Investigations of the corrosion resistances of magnetron sputtered multicomponent materials : A study on high entropy alloys, high entropy sublattice ceramics, and metallic glasses
Abstract: The corrosion resistances of sputter deposited AlCrNbYZrN, CrNbTiTaWC and TaNiSiC alloys were assessed in acidic and alkaline environments. Compositionally complex alloying strategies, and whether they produced coatings with superior corrosion resistances, were explored. Another priority was to determine if the formation of a random solid solution was necessary for the design of corrosion resistant multicomponent alloys. The results showed that successful surface passivation/oxidation was the key factor determining corrosion resistance. The corrosion resistance was maximised when coating porosity was minimised, spontaneously passivating elements were used, and the oxide solubility in the electrolyte was low. The beneﬁts of multicomponent alloying were demonstrated through the protective effect of oxides of more stable elements (Cr, Nb, Ta) reducing the loss of less corrosion resistant elements (Al, Ti). The supersaturation of nitrogen, carbon and/or silicon affected coating morphologies, surface repassivation rates, and even directly corrosion resistances, e.g. when silicon oxides were formed on TaNiSiC ﬁlms.The N content (0-50 at.% N) in AlCrNbYZrN ﬁlms affected the ﬁlm porosity, the absence of which led to high corrosion resistances in 1.0 M HCl. The formation of a single phase did not govern the corrosion resistances. It was more important that the coatings were dense, as this permitted the formation of an impervious passive layer. The addition of 8 at.% C to nearequimolar CrNbTiTaW alloys affected surface repassivation rates of ﬁlms whose oxide had been partially removed by chronoamperometry in a 1.0 M HCl electrolyte. However, the effect of carbon on the corrosion properties was minor. A comparison of the nearequimolar CrNbTiTaW, Nb and TaW alloys with their native oxides revealed comparable corrosion resistances in 0.01 M HCl + 0.1 M NaCl. The nearequimolar CrNbTiTaW, Nb, and TaW alloys also showed no evidence of pitting in 1.0 M HCl. The high corrosion resistances in CrNbTiTaW alloys was due to the presence of extremely stable surface oxides abundant in Cr, Nb, and Ta. TaNiSiC ﬁlms with Si contents above 12 at.%, and C contents lower than 11 at.% showed high corrosion resistances in 10 mM sodium borate due to the formation of silicon oxides, even after three polarisations from −0.7 to +1.5 V vs. Ag/AgCl (3 M NaCl) and 50+ hours of immersion. Microscopy of the Si-rich TaNiSiC ﬁlm showed negligible differences in surface condition before and after corrosion in sodium borate, while ﬁlms with lower Si contents or higher Ta contents were more severely corroded.
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