Recent requirement for increasing operation temperatures in energy conversion system has let to an increase in the demand for improving materials at higher temperatures. Coating is one method used to meet these demands by retaining substrate mechanical properties and giving improved high temperature oxidation and corrosion resistance. This review introduces some recent results we obtained, and concepts for next generation coatings are discussed.
Titanium is one of the candidate materials for overpacks as a high corrosion resistance metal. Hydrogen embrittlement is a main cause of the damage of long term integrity of titanium overpack. It is not well known about the corrosion resistance and hydrogen absorption behavior of titanium under anaerobic condition. In this study, the completely sealed ampoule test and the immersion test of titanium was carried out in aqueous solution and bentonite in order to obtain reliable data about the hydrogen generation rate and the ratio of hydrogen absorption in titanium. As the results of the tests with changing the environmental factors, obvious higher corrosion rates were observed at high carbonate (1 M) and high pH (pH 13) conditions due to the increase in the anodic reaction rate. In other conditions, corrosion rate of titanium were estimated to be in the order of 10-3∼10-2 μm/y. Almost all (<98%) of the hydrogen generated by corrosion was absorbed into titanium. Assuming that the time evolution of the hydrogen content in titanium follows linear law to make conservative assessment, the absorbed hydrogen content was estimated to be of 400∼500 ppm in 1000 years.
Crevice corrosion damages of a flat heald, 13Cr stainless steel components used for water jet room (WJL), was investigated in this paper. The healds had been operating in highly humid atmosphere of sprayed tap water, bearing heavy deposits composed of PAAE resin on their surface. Conventional group of heald (heald C) has used to serve for about two years, while a newly group of heald (heald X) suffered significant localized corrosion earlier in one or two months. Determined repassivation potential for crevice corrosion, ER,CREV, was expressed as ER,CREV(mV vs. SHE)=-294 log[Cl-](mg/L)+832 (1) in the Cl- concentration, [Cl-], from 20 to 1000 mg/L for metal/metal-crevice of the healds X and C at 30°C. ER,CREV for metal/deposit-crevice was confirmed at 16 mg/L Cl- to be close to the equation (1). Residual chlorine (R-Cl) concentration in the tap water increased as high as 0.4 mg/L, when spontaneous electrode potential, ESP, for the heald X might be believed to reach 570 mV vs. SHE (ESP,X). Based on comparison of ER,CREV as equation (1) with ESP,X crevice corrosion can be expected to occur above 8 mg/L Cl-. Conclusively, Cl- concentration of 14 mg/L in the tap water in summer could explain real crevice corrosion for the heald X. More ennoblement in ESP for the heald X than the heald C could be attributed to larger amount of precipitated Cr-carbide. To effectively prevent the crevice corrosion, R-Cl concentration in the tap water to be used for WJL should be kept below 0.04 mg/L.
Electrochemical measurements and surface observations have been made to clarify the effects of debris and residual carbon film in circulated water on corrosion behavior of copper tube for air conditioning system. Proper usage of water treatment agent has kept tube surface corrosion-resistant, regardless of surface with or without residual carbon film. It is electrochemically and morphologically indicated that debris such as corrosion product in circulated water causes damages of the tube surface and then leads to occurrence of localized corrosion such as pitting attacks in some situations.