Takumi Haruna

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− ...

Theoretical Prediction of the Scan Rate Dependencies of the Pitting Potential and the Probability Distribution in the Induction Time

The point defect model (PDM) has been used to derive the dependence of the pitting potential on the voltage scan rate. Relationships derived from the PDM predict that the observed pitting potential is a linear function of the square root of voltage scan rate at low scan rates, which agrees with experimental data reported in the literature. Furthermore, the critical concentration of condensed cation vacancies that give rise to passivity breakdown, as estimated from the sweep rate dependence of the pitting potential, is found to be in good agreement with that estimated from structural considerations. The PDM is also used to predict the probability distribution function in the induction time for pitting (t{sub ind}), that is, the survival probability, by assuming that the maximum diffusivity of the cation vacancy in a population of specimens is log-normally distributed. This calculated external distribution in the induction time is found to be in reasonable agreement with experimental induction time data for passivity breakdown on multiple specimens of single crystal (100) Ni and polycrystalline nickel buffered chloride solutions.

Initiation and propagation of IGSCC for sensitized Type 304 stainless steel in dilute sulfate solutions

Initiation and propagation of IGSCC for sensitized Type 304 stainless steel in dilute sulfate solutions have been investigated by the slow strain rate test (SSRT) with a CCD camera system. An increase in concentration of sulfate ion was found to cause decrease in crack initiation strain and fracture strain, resulting in the increase in the IGSCC susceptibility. Statistical analysis made it clear that the increase in concentration of sulfate ion had little effect on crack initiation frequency, and crack initiation time obeyed the exponential probability distribution, but the crack velocity obeyed the Weibull probability distribution. With increase in the concentration of sulfate ion, the crack velocity increased, and the number of cracks decreased. On the contrary, the mean crack velocity decreased and the number of cracks increased with time. The dependence of the crack velocity on the concentration of sulfate ion and the number of active cracks under potentiostatic SSRT conditions was successfully explained by a multiple cracks model combined with the mass transport model.

Initiation and propagation of stress corrosion cracks for type 3041 stainless steel in chloride solutions containing thiosulfate

Abstract The stress corrosion cracking (SCC) of Type 304L stainless steel in chloride solutions containing various concentrations of thiosulfate ions (S2O32−) has been studied by using a slow strain rate testing apparatus with a dynamic observation system. It was observed that intergranular cracks were initiated from pits and propagated with the evolution of bubbles. The SCC was found to occur > 10−3 M S2O32−, and the variation in the concentration of S2O32− from 10−3 to 10−1 M did not shown any marked difference in SCC susceptibility evaluated by maximum stress. According to the analysis of the cracking behavior from the observed images, crack propagation rate was found to increase slightly with increase in the concentration of S2O32−. The increase is considered to be caused by the change in the corrosion potential with the concentration. Apparent critical stress intensity factor for SCC was less scattered around 2–3 MNm − 3 2 and slightly increased with increase in the concentration. The strain where either a pit or a crack was initiated first and the number of pits and cracks decreased remarkably and increased with increase in the concentration, respectively. S2O32− mainly affects the initiation process of the pit and the crack more than the crack propagation process.

The effect of potential on initiation and propagation of stress corrosion cracks for type 304l stainless steel in a chloride solution containing thiosulfate

The effect of applied potential on the stress corrosion cracking (SCC) of Type 304L stainless steel in a chloride solution containing thiosulfate ions (S2O32−) has been investigated by using a slow strain rate testing apparatus with a dynamic observation system. It was found from the SCC test that the material suffered SCC in the potential region from −360 to −410 mVAgAgCl, and that maximum stress decreased with increase in the potential. At any potential in that potential region, moreover, polarity of the current changed from negative to positive during the test, and strain where the current was zero decreased with increase in the potential. According to the detailed analysis of the cracks from the observed images, the following findings were obtained: The number of pits and cracks increased with increase in the potential; Strain where either a pit or a crack was detected first were decreased with increase in the potential, and was almost the same value as the strain where the current was zero; Apparent critical stress intensity factor for SCC decreased with increasing potential; crack propagation rate slightly increased with increasing potential. These findings suggest that the potential accelerates the initiation process of pits and cracks more than the propagation process of the crack.

A HSAB concept applied to inhibition effect of anions on IGSCC of sensitized type 304 stainless steel

Abstract The effect of concentration and type of anions on stress corrosion cracking of sensitized type 304 stainless steel was investigated by slow strain rate test (SSRT) with a dynamic cracking observation system. It was found that difference of anion type added into pure water exerted a significant effect on crack initiation time and mean crack initiation frequency. Some anions, such as OH− and B4O72−, can suppress, while others, such as S2O32− and SO42−, accelerate crack initiation. The effect of anion type on the crack initiation behavior was explained on the basis of the passive film model proposed by Okamoto and Shibata and a HSAB (hard and soft acids and bases) concept. The anion having a larger hardness value was found to show a longer crack initiation time or to delay the crack initiation. The anion with a larger hardness, that is expected to show a stronger affinity to proton, is considered to accelerate deprotonization, and to enhance the repassivation process. Consequently, the harder anion is expected to induce a more perfect passive film more rapidly, and then to retard the crack initiation more effectively.

Inhibition effect of the borate ion on intergranular stress corrosion cracking of sensitized type 304 stainless steel

Abstract The intergranular stress corrosion cracking (IGSCC) behavior of sensitized type 304 (UNS S30400) stainless steel (SS) in dilute sodium borate (Na2B4O7) solutions at 95°C was examined by slow strain rate testing (SSRT) using a dynamic observation system. The borate ion (B4O72−) suppressed IGSCC susceptibility of sensitized type 304 SS by delaying the crack initiation time and reducing the crack initiation frequency (CIF). The inhibition effect of the borate ion (B4O72−) on crack initiation may have resulted from its buffer effect on local acidification at crack nuclei and its inhibitive nature in reacting with metal ions to form a protective film. However, B4O72− concentration provided no remarkable inhibition effect on crack velocity (CV), which depended mainly upon solution conductivity, within the scattered values. Hydroxyl ions (OH−) also suppressed initiation of IGSCC, but not as effectively as B4O72−. The crack initiation time obeyed the exponential probability distribution, and the CV obeye...

Potential-pH region for environment-assisted cracking of gamma titanium aluminide

Abstract Environment-assisted cracking (EAC) of γ-TiAl has been investigated from electrochemical viewpoint. The material whose potential was controlled was tested in aqueous sulfate solutions of various pHs at room temperature by using a slow strain rate tensile testing apparatus. The TiAl forged at 1373 K and annealed at 1073K for 7.2 ks was used in the EAC test. It was found in the alkaline and the natural solution that maximum stress was independent of the potential in higher potential region, and decreased with decrease in the potential in the lower potential region. In acidic solution, the maximum stress changed with the potential as follows; the maximum stress was independent of the potential above −200 mV SHE , decreased with decrease in the potential between −200 and −1000 mV SHE , and increased with decrease in the potential below −1000 mV SHE . The experimental EAC results was discussed on the basis of the potential-pH diagram of Al and Ti/water system. It was revealed that the diagram of Ti/water system can predict the EAC susceptibility of TiAl, although the diagram of Al/water system shows almost no relation to the susceptibility. Therefore, the EAC of TiAl is controlled by the stability of Ti oxide, hydride, and ion on the solution/base material interface, which affect the entry of hydrogen into the material.

Contribution of solution pH and buffer capacity to suppress intergranular stress corrosion cracking of sensitized type 304 stainless steel at 95°C

Abstract The effects of solution pH and buffer capacity on intergranular stress corrosion cracking (IGSCC) of sensitized type 304 stainless steel ([SS] UNS S30400) was examined at 95°C by slow strain rate technique (SSRT) with an in-situ cracking observation system. It was found that an increase in solution pH or buffer capacity increased crack initiation time and decreased mean crack initiation frequency, but exerted almost no effect on crack propagation. This inhibition effect on IGSCC initiation was explained as resulting from a retarding effect of solution pH and buffer capacity on the decrease in pH at crack nuclei caused by the hydrolysis of metal ions dissolved when the passive film was ruptured by strain in SSRT.

Analysis of Initiation and Propagation of Stress Corrosion Cracks in Sensitized Type 304 Stainless Steel in High-Temperature Water

Abstract A slow strain rate testing apparatus equipped with an autoclave as well as with an in-situ crack observation system has been newly developed. Using this system, the growth behavior of mult...