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Abstract  

The oxidation of a precipitation-hardening (PH) steels is a rather unexplored area. In this study an attempt is made to estimate the oxidation mechanism and the kinetics that take place up to 850 °C. For this purpose specimens of the material under examination were isothermally heated at 725, 775, 800, 825, and 850 °C for 12 h in O2 atmosphere. The as-treated samples were examined with SEM and XRD, while kinetics were based on thermogravimetric (TG) results. From this examination it was deduced that the oxidation of this steel is accomplished at minimum three steps, following the changes of the scale morphology and the kinetics. After 850 °C although that the oxidation rate increases, the scale morphology does not change. From the calculations of the rate constant k p and the activation energy for the phenomena below 850 °C, it was deduced that the oxidation phenomena during this stage provides another barrier to the deterioration of the ferrous material.

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Zinc coatings for oxidation protection of ferrous substrates

Part I. Macroscopic examination of the coating oxidation

Journal of Thermal Analysis and Calorimetry
Authors: G. Vourlias, N. Pistofidis, K. Chrissafis, and G. Stergioudis

Abstract  

The oxidation resistance of ferrous materials at elevated temperatures is limited. For that purpose the performance of zinc coatings deposited with hot-dip galvanizing, pack cementation and thermal spraying was considered. In the present work the oxidation resistance of these coatings at 400�C was estimated with light microscopy, thermogravimetric analysis and X-ray diffraction. From this examination it was deduced that in every coating a scale that was mainly composed of ZnO was formed, while Fe oxides were also detected in galvanized and pack coatings. However, the presence of the Fe/Zn phases inside the galvanized and pack coatings led to the formation of cracks, which could expose the substrate and thus destabilize the coating. This phenomenon was not observed in the thermal sprayed coatings, where the Fe/Zn phases were absent. In any case these cracks are not likely to jeopardize the resistance of the coating because zinc is anodic to steel. Hence, from the above examination it was deduced that the behavior of zinc coatings would be sufficient at 400�C.

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Zinc coatings for oxidation protection of ferrous substrates

Part II. Microscopic and oxidation mechanism examination

Journal of Thermal Analysis and Calorimetry
Authors: G. Vourlias, N. Pistofidis, E. Pavlidou, and K. Chrissafis

Abstract  

A common phenomenon in the process industries is the oxidation of the exterior surface of steel pipes used in superheated steam or hot oils networks. For their protection different coatings could be used. In the present work the performance of zinc coatings deposited with hot-dip galvanizing, pack cementation and thermal spraying was considered, in order to protect industrial equipment up to 400�C. For that purpose coated carbon steel coupons were exposed at 400�C and their behavior was examined with light microscopy, scanning electron microscopy and X-ray diffraction. Thermogravimetric analysis was also used in order to observe in situ the oxidation phenomena. From this investigation it was deduced that in every coating a scale is formed that is mainly composed of ZnO, while Fe oxides were also detected in galvanized and pack coatings. The growth of this scale took place at the metal/scale interface. Moreover, as far as it regards the kinetics of the oxidation, it was concluded that the increase of the mass of the specimens is a function of the square root of the exposure time, which means that the scale formed is rather protective for the underlying zinc. From the above observation it seems that the behavior of zinc coatings would be excellent at 400�C. However, the presence of the Fe/Zn phases inside the galvanized and pack coatings led to the formation of cracks, which could expose the substrate and thus destabilize the coating. This phenomenon does not take place in the thermal sprayed coatings, where the Fe/Zn phases are absent.

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Abstract  

In the present work the oxidation behavior of ageing treated steel was examined up to 1000°C in different environments (O2 and CO2) and with different heating rates. The examination was conducted by means of thermogravimetric analysis, scanning electron microscopy and X-ray diffraction. In this study it was deduced that in the case of O2 an oxide scale is formed on top of the steel. The oxidation is uniform and the growth of the scale is more intent at low heating rate. It consist of different Fe, Mn, Mo and Cr oxides which are adjusted in the form of layers. This phenomenon was explained by the different diffusion coefficients of each metal in the already formed scale. Regarding the oxidation in CO2, the scale formation takes place at a lower temperature than in the case of O2. Hence the examined substrate is more vulnerable in CO2.

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Abstract  

A zinc deposition method that could be used instead hot-dip galvanizing is pack cementation, where the substrate is heated immersed in a powder mixture containing Zn and a halide activator (NH4Cl). In the present work the mechanism of this process is examined, along with the effect of temperature and heating time on the coating thickness and structure. For this purpose the coating was deposited and characterized with SEM, while the deposition mechanism was investigated with DSC. From the above examination it was deduced that the deposition of Zn takes place with a multiple-step mechanism, which involves several reactions in the gaseous phase including the formation of volatile zinc halides and finally the diffusion of zinc in the crystal lattice of the ferrous substrate. This procedure is accomplished at about 300°C and leads to the growth of a coating composed by two layers referring to Γ and δ phase of the Fe-Zn system. The coating deposition rate seems to be controlled by the zinc diffusion as its determination at 300 and 350°C showed, where it was deduced that the coating thickness is a linear function of the square root of heating time. However the coating structure is not affected by the heating time and temperature.

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Journal of Thermal Analysis and Calorimetry
Authors: N. Pistofidis, G. Vourlias, D. Chaliampalias, E. Pavlidou, K. Chrissafis, G. Stergioudis, E. Polychroniadis, and D. Tsipas

Abstract  

Zn pack coating formation takes place in three steps as differential scanning calorimetry shows. The initial step (at 193.9C) is endothermic and involves the transformation of α-NH4Cl to β-NH4Cl and the NH4Cl decomposition to NH3 and HCl. During the second step (at 248.6C), which is exothermic, Zn2+ salts are formed and most probably ZnCl2. Finally at 264.1C (endothermic reaction) it seems that ZnCl2 is decomposed to form Zn that is deposited on the ferrous substrate. The as-cast Zn diffuses in the iron substrate forming the gamma and delta phase of the Fe–Zn phase diagram. Al2O3 is not involved in the above-mentioned mechanism and acts only as filler.

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Journal of Thermal Analysis and Calorimetry
Authors: G. Vourlias, N. Pistofidis, D. Chaliambalias, K. Chrissafis, El. Pavlidou, and G. Stergioudis

Abstract

Zinc coatings on ferrous substrates are possible to be applied with thermal spraying. In the present work the corrosion behavior of zinc thermal sprayed coatings deposited on low carbon steel St-37 was examined in a simulated marine atmosphere (salt spray chamber-SSC) and in a dry atmosphere at elevated temperature (400°C). The corrosion progress was examined by means of optical microscopy, scanning electron microscopy, X-ray diffraction and thermogravimetric analysis. From this investigation it was deduced that in the SSC the coating is uniformly corroded, while the main corrosion products are hydrated zinc oxides and chlorides. By contrast at 400°C only a thin, compact and continuous film of ZnO is formed on top of the coating, which remains adherent to the ferrous substrate.

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Journal of Thermal Analysis and Calorimetry
Authors: N. Pistofidis, G. Vourlias, E. Pavlidou, K. Chrissafis, G. Stergioudis, E. K. Polychroniadis, and D. Tsipa

Abstract  

The effect of Pb and Sn on the structure of zinc hot-dip galvanized coatings on steel wires is examined. The coating quality is often low because of the Sandelin effect. An improvement could be expected if 1.0 mass% Pb is added to the Zn bath. In this case the η phase formation is enhanced, while the coating thickness is reduced. The Sn addition at the same concentration promotes the formation of the η phase but it does not affect the thickness.

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