Single-loop Electrochemical Potentiokinetic Reactivation Research Articles

Correlation between microstructure and corrosion behavior in an AISI 316L steel pipeline exposed for 100,700 h at 640°C in a petrochemical plant

AbstractThe microstructure and electrochemical properties of 316L stainless steel were investigated after two conditions: aged at 640°C for 100,700 h and solution annealed at 1050°C for 2 h. While the aged samples were obtained from a pipe of a petrochemical reactor plant that was in service, the solution annealing was carried out in a conventional laboratory furnace. After aging, the precipitates present in decreasing order of quantity were sigma, Laves phase, and M23C6. After solution annealing, the microstructure was full austenitic. These results were in agreement with equilibria phase simulation with Thermo‐Calc software. Intergranular corrosion susceptibility, evaluated by means of the single loop electrochemical potentiokinetic reactivation technique and Practice A of ASTM 262, indicated a preponderant role for the sigma precipitate. The pitting potential (Epit) was evaluated through potentiodynamic polarization curves in 0.6 M NaCl and electrochemical impedance spectroscopy was performed at the corrosion potential to complement the information about the corrosion resistance.

Contradictory Results from Single Loop Electrochemical Potentiokinetic Reactivation Test and Oxalic Acid Test for Intergranular Corrosion in 304L Stainless Steels Attributed to Si Grain-Boundary Segregation

The intergranular corrosion susceptibility of AISI 304L stainless steel can be measured in the field using the single loop electrochemical potentiokinetic reactivation test. However, the present work questions the reliability of the test, with nominally the same as-received samples given a wide range of degrees of sensitization, enough for opposite conclusions and potentially false positive results. More worryingly, when comparing the results to the oxalic acid etch test, a false negative result of the SL-EPR test was obtained on some heat-treated samples. This could entail serious repercussions and justifies a thorough investigation into this issue. The most likely explanation for the unexpected behavior of one batch of 304L, is its moderate silicon enrichment at 0.4 wt%. As long as the stainless steel does not exceed the passive region, as is the case for the SL-EPR test, the moderate silicon additions help suppress intergranular corrosion, but in the strongly oxidizing environment of the oxalic acid test the stainless steel becomes transpassive and the silicon results in accelerated corrosion of the grain boundaries.

Corrosion Characteristics of Laser-Engineered Net Shaping Additively-Manufactured 316L Stainless Steel

The single-loop electrochemical potentiokinetic reactivation (EPR) test was used to quantify the degree of sensitization for 316L stainless steel (SS) produced by laser-engineered net shaping (LENS®) additive manufacturing (AM). EPR tests were also used to monitor how the sensitivity of the as-deposited and heat-treated AM material compared to wrought 316L SS. A solution-annealing heat treatment that recrystallized the material was also applied to AM-316L SS. The EPR tests showed that as-deposited AM-316L SS displayed low levels of sensitization. Heat-treated AM-316L SS demonstrated some improvements in resistance to corrosion, but overall did not perform as well as wrought 316L material. Optical micrographs of the AM-316L SS samples collected after conducting the EPR tests also showed localization in the as-deposited AM-316L SS material; however, these features were reduced after heat treatment, suggesting greater resistance to corrosion.

Single Loop Electrochemical Potentiokinetic Reactivation Behaviour of Continuously Cooled SUS329J4L Duplex Stainless Steel

Sigma phase precipitates in duplex stainless steel during slow cooling from high temperature, which generally induces great loss in both corrosion resistance and impact toughness. To develop a nondestructive method for detecting the presence of sigma phase in duplex stainless steel for on-site use in the quality management of ready-made large machines, the single loop electrochemical potentiokinetic reactivation (SL-EPR) behaviour of SUS329J4L duplex stainless steel continuously cooled from the solution annealing temperature with various rates by adjusting fluxes of cool nitrogen gas were investigated under several polarization conditions. The reactivation current density of SL-EPR test was used to detect the precipitated σ phase and compared with the ferric chloride test, the modified DL-EPR test and the impact test, as well as observation of the dissolution morphology. As the result, the high reactivation current density in the SL-EPR polarization was obtained on the cooled SUS329J4L steel at WQ ~ 0.02 K/s in solution of 0.5 M H2SO4 + 0.01 M KSCN + 0.5 M NaCl at potential sweep rate of 0.83mV/s. In the polarization, the potential holding at 0.3 V vs. SCE for 300 s before sweeping the potential to negative side is necessary to stabilize the passive film and thus improved the detection sensitivity of sigma phase in duplex stainless steel. As comparison, the modified SL-EPR test which checks the presence of sigma phase has a quasi-equivalent detection sensitivity to impact test on the WQ ~ 0.02 K/s cooled SUS329J4L steel, with higher sensitivity in SL-EPR method than the modified DL-EPR method.

Low temperature thermal ageing embrittlement of austenitic stainless steel welds and its electrochemical assessment

The low temperature thermal ageing embrittlement of austenitic stainless steel welds is investigated after ageing up to 20,000 h at 335, 365 and 400 °C. Spinodal decomposition and G-phase precipitation after thermal ageing were identified by transmission electron microscopy. Ageing led to increase in hardness of the ferrite phase while there was no change in the hardness of austenite. The degree of embrittlement was evaluated by non-destructive methods, e.g., double-loop and single-loop electrochemical potentiokinetic reactivation tests. A good correlation was obtained between the electrochemical properties and hardening of the ferrite phase of the aged materials.

Analysis of sensitization phenomenon in friction stir welded 304 stainless steel

This study evaluates the degree of sensitization (DOS) of 304 stainless steel joined by friction stir welding (FSW). Single-loop electrochemical potentiokinetic reactivation tests were performed using a 0.5 mol/L H2SO4 + 0.01 mol/L KSCN solution. Sensitization was promoted by exposition of the stainless steel at temperatures between 400°C and 850°C. The microstructure was characterized using optical microscopy to identify the weld zone and the base metal. The samples treated at 550°C showed the most severe intergranular corrosion. The DOS was lower in the weld zone than in the base metal after heat treatments. This reduction in the DOS for the weld zone indicates that FSW is a beneficial process in joining stainless steel.

Effect of KSCN and its concentration on the reactivation behavior of sensitized alloy 600 in sulfuric acid solution

The effects of KSCN addition and its concentration on the reactivation behavior of sensitized Alloy 600 in an H 2SO 4 solution were investigated. Single-loop electrochemical potentiokinetic reactivation (SL-EPR), potentiostatic and potential decay tests were conducted in H 2SO 4+KSCN solutions, including intermittent adjustment of the electrolyte composition and concentration. The SL-EPR results showed that the maximum current densities of the reactivation peaks increased with an increase in KSCN concentration from 0 to 0.05 M in the 0.5 M H 2SO 4 solutions. A decrease in the peak current density was observed, however, if the KSCN concentration was increased to 0.5 M. The potentiostatic test results at +400 mV SCE showed that sensitized Alloy 600 could be passivated in plain H 2SO 4 and KSCN solutions as well as in mixed H 2SO 4+KSCN solutions. Potentiostatic tests at +30 mV SCE revealed that the presence of low KSCN concentrations made the passive film less stable in H 2SO 4 solutions, but it enhanced passivation at 0.5 M. The adsorption and desorption of SCN − and its subsequent redox reaction at different concentrations was discussed in relation to the reactivation of passive film formed on sensitized Alloy 600.

Development of a single loop EPR test method and its relation to grain boundary microchemistry for alloy 600

A single loop electrochemical potentiokinetic reactivation test method has been developed for alloy 600 that produces good passivation on all the surfaces, good etching during the reactivation scan and no appreciable pitting. It is able to quantify and discriminate between samples with a wide range of degree of sensitization. The Pa value correlates well with the minimum level of chromium in the depletion regions at the grain boundaries. It has been shown that the width of the attacked regions is much larger than the width of chromium depletion regions and it does not show any direct correlation with either depth or width or with a volume parameter of chromium depletion regions. It has been shown that the chromium carbides are not attacked during the test and that the intragranular regions attacked during the test are the sites of chromium carbides in the grain matrix. A modified Pa parameter is shown to be sensitive down to 7.5 wt% chromium in the depletion regions and indicates that the intragranular carbides have shallower depletion profiles than those at grain boundaries. Comparison of the results of the single loop and the double loop tests showed a good correlation.

Improvement of Intergranular Corrosion Resistance of Type 316 Stainless Steel by Laser Surface Melting

Laser surface melting (LSM) on sensitised austenitic stainless steel with various degrees of cold work (5, 10, 15, 20, and 25%) was carried out by using a continuous wave CO2 laser with a maximum power 450 W. The stainless steel was sensitised at 898 K for 10, 20, and 50 h durations. The ASTM A262 Practice A test was used to examine the microstructures of the specimens before and after LSM, while the single loop electrochemical potentiokinetic reactivation (SL-EPR) technique was used to quantify the degree of sensitisation (DOS). A trend is noted between the level of cold work in the stainless steel and the degree of desensitisation by LSM and it is inferred that laser surface melting can be effectively used as a method to improve the intergranular corrosion resistance of austenitic stainless steel.

Effect of electrolyte composition on the electrochemical potentiokinetic reactivation behavior of Alloy 600

The effects of electrolyte composition and concentration on the reactivation behavior of Alloy 600 in H 2SO 4+KSCN solution were investigated by conducting single-loop electrochemical potentiokinetic reactivation (SL-EPR) tests. The experimental results revealed that two anodic peaks existed, which corresponded to pitting corrosion and matrix corrosion, respectively, in the reactivation polarization curve of non-sensitized Alloy 600. Alternately, for sensitized Alloy 600, an additional peak associated with grain boundary corrosion appeared in the potential range of +60 to −10 mV SCE within the reactivation polarization curve. The shape and current density of the reactivation curve of Alloy 600 with different degrees of sensitization varied with the electrolyte composition adjustments. A maximum peak anodic current density associated with grain boundary corrosion occurred when the concentration ratio of H 2SO 4/KSCN was 10 for the sensitized low carbon Alloy 600.

The electrochemical potentiokinetic reactivation behavior of Alloy 600

Abstract The characteristics of reactivation polarization behavior of Alloy 600 in 0.5 M H 2 SO 4 +0.005 M KSCN solution were investigated. The feasibility of single-loop electrochemical potentiokinetic reactivation (SL-EPR) test technique for the evaluation of the degree of sensitization (DOS) was also explored. The experimental results showed that both sensitized and unsensitized materials exhibited reactivation curves with two separated anodic peaks. The results of reactivation termination tests showed that the anodic peak which appeared at −50 mV (SCE) was associated with the depassivation of grain matrix, while that at +100 mV (SCE) was corresponding to the grain boundary corrosion for sensitized Alloy 600. The anodic peak at more noble potential in the EPR curve might also reflect the occurrence of pitting corrosion of Alloy 600 in the testing electrolyte.

Desensitization of type 316 stainless steel by laser surface melting

Laser surface melting (LSM) on sensitized austenitic stainless steel of cold‐work 25 per cent was carried out by using a continuous wave CO2 laser of power 5kW and traverse speed 20mm/sec of the beam. The stainless steel was sensitized at 898K for 10, 20 and 50hr durations. ASTM A262 Practice A test was used to examine the microstructures of the specimens before and after LSM while single loop electrochemical potentiokinetic reactivation (SL‐EPR) technique was used to quantify the degree of sensitization (DOS). A trend is noted between the cold‐work of stainless steel and the degree of desensitization by LSM.

Sensitization of High-Nitrogen Austenitic Stainless Steels by Dichromium Nitride Precipitation

Abstract High-nitrogen (N) stainless steels (SS) are receiving increased attention because of their strength advantages over carbon (C)-alloyed materials, but they have been found susceptible to dichromium nitride (Cr2N) precipitation during thermal exposure between ∼ 600°C and 1,050°C. Sensitization susceptibility of a high-N, low-C austenitic SS by Cr2N precipitation at 700°C and 900°C was determined using the single-loop electrochemical potentiokinetic reactivation (EPR) test. High-N SS was found susceptible to sensitization caused by grain boundary (GB) precipitation of Cr2N, with the degree of sensitization increasing systematically with aging time at 700°C. Sensitization of high-N materials did not require the concomitant precipitation of chromium (Cr)-rich metal carbide (M23C6). Materials aged at 900°C were not sensitized, although the rate of precipitation was greater than at 700°C. This indicated the minimum Cr level in the Cr-depleted zone of the matrix associated with nitride precipitation at 9...

Distributions of Cr Depletion Levels in Sensitized AISI 304 Stainless Steel and Its Implications Concerning Intergranular Corrosion Phenomena

An electrochemical method has been employed to obtain the population distribution of minimum grain boundary Cr concentrations for sensitized AISI 304 stainless steel (S30400). The detailed information about the extent of Cr depletion provided by the distribution is compared to the average degree of sensitization (DOS) evaluated by the standard and a modified single loop‐electrochemical potentiokinetic reactivation (SL‐EPR) technique. The distribution of Cr levels on a grain by grain basis is shown to provide information about intergranular stress corrosion cracking (IGSCC) susceptibility that cannot be provided by EPR methods. Sensitized 304SS tested in an environment relevant to nuclear reactors has been shown to fail by IGSCC when the grain boundary Cr concentration is depleted to below a critical level of ≈13.5%. In a separate study, it has been shown that more than 23% of the grain boundaries must be depleted to the relevant critical level in order to observe macroscopically brittle behavior. The present study combines these two criteria and defines a sensitized material's IGSCC susceptibility by revealing the extent of Cr depletion as well as the quantity of depleted grain boundaries.

Electrochemical evaluation of sensitization in austenitic stainless steels using miniaturized specimens

An electrochemical testing system was developed to evaluate the sensitization of neutron-irradiated austenitic stainless steels using miniaturized disk-type specimens, 3 mm in diameter and 0.25 mm thick. The system consists of a specimen holder in which a miniaturized specimen is mounted as the working electrode, a test cell designed to handle radioactive materials and waste, a computer-controlled potentiostat/galvanostat and a surface preparation equipment. Sensitization of a thermally-aged Ti-modified austenitic stainless steel was successfully detected by the single-loop electrochemical potentiokinetic reactivation (SL-EPR) method.

Evaluation of Radiation-Induced Sensitization of Austenitic Stainless Steels Using Electrochemical Potentiokinetic Reactivation Technique

Abstract The electrochemical potentiokinetic reactivation (EPR) test technique was applied to the determination of radiation-induced sensitization in a neutron-irradiated (420°C, 9 displacements per atom [dpa]) titanium-modified, austenitic stainless steel. Miniaturized specimens (3 mm diam by 0.25 mm thick) in both the solution-annealed and 25% cold-worked conditions were irradiated and tested by the single-loop EPR test technique. The degree of sensitization (DOS) was calculated in terms of the reactivation charge (Pa). The Pa value was increased by the irradiation as compared to control specimens thermally aged at the irradiation temperature. Grain boundary etching was observed on the specimen surface after EPR testing. Subsequent x-ray microanalysis by analytical electron microscopy (AEM) revealed that chromium depletion at grain boundaries had occurred. These results suggest the occurrence of radiation-induced sensitization associated with chromium depletion at grain boundaries. However, the post-EPR...