Anodic metal dissolution produces hydrated metal salts concentrated at the anode interface and modifies the ion transport in the interfacial diffusion layer to be anion-selective or cation-selective. The anion-selective diffusion layer formed with monovalent chloride or hydroxide contributes to the formation of a chloride film and leads to either the chloride-film-induced passivation if the chloride is insoluble (e.g. Ag/AgCl) or the transition from the active state to the polishing state dissolution if the chloride is soluble (e.g. Fe/FeCl2). The cation-selective diffusion layer formed with multivalent phosphate or sulfate ions leads to the formation of an oxide film, and thereby to the oxide-film-induced passivation of metal anode (e.g. Ni/NiO). Generally, rust layers are anion-selective in acidic solutions and cation-selective in basic solutions. Adsorption of multivalent oxoanions often changes the rust layer from an anion-selective to a cation-selective layer. The anion-selective rust layer accelerates the localized corrosion of metals; whereas, the cation-selective rust layer inhibits the localized corrosion. The bipolar rust ion-selective layer suppresses the anodic metal dissolution and leads to metal passivation. The cathodic oxygen reduction is allowed to proceed on p-type rust layers but is inhibited on n-type rust layers.
Our recent studies on corrosion of iron in anhydrous organic solvents containing electrolytes are reviewed. Iron corrodes in an anhydrous methanol containing 0.1M LiClO4 forming a precipitate of ferrous methoxide Fe(OCH3)2 on the iron surface. Similar electrochemical corrosion of iron to that in an aqueous solution occurs in anhydrous acetonitrile and dimethylformamide solutions of carboxylic acids. The anodic process of iron corrosion in the anhydrous methanol solution is stimulated by adding complexing agents to form soluble complexes with Fe2+ at the surface. Accelerated corrosion of iron occurs in anhydrous methanol and acetonitrile solutions containing FeCl3 and is suppressed by the addition of adsorption-type corrosion inhibitors effectively.
Intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) of the laser-modified surface of Alloy 600 containing 0.04%C or 0.06%C have been investigated. YAG (Yttrium-Aluminum-Garnet) laser beam was used as a heat source for melting or alloying of the surface of the thermally sensitized Alloy 600. The resistances of IGC and IGSCC were evaluated by Streicher test and creviced bent beam test in high temperature water, respectively. The results obtained are as follows: (1) The IGC and IGSCC resistances of the laser-melted surface of Alloy 600 depend on carbon content of the base metal. (2) In the case of 0.04%C Alloy 600, corrosion resistance slightly increases by laser surface melting. And in the case of 0.06%C Alloy 600, corrosion resistance decreases by laser surface melting. (3) The IGC and IGSCC resistances of the sensitized 0.06%C Alloy 600 remarkably increase by the laser surface alloying, in which Cr, Mo, Ti, Nb-rich filler wires are used.
Sulfidation experiments of Ti-Al intermetallic compounds (Ti3Al, TiAl, and TiAl3) and pure titanium were carried out at 1073K and 1173K, and influence of aluminum content and sulfur partial pressure on the sulfidation amount was investigated. It was found that sulfidation amount decreases with increasing aluminum content. This is thought to be due to formation of Al2S3 in the scale and/or formation of the layer enriched with aluminum at the scale/alloy interface. It was shown that sulfidation amount decreases with decreasing of sulfur partial pressure. And then surface morphology changes from the fiber-like structure into the faceted structure with decreasing sulfur partial pressure. In the relatively low sulfur pressure and high aluminum content region, surface morphology shows fine granular or acicular structure.
Sulfidation behavior of γ-TiAl alloy was investigated at 1173K for up to 360ks in H2S-H2 gas mixture. The process of scale formation was discussed from measurement of corrosion amount, observation of cross-sectional microstructure, identification of reaction products by X-ray diffraction and calculation of dissociation pressure of sulfides. The scale became duplex structure. In the outer scale, titanium sulfides (TiS, Ti3S4, TiS2) and aluminum sulfide (Al2S3) were identified, while in the inner scale, titanium sulfides (TiS, Ti3S4) were identified. And the TiAl3 phase was formed at scale/matrix interface. Corrosion rate changed at the test time of approximately 5.4ks, and at same time, inner scale and TiAl3 layer began to form. Titanium sulfide (TiS) was clarified to be more stable than aluminum sulfide (Al2S3) by calculation of dissociation pressure. From these results, the scale was considered to be formed in two steps. That is, the outer scale was formed during transient period, and then titanium was sulfidized alternatively, forming of the inner scale which mainly consists of titanium sulfide and the TiAl3 occur during steady state.
To clarify the caustic Intergranular Stress Corrosion Cracking (IGSCC) mechanism of alloy 600, some metallurgical factors were extracted from the various IGSCC contributing factors, and several kinds of experimental studies were conducted. Caustic IGSCC susceptibilities of mill-annealed alloy 600 (600MA), sensitized one (600FS) and thermally treated one (600TT) which have different grain boundary characteristics were examined using U-bend, C-ring and SSRT tests. Grain boundary characteristics, such as carbide precipitation, impurity segregation, intergranular slipping behavior, and electrochemical dissolution, were examined using Auger Electron Spectroscopy (AES), Transmission Electron Microscopy equipped with Field Emission Gun, and electrochemical measurements, respectively. The C-ring study showed that the caustic IGSCC resistance were improved with an increase in aging time. From the AES and U-bend studies, it was revealed that the intergranular segregations were sufficient, but not necessary for the caustic IGSCC. The slips at the grain boundary were easy to detect in the alloy 600MA and alloy 600FS, while the formation of slips were suppressed in the 600TT material. Electrochemical study showed that the chromium dissolution was increased with the increases in temperature and pH under the caustic condition. From the above test results, it was concluded that the slipping behavior, the chromium dissolution of intergranular carbide and intergranular purification due to the incorporation of impurities into the intergranular carbide could work together as the contributing factor to caustic IGSCC.
The effect of ruthenium on corrosion of stainless steel in nitric acid solutions was evaluated through immersion test, electrochemical mesurement, quantitative analysis of RuO4 or NOx and corrosion test under gamma-irradiation. Under non-irradiation, Ru3+ or RuNO3+ was oxidized to RuO4 in concentrated nitric acid solutions at high temperature, and the corrosion of stainless steel was promoted first through by the reduction of RuO4 to a lower valent state compound like RuO2. If the reduction product of RuO2 was deposited on stainless steel, then the corrosion was further promoted through a catalytic function of the deposit for a reduction of nitric acid to HNO2. It was found that the promotion of corrosion with ruthenium was extreamly reduced by gamma-irradiation (1×106R/h), due to inhibition of the oxidation of RuO4 from Ru3+ or RuNO3+ by formation of nitrous acid resulting from radiolysis of nitric acid solution.