Influenza degli elementi di lega nei bagni di zincatura sulla formazione delle fasi intermetalliche e sulla resistenza a flessione dei rivestimenti a caldo

In almost all industrial applications metallic materials are the most widely used and they are employed in a wide range of applications. These materials work in continuous contact with the surrounding environment; in fact, in this environment they fulfil the functions for which they are designed and manufactured. The interaction between the metallic material and the environment in which it operates is a chemical one and it is known as corrosion. Hot-dip galvanising is the most effective and widely used protection technique against corrosion. This technique is valuable because it has a dual action: firstly, a barrier is created that insulates the metal from the aggressive environment, and secondly, the zinc in the coating has a lower electrochemical potential than iron; therefore, the substrate is protected by the Zn-based coating even if this latter is damaged. The relationship between the mechanical properties of the coating and the parameters of the hot dip galvanising process can be understood by analysing the kinetics of coating developments. Indeed, the mechanical characteristics can be profoundly modified by intermetallic phases. For this reason, to supervise the phases present in the composition of the coating is necessary for improving the mechanical properties. In the scientific literature, there are some models that consider the kinetic development of intermetallic phases. Considering both the phenomena of interdiffusion between zinc and iron atoms, and the stability of the phases for a precise chemical composition, to anticipate the thickness of the phase is substantial. The formation and propagation of cracks can be anticipated through a damage model, which should be developed by analysing the damage micro-mechanisms of the intermetallic phases. In this activity, the hot dip galvanizing process was applied to low carbon steel samples, considering different bath compositions. Then, the obtained coatings were studied by optical microscopy, analysing and identifying the formation of different intermetallic phases, depending on the composition of the bath. After, bending tests were carried out in the laboratory using the Duncan mechanism, which was suitably mounted on a 100 kN electromechanical machine. Five different protective zinc coatings were characterised and compared, which derived from immersion in different baths. Specifically, the five baths used were: pure Zn, Zn - Pb 1%, Zn - Al 5%, Zn - Sn 3% and Zn - Ti 0.5%; moreover, five different immersion times were considered: 15, 60, 180, 360 and 900 seconds. The temperature used in the galvanizing phase was instead kept constant at about 460 °C for all the investigated conditions. The obtained thicknesses and the various damage caused by the stress were then compared and analysed, through various comparisons carried out by means of optical microscope observations on the sections of the different galvanised specimens. Therefore, through this study, it was possible to identify the various damage mechanisms of the intermetallic phases as a function of the dipping time and the bath composition. It was found that the damage mechanisms were mainly represented by nucleation and growth of radial cracks in correspondence of the hardest and most fragile phases of the coating. It was also possible to evaluate the importance of ductile phases presence and how their thickness growth tended to stop the propagation of defects. Finally, replacing toxic elements such as Lead in favour of elements such as Tin, Aluminium and Titanium was preliminary evaluated; however, further studies are necessary in order to improve the homogeneity and uniformity of the zinc coating for avoiding possible problems of coating adhesion and detachment.