Ju-Tung Lee

Electrochemical behavior of the simulated heat-affected zone of A516 carbon steel in H2S solution

Electrochemical behavior of the simulated grain-coarsened heat-affected zone (HAZ) of A516 carbon steel in an H2S containing acidic chloride solution was investigated by electrochemical impedance spectroscopy (EIS) measurement at open circuit potential at 25 °C. EIS measurement was made for the specimens, prepared with three different heat input conditions (15, 30 and 45 kJ.cm−1) by thermal simulation with a Gleeble machine, in the H2S containing solution. The results show that the corrosion process of the simulated grain-coarsened HAZ can be divided into two stages regardless of the heat input condition. At an early stage, the impedance which is characterized by one large semicircle in the Nyquist plot increases with immersion time. At a later stage, the precipitation of sulfide film modifies the surface properties and leads to a variation in the EIS data by revealing a capacitive effect in the low frequency region. The polarization resistance initially increases with immersion time but then decreases gradually. Equivalent circuits containing two or three time constants are proposed to represent the corrosion processes at different stages.

The electrochemical and corrosion behavior of austenitic stainless steel containing Cu

Abstract The influence of Cu additions (0.4-3.0 wt%) on the electrochemical and corrosion behavior of 18-8 austenitic stainless steel was investigated. Both potentiodynamic polarization curves and potential decay curves, which were dependent upon the Cu content and the dissolved oxygen in the solution tested, were determined in both de-aerated and aerated 0.1 M H 2 SO 4 solution. Surface characterization was examined by Auger electron spectroscopy (AES) and X-ray photo-electron spectroscopy (XPS). Cyclic polarization curves determined in 3.5 wt% NaCl solution showed that the pitting protection potential (E pp ) of the stainless steels tested increases with increasing Cu content. The constant extension rate test (CERT) in boiling 42 wt% MgCl 2 solution revealed that Cu had a detrimental effect on the stress corrosion cracking (SCC) resistance. The role of Cu on the corrosion behavior of the austenitic stainless steel observed in this investigation is discussed.

The effect of heat treatment and applied potential on the stress corrosion cracking of alloy 600 in thiosulfate solution

Abstract The effects of heat treatment and applied potential on the stress corrosion cracking (SCC) behavior of Alloy 600 in thiosulfate solution were investigated. Electrochemical Potentiokinetic Reactivation (EPR) tests and modified Huey tests showed that a thermal stabilization treatment (990°C/5 min + 705°C/15 h) could climinate the sensitization of Alloy 600 and result in a lowering of the degree of sensitization as compared with other heat treatments. Electrochemical polarization curves showed that Alloy 600 could passivate in thiosulfate solution and exhibited two anodic peaks located around −500 and +100 mV (SCE), respectively, independent of the heat treatment. The polarization curves also showed that Cr played an important role in the electrochemical behavior of Alloy 600 in thiosulfate solution. Under applied potential conditions, intergranular stress corrosion cracking (IGSCC) was observed in sensitized and stabilized Alloy 600 in 0.01 M thiosulfate solution at 95°C. The severity of IGSCC was less after the stabilization heat treatment than after the sensitization heat treatment. In these conditions the susceptibility of IGSCC was strongly affected by the applied potential. In general, the crack initiation time decreased with increasing applied potential in the anodic potential region.

The influences of microstructure and composition on the electrochemical behavior of a516 steel weldment

Abstract ASTM-A516 (Grade 70) low carbon steel weldments have been characterized in terms of the effect of metallurgical stimuli on electrochemical behavior. Potentiodynamic polarization measurement and electrochemical impedance spectroscopy (EIS) were made for the specimens, prepared with three different welding heat input conditions (15, 30 and 45 kJ cm −1 ) either by submerged arc welding or by weld thermal simulation with a Gleeble machine, in NACE solution (5 wt% NaCl + 0.5 wt% CH 3 COOH) of pH 2.8 at 25°C. The polarization resistances of the base metal, thermal simulated heat-affected zone and weld metal obtained using linear polarization method and EIS were compared. The results show that weld metal is distinguished in having the greatest polarization resistance. Furthermore, the polarization resistances of the weld metal and simulated grain-coarsened HAZ increased with increasing welding heat input. The electrochemical behavior of the weld metal is dependent upon the volume fraction ratio of grain boundary ferrite (GF) to acicular ferrite (AF), while that of the thermal simulated grain-coarsened HAZ is associated with the coarsening of Widmanstatten ferrite (WF).

Cracking characteristics of A516 steel weldment in H2S containing environments

Abstract Sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC) are two well-known phenomena that take place in steels situated in H 2 S containing environments. This investigation presents the cracking characteristics of A516 pressure vessel steel weldments prepared by submerged arc welding, under various welding heat inputs (15, 30 and 45 kJ cm −1 ), following immersion in NACE TM-01-77 solution (5 wt.% NaCl + 0.5 wt.% CH 3 COOH) saturated with H 2 S gas. The results of slow strain rate testing, performed at a strain rate of 5 × 10 −7 s −1 , reveal the presence of SSC, HIC, and quasi-cleavage fracture in the base metal. Metallographic examination of the fractured specimens reveals that HIC initiates and/or propagates in both the ferrite and pearlite phases and along the ferrite/pearlite interface in the base metal. An increase in the welding heat input causes increases in the amount of grain boundary ferrite and Widmanstatten ferrite in the weld metal and heat-affected zone (HAZ), respectively, and subsequently results in decreases in the cracking resistances of the weld metal and HAZ n H 2 S containing environments.

Effect of nitrogen on the electrochemical behavior of 301LN stainless steel in H2SO4 solutions

The effect of nitrogen on the electrochemical behavior of 301 stainless steel in sulfuric acid solutions was investigated. The results of electrochemical potentiokinetic reactivation tests show that the degree of sensitization of 301LN stainless steel is less than that of 301 stainless steel. The alloying of nitrogen into 301 stainless steel causes a decrease in the passive current density and an increase in the polarization resistance in 10% sulfuric acid solution. Electrochemical impedance spectroscopy data also indicate that the polarization resistance increases with increasing nitrogen content in the steel at open circuit potential and at + 400 mV versus a saturated calomel electrode. Potential decay measurements show that the passive film formed on 301LN stainless steel is more stable than that on 301 stainless steel. The results of X-ray photoelectron spectroscopy reveal that NH3 formed on the surface of the nitrogen containing 301 stainless steel after immersing in sulfuric acid at +400 mV for2 h.

Microstructure and electrochemical behavior of laser treated FeCr and FeCrSiN surface alloyed layers on carbon steel

Abstract Laser surface alloyed FeCr and FeCrSiN layers on carbon steel were produced by laser irradiation involving preplaced Fe/Cr and Fe/Cr/Si 3 N 4 powders, respectively. The effects of Cr content (13–25 wt.%) and microstructure on the corrosion behavior of the laser treated FeCr and FeCrSiN alloys in deaerated neutral 3.5 wt.% NaCl solution were studied. Potentiodynamic polarization tests and electrochemical impedance spectroscopy measurements were used to evaluate the corrosion resistance of the surface layers. The results showed that the FeCrSiN treated layer with a fine duplex phase microstructure had a higher pitting potential than the FeCr treated layers. The passive film resistances increased and the passive current density decreased with increasing Cr content in the FeCr layer.

Microstructure and electrochemical behavior of laser cladded Fe-Cr-Mo-Si-N surface alloys on carbon steel

Abstract Laser surface cladding was used in this study to introduce Fe-Cr-Mo, Fe-Cr-Si-N and Fe-Cr-Mo-Si-N stainless steel layers on carbon steel to improve its corrosion resistance. The cladding materials used were Fe/Cr/Mo, Fe/Cr/Si3N4 and Fe/Cr/Mo/Si3N4 powders which were premixed before laser irradiation. The chemical composition, the microstructure and the electrochemical behavior of the cladded layers were of particular interest and were analyzed. Cyclic potentiodynamic polarization tests and electrochemical impedance spectroscopy measurements were performed to evaluate the corrosion resistance of the cladded layers in a deaerated 3.5 wt.% NaCl solution at pH 4. Experimental results indicated that the Fe-Cr-Mo cladded layers exhibited a lower passive potential range with a higher anodic passive current density, while the Fe-Cr-Si-N and Fe-Cr-Mo-Si-N cladded layers both with a duplex microstructure evidenced a higher passive potential range with a lower anodic passive current density in Cl−-containing solution, as demonstrated by the cyclic potentiodynamic polarization test. The polarization resistance of the laser cladded Fe-Cr-Mo-Si-N layer was higher than that of Fe-Cr-Si-N as determined by electrochemical impedance spectroscopy.

Improvement of electrical and reliability properties of tantalum pentoxide by high-density plasma (HDP) annealing in N 2 O

This study aims to improve the electrical characteristics and reliability of low-pressure chemical vapor deposited (LPCVD) tantalum pentoxide (Ta/sub 2/O/sub 5/) films by a new post-deposition annealing technique using high-density plasma (HDP). Experimental results indicate that excited oxygen atoms generated by N/sub 2/O decomposition from HDP annealing can effectively reduce the carbon and hydrogen impurity concentrations and repair the oxygen vacancies in the as-deposited CVD Ta/sub 2/O/sub 5/ film, thereby resulting in a remarkable reduction of the films leakage current. Two other post-deposition annealing conditions are compared: HDP O/sub 2/ annealing and conventional plasma O/sub 2/ annealing. The comparison reveals that HDP N/sub 2/O annealing has the lowest leakage current and superior time-dependent dielectric breakdown (TDDB) reliability.

Surface modification of carbon steel with laser treated nitrogen-containing stainless steel layers

Abstract Laser surface alloying was used in this study to introduce nitrogen-containing stainless steel layers on carbon steel to improve its corrosion resistance. The alloying materials used were FeCrNiSi 3 N 4 and FeCrMnSi 3 N 4 powders which were pre-mixed before laser irradiation. The chemical composition, the microstructure and the electrochemical behavior of the alloyed layers were of particular interest and analyzed. Potentiodynamic polarization tests were performed to evaluate the corrosion resistance of the laser treated alloys in deaerated 3.5 wt.% NaCl solution at pH 4. Two austenitic stainless steels, namely 316L and P900N, were tested for comparison. Experimental results indicated that nitrogen contents in the laser treated FeCrNiSiN and FeCrMnSiN alloys could reach 0.38 and 0.83 wt.% respectively. The FeCrNiSiN layer exhibited higher resistance to pitting corrosion in Cl − containing solution as compared with those of 316L and P900N stainless steels. However, the pitting corrosion resistance of FeCrMnSiN layer was inferior to those of the 316L and P900N stainless steels.