Wastage of materials manufactured by much effort is social loss, also, prevention of corrosion is important issue to effective use of energy and prevention of pollution. Furthermore, the corrosion can be directly related to safety life on recent people living contact with many machines and plants. High temperature corrosion arise on the social base equipments supporting human life such as energy conversion plants, chemical reactor,. pollution prevention equipment and transportation system, and corrosion phenomenas are somewhat complicated due to influencing man kinds of environmental and material factors. This paper describes basics of main corrosion reactions, mixed oxidant corrosion, molten salts corrosion and hot corrosion in actual equipments etc. aiming of easy understanding in students and non-professional people.
Rotating corrosion tests of carbon steel were conducted in 40 natural fresh water samples collected from various regions. In these tests, five corrosion types were observed. That is, (1) No corrosion, (2) Pitting corrosion only, (3) Uniform corrosion, (4) Pitting in uniform corrosion and (5) Densely distributed localized corrosion. A kind of Mahalanobis-Taguchi System (MTS) evaluation was applied to discriminate the corrosive water samples from non-corrosive ones. And also, MTS applicability to identify the dominating factors of carbon steel corrosion in freshwater environment was evaluated by determining optimum set of variables that gives minimum number of misjudgment. From MTS evaluations using 5 or more water quality parameters, it is concluded that corrosive samples except for 3 particular samples that caused “Pitting Corrosion Only” can be discriminated from non-corrosive samples. And by using 7 proper variables considering interaction between water quality parameters, all the samples including 3 pitting samples mentioned above can be properly identified to be corrosive. From the optimum set of variables, it is suggested that the interaction variable of [SO42-*M-alkalinity] and Silica-related variables may have major effect on the pitting corrosion of carbon steel in freshwater environment.
In order to prevent the damage of coating layer on marine structures by sea urchins, we studied the influence of coating hardness on the damage of anti-rust paint coating by a kind of sea urchins called ‘tawashiuni’ (Echinostrephus molaris). That is, we prepared steel test plates which have different coating hardness respectively, fed sea urchins on the test plates for three months and observed damage conditions of anti-rust coating layers. As a result, it was observed that coating damage reached base steel plate with regard to tar-epoxy-coating and epoxy-coating (Micro Vickers Hardness (MVH) after the tests were about 10 MVH and 18 MVH respectively.) and the rate of coating damage occurrence (ratio of the number of sea urchins which produced coating layer damage to the number of sea urchins observed.) was over 50%. On the other hand, in the case of epoxy-glass-flake paint which is the hardest coating among commercialized anti-corrosion paintings (MVH after the test was approximately 30) the occurrence rate of coating damage was 10% or so, and the damages were almost like scratches and did not reach base steel plate. Therefore it was considered that the coating layer damage, which reach base steel plate by ‘tawashiuni’, could be protected with epoxy-glass-flake paint although it is expensive. Further it was suggested that the coating layer damage by sea urchins hardly occur if MVH is not less than 40.
Titanium hydrides tend to suffer fracture when their thicknesses reach a critical thickness. Morphology and mechanical property of the hydrides are, however, not well known. The study aims to reveal the hydride morphology and fracture types of the hydrides. Chevron shaped plate hydrides were found to be produced on the surface of pure titanium (Grade 1) and Grade 7 titanium absorbing hydrogen. There were tree types of fracture of the hydrides, i.e., crack in hydride layer, exfoliation of the layer and shear-type fracture of the hydride plates, during the growth of the hydrides and deformation. We next estimated the true stress-strain curves of the hydrides on Grade 1 and 7 titanium using the dual Vickers indentation method, and the critical strain causing the Mode-I fine crack by indentation. Fracture strength and strain of the hydrides in Grade 1 titanium were estimated as 566 MPa and 4.5%, respectively. Those of the hydride in Grade 7 titanium were 498 MPa and 16%. Though the fracture strains estimated from the plastic instability of true stress-strain curves were approximately the half of those estimated by finite element method, the titanium hydrides were estimated to possess some extent of toughness or plastic deformation capability.