The research statuses on SABI [rust] chemistry and protective rust layer on weathering steel, compatibility between corrosion resistance improvement and surface hot shortness by Cu in recycling ferrous scrap steels and either side approaches of mechano-chemical and chemo-mechanical reactions in environmental degradation have been reviewed from the viewpoint of looking back over corrosion science and technology in the 20th century. Recent research fruits on the three topics mentioned above are referred to propose future problems.
This paper is concerned with materials used for the seal components of mechanical seals. Mechanical seals are used for rotary seals of fluid machinery in various industries. Their service conditions, such as sliding rate, temperatures and pressure, have been changed to severe in recent years. The sliding surfaces are the most critical part of a seal, and are mainly required tribological properties. Sliding surfaces are subject to chemical attack under the various fluid conditions in the service. Therefore corrosion properties must be taken into consideration in selecting materials for the seals.
The influence of water quality factors such as silica (SiO2) and chloride ion (Cl-) on a stability index of water and corrosion rate of steel was investigated in a solution with polyacrylic acid (PAA) as a corrosion inhibitor. In corrosion weight loss tests, the corrosion rate in PAA additive system decreased in comparison with that in PAA non-additive system as reported in previous paper and was dependent on [SiO2] and [Cl-]. In solutions with SiO2 and Cl- of PAA additive system, the corrosive property become weak as the stability index was smaller. In numerical value analysis, SiO2 had an ability of corrosion inhibition in PAA additive system as well as non-additive one, and then, tended to change the stability index at chemical equilibrium state. The influence of Cl- in solutions with SiO2 of PAA additive system differed from that of non-additive system : The corrosion rate was affected by Cl- and the stability index was not.
Two types of steel, i.e., CrMoV steel and 3.5 NiCrMoV steel, are commonly used for the rotor materials in condensing turbines. Test specimens for SCC test made of these two types of steel were inserted into capsule and the capsule was installed in a branching pipe from the extraction pipe of an actual turbine in the wet zone. The results were as follows: (1) The intergranular crack was observed and the crack propagation rate was about 3.4×10-4mmh-1 for CrMoV steel and 1.4×10-4mmh-1 for 3.5NiCrMoV steel at 363K of this test temperature, respectively. (2) A comparison of the crack propagation rate determined using a published prediction formula with the value determined through tests in an actual turbine showed that the actual measured value was high. When determining the stress corrosion cracking inspection interval in the locations of small condensing turbines at which the temperature is comparatively low, the prediction formula shall be set to considerably conservative evaluation side. (3) The three-point bend test pieces used to examine SCC initiation were exposed for up to 27, 000 hours, however, no SCC initiation was observed in either of the test specimens.
The effect of nitrogen on localized corrosion resistant properties and repassivation behavior of nitrogenbearing 316L stainless steels was investigated in the 3.5%NaCl, 0.1kmol m-3 Na2SO4 and 0.5kmol m-3 H2SO4 solutions. Interstitial nitrogen in the steel is converted to ammoniacal state with the efficiency of 100% when the metal dissolves inside a crevice. It has been proven that the molar amount of dissolved ammonia was equivalent to the molar amount of nitrogen contained in a dissolved portion of the steel. The result is in agreement with the report where it suppresses the decrease in pH, leading to the conditions for repassivation, since conversion of nitrogen into ammonia is a proton-consuming process. Neither nitrate-N (NO3-N) nor nitrite-N (NO2-N) was detected by chemical analysis although the measured potential lies in the stability region of NO3-N in the E-pH diagram. A scratching electrode method was applied for evaluating the repassivation kinetic of nitrogen-bearing 316L stainless steels in 0.1kmol m-3 Na2SO4. The technique developed in this study allowed us to measure the recovery process of passive film immediately following the exposure of bare metal surface. The repassivation process was faster with increasing dissolved nitrogen content in the steel.
The effect of prestrain on intergranular corrosion was studied to infer the relationship between the preferential dissolution of grain boundary and lattice defects of sensitized Inconel 600 alloy in H2SO4-NaCl solution. The polarization curves showed that prestraining decreased passive current densities and shifted starting potentials of trans-passivity (Etps) to less noble side. The intergranular corrosion rates at Etps +51mV decreased with increasing prestrain. Those values at 980mV decreased up to the prestrain of 10% but showed a tendency to increase with higher prestrain. This relationship between the intergranular corrosion rates and prestrain found under no stress also stood under the constant load of 90% proof stress for 0% prestrained specimen. But the intergranular corrosion rates under the applied stress of 90% proof stress for each prestrained specimen increased with increasing prestrain because of the introduction of continuous plastic deformation at intergranular corrosion tips. EPR values increased with increasing prestrain. After measurements of polarization curves, sensitized specimens showed intergranular corrosion and 30% prestrained specimen of solution annealed condition showed localized one as well. Therefore, it is thought that the intergranular corrosion in an active region is accelerated by prestrain. It is concluded that prestraining accelerated the intergranular corrosion in an active region but suppressed it in a trans-passive one.
The oxidation behavior of CrN, TiCrN and TiAJN coatings deposited on a steel substrate by ion plating was studied between 973 and 1173K in atmospheric air. The oxidation resistance increased in the order of TiAlN, TiCrN and CrN, which were oxidized to TiO2 and α-Al2O3, TiO2 and Cr2O3, and Cr2O3, respectively. The α-Al2O3 layer formed on the TiAlN coating was too thin to resist oxidation, so that the oxidation resistance of TiAlN decreased rapidly at 1173K. The Cr2O3 layer formed on the TiCrN or CrN coating was comparatively thick, so that the oxidation resistance of these coatings was better than that of the TiAlN coating over entire testing temperatures. Particularly, the CrN coating displayed excellent oxidation resistance.