Recently high-Cr ferritic stainless steels have been adopted as exterior parts of buildings especially at waterfronts. In this study, atmospheric corrosion resistance of various kinds of stainless steels was evaluated in various environments. Correlation between the atmospheric corrosion resistance and laboratory corrosion tests and a method for evaluating atmospheric corrosion resistance were investigated. Results are as follows. 1) Atmospheric corrosion resistance of stainless steels was improved by increasing pitting index (P. I.; ferritic stainless steel: Cr+3.3Mo, austenitic and dual phase stainless steels; Cr+ 3.3Mo+16N). 2) Ferritic stainless steel that have the same P.I. value as austenitic and duplex stainless steels showed better corrosion resistance than the austenitic and duplex stainless steels. The pitting index of the steels is useful for estimating corrosion resistance for each type of steel. 3) Corrosion resistance in atmospheric exposure tests had a good correlation with pitting potential. If pitting potentials of stainless steels were more than critical pitting potentials (Vcrit) which correspond to each exposure site, the steels did not have rust. The Vcrit values can be used as one of the criteria for selecting steels to site a service environment. 4) In a severe corrosive environment such as a coastal area, Mo-bearing high-Cr ferritic stainless steels are required.
The influence of stress ratio on the retardation of surface fatigue crack growth of mild steel in sea water was investigated by varying the testing frequency from 0.2Hz to 10Hz and the temperature from 288K to 298K at two stress ratios of 0.32 and 0.5. For both stress ratios, there appeared to be a frequency at which the rate of crack growth showed maximum. This frequency increased with the increase of the temperature, although the maximum of the crack growth rate decreased with the increase of the temperature. Furthermore, the rate of crack growth increased with the increase of the testing frequency up to a certain frequency, but it started to decrease with the further increase of the testing frequency. At lower testing frequencies and for the two stress ratios employed here, there appeared to be a critical frequency at, or below which the rate of crack growth in sea water was so low that it could be regarded as virtually zero. This critical frequency increased with increase of the sea water temperature. At temperatures in the range between 288K and 298K, the critical frequency was higher at the stress ratio of 0.5 than that of 0.32. These observations can be explained reasonably by crack tip blunting caused by dissolution in sea water.
Recently, the authors have demonstrated that iron-oxidizing bacteria have been shown to be involved in the severe corrosion of ductile cast iron pipes buried in acid-sulfate soils. However, direct evidence for the involvement of iron-oxidizing bacteria in corrosion processes has not yet well been clarified. In the present study, the involvement of iron-oxidizing bacteria in the corrosion of a ductile cast iron exposed to the Thiobacillus ferrooxidans (a species of iron-oxidizing bacteria) inoculated medium was examined by using scanning vibrating electrode technique (SVET) together with fluorescent microscope. It has been observed that the anodic regions correspond to the location of Thiobacillus ferrooxidans; providing further evidence that severe corrosion attacked regions can be caused by the action of Thiobacillus ferrooxidans in the initial stage of corrosion processes.
In order to study the mechanism of environmental assisted cracking of metallic materials at high temperatures, a new heat resistant acoustic emission (AE) sensor was developed and utilized to measure the acoustic and mechanical properties of the SUS 304 stainless steel at temperatures lower than 600°C. The capacitive type AE sensor with an amorphous alumina film deposited by an electron-cyclotron resonance laser PVD method was found to measure the out-plane displacement in the broad frequency band, but show a slight response delay due to the relatively large sensor area. Both the phase velocity and wave attenuation of the SUS 304 steel at elevated temperatures were successfully measured utilizing the Q-switched YAG laser and the developed AE sensor. A new signal processing for the source inversion of the dissipative elastic waves, including the wave attenuation, the time retardation function of the sensor and the analytical transfer function of the hot medium, was proposed.
Case studies were conducted on CO2 corrosion of carbon steel in various areas of industrial applications, i.e., steam return lines, heat exchanger tubes of a steam heating system, steam pipings, blast furnace gas lines, town gas lines, a CO2 recovery system and an oil and gas well. Corrosion of steam return lines proceeds under very low CO2 partial pressures, e.g., 10-5-10-4atm., but at a high corrosion rate reaching several mm/y due to the presence of oxygen. The corrosion product films, FeCO3, are stable when pH of the water phase is higher than a critical value, resulting in a low corrosion rate. When the pH value is a little lower than critical, the corrosion rate is lowered by the formation of partially protective films. Very high rates of corrosion are usually not accompanied by corrosion products. Mesa corrosion characterized by sharp lines deviling the corroded and un-corroded area presumably occurs under conditions of partial protection where localized areas of stable corrosion product films remain intact for a long time by further stabilization of the films caused by pH increase of the surface resulting from galvanic current of macro-galvanic cells.
Acid deposition causes the forest decline and the acidification of lakes and marshes. Furthermore, historic stone monuments, which were made of marble, limestone, and so on, have been reported to be deteriorated by acid deposition and concrete structures may be deteriorated as well. There are few studies on the effects of acid rain on mortar and concrete, though the effects of CO2 and SO2 have been studied. Deterioration of concrete structures by acid rain is guessed to proceed as follows. (1) Dissolution of aggregate, hydrated and unhydrated materials at the surface. (2) Conversion of CaCO3, which is formed by carbonation of hydrated materials, to water soluble salts. (3) Elution of the salts at the surface and development of stress by deposition of the salt beneath the surface. (4) Cracking by formation of corrosion materials such as ettringite, which occupy a large volume. (5) Cracking and spalling by the corrosion of reinforcing steel, which is lead to by neutralization and penetration of chloride and sulfate. At present, there is no obvious evidence showing that deterioration of well-constructed concrete structures is accelerated by acid rain, though the investigation is limited.