Papers by Author: N. Pistofidis

Abstract: In the present work the phases of the zinc coatings deposited with hot-dip galvanizing, pack cementation and wire flame spraying are examined with Scanning Electron Microscopy and Transmission Electron Microscopy. The different phases which are observed are identified with the combined results of electron and X-Ray diffraction. From the results it is concluded that pack cementation coatings are consisted by two different layers while hot dip galvanized coatings are composed by the same phases and additionally two extra phases of the Fe-Zn phase diagram. Flame sprayed coatings are composed by pure zinc, in the form of thin lamellae, together with nanocrystaline zinc oxide which is formed from the oxidation of liquid metallic droplets during the spray procedure.

Abstract: Zinc hot-dip galvanizing is one of the most effective methods for the corrosion protection of ferrous substrates. However, the failure of zinc coatings is possible when exposed to harsh environments for rather long periods. The application of a thin diamond like carbon (DLC) film on the top of the zinc coating might be a promising method for promoting their corrosion resistance. In the present work, a DLC thin film was deposited on zinc galvanized coatings by Plasma Enhanced Chemical Vapor Deposition. The as-formed film was composed of nanostructured and amorphous areas. The electron diffraction patterns acquired from the nanograins correspond to carbon phases with d-spacing ranging from diamond to graphite. Additionally, after 18 days of exposure in a simulated marine atmosphere, the DLC coated samples were proven to be more resistant than the naked galvanized coatings indicating its potential to improve the corrosion resistance of galvanized ferrous materials.

Abstract: One of the most effective methods for the protection of ferrous substrates from corrosion is zinc hot-dip galvanizing. Although this method has many advantages, it is characterized by a very negative effect on the environment. In the present work Zn coatings were formed with thermal spraying, pack cementation and fluidized bed reactor, which are friendlier to the environment. Their microstructure was characterized with X-ray diffraction and scanning electron microscopy, while their corrosion performance was estimated with exposure in a salt spray chamber. From this investigation it was deduced that CVD coatings are composed by two layers referring to Γ-Fe11Zn40 and δ-FeZn10 phase of the Fe-Zn phase diagram. By contrast the thermal coatings are very porous and composed by pure Zn. However, the corrosion performance of all coatings is similar. This conclusion is very important because it verifies that hot-dip galvanizing could be replaced by the other coating methods.

Abstract: This work aims to investigate the feasibility of Zn-Al deposition on low alloy steels at temperatures from 400 up to 440oC by pack cementation process aiming to increase their corrosion resistance. A series of experiments were undertaken to investigate the effects of pack powder composition and the deposition temperature of the process. It was observed that the parameters of zinc content and temperature affect only the coating deposition speed, but not the phase composition of the as produced coating. Al forms an overlying layer that seals the zinc coating. In any case, the deposition of successive layers of Zn and Al is feasible with pack cementation. The corrosion performance of Zn-Al coatings formed with alternative methods is already studied and proved to be resistant in harsh environments. So the herein studied coatings are expected to be corrosion resistant. Furthermore as Al is much more resistive than Zn, these coatings are more effective than pure Zn ones.

Abstract: Thick sheets of steel were galvanized in a bath containing Al additions. A Fe2Al5 layer is formed at the substrate steel sheet, which leads to a desirable transient inhibition of Fe-Zn reactions. Thus the more protective (eta) phase rich in zinc is favored. However an appreciable intergranular diffusion and a gradual formation of internal and surface oxide particles influence the growth and stability of the inhibition layer. The location of some oxide particles at the Fe2Al5 surface or inside of this layer, led to conclude that oxide particles might cause Fe-Zn outburst growths to form. This is because zinc diffuses along the oxide particle/Fe2Al5 interface. Moreover the mechanism of oxide formation causes a local depletion of the atoms concentration in the bath in the vicinity of the formed oxide. This in turn diminishes the probability of the formation of the Fe2Al5 layer. So the whole mechanism provides a fast diffusion bath for Zn, which reacts with Iron atoms forming Fe-Zn phases. The formation of the phases, were determined by XRD measurements (PHILIPS diffractometer CuKα radiation) while the dispersion of the elements was examined by SEM (20kV JEOL 840A equipped with an OXFORD ISIS 300 EDS analyzer.