Localized corrosion mechanisms of Al-clad steel wire strand under sealing tape in the coastal area were studied mainly by laboratory accelerated corrosion testing. The results are summarized as follows: (1) The same-type localized corrosion of Al-clad steel wire strand found in the field was also observed during laboratory testing. (2) Alkaline areas coexisted with acid areas at corroded regions for both the laboratory-tested specimen and the corroded strand in the field. (3) The corrosion might be a kind of “crevice corrosion” caused by destruction of passivation film of Al layer in the crevice between sealing tape and Al-clad strand. In the crevice, chloride ions might be concentrated and oxygen supply might be restricted. (4) Regarding this “crevice corrosion”, a reduction of oxygen as a cathodic reaction of corrosion cell proceeded within the crevice, whereas in the typical crevice corrosion, such as that of stainless steel in chloride solution, cathodic reaction proceeds outside the crevice. (5) This difference might be due to the passivation film stability difference between Al and stainless steel. The passivation film of Al can be destroyed under high oxygen conditions where oxygen reduction could proceed sufficiently as a cathodic reaction of crevice corrosion cell.
Investigations have been made on relationship between concentration of oxygen in NaCl solution and pitting attacks of aluminum alloy 5052 for can stocks. The value of anodic polarized resistance of 5052 was measured in 5000ppm NaCl solution, in which the oxygen concentration was continuously changed from 1ppb to 3ppm, by the technique of area polarization method. This technique characteristically shows rapid and continuous measuring system. It is found that the oxygen concentration at occurrence of the pitting attacks of 5052 shows the value of 900ppb in the 5000ppm NaCl solution.
Nondestructive exploratory test of corrosion of steels embedded in cement paste and mortar was carried out by the method based on γ-ray back-scattering. The possibility of such test was investigated with the theoretical calculation. From these results of the test, the difference (ΔD) of back-scattering dose rate of corroded and uncorroded specimens of bare-steels or cement paste- and mortar-embedded-steels was found and increased with angle (ω) of Compton scattering. A relationship between ΔD and ω was observed. The relation of ΔD with ω by the test corresponded to that by the theoretical calculation. For the steels embedded in cement paste and mortar, the value of ΔD decreased and became irregular depending upon i) the thickness and component of cover and ii) the shape of steel. The subject in future will be the investigation for a method of the exploratory test by γ-ray back-scattering by consideration of above environmental conditions.
Field tests were done at Onikobe and Minamikayabe geothermal sites and the effect of flow velocity on flow-assisted corrosion damage was clarified as follows: Corrosion rates of stainless steels were very small in the alkaline liquids of Onikobe brine A and Minamikayabe brine, and the flow effect was also very small. The rates of mild steels were large in short test duration, but decreased to 0.01mm/y after 46 hours. The flow velocity at which corrosion rates of mild steels reached to the maximum was larger in Minamikayabe geothermal brine than in Onikobe brine A, because the former contained hard solid particles and less dissolved oxygen than the latter. In Onikobe acid geothermal brine B, corrosion rates of mild steels increased with velocity at 0-0.5m/s, but were constant between 0.5m/s and 4.1m/s. However, in case that the brine contained 5mg/l dissolved oxygen, the rates increased with velocity beyond 0.5m/s.
Scanning acoustic microscope (SAM) is a useful nondestructive technique for obtaining inside information by observing from the surface which cannot be seen by conventional microscopic techniques. In the present article application of SAM to nondestructive evaluation of typical localized corrosion damages is described and advantages of the technique to conventional microscopic techniques are emphasized. Furthermore discussion is addressed to several problems which should be solved before SAM establishes itself as an indispensable corrosion evaluation technique.
Methods and effects on metal substrate to be coated on some of mechanical surface preparation, especially blasting, are described. And, after introducing recent action of ISO/TC35/SC12 on this subject, correspondence as Japanese industries for the action of ISO is appealed.