Shigeo Tsujikawa

Characterization of Sol‐Gel‐Derived TiO2 Coatings and Their Photoeffects on Copper Substrates

In accordance with the corrosion protection of Cu by n-TiO 2 coating under illumination, the relation between the structural characterization and the photoelectrochemical properties of sol-gel-derived TiO 2 coating has been investigated utilizing glow discharge spectroscopy, x-ray diffraction, and x-ray photoelectron spectroscopy analysis techniques. As a result of the well-known shrinkage of the get under heat-treatment, the thickness of the TiO 2 coating decreased with increasing heating temperature, about a 50% decrease from 200 to 800°C. Amorphous TiO 2 get was found to be crystallized above 400°C, which gave rise to a great enhancement of the photocurrent of the TiO 2 coating. In relating these results to the photoelectrochemical behavior of TiO 2 -coated Cu, it was revealed that both the significant change in the photocurrent and the existence of Cu oxides in the coating were not the direct reasons which accounted for the critical temperature for TiO 2 to impose its photoeffect on the Cu substrate. The dramatic change in the photopotential of TiO 2 -coated Cu would be explained by the change of Schottky barrier at the TiO 2 /Cu interface in terms of the Fermi level pinning at the Ti 3+ defect level. Nevertheless, the increase in the photocurrent of TiO 2 coating was beneficial to move the photopotential of TiO 2 -coated Cu to a much less noble level. The degradation of the photoeffect of TiO 2 coating heated above 800°C was due to the significant diffusion of Cu into the coating.

Alternative for evaluating sour gas resistance of low-alloy steels and corrosion-resistant alloys

Abstract Thiosulfate ion was used as a substitute for hydrogen sulfide (H2S) to simulate stress corrosion cracking (SCC) of corrosion-resistant alloys (CRAs) and sulfide stress cracking (SSC) of high-strength, low-alloy steels. Several SCC tests using a variety of stress application techniques showed the brine containing thiosulfate exhibited similar severity to brine containing H2S in regard to SCC when plastic strain was applied to the CRAs. Materials that exhibited SCC susceptibility in brine containing thiosulfate agreed well the SCC susceptibility of those in brine containing H2S. Types 304 (UNS S30400) and 316L (UNS S31603) stainless steels and duplex stainless steel exhibited in both environments. However, high-nickel austenitic alloys such as alloys 904L (UNS N08904) and 825 (UNS N08825) did not. A 10−3 to 10−2 mol/l S2O32− addition in 20% NaCl aqueous solution at 353 K corresponded to H2S of 0.1 to 1 MPa at 473 K. The SSC susceptibilities of high-strength, low-alloy steels in a 10−3 mol/l S2O32− ...

The effect of marine fouling on the ennoblement of electrode potential for stainless steels

Abstract Electrode potentials for 18 kinds of stainless steels and commercially pure titanium were measured in live sea water pumped up into the laboratory room through a 100m-long pipe, so that the specimens were fouled by only the brown slime. After 10 days of immersion, the electrode potentials for all the specimens attained to about 400mV(SCE). Deposited slime was examined under a scanning electron microscope and analyzed in terms of fluorescence. It was found that the slime was composed of living sea diatoms. Variations of electrode potential for stainless steel which had been immersed for 35 days were measured with successive removal of the deposited layer. The electrode potential remained noble after removal of the slime layer, while it decreased drastically after subsequent removal of a thin organic film under the slime layer. The ennoblement of the electrode potential is caused not by the metabolism but by the metabolite of the sea diatoms included in the thin organic film.

Investigation of Alpha Prime Precipitation in Aged Duplex Stainless Steel

A ferritic/austenitic duplex stainless steel was aged at 450 o C for up to 5000 h. Electrochemical behavior of the aged steel was examined in 0.1∼0.5M H 2 SO 4 , solution containing 0.01 M KSCN by electrochemical potentiokinetic reactivation and scanning vibrating electrode techniques (SVET). An attempt was made to relate the electrochemical behavior to the Cr concentration in the Fe-rich region formed by aging, which was determined by a laser atom probe. In the early stage of aging, while neither reactivation nor change in activation behavior is observed, the passivation current increases as aging proceeds

Repassivation method to determine critical conditions in terms of electrode potential, temperature and NaCl concentration to predict crevice corrosion resistance of stainless steels

Abstract Repassivation potentials, E R , for crevice corrosion of metal/metal-crevices were measured with more than 50 kinds of stainless steels and nickel base alloys in solutions containing 3×10 −1 , 3 and 20% NaCl at 80 °C. Critical temperature and critical Nacl concentration for repassivation of the crevice corrosion were determined after successive decreasing of temperature and/or NaCl solution under an electrode potential kept at −0.20 V,SCE. The obtained boundary conditions for repassivation depend on both the temperature and NaCl concentration (Type A) for high alloys containing Ni and Mo as expressed by Mo (§)+13.5 log Ni (§) 26 while these depend mainly on NaCl concentration (Type B) for the other alloys from ferritic to austenitic stainless steels. These differences could be explained from the temperature dependence of repassivation potentials for the two type of alloys.

Repassivation Method to Predict Long Term Integrity of Low Alloy Titanium for Nuclear Waste Package

Long term corrosion resistance is evaluated for G-2 and G-12 titanium as a candidate of the high-level nuclear waste packages. The repassivation potential for crevice corrosion, E R , in comparison with the well known spontaneous potential, E SP , of a metal passivated in given environment, allows conclusions to be drawn whether it performs “permanent” or does not. This repassivation method is extended to determine critical conditions in terms of NaCl concentration and temperature for specimens kept at an electrode potential which is more noble than E R and is included in E SP ranges. Thus obtained NaCl concentration - temperature - crevice corrosion map could predict critical conditions for the titanium used in geologic formations.

Simulation of The State of Carbon Steel n Years After Disposal with n Years of Corrosion Product on its Surface in a Bentonite Environment

The use of bentonite as buffer and carbon steel as overpack material for the geological disposal of nuclear waste is under investigation. To better assess the long term integrity of the carbon steel overpack, a quantitative analysis of the corrosion behavior on the steel surface for time frames beyond that of feasible empirical determination is required. The state n years after disposal, consisting of Carbon Steel/Corrosion products + Bentonite/Water, was simulated and the corrosion behavior of the carbon steel in this state investigated. The following facts became apparent. Both the corrosion rate and the non-uniformity of it increased with increase in the corrosion product content in the compacted bentonite. When the corrosion product layer was formed between the carbon steel and the bentonite, it enabled the corrosion potential and increased the corrosion rate.

Moire method to measure penetration depth profiles on unevenly corroded metal surfaces

Abstract A system based on the moire method was successfully applied to in situ measurement of crevice penetration for high purity 18Cr-14Ni steels in NaCl solutions at 80°C. For the growing picture element, PE (II), the dissolution rate in the first stage (stage I), V I , and the dissolution rate in the stationary growing stage (stage II), V II , were measured, and current densities, i I and i II , corresponding to V I and V II , respectively, were determined. The effect of NaCl concentration on dissolution behavior at a potential just above repassivation potential ( E R ), E R 1 , was studied in terms of the effective corrosion current density inside the crevice, i eff * , which was found to be constant regardless of bulk NaCl concentration, which could contribute to the concentration difference of ions inside the crevice and in bulk solution.

Crevice corrosion -- NaCl concentration map for Alloy 625 at elevated temperature

The repassivation potentials, Er, for metal/metal-crevice of Alloy 625 were determined in 0.3--10% NaCl solutions for temperatures up to 250 C. The ER were found to be the least noble at temperatures around 100 and 125 C. The Er became more noble as temperature increased; this tendency was particularly strong in diluted solutions. Based on the experimental data, a crevice corrosion map showing the critical condition in terms of temperature and NaCl concentration was presented. As for the map, a wide repassivation region was found in elevated temperatures, similar to that of commercially pure titanium, C.P.Ti.

Crevice corrosion - NaCl concentration map for grade-2 titanium at elevated temperature

The repassivation potential, ER, for metal/metal-crevice of Commercially Pure Titanium, C.P.Ti, was determined in NaCl solutions at temperatures up to 250C. The ER has its least noble value near 100C and becomes more noble as the temperature increases. As shown in previous research, the shrinkage of the repassivation region should continue with increasing temperatures. However, in conducting this same experiment at temperatures higher than 100C, an examination of the NaCl concentration - temperature - crevice corrosion map verifies that the repassivation region began to expand again when the temperature exceeded 140C. This expansion continued as the temperature continued to increase.