upwards, oblong and perpendicular to the surface are clearly recognizable.

    In most cases, albeit with significant exceptions as will be illustrated below, in the galvanizing coating there is a last and outermost surface layer, called layer η (eta), which is made up of zinc with a composition coinciding, in practice, with that of the bathroom. It is the result of the last interaction with the molten zinc before the extraction of the pieces and is deposited by dragging. For traditional baths, it is almost pure zinc, as it has a maximum iron content of about 0.008% at room temperature.

    The case of a galvanizing bath consisting of a technological zinc alloy (with the addition of tin and nickel, for example) is different, in which the η layer will have a composition obviously influenced by the presence of the other elements in the alloy. Note that the zinc bath according to the Italian and international standard UNI EN ISO 1461 certainly cannot contain less than 98% zinc.