OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 2 2022 a b Fig. 5. Oxidation resistance of coatings at a temperature of 900 оС in air (a) and X-ray patterns of the samples surface after the oxidation resistance test (b) Ta b l e 3 Corrosion parameters of coatings Parameters Samples AISI 304 Cr5 Cr10 Cr15 Ecorr, V –0.777 –0.646 –0.603 –0.489 Icorr, μA/сm2 42.24 20.66 14.80 11.47 Figure 5, a shows the results of cyclic testing of Fe-Cr-B coatings for oxidation resistance at a temperature of 900 °C. The weight gain of samples with coatings, according to the results of 100 hours of testing, ranged from 17 to 51 g/m2. The smallest gain was observed for the Cr15 sample, and the largest for Cr10, however, in this case, the weight gain is not an unambiguous criterion for the oxidation intensity. Thus, the inset to Figure 5a shows that uncoated AISI 304 steel gained weight in the fi rst test cycle, and monotonically substrate weight loss in subsequent cycles. This cannot be explained by the delamination of oxide layers, as was the case in [26], due to the presence of samples in ceramic crucibles during the oxidation resistance test. Therefore, the only explanation for the observed weight loss of AISI 304 steel can be the burnout of carbon, phosphorus and sulfur included in its composition (Table 2). It is noteworthy that in the above work, for 100 hours of testing at 900 °C, the weight gain of AISI 304 steel was only 6.5 g/m2, and in [27] – 22.2 g/m2 for 90 hours. The oxidation rate of the Cr5 sample was the highest among the coatings up to 65 hours, and then the weight gain stopped, that can be explained by the action of two differently directed processes: weight loss by the substrate and weight gain of the coating. Thus, the oxidation resistance of the Cr5 coating can be qualifi ed as the worst. The Cr15 coating had the best oxidation resistance. The gain in the process of high-temperature oxidation is due to the fi xation of oxygen on the surface of the samples with the formation of magnetite Fe2O3 and hematite Fe2O3 (Fig. 5, b). According to X-ray data, ferrochrome Fe0.52Cr1.36 was also observed on the surface of the samples after the oxidation resistance test, the intensity of the refl ections of which monotonically increased from the sample Cr5 to Cr15. It is explained by a decrease in the thickness of the oxide layer and confi rms the improvement in the oxidation resistance of coatings with an increase in CrB2 in the anode mixture. In general, the use of electrospark Fe-Cr-B coatings makes it possible to increase the oxidation resistance of AISI 304 stainless steel from 5 to 15 times.
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