Loop Electrochemical Potentiokinetic Reactivation Method Research Articles

Influencing mechanisms of C content and aging treatment on intergranular corrosion sensitivity of 316 stainless steel weld metals

For analyzing the intergranular corrosion sensitivity of 316 stainless steel weld metals with different C contents, the intergranular corrosion behaviors of the weld metals under as-welded and different as-aged states were evaluated by both the H2SO4–CuSO4 solution corrosion method and the double loop electrochemical potentiokinetic reactivation method, respectively. The results indicate that after long term aging treatment at 550 °C, the δ ferrite in high C weld metal has a higher transformation fraction than that in low C weld metal, which results in a higher intergranular corrosion sensitivity for high C weld metal. However, after the long term aging treatment at 600 °C, the δ ferrite in high C weld metal transforms completely in the early stage aging, and with increasing of the aging time, the desensitization process occurs in the Cr-poor region of high C weld metal, which improves the intergranular corrosion resistance, and results in a higher intergranular corrosion sensitivity for low C weld metal. Therefore, under the service environment of considering the intergranular corrosion performance of the weld metal, the welding material with the appropriate C content should be selected according to the service temperature.

Influence of precipitation on the intergranular corrosion sensitivity of aged Alloy 31 using a modified electrochemical potentiokinetic reactivation method

The influence of precipitation on the intergranular corrosion sensitivity of Alloy 31 aged at 950 ℃ for 0–96 h was investigated by microstructure analysis and the modified double loop electrochemical potentiokinetic reactivation method. The results reveal that the kinetics of the primary precipitated sigma phase agree with the Yamamoto equation. The degree of sensitization increases rapidly to 4 h, then decreases to 8 h with self-healing, but no further self-healing occurs after that. The degree of sensitisation depends on the Cr and Mo-depletion zone neighbouring the precipitated sigma phase.

Electrochemical investigation of in-service thermal aging in two CF8M cast stainless steels

In-service thermal aging of CF8M cast austenitic stainless steel was investigated in materials removed from the steam generator inlet and crossover elbows of the Ringhals 2 pressurized water reactor nuclear power plant unit after approximately 92kh of full operating time. The thermal aging of these materials was investigated using the double loop electrochemical potentiokinetic reactivation method, coupled with indentation hardness measurements and microstructural characterizations, to identify correlations between the electrochemical behavior and traditional methods of investigating thermal aging embrittlement effects in cast stainless steels. While this electrochemical method can be easily employed to quantify thermal aging effects in materials aged at higher temperatures (e.g. greater than 350∘C), this study highlights the difficulties encountered when electrochemically evaluating the aging of materials exposed to nuclear power plant operating conditions.

The Mechanism on the B Addition to Regulate Phase Precipitation and Improve Intergranular Corrosion Resistance in UNS S31254 Superaustenitic Stainless Steels

The effects of boron on the microstructure and intergranular corrosion behavior of UNS S31254 (S31254) austenitic stainless steels were investigated. The sensitivity to intergranular corrosion in the samples was determined by an altered double loop electrochemical potentiokinetic reactivation method. The microstructure characterization showed that the S31254 steels only precipitated sigma (σ, Fe-Cr-Mo-Ni) phases during the aging process, but boron addition decreased the amount of σ phase precipitation, reduced the size of the σ phases, and promoted the formation of small Laves phases at the grain boundaries. The precipitated σ phases lead to surrounding zones depleted of Cr and Mo, leading to intergranular corrosion. The S31254 alloy with added boron slows down the intergranular corrosion, due to microstructure manipulation at the grain boundaries.

Investigation on ultra-pure ferritic stainless steel 436L susceptibility to intergranular corrosion using optimised double loop electrochemical potentiokinetic reactivation method

ABSTRACTThe susceptibility of 17Cr ferrite stainless steel to intergranular corrosion (IGC) was investigated using the double loop electrochemical potentiokinetic reactivation (DL-EPR) test and the microstructural characterisation. The results show that the optimised DL-EPR test condition for 17Cr ferrite stainless steel is l M H2SO4 + 0.006 M KSCN solution with a scan rate of 0.1 V min−1 at 25°C The severe IGC occurs in the temperature range of 500–650°C and the nose temperature locates at approximately 600°C. The Ir/Ia value rises up to 16.99% when heat treated at 600°C for 30 min. After aging treatment, M23C6 precipitates are detected using transmission electron microscopy, resulting in the presence of the Cr depletion zone. However, the degree of IGC for 436L is extremely limited due to the Cr element diffusion form the matrix despite prolonging the aging time to 5 h.

Optimization of the Double Loop Electrochemical Potentiokinetic Reactivation Method for Detecting Sensitization of Nickel Alloy 690

Nickel alloy 690 (UNS N06690) is one of the current choices for nuclear power plant steam generator tubing. The severity of certain stress corrosion cracking submodes in Ni-Cr-Fe alloys, such as Alloy 690, may be a function of the alloy sensitization condition. Sensitization results from chromium carbide precipitation at or near grain boundaries when the alloy is exposed to temperatures from 500°C to 800°C, which causes local chromium depletion in the adjacent matrix. The objective of this work was to study the parameters that control the double loop electrochemical potentiokinetic reactivation (DL-EPR) method and to develop an optimized test routine for Alloy 690. Alloy 690 specimens were tested under different thermal treatments, tailored to obtain a variety of concentrations of chromium in solid solution near the grain boundary (GB) region. The DL-EPR method was applied to Alloy 690 in solutions with different concentrations of sulfuric acid and potassium thiocyanate, at 30°C and 50°C. The optimal condition for detecting sensitization of Alloy 690 was 0.5 M H2SO4 + 0.001 M KSCN, at 30°C. Optical micrographs confirmed an intergranular type of attack under thermally aged (sensitized) conditions. The ratio of the reactivation to the activation peak intensities (Ir/Ia) obtained in the DL-EPR test agreed well with mass loss obtained in boiling nitric acid solution with Cr(VI) additions (modified Huey test). Chromium depletion profiles near the GB of thermally treated specimens under different conditions were measured with the energy dispersive x-ray spectroscopy technique, performed on a transmission electron microscope. A model available in the literature was fitted to those results. The Ir/Ia ratio for Alloy 690, obtained when using the optimal DL-EPR testing conditions, correlated well with the Cr depletion zone width and depth near the GB.

Aging Degradation of Austenitic Stainless Steel Weld Probed by Electrochemical Method and Impact Toughness Evaluation

The present study discriminates the spinodal decomposition and G-phase precipitation in stainless steel welds by double loop electrochemical potentio-kinetic reactivation method and correlates it with the degradation in toughness property. The welds produced with different heat inputs were aged up to 10,000 hours at 673 K to 723 K (400 to 450 °C) and evaluated subsequently for the degree of sensitization (DOS) and impact toughness. The DOS values obtained were attributed to the spinodal decomposition and precipitation of G-phase. Study shows that the DOS correlates well with the impact toughness of the 304LN weld. Prolonged aging at 673 K and 723 K (400 °C and 450 °C) increased the DOS values while the impact toughness was decreased. The weld fabricated at 1 kJ/mm of heat input, produced higher DOS, compared to that at 3 kJ/mm. The geometrical location along the weld is shown to influence the DOS; higher values were obtained at the root than at the topside of the weld. Vermicular and columnar microstructure, in addition to the spinodal decomposition and G-phase precipitation, observed in the root side of the weld appear risky for the impact toughness.

Detection of susceptibility to intergranular corrosion of aged super austenitic stainless steel S32654 by a modified electrochemical potentiokinetic reactivation method

The double loop electrochemical potentiokinetic reactivation method was modified and applied to detect the susceptibility to intergranular corrosion of super austenitic stainless steel S32654 aged at 1000 °C. The precipitation behavior was clarified, and the corresponding intergranular corrosion mechanism of aged S32654 was proposed. The microstructure demonstrated that σ phase, Cr2N and π phase nucleated at the initial stages of aging and gradually grew larger with increasing aging time. The degree of sensitization increased rapidly in the early stage of aging and then showed a slow increasing rate during long term aging stage, and no self-healing has been observed. Such modified double loop electrochemical potentiokinetic reactivation method could successfully characterize the relationship between microstructure evolution and intergranular corrosion susceptibility of S32654. The mechanism of intergranular corrosion was attributed to the Cr and Mo depletion adjacent to σ phase and π phase and the Cr depletion around Cr2N.

Evaluation of aged duplex stainless steel UNS S32750 susceptibility to intergranular corrosion by optimized double loop electrochemical potentiokinetic reactivation method

Optimized double loop electrochemical potentiokinetic reactivation (DL-EPR) in combination with microstructure observation were successfully applied to study the sensitization of super duplex stainless steel UNS S32750 aged at 900°C for different duration to intergranular corrosion (IGC). The results indicated that the degree of sensitization increased gradually with the aging time increased from 10min to 4h due to the sigma phase precipitation, and Ir:Ia value reached the maximum value of 17.1%. However, further increasing aging time slightly decreased the sensitization due to the healing effect incurred by the diffusion of chromium from adjacent grains to chromium-depleted zone.

Evaluation of intergranular corrosion susceptibility of UNS S31803 duplex stainless steel with an optimized double loop electrochemical potentiokinetic reactivation method

Abstract In this study, an optimized double loop electrochemical potentiokinetic reactivation (DL-EPR) test was applied to evaluate the degree of intergranular corrosion (IGC) susceptibility of duplex stainless steel UNS S31803. The measurement in a solution of 2 M H 2 SO 4 + 1 M HCl at 30 °C with a scan rate of 1.66 mV s −1 can successfully be used to analyze the interactions between precipitation, chromium depletion, and IGC sensitization of the UNS S31803 steel, which was aged between 30 min and 48 h at 800 °C. To gain an insight into the phase transition, the specimens were analyzed by scanning electron microscopy (SEM) and electron dispersive X-ray spectroscopy (EDX). The results indicated healing due to re-diffusion of Cr and Mo from the σ phase to the γ 2 phase.

Influence of cold rolling on sensitization and intergranular stress corrosion cracking of AISI 304 aged at 500 °C

Sensitization and intergranular stress corrosion cracking (IGSCC) behavior of deformed AISI 304 prior to aging at temperature 500 °C were investigated. Sensitization was evaluated by double loop electrochemical potentio-kinetic reactivation method and IGSCC studies were carried out using slow strain rate tests in polythionic acid (PTA). The degree of sensitization (DOS) obtained for undeformed specimens was very low as compared to that of deformed prior to sensitization. The undeformed, sensitized AISI 304 was resistant to IGSCC in PTA solution. The specimens deformed by 20 and 40% reduction in thickness were undergone IGSCC (brittle cracking). The deformation >40% prevented the IGSCC, even though the % DOS was significantly high.