Effects of alloying elements of weathering steel such as Cr, Cu, Ni, Ti and Nb on the corrosion behavior of carbon steel, especially the rust layer formation, were investigated. The alloying elements were physically deposited or implanted onto the surface of carbon steel coupons in order to elucidate roles of alloying elements that present near/on the surface. The coupons were exposed at accelerated atmospheric corrosion condition, thereafter the weight losses were determined and the rust layers were microscopically or physically analyzed. It was found that alloying elements are present in rust layers and influenced forms of rust layers. In the corrosion test condition excluding chlorides, microscopic observation of the rust layer using polarized reflected light revealed that continuous optically isotropic layer was established on the coupons deposited/implanted with Cu. As for coupons deposited/implanted with Cr or Ti, significant effects of doping process on the corrosion rates were recognized. In the corrosion test condition including chlorides, adhesive rust layer with fewer voids was formed. On the other hand, on the coupons with other elements deposition/implantation, less adhesive rust layers with voids were observed.
To estimate concentration and thickness of water film formed on metal surface in atmospheric environment, relative humidity (RH) data in equilibrium with various concentrations of strong electrolyte solutions were calculated with available thermodynamic data. The activity coefficient of water (fw(X)) for the solution with molar fraction of water (X) could be given as a function of ionic strength of the solution, which was determined by mean activity coefficient data in literatures for electrolytes in the solution. RH values obtained as RH(%)=100×fw(X)·X were fitted well with measured values of these solutions in each literatures, for example NaCl, MgCl2, Na2SO4, Mg(NO3)2 and so on. RH values for solutions with various contents of Na-MgCl ternary system were also calculated as NaCl-MgCl2 mixed solution. The calculated RH values were also fitted well with measured data. Using the relationship between density and concentration of the solution, the thickness of water film was also calculated. When the sea salt is put on 1g/m2, while the thickness of water film reaches 3-30μm in the range of relative humidity is larger than 75%, it remains 0.3-0.7μm in the range of relative humidity is smaller than 75% where NaCl solid state deposition occurs.
Pyrochemical reprocessing using molten salts (RIAR process) has been recently developed for recovering UO2 from spent nuclear oxide fuel. It is essential to improve the corrosion resistance of equipments such as electrolyzer because the process includes severe corrosion environment. In this study, the corrosion resistance of ceramic materials was discussed through the thermodynamic calculation and corrosion test. The corrosion test was basically carried out by reacting ceramic specimens with alkali molten salt under chlorine gas. In addition, the effects of oxygen, carbon and major fission product's chlorides on ceramics corrosion were evaluated in molten salt under chlorine gas. In eleven kinds of ceramic materials, silicon nitride, mullite and cordierite had a good corrosion resistance which was 0.1mm/y or less in above conditions. Cracks on the materials were not observed and bending strength did not drop remarkably after corrosion test of 480h in molten salt under Cl2-O2 bubbling conditions. These ceramic materials are promising to apply to the RIAR process.
Stress corrosion cracking (SCC) and its protection method of Invar alloy for LNG piping were studied. SCC behavior of Invar alloy was investigated in simulated marine atmosphere and in chloride solutions. TG-SCC was initiated by pitting corrosion in the atmosphere with adhered airborne salt and by crevice corrosion in chloride solutions. This result was discussed from the relationship among the pitting potential (VC′), the re-passivation potential for crevice corrosion (ER, CREV) and the corrosion potential (Ecorr). In a solution containing more than 200ppm chloride ion, ER, CREV was almost the same as Ecorr (-0.5/-0.4V vs. SCE), which means that the crevice corrosion is easy to occur. Ecorr became more noble than VC, showing the occurrence of pitting corrosion, because of the ennoblement of Ecorr by the thin condensed electrolyte layer which contained adhered airborne salt. Therefore, the breakdown of the passive film was considered to be closely related to SCC susceptibility of Invar alloy. The protection method of SCC was evaluated. Sand blasted Invar alloy showed a good SCC resistance. Inorganic zinc-rich painting after sand blasting also prevented Invar alloy from SCC even if there were defects of painting.
A 1, 2-bis (triethoxysilyl) ethane (BTSE) is coated to build up self-assembled monolayers (SAM) on the surface of iron for protecting from in-door atmospheric corrosion. To close the terminals of SAM molecule, a water repellent agent octadecyltrimethoxysilane (ODTMS) is coated further on BTSE coated film. For corrosion test to evaluate corrosion-resistant property, accelerated corrosion test was performed in a thermohygrostat kept at 40°C and 40% relative humidity. Also, anodic polarization measurement was performed. It is found that the ODTMS coating on BTSE coated film shows high corrosion resistance by anodic polarization measurement and an accelerated corrosion testing.