In order to implement Sustainable Society we have to solve the exiting complex issues such as energy, mineral and food resources as well as environments on this planet. As the multidisciplinary research efforts, Sustainability Science which could provide us with (1) the keen insight into the complex issues and (2) the problem solving potentials has been proposed by World Congress over existing sciences. Materials science is expected to play a critical role in Sustainability Science in making the efficient life cycle design of industrial products from resources to dismantling. Since the lifecycle performance of industrial products is a key factor in connecting Materials Science with Sustainable Society, the time dependent properties such as corrosion and corrosion fatigue of the products during their services have to be predicted and/or avoided by lifecycle safety design and the sensing system.
Hot water supply aluminized steel pipe (125 A) corroded and leaked in about six years. The average corrosion rate is estimated to be 0.76 mm/year for the use period. It is very large as the corrosion rate of the carbon steel pipe. The corroded pipe was analyzed. The corrosion examination on the defective aluminized steel test piece with the hot water test tank at 60℃and the corrosion test on the defective aluminized steel test piece in hot water supply piping system were done. The base steel of aluminized steel test piece corroded in each aluminum plating defect part. Moreover, corrosion potentials of the aluminized steel (surface pure Al layer, Fe-Al alloy layer and base steel) were measured. The corrosion potentials of surface pure Al layer and Fe-Al alloy layer are more noble than base steel at 60℃ and room temperature. As a result, it is thought that this corrosion depends on the localized corrosion generated by the reverse electrical potential between Al (particularly Fe-Al alloy) and base steel.
From the viewpoint of oxide stabilizer, the action of inhibitor was studied by measuring the corrosion potential and corrosion rate of carbon steel in tap waters containing various kinds of inhibitors. According to corrosion potentials (V vs. SSE) inhibitors were classified into three categories; types A: -0.1～0.1 V, type B: -0.3～-0.2 V, and type C: -0.7～-0.5 V. Type A acted as a passivator, keeping the most noble potential which meant γ-Fe2O3 stable. The corrosion potential of type B corresponded to that of the electrode covered with Fe3O4 film. Type C showed less noble potential at first, but the potential was shifted to the noble direction to be brought close to that of type B gradually. This behavior was considered because of enhancing the inhibitive function of the inner oxide film with time. It is concluded that the actions of inhibitors is to stabilize the oxide film on metal.
Ant nest corrosion is one of the local corrosion which occurs in the pipes of air equipments. Because DHP-Cu (Deoxidized High Phosphorous Copper) is generally used as material of those pipes and heat exchangers, ant nest corrosion of DHP-Cu has been mainly studied. According to a few experimental results included in those studies, corrosion rate of DHP-Cu differed from that of OFC (Oxygen Free Copper). In this report, corrosion resistance to ant nest corrosion of two materials was closely investigated. It was found that the corrosion rate of OFC was slower than that of DHP-Cu. The reason for this difference could be explained as follows: electron probe microanalysis on a cross-section of corroded DHP-Cu revealed that P (phosphorus) was oxidized. Since oxidized P, that is, phosphate generates H+(ex. P+4H2O→H2PO4−+6H++5e−), pH in the corroded pore of DHP-Cu easily drops. As is well known, ant nest corrosion evolves around the broken oxide film (cuprous oxide). If pH drops, the dissolution of cuprous oxide will be promoted and then corrosion rate will increase. Based on the mechanism above mentioned, ant nest corrosion rate of OFC is considered to be slower than that of DHP-C because the former's pH is difficult to drop.
The hot corrosion behavior of JIS SS400 and JIS SUS304 steels in BaO2 salt has been examined by measuring the corrosion mass loss, observing the corrosion morphology and analyzing corrosion product. Because solid BaO2 melts at 723 K and the molten BaO2 dissociates into oxygen gas and solid BaO in the temperature region higher than 1073 K, particular attention was given to the change in the corrosion behavior with such an environmental change. In the molten BaO2 at a temperature lower than 973 K, the SS400 steel formed an oxide scale consisting of FeO and Fe3O4 as the inner scale. In the same environment, on the other hand, the SUS304 steel formed a thick scale mainly consisting of BaFe2O4. In the molten BaO2, the SUS304 steel showed a maximum corrosion mass loss at 773 K. The corrosion mass loss for SUS304 steel at this temperature was about 9 times as large as that for the SS400 steel. In the solid BaO in the temperature range higher than 1073 K, the corrosion rate for the SS400 steel remarkably increased with an increase in the temperature. In this case, the SS400 steel formed an inner scale consisting of FeO and Fe3O4, and an outer scale consisting of BaO and BaFe2O4. On the other hand, the corrosion rate for the SUS304 steel in the same environment was far lower than that for the SS400 steel. In this case, the SUS304 steel formed a thin scale which contained an inner scale consisting of Cr2O3･FeO.