IMPACT OF HYDROCARBONATE TREATMENT ON CORROSION RESISTANCE OF COPPER, NICKEL, AND STAINLESS STEELS IN THE NAOH MELT

The paper focuses on the research into impact of hydrocarbonate treatment in the boiling 1 М solution of NaHСО on the corrosion resistance of copper, nickel, and low-carbon chromium-nickel stainless steels during their corrosion testing in the NaOH melt at the range of temperatures from 400 to 500С. The hydrocarbonate treatment of materials was conducted for two hours following holding them (treatment) in the NaOH melt preliminary dewatered and deaerated by Ar in intervals multiple of 96 h. The total duration of corrosion tests was 288 h. The microstructure and phase composition of surface layers that formed on materials under study during corrosion tests in the NaOH melt coupling with hydrocarbonate treatment were investigated using X-ray phase analysis and electron microscopy methods. It has been found that the hydrocarbonate treatment of materials neither affects the total corrosion rate for the materials under study in the NaOH melt in the given temperature range. It has been discovered that the hydrocarbonate treatment of nickel containing three oxide phases - NiO, Ni(OH) и γ-NiOOH - after being held in NaOH melt impacts the proportions of the oxide phases. Nickel oxyhydroxide (NiOOH) is unstable in aqueous weakly alkaline solutions bound to and is spontaneously reduced to nickel dioxide (NiOOH → Ni(OH)) resulting in the formation of a passive film on the nickel surface. The film consists of two oxide phases (NiO и Ni(OH)) and has high protective properties. In the course of the hydrocarbonate treatment of copper, which after having been held in the NaOH melt contains a 2-layer film of oxides Cu/СuО/СuО in the surface layer, oxide and carbonate layers with higher protective properties do not form. The hydrocarbonate treatment of stainless steel containing 17.5 and 18.5 % Ni and (6.0 - 6.5) % Mo demonstrating increased corrosion resistance in the NaOH melt at temperatures not exceeding 500С (as nickel does) does not affect the corrosion resistance of the alloy elements. Doping the steel of the given composition with such alloying elements as copper, manganese, and silicon, which might cause local depassivation of steel under certain conditions, does not affect the protective properties of the passive steel film forming in the NаOH melt. Such film consists of oxides (hydroxides) of predominantly corrosion resistant chromium (CrO) and nickel compounds (NiO, Ni(OH)) or their mixed oxides NiCrO (NiO∙CrO), as well as ferrous oxides FeO γ-FeO. As the nickel and molybdenum content in the steel decreases (to 13.0 % and 2.0 % respectively), or temperature of the NаOH melt increases up to 600С, more defective porous oxide layers form on the steel surface. They contain a larger portion of less stable ferrous (II, III) and nickel (II) oxides: FeO, NiO, FeO as well as a small amount of mixed oxides NiCrO (NiO∙CrO) leading to increase in corrosion rate.