Issue 48

C. Bellini et alii, Frattura ed Integrità Strutturale, 48 (2019) 740-747; DOI: 10.3221/IGF-ESIS.48.67 741 The protection action of HDG coating can protect the substrate also in the presence of short cracks which eliminate the barrier effect. In this case, the galvanic couple between zinc and iron is able to protect the substrate allowing corrosion of the zinc coating. The corrosion product of zinc, characterized by white colour, is often able to repair the crack generating a new barrier. The formation of the coating is governed by both physical and chemical parameters. Some physical parameters are bath temperature, immersion time, pre-galvanizing surface temperature, etc. Main controlling chemical parameters are the steel and the bath chemical compositions, flux chemical composition, and so on. The formation of hot dip galvanizing coatings is due to interdiffusion of zinc and iron atoms between zinc-based melting and iron-based specimens dipped in the bath. Due to different chemical composition, the interdiffusion of atoms generates a non-uniform coating characterized by different intermetallic phases. In the rich zinc zone (close to the surface of coating) the presence of a η phase is due to the wettability of melting zinc and the chemical composition is very close to the chemical composition of the bath. Towards the inner zone the content of iron increases, generating a different zinc-based phase characterized by a columnar morphology ζ phase). This phase is due to the interdiffusion phenomena between iron and zinc atoms in the radial direction, which is the direction of coating growth, and it can lose the columnar morphology if the coating will be exposed at high temperature for a long time. It happens for instance, at high dipping time, where the secondary diffusion phenomena (not only in the radial direction but also in transversal direction) takes place. The ζ phase is characterized by a monoclinic unit cell and an atomic structure that contains a Fe atom and a Zn atom surrounded by 12 Zn atoms at the vertices of a slightly distorted icosahedron. The icosahedra link together to form chains and the linked chains pack together in a hexagonal array [4]. Increasing the content of iron, a new phase is observed in the inner zone of the coating. This phase is often named as δ phase. It is a brittle one, with Fe content up to 11.5 wt% and characterized by a hexagonal crystal structure. Finally, for higher Fe contents a presence of another brittle phase, often named as Γ, could be observed, but it is characterized by a very thin layer, sometimes with a thickness of about a few atoms. The Γ phase is characterized by Fe content up to 29 wt% and by an FCC structure. Hot dip galvanizing processes are very important in order to control the properties of coatings. In the galvanized steel strip, an innovative HDG process is able to control the thickness of an adherent coating film by impinging a thin plane nitrogen gas jet. This technique has been adopted in some continuous hot-dip galvanizing process [5]. Presence of different alloy elements in the bath allows to optimize the HDG processes in order to reduce the scraps but it permits also to change the traditional intermetallic phases, for instance improving the corrosion resistance and the mechanical properties [7]. For instance, stable reaction products that allow improving corrosion resistance have been observed on electrochemically passivated HDG surface [4-6]. Other authors analysed the influence of alloy elements in terms of microstructural phases compositions [8]. As observed by Shibli (2006), the presence of silicon in the coated steel strongly influences coatings formation and their properties. The mechanisms of silicon interaction with galvanizing reactions can be summarizing as follows (Figure 1) [8]:  Galvanizing reactions move Feα toward Γ phase;  Silicon does not move toward Γ phase because its solubility in Γ is very low. As consequence silicon increases its contents at Feα-Γ interface;  α-Fe, rich in silicon, breaks the interface, and particles enter in the δ phase;  Particles dissolving in the δ phase increases the thickness. In order to improve the coatings performances, it is possible to introduce different techniques also in pre-galvanizing phase [9]. For example, traditional pre-galvanizing treatment can be optimized by replacing conventional flux by using vegetable oil like the linseed oil or using mineral oil. The presence of mineral oil protects the substrate because is a sort of barrier against oxygen. Furthermore, the addition of hydrochloric acid in the oil leads to an improvement of coated areas and their adherence. Finally, the addition of natural fatty acid, used in the flux operation, leads to good galvanizations too, due to its light acidity as observed for the additions of hydrochloric acid [9]. The intermetallic phases growth and their performances are strongly influenced by the chemical composition of the bath. Addition of the strontium improves both the adhesive strength and the corrosion resistance of hot-dip galvanized coating [10]. The presence of SiO2:Na2O molar ratio of silicate solution leads to a decrease in the corrosion rate, increasing both the polarization resistance and total impedance values [11]. Sometimes for improving the corrosion resistance of the zinc coating, or just for changing the colour of the galvanized surface, painting can be used. In order to improve the adhesion on the galvanized surface, an organofunctional silane deposition on hot-dip galvanized can be performed [12].