Various types of materials, stainless steels, cast irons, Ni-Resist cast irons, copper alloys, etc, are applied to seawater pumps. And the pump materials are changing with the times. Seawater contains a large amount of minerals salts, mainly chloride, therefore, its conductivity is high, and these lead to high corrosiveness. For this reason, the maintenance and management of machinery and equipment, as well as structures, which handle seawater are important issues in terms of corrosion protection. The author et al. has carried out intensive R&D on the corrosion and corrosion protection of seawater pumps. This paper offers explanations on our notable R&D on seawater corrosion, including galvanic corrosion, differentialflow-rate-cell-corrosion, SCC of Ni-Resist cast irons and the development of welded structure duplex stainless steel seawater pumps.
Corrosion environments of each part in steel structures are often significantly different. Therefore, to maintain economically the structures, it is necessary to evaluate properly the environment and to predict the time-depen dence of the corrosion damage. This research focused on comprehensively evaluating the corrosive environment of each part in paint coated steel structures using mean corrosion depth of uncoated steel plate mounted on the steel members. Atmospheric exposure tests were carried out on the uncoated specimens in four exposure sites where each corrosion environment is significantly different. Based on the test results, a high practical method of evaluating mean corrosion depth using the thickness of corrosion product layer of the mounted plates was proposed without removal of the corrosion product. In addition to this method, an evaluation method for time-dependence of corrosion depth of each part in the structures after the paint coating has deteriorated was also proposed.
Anodically electrolyzed dilute NaCl or Na2SO4 solution (NaCl or Na2SO4 acidic electrolyzed water) shows high redox potential and dissolved oxygen concentration, also the former generates free chlorine. We have evaluated the characteristics of both acidic electrolyzed water, and immersed iron in an acidic electrolyzed water, HCl, or H2SO4 of the same pH (=2.48). Then compared dissolution rate, discussed effect of free chlorine or dissolved oxygen on corrosion behavior. As a result, corrosion rate of iron immersed in NaCl acidic electrolyzed water is as much as 3.5 times that in HCl at the same pH, furthermore, free chlorine in NaCl acidic electrolyzed water accelerates the dissolution rate and increases the surface roughness of iron. Corrosion rate in Na2SO4 acidic electrolyzed water is as much as 3.4 times that in H2SO4 at the same pH. Dissolved oxygen is major factor for iron dissolution, however, it is likely another factor may promote iron dissolution except dissolved oxygen. Immersed iron with thermal-oxide film in NaCl or Na2SO4 acidic electrolyzed water, thermal-oxide film was dissolved with underlaying iron, the surface, exhibited natural-oxide film, was as same as that was immersed in HCl similarly. We can conclude that acidic electrolyzed water has the possibility to use etching or pretreatment instead of HCl or H2SO4 conventionally used.