Protective coatings for corrosive wear and hot corrosion resistance

Abstract

Corrosive-wear is a process of degradation resulting from synergetic damage by mechanical wear and chemical or electrochemical corrosion. In this study, the unbalanced magnetron sputtering deposition technique was employed to produce S- phase coatings (nitrogen supersaturated austenitic stainless steel) on the stainless steel substrates in order to improve the corrosive-wear resistance of stainless steels. AISI 316L stainless steel was used as both target and substrate materials in the present sputtering deposition experiment. A range of S-phase coatings were produced under six different nitrogen flow rates. The actual nitrogen contents were identified as 1.2, 5.5, 7.9, 15.3, 28.6 and 36.4 at%N. The coatings were characterised by using XRD, SEM and AFM. The coating containing lowest nitrogen (1.2 at%N) showed a full a phase with bcc crystal structure. The coatings with 5.5 and 7.9 at%N showed a mixed fee and bcc structure (α+γ). The coatings with high nitrogen contents (15.3, 28.6 and 36.4 at%N) showed γ phase with highly distorted fee crystal structure. AFM observation of the coating surface showed that the grain size of the coating was approximately 500 nm. The thickness of the coating was identified as 8 μηι by SEM observation. The surface hardness of the coatings increased steadily with increasing nitrogen content. Passivation behaviour of the coatings in sulfuric acid (0.5 M) was studied by performing immersion test in an open circuit. Nitrogen content influenced the passivation ability of the coatings in the way of varying the duration time in active region. The coatings containing 28.6 and 36.4 at%N were shown longer active region duration than the coatings with low nitrogen content. SEM observation proved that these two coatings were corroded severely after immersion test. The passivated coating surface was analysed by XPS. The spectra of the passive film forming elements indicated that the passivated surface was rich in N and Cr. The analysis for the binding states of the main elements suggested that N atoms supersaturated in the fee lattice, existing as clusters with Cr atoms, which resulted in the binding energy very similar to CrN. Corrosive-wear tests were conduced in a newly designed corrosive-wear tester, which allows specimen to be exposed to a corrosive electrolyte and undergoing sliding wear simultaneously. The corrosive-wear resistance of the coatings was evaluated by the metal removal during the test. The results showed that the corrosion-wear resistance increases remarkably as the nitrogen content increases from 0.3 to 5.5 at%2V, and then increases slowly as the nitrogen content further increases to 15.3 at%N. However, the S-phase coatings containing 28.6 and 36.4 at%N are of brittle nature and do not perform well in corrosive-wear tests. Repassivation ability of the S-phase coatings after corrosive-wear was expressed as the electrode potential versus the exposure time. It was found that an inflection point separated repassivation process into two stages. The inflection point of the coatings tends to occur earlier as the nitrogen content increases. S-phase coatings showed a faster repassivation rate than the uncoated AISI 316L. An empirical model describing the effects of mechanical parameters and material properties on the metal volume removed by the corrosive-wear was established. A physical model describing the two-stage repassivation process was established. An equation based on the kinetics of the passive film growth after corrosive-wear test was established in order to express this two-stage repassivation process. To investigated intermetallic coating for hot corrosion resistance, FeAl coatings structured with single B2 phase were successful deposited on the AISI 304, 310 stainless steels and as-cast FeAl substrates using unbalanced magnetron sputtering deposition technique. A thermal spring balance system was established specially for performing hot corrosion tests. Hot corrosion tests were carried out at 850°C for 30 h with the presence of mixed salts Na2SO4+NaCl. All uncoated and FeAl coated specimens revealed approximately parabolic kinetics in the hot corrosion tests. FeAl coated AISI 304 and 310 stainless steels and cast FeAl specimens have improved hot corrosion resistance in the first 10 h exposure time. It is believed that the improvements in the corrosion behaviour can be attributed to the formation of a scale with very fine AI2O3 grains. The application of innovatively designed electro-spark deposition (ESD) equipment with a rotating wheel-electrode was investigated. Metallurgical bonded alloy coatings with very fine-grained structure were produced on stainless steels and Ni based alloys. The ESD processing conditions were optimised in order to obtain desired coating quality. A combination of processing parameters including spark discharge energy, frequency and electrode movement was therefore determined to produce coatings with reasonably good thickness, uniformity and smooth surface. XRD characterisation showed that the A1 coating consists of a complex phase mixture including metal, intermetallic compounds. The mechanisms involved in the coating process were discussed based on the SEM observation. Hot corrosion tests at 900°C with mixed salts of Na2SC>4 + 10%NaCl indicated that both NiCr and A1 coatings improved the corrosion resistance for all substrates under study. SEM observation, combined with EDX analysis, indicated that a thick layer of AI2O3 still existed after hot corrosion test. A dispersed Y2O3 particle phase was successfully added into the coating layer, which showed further improvement in hot corrosion resistance. EDX analysis showed that the dispersed Y2O3 particles retarded the outward diffusion of the substrate elements and promoted the adhesion of AI2O3 layer on the coating surface.