The hot corosion kinetics curves of 102G,20G and various coatings are shown in Fig.1 and Fig.2, respectively. The effect of coating sealing on hot corrosion pe-formance is presented in Fig.3. Figs.4 present the EPMA images of hot corrosion test on 102G steel, FeCrAl and NiCr-50% Cr3C2coatings, respectively.
Fig.1 Hot corrusion kinetics curves of 102G,20G
Fig.2 Hot corrosion kinetics curves of coatings
Fig.3 Effect of sealing on the hot corrosion performance of coatings
Although the hot corrosion resistance of boiler tubes protected by thermally sprayed coatings can be improved considerably, the protective effects of various coatings are quite different(shown in Fig.1and Fig.2). The hot corosion performance of tested coatings was 16.5 to 73.3times as high as that of 20G steel,13.8 to 61.3 times as high as that of 102G steel. After 250h hot corosion test, whole 20G sample was almost cormoded and more than two third of 102G samples suffered a severe attack of corrosion. It can be seen from Fig.4 that sulphur penetrated into the interface between 102 G surface and its cornodent, and rapid corrosion still took place on the suface of 102G steel substrate. The reason is that the content of protective scale-forming alloying elements is low,a porous oxide scale is produced on the surface of 20G and 102 G at tested high temperature and sulphur can penetrate into the fresh surface of 20G and 102G, causing a rapid attack of cormosion. Among the tested coatings, FeCrAl(A1) and NiCrAIMbFe-Cr3C2(A7) coatings exhibit poor hot corosion resistance in comparison with other coatings. This can be explained that in these two coatings, some amount of Al element exists; Al2O3, and AlS would form under thecondition of oxidation and sulphate corrosion, which would cause stable Cr2O3, to degenerate and lead to local hot corrosion uaking place in the coatings. Sulphur and oxygen from the outside of the coatings would penetrate into substrates along the rapid corroded path and cause internal corrosion of local substrates.
Fig.4 EPMA images of hot corosion tested on 102G steel coatings
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