The high-temperature corrosion behavior of four valve steels (X33CrNiMn23-8, X50CrMnNiNbN21-9, X53CrMnNiN20-8 and X55CrMnNiN20-8) in combustion gases of fuel oil, containing different concentrations of bio-components (5 and 10 wt%) has been studied under thermal shock conditions. It was found that the addition of bio-components decreased the corrosion resistance of steels investigated. It was also found that the X33CrNiMn23-8 steel containing the highest chromium concentration, behaved in the investigated atmospheres much better than three remaining steels due to the formation of highly protective chromia scale.

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The oxidation resistance of two X33CrNiMn23-8 and X50CrMnNiNbN21-9 steels, utilized for the production of valves in Diesel engines has been studied under thermal shock conditions. It has been found that X50CrMnNiNbN21-9 steel undergoes under such sever conditions dramatic degradation, due to extensive cracking and scale spallation.The second X33CrNiMn23-8 steel, with higher chromium concentration behaved much better, due to the formation of the protective chromia scale. It has been found also that both steels under investigation, covered with metalceramic hybrid coating, show excellent oxidation resistance.

The influence of two-layer hybrid coatings on the oxidation behavior of X33CrNiMn23-8 steel, utilized in valves of Diesel engines, have been studied as a function of temperature (873 -1273 K) and oxygen pressure (1-10 5 Pa), using modern microthermogravimetric techniques. Phase composition of the oxidation products (scale) was investigated by X-ray diffraction (XRD), and the morphology and chemical composition of reaction products by electron probe micro analysis (ΕΡΜΑ) and scanning electron microscopy (SEM) with energy disperse X-ray analyzer (EDX). It has been found that uncoated material shows very good oxidation resistance under isothermal conditions, comparable with that of chromia formers, due to the formation on its surface of Cr 2 0 3 scale. However, beneath the scale, steel undergoes rapid degradation, as a result of crack formation; the deeper the penetration, the longer is the oxidation time. On the other hand, no cracks were observed in the coated steel and the oxidation rate was even slightly lower than that of uncoated material.

The oxidation kinetics of four Fe-Cr-Mn-Ni based steels, utilized in automobile industry, have been studied as a function of temperature (973-1273 K) and oxygen partial pressure (5-10 5 Pa). It has been shown that the rate of corrosion of these steels under isothermal conditions is determined by diffusion of reagents through the scale, which phase composition and morphology depend first of all on chromium and to some extend also on nickel and manganese contents. The highest oxidation resistance at high temperatures is observed in the case of the X33CrNiMn23-8 steel, containing highest chromium concentration, equal 23.4 wt.% and nickel equal 7.8 wt.%. The remaining three steels with virtually the same chromium content ( 20 wt.%), but lower than that in the first one, show comparable oxidation resistance. Small differences in the oxidation rates of these three steels may be related to different nickel and manganese contents. It has been found also that the rate of corrosion of all steels under investigation does not depend under steady state conditions on oxygen partial pressure.