Timber is still an important construction material in many small and medium-size marine structures owing to good properties with respect to strength, toughness, elasticity/resilience, weight, shaping, and cutting. The PIANC WG31 revised the PIANC WG17 report with the title “Inspection, maintenance and repair of maritime structures exposed to material degradation caused by a salt water environment”, and published the revision in 2004. In the revision, the chapter of timber was rewritten based on the data from foreign countries. In this report, the outlines of the degradation, inspection, maintenance and repair of timber in a marine environment are presented in reference to the revision.
Pb-Sn-X alloys (X: additives) are interesting materials as grid of lead acid battery with corrosion-resistance. In this study, Pb-Sn alloys of 0-2.0mass% Sn were prepared by casting, and then the corrosion tests with electrify/immersion cycling were carried out in 4.50kmol/m3 H2SO4 at 348K for 604.8ks. The test resulted in the formation of a peculiar corrosion scale on the electrode surface and black corrosion powders suspended in the electrolyte solution. The mass change of the specimen electrode during the corrosion test, which was caused by formation of the scale composed of PbO2 and PbSO4 and its peeling away, increased with the increasing Sn content ranging from 0 to 1.0mass%. On the other hand, the amount of the corrosion powders suspended in the electrolyte decreased with the increasing Sn content. Consequently, the Pb-Sn alloy of about 2.0mass% Sn had good corrosion-resistance to suppress both the mass change of the sample electrode and the amount of the corrosion powders. Moreover, the corrosion data suggest that not only the crystal structure but also the current efficiency of oxygen generation influenced the formation rate of the corrosion scale. That is, the increasing of Sn content caused a remarkable decrease in the current efficiency of oxygen generation and activated the production of PbO2, PbSO4 and the like. The coarsening of the crystal grain was also effective in suppressing the intergranular corrosion. Therefore, the formation rate of the corrosion scale was synthetically determined by both effects of the grain size and the current efficiency of oxygen generation. In addition, the oxygen generation, which accelerates the peeling away of the corrosion scale, was related to the formation of the powders suspended in the electrolyte.
Corrosion resistance of newly developed 5Cr-0.3Cu-0.3Ni steel in LNG fired smokestack environment was evaluated. On the basis of analytical survey of the corrosion environment, the authors deduced that corrosion could hardly occur during stationary operating conditions, but dominantly proceed during start and stop of boilers, which induces cyclic wet and dry conditions with dew dissolving CO2 and other soluble deposits. Results obtained with the alternating wet and dry CO2 gas corrosion test developed by the authors, showed good agreements with those with field exposure test in terms of corrosion rates, corrosion losses per wet and dry cycle, and morphology. It was confirmed that the 5Cr-0.3Cu-0.3Ni steel itself and its welded joints with austenitic welding material showed sufficient corrosion resistance in LNG fired smokestack environment. It was found that 5Cr efficiently reduces the corrosion rate and that 0.3Cu-0.3Ni improves adhesive property of corrosion products.
It has been observed by divers who were engaged in maintenance and management of marine structures that their surface coating was damaged by sea urchins. There are no reports with regard to such damages in the past. Therefore, survey was performed for sea urchins to survive on marine structures and for the damage of surface coating. As a result, 4 kinds of sea urchins such as ‘tawashi-uni’ (E. molaris) and ‘murasaki-uni’ (A. crassispina) were observed and the coating damage was observed at the location where 3 sorts of sea urchins such as ‘tawasi-uni’ (E. molaris) were living. Pieces of coating as well as feed creatures were detected in their digestive canals. The ‘tawasi-uni’ (E. molaris) more frequently observed on the damaged coating surface are usually said that they stay, in almost all cases, in holes dug in rock beds at a sea coast and eat pieces of sea weeds which flow into the holes. Because they cannot dig holes in metal of ocean structures, it is considered that they move on the surface of marine structures, biting sea weeds and barnacles on those as feeds. At the time, their hard teeth may damage the coating. The survey showed that high frequency of coating damage was observed at the points of ‘tawasi-uni’ (E. molaris) existence. But teeth of ‘murasaki-uni’ (A. crassispina) are as hard as those of ‘tawasi-uni’ (E. molaris), and it is not clear why ‘murasai-uni’ (A. crassispina) has more influence on it.
External stress corrosion cracking induced by sea salt particles (ESCC) will be expected in the stainless steel canister for interim storage of spent nuclear fuels in air using concrete casks. Initial corrosion behavior as the precursory phenomena of ESCC and the long term ESCC behavior were examined in this study to clarify the ESCC characteristics of candidate canister materials. To study the initial corrosion behavior, several kinds of tests were conducted in response to the processes from deliquescence of sea salt particles to corrosion initiation. It was found that the critical relative humidity (RH) which leads to corrosion was lower than the RH at which sea salt particles absorb water and have an electric conductivity, when the surface chlorine concentration was 5g/m2 as Cl. Crack-like corrosion occurred within 12h at the test condition of RH=35% at 353K with surface chlorine concentration of 10g/m2 as Cl. On the contrary, no ESCC failure was observed for more than 30, 000h in the candidate canister materials at the same test condition. Thus it was suggested that the high ESCC resistance of the candidate materials stemmed from the high resistance to crack extension and/or the slow process in which crack-like corrosion transformed to ESCC cracks.