Hot-Dip Galvanising and Pack Cementation in Comparison (Microscopical Study of Zinc Coatings). The microstructure of hot-dip galvanised and zinc pack coatings was examined with light and electron microscopy. It was deduced that hot-dip galvanised coatings are composed by gamma (γ), delta (δ), zeta (ζ) and eta (η) phase of the Fe-Zn system, while pack coatings are composed by gamma (γ), delta (δ) and spherical inclusions with about 50 % Zn and 50 % Fe. The morphological differences of the two coating types imply that the outer surface of the pack coatings is harder than the outer surface of the galvanised coatings, while their corrosion performance is expected to be similar.
Studying Zinc Coatings on the Microlevel
Atmospheric corrosion is a main disadvantage of ferrous materials. Therefore, many methods have been studied to overcome it , one of which is the application of zinc coatings . A number of different procedures are commercially available for that purpose. The most widely used is hot-dip galvanising (HDG) . In this case, the ferrous substrate is coated by immersion in molten zinc. The effectiveness of HDG is undisputable. However, its high environmental impact imposes the investigation of alternative coating techniques such as pack cementation (PC) . In this case, the coating is formed by heating the substrate up to 400° C covered with a mixture of powders containing Zn.
The aim of the present work is the comparative microstructural and morphological study of zinc coatings formed with the above-mentioned methods. This investigation could give useful data on the prediction of the mechanical properties and the anticorrosive performance of these coatings.
Specimen Preparation : HDG and PC
Coupons of steel St-37 were coated with Zn using HDG and PC. Steel St-37 has the typical sub-eutectoid structure of the Fe- C system (Fig. 1a) which is prone to diffusion of other atoms in its crystal lattice. Prior to the coating procedure the coupons were degreased in a non-ionic tenside containing H3PO4 and deoxidized in 16 % HCl.
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This process creates also a rough profile (Fig. 1b) enhancing coating adhesion. Some of the as-prepared specimens were fluxed in 50 % ZnCl22NH4Cl and galvanised at 450°C with dipping time 180 sec. The rest of the samples were coated with PC. For that purpose they were sealed in crucibles with a powder mixture containing 30 % Zn, 5 % NH4Cl and 65 % Al2O3 and placed in a furnace (Fig. 2) at 400° C for 60 min.
Cross sections of the as-coated specimens were initially observed with an optical microscope Olympus BX60 associated with a digital camera CCD JVC TK-C1381. However, the main examination was accomplished with a 20 kV JEOL 840A Scanning Electron Microscope (SEM) with an Oxford Isis 300 EDS analyzer. Finally, some of the samples were also examined with Transmission Electron Microscopy (TEM) with a 100 kV Jeol 100CX TEM.
Different Phases in Zinc Coatings
In the HDG samples, four phases are observed (Fig. 3). The one close to the steel is composed by about 25 % Fe and 75 % Zn. Similar morphology is also observed for the next one (10 % Fe-90 % Zn). However, the most impressive morphology is presented by the third phase (5 % Fe and 95 % Zn), which is composed by columnar crystals grown almost perpendicularly to the steel substrate. Finally, the outer layer of the coating is formed by a zone of almost pure Zn. This data implies that the observed phases refer respectively to the gamma (α), delta (δ), zeta (ζ) and eta (η) phase of the Fe-Zn system. Zeta phase was also identified by TEM (Fig. 4a).
By contrast to HDG coatings, the pack coatings are composed by three phases (Fig. 5a). Two of them are in the form of layers (gamma and the delta, Fig. 4b and 4c), while the third is in the form of spherical inclusions with about 50 % Fe and 50% Zn. Zeta and eta phase were not formed. Hence, the outer surface of the pack coatings should be much harder, because the delta phase is harder than eta . As a result, pack coatings are more wear resistant than HDG while the same corrosion performance is expected .
Several cavities are also present which could have a negative effect on the adherence of the coating with the substrate and its integrity when torsional stresses are applied .
From the above investigation it turned out that:
• Hot-dip galvanised coatings are composed by gamma, delta, zeta and eta phase of the Fe-Zn system, while pack coatings are composed by gamma and delta, along with spherical inclusions with about 50 % Zn and 50 % Fe.
• The outer surface of the pack coatings is harder than the outer surface of the HDG coatings.
• Several cavities were observed in the pack coatings with a negative effect on the adherence of the coating to the substrate.
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Keywords: application of zinc coatings Different Phases in Zinc Coatings HDG HDG coatings Hot-Dip Galvanising method against atmospheric corrosion microstructural study of zinc morphological differences of Zn and Fe morphological study of zinc Pack Cementation Scanning Electron Microscopy TEM Transmission Electron Microscopy Zinc zinc pack coatings