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.