Yasuyuki Katada

Role of nitrogen on the corrosion behavior of austenitic stainless steels

Abstract The role of nitrogen in the mechanisms of localized corrosion resistance and repassivation were electrochemically investigated using a nitrogen-bearing austenitic stainless (SUS316L) steel in 0.1 and 0.5 M Na 2 SO 4 solutions and a 3.5% NaCl solution. Almost 100% of the nitrogen compounds dissolved into the bulk solution after crevice corrosion were transformed into NH 3 . That is, the mole amount of ammonia in the solution was approximately equivalent to the mole amount of nitrogen dissolved in the steel. This suggests that NH 4 + consuming H + in the pit controlled the local decrease of pH and promoted the repassivation. NO 3 –N and NO 2 –N were not detected by chemical analyses in the high potential and thermodynamically stable zone as NO 3 − in the potential–pH diagram. The repassivation in nitrogen-bearing SUS316L steel in a 0.1 M Na 2 SO 4 solution was studied using the scratching electrode technique, which measured the partially destroyed passivation films on the steel. This technique showed that nitrogen dissolved in the steel has a strong repassivation capacity.

The influences of microstructure and nitrogen alloying on pitting corrosion of type 316L and 20 wt.% Mn-substituted type 316L stainless steels

Abstract The effects of nitrogen alloying on pitting corrosion of type 316LN and 20 wt.% Mn-substituted type 316LN stainless steels were investigated by potentiodynamic polarization tests in Cl − ion-bearing neutral and acidic solutions. Pitting resistance was markedly improved through the nitrogen alloying in both types of alloys, compared with the nitrogen-free alloys. It was confirmed that the added nitrogen was solid-solutioned in the austenitic phase without forming any nitrides under 20 min heat treatment at 1150°C. From the in situ observation on the initiation and growth of pits, pitting was found to occur consistently at the sites of inclusions. The pitting corrosion behaviors in both types of alloys were discussed with respect to the role of sulfides as initiators of pitting corrosion, and the changes of repassivation properties due to the nitrogen alloying in the alloys.

Fabrication of High Strength High Nitrogen Stainless Steel with Excellent Corrosion Resistance and Its Mechanical Properties

A project called STX-21 started in 1997 at National Research Institute for Metals (NRIM) for research and development (R&D) of ultra steels for the next century. One of the themes was ‘‘Development of steels for structures in marine and offshore environments.’’ The objective of this research is to develop a resourcesaving type austenitic stainless steel with highly corrosion-resistant properties in seawater. Two key techniques were adopted for this research. One is high nitrogenaddition, and the other purification of the material. In this study the first Japanese pressurized-electro-slag remelting (P-ESR) furnace was developed at NIMS to attain both high nitrogen-addition and purification of materials simultaneously. By using this facility, high nitrogen-bearing stainless steel of more than 1mass% was successfully fabricated without the addition of manganese. In the present study,

The effect of temperature on fatigue crack growth behaviour of a low alloy pressure vessel steel in a simulated BWR environment

Abstract Fatigue crack growth tests for a low alloy steel equivalent to ASTM A533 Grade B Class 1 were conducted under a simulated BWR environment in a temperature range from room temperature to 320°C. The frequency and the stress ratio were 1 cpm and 0.1, respectively. Two types of CT specimens with L-S and T-S orientations were employed. In situ monitoring crack length was continuously carried out by compliance method using LVDT. Complicated temperature dependences of fatigue crack growth rates were observed: the minimum growth rates were at 175°C, while those at 100°C showed the maximum enhancement by the environmental effect, which was approximately four times the maximum compared to the current air default line in ASME Code Section XI. Even the maximum enhancement of the crack growth rates at 100°C did not exceed the current water default lines. The discontinuities of the temperature dependences of fatigue crack growth rates under a simulated BWR condition were basically consistent with those under simulated PWR conditions.

The effect of VEGF-immobilized nickel-free high-nitrogen stainless steel on viability and proliferation of vascular endothelial cells.

Using ester bonds, vascular endothelial growth factor-A (VEGF-A) was immobilized on the surface of a novel biometal, nickel-free high-nitrogen stainless steel (HNS). The biological activity of immobilized VEGF-A was investigated after the culture of human umbilical vein endothelial cells (HUVECs) on the substrate. Immobilization of VEGF-A onto the HNS surface was performed using trisuccinimidyl citrate (TSC) as a linker. Firstly, UV irradiation was employed to amplify hydroxyl groups on the HNS surface. Next, the HNS was dipped into TSC/dimethyl sulfoxide solution at room temperature. From the results of water contact angle measurement and X-ray photoelectron spectroscopy (XPS) analysis, TSC was found to be immobilized on the HNS surface via ester bonds. Quantitative analysis demonstrated that immobilized VEGF-A remained even after immersion in culture medium for 7 days; however, it was gradually deimmobilized by hydrolysis of the ester bonds at the TSC-metal interface. As a result, VEGF-A-immobilized HNS significantly contributed to the stimulation of HUVEC growth for the initial stage of culture, even though the gradual reduction in growth stimulation of HUVECs occurred by the sequential deimmobilization of VEGF-A, which was caused by the hydrolysis of the ester groups. Therefore, VEGF-A-immobilized HNS could be applied as a basic material for coronary stents.

Weak Beam TEM Study on Stacking Fault Energy of High Nitrogen Steels

The nature of the high work-hardening rate of nitrogen bearing steels was examined focusing on the stacking fault energy (SFE). The dislocation configuration and the width of dissociated dislocations were evaluated in various kinds of austenitic stainless steels with and without nitrogen, using the weak beam method. Nitrogen addition resulted in changing the dislocation configuration from tangled to planar. Nitrogen was, however, found to increase the SFE rather than decrease as reported previously and the SFE can be formulated as a function of chemical composition, SFE(mJ/m2) = 5.53 - 0.16 (wt%Cr) + 1.40 (wt%Ni) + 17.10 (wt%%N). These results indicate that dislocation planarization by nitrogen addition is inadequately explained in terms of SFE.

UV irradiation enhances the bonding strength between citric acid-crosslinked gelatin and stainless steel.

The effect of ultraviolet ray (UV) irradiation on the bonding strength between low carbon stainless steel 316 (SUS316L) and trisuccinimidyl citrate (TSC)-crosslinked alkali-treated gelatin (AlGelatin-TSC) was investigated. The UV irradiation effectively generated hydroxyl groups on the surface of SUS316L. The bonding strength between AlGelatin-TSC and SUS316L before UV irradiation was 0.345±0.007 MPa, and upon UV irradiation it increased to 0.750±0.069 MPa. In order to explain this enhanced bonding strength, the surface of SUS316L was examined using its water contact angle and X-ray photoelectron spectroscopy. Furthermore, the N 1s peaks derived from the TSC succinimidyl group were assigned to the surface of SUS316L after the immobilization of the TSC. This indicates that ester bond formation between the TSC active esters and the SUS316L hydroxyl groups contributed to the enhanced bonding strength. Therefore, UV irradiation and subsequent TSC immobilization is a simple way to functionalize biometal surfaces with various structures. This has practical applications for medical devices such as drug-eluting stents, dental implants, and metallic artificial bone.

Poly-(L-lactic acid) and citric acid-crosslinked gelatin composite matrices as a drug-eluting stent coating material with endothelialization, antithrombogenic, and drug release properties†

Biodegradable composite matrices comprising poly-(L-lactic acid) (PLLA) and citric acid-crosslinked alkali-treated gelatin (AlGelatin) with endothelialization, antithrombogenic, and drug release properties were prepared. The characterization of composite matrices with various mixing ratios was performed by evaluating their swelling ratio, endothelial cell culture, antithrombogenic tests, and drug release behavior. Tamibarotene (Am80), which specifically inhibits smooth muscle cell proliferation, was employed as the drug. The swelling ratio of composite matrices decreased as the PLLA content decreased. The number of endothelial cells cultured on the surfaces of composite matrices was maximal at the PLLA/AlGelatin-TSC ratio of 80/20. Antithrombogenic tests revealed that the levels of platelets and fibrin network formation decreased as the AlGelatin-TSC content increased. The Am80 release test indicated that the release rate decreased as PLLA content increased. Using the resulting composite matrix, Am80-eluting stents possessing a smooth surface and a coating thickness of ∼15 μm were successfully obtained. Am80 was continuously released from the resulting stent at ∼40%, up to 28 days without burst release. Therefore, Am80-eluting stent with its antithrombogenic and endothelialization properties has great potential for clinical use.

Quantitative biocompatibility evaluation of nickel‐free high‐nitrogen stainless steel in vitro/in vivo

Coronary stents must not provoke an inflammatory response; however, some kinds of ions that are released from biometals induce biological reaction. In the present study, we quantitatively evaluated biological reaction of nickel-free high-nitrogen stainless steel (HNS) by endothelial cell culture, and a bioimaging system using NF-κB/luciferase transgenic mice to confirm the potential of HNS for the application of coronary stent. Endothelialization was greater with HNS than with commercial stainless steel (SUS316L). In vivo inflammatory response of HNS was lower than that of SUS316L. These differences may be related to the amounts of nickel ion eluted from the stents, as HNS did not elute nickel ion. These data suggest that HNS may be useful as a material for coronary artery stents.

Corrosion Fatigue Behavior of Low-Alloy Pressure Vessel Steels in High Temperature Water Under Multi-Factor Conditions

The corrosion fatigue resistance of pressure vessel steels in simulated light water reactor coolant water is greatly influenced by the mechanical factor such as strain rate, environmental factor such as temperature and dissolved oxygen (DO) concentration in water, and material factor such as sulfur content in steels. In most of previous work, the mechanical or environmental conditions were usually fixed as a constant throughout an individual test. However, these factors may change frequently during actual operations in power plants. So, it is of great interest to investigate the effects of change of the above factors on fatigue resistance of components materials in service environments and to develop appropriate methods for evaluating the environmental effects on fatigue damage. The present work was to investigate the low cycle fatigue (LCF) behavior of low-alloy pressure vessel steels in high temperature water. Special attention was paid on the influence of strain rate change in an individual LCF test on fatigue resistance of the steels. The alternate sequence of strain rate in a test was also considered. Moreover, the influence of DO, temperature and sulfur content in the steels was investigated. The detailed cracking and fractographic features were examined to assist the understanding of underlying corrosion fatigue mechanisms.