Hydrogen embrittlement susceptibility of steels and its mechanism were evaluated in terms of environmental and metallurgical aspects. Hydrogen entry and its effects on embrittlement susceptibility were investigated under atmospheric exposure and gaseous hydrogen environments. Investigations on hydrogen entry under atmospheric exposure by hydrogen permeation tests indicate that environmental factors such as temperature, humidity and sea salts on the steel surface strongly affected hydrogen entry and delayed fracture susceptibility of high strength steels. Evaluation by Slow Strain Rate Test (SSRT) in both gaseous hydrogen environments and electrochemical cathodic charging shows that hydrogen embrittlement susceptibility depended upon surface hydrogen contents. Hydrogen embrittlement mechanism of high strength low alloy steels and the countermeasures were investigated from the viewpoints of precipitations (secondary phases) in the steel. Nano-scale carbides increased trapped hydrogen content. Carbides along grain boundary accelerated intergranular hydrogen cracking. Inclusions on the steel surface played pitting initiation sites, at which hydrogen cracking occurred. Controls of such secondary phases enabled developments of high strength steels withstanding hydrogen embrittlement.
The early stage of copper patination, which occurs when copper is exposed to the atmosphere, was investigated by exposing copper plates for one month in urban, rural/coastal, hot springs, suburban, and volcanic areas. The exposures experiment started in summer or autumn. The patinas that formed during the one-month exposure were characterized using X-ray diffraction (XRD), X-ray fluorescence analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and glow discharge optical emission spectroscopy (GDOES). The XRD patterns revealed that cuprite (Cu2O) and posnjakite (Cu4SO4(OH)6•H2O) formed on the copper exposed in all areas, except the hot springs area. This is the first report of posnjakite forming on copper exposed for one month in benign environments. The XRD pattern revealed only cuprite on the copper exposed in the hot springs area although copper sulfide (Cu2S) was found in the cathodic reduction curve. The sulfur 2p XPS spectra of copper exposed in the urban, rural/coastal, and volcanic areas mainly showed a sulfate component, whereas that of copper exposed in the hot springs area showed both sulfide and sulfate components. The former reflected the formation of posnjakite ; the latter reflected the oxidation of sulfide during exposure. In contrast, the chlorine 2p XPS spectra revealed that the chlorine existed as chloride. The SEM observation revealed that the surface morphology differed between exposure sites. GDOES depth profiling analysis revealed a difference in the distributions of sulfur and chlorine in the early stage of copper patination. The sulfur was located in the upper part of the patina, whereas the chlorine penetrated the patina. The oxidized sulfur species lowered the pH of the surface electrolyte, and this accelerated the dissolution of cuprite. They also formed posnjakite, which roughened the surface. The reduced sulfur species lowered the pH of the surface electrolyte and formed copper sulfide. The chlorine dissolved the cuprite forming copper chloride complex. This changed the chlorine depth profile.
A new method to estimate the degree of a corrosion attack on silver taking into account seasonal variations of temperature and relative humidity was developed in order to evaluate the corrosivity of an atmospheric environment. In addition to measuring the thickness of corrosion product on silver using a resistance-type corrosion sensor, temperature and relative humidity were measured in a machine room for a certain period. Based on these measured data, we investigated a method to estimate the thickness of corrosion product on silver that forms in one year. First, the temperature in the machine room was estimated for a one-year period by adding the seasonal component of temperature at the nearest monitoring site of the Japan Meteorological Agency (JMA), the difference in temperature between the machine room and the JMA site, and the irregular component of temperature in the machine room. The relative humidity was estimated by substituting the estimated temperature in the machine room and the absolute humidity at the JMA site into the Sonntag formula. Second, a corrosion gas coefficient was determined by using the thickness of corrosion product on silver and the temperature and relative humidity in the machine room for the measurement period. Third, the thickness of corrosion product on silver for a unit period was calculated by substituting the corrosion gas coefficient and the estimated temperature and estimated relative humidity for a unit period into the already proposed estimation equation of silver corrosion under a constant environment. The thickness of corrosion product on silver for a yearly period can be estimated by accumulating the thickness of corrosion product on silver for the unit period. With a preferable measurement period of three months, except for the winter season, the yearly thickness of corrosion product on silver can be estimated within a 10 percent margin of error.