Application of the 57Fe Mössbauer spectroscopy for the study on rust formed on steels has been discussed. Principle and method of the spectroscopy have briefly described and the results from spectroscopy, so called, Mössbauer parameters have been presented. The problems and characteristic features when we apply this spectroscopy to investigate the corrosion products (rust) on the steels, which are mainly iron oxides and hydroxides, have been discussed.
The applicability of the electrochemical corrosion potential (ECP) method to the boiler equipment during the long-term shutdown and lay-up units was experimentally investigated for three boiler equipment materials. It is found that the ECPs of all materials show stable values in a few hundred hours after the test, and that oxide films formed in the simulated AVT and CWT waters show a good stability in the following ECP measurement. Under conditions of the coexistence of Cl- and dissolved oxygen, the corrosion potential of the STB 410 carbon steel shifts to the less noble side showing the severe corrosion, although that of the SUS 304 steel is shifted to the noble side showing the high corrosion resistance. The results obtained by the ECP method have the good correspondence with those of the corrosion immersion test. The temporary decrease of the corrosion potential due to the addition of hydrazine to the waters may be resulted from both effects of the reducing reaction of oxide films and the decomposition of hydrazine. From the fundamental results obtained, it is clear that the corrosion of boiler equipment materials during the lay-up period could be detected by ECP method.
Environmental factors affecting hydrogen absorption into steels have been investigated by hydrogen permeation tests under atmospheric exposure and simulated environmental conditions. Hydrogen absorption into steels under atmospheric exposure depended upon time in day, seasons and exposure locations. It was confirmed that main environmental factors deciding hydrogen absorption were temperature, humidity and salt concentration by simulated environmental tests. The hydrogen absorption into steels was caused by the hydrolysis accelerated by the combination of Fe3+ ion and Cl- ion in water film on the steel surface. Fe3+ ion which was supplied during corrosion, increased by temperature, humidity and salt on the steels. On the other hand, Cl- concentrated in water film at the lower humidity. The opposite reliability on humidity between the corrosion rate and the Cl- concentration, resulted in the highest hydrogen absorption value in the specific relative humidity range, 50-60%.
The application of impressed current cathodic protection to the inner surface of the steel aqueduct bridge in which river water flows has been studied by site tests. The site tests were conducted on two aqueduct bridges, which bring the river water for tap water supply. The span of the test site was 5 meters. The each inner surface of the aqueduct bridges is covered with tar epoxy resin. However, the painting has failed through 40 years of use. Many painting blisters and tubercles were observed on the inner surface. Even under these imperfect painting conditions, the cathodic protection was applicable to one of the two aqueduct bridges. The main reason why the cathodic protection was successful was the improvement of current distribution by the remaining paint coating. The potential measured around the inner surface became uniformly more negative than the protective potential at a cathodic current density of 16mA/m2 in the test.
Phosphonic acid is used as a corrosion inhibitor in cooling water in chemical plants. To determine the optimum conditions for its use, a corrosion test was carried out using a flowing solution, in conjunction with a jet-in-slit testing apparatus. An immersion test was also carried out for comparison. The test involved the exposure of carbon steel to a solution of amino-trimethyl phosphonic acid (ATMP) and zinc sulfate, to simulate industrial water. The anticorrosive effect was the highest at an ATMP concentration of 150ppm and a molar ratio of Zn2+/ATMP of 4, under flowing conditions, but, in the immersion test, this ratio was set at 2. In general, the presence of NaCl and NaClO accelerate corrosion, in that they function as anticorrosive agents, under both immersion and flowing conditions, when an inhibitor is present at optimum concentrations. The film that forms on the material surface is a complex film, consisting of ATMP and Zn2+. The presence of NaCl makes the film homogeneous and NaClO improves the adhesion of the film to the surface of the material, by affecting the early stage of film formation.