Electrochemical Potentiokinetic Reactivation Research Articles

Use of DL-EPR Test to Assess Sensitization Resistance of AISI 409M Grade Ferritic Stainless Steel Joints

The susceptibility of 409M grade ferritic stainless steels to sensitization due to welding was investigated. Joints were fabricated by gas tungsten arc welding, friction stir welding, laser beam welding, and electron beam welding processes. Double loop electrochemical potentiokinetic reactivation test was carried out for determining the degree of sensitization of welded joints. The experimental result reveals that, the friction stir welded joint is less prone to sensitization, when compared to the other joints.

Methods for making alloy 600 resistant to sensitization and intergranular corrosion

Thermomechanical processing (TMP) and strain annealing were performed on alloy 600. The degree of sensitization (DOS) and susceptibility to intergranular corrosion (IGC) were assessed by electrochemical potentiokinetic reactivation and weight loss tests respectively. The grain boundary character distribution was established by orientation imaging microscopy. It was shown that as the fraction of special boundaries increased with TMP/strain annealing, the DOS and IGC rates decreased drastically. Alloy 600 with fraction of special boundaries above 85% showed only uniform corrosion. The DOS and IGC values indicating resistance to sensitization and IGC and the methods to achieve such values have been highlighted.

Evaluation of the Susceptibility to Intergranular Corrosion for a Novel Cr-Mn-N Austenitic Stainless Steel Using DL-EPR

The sensitization and intergranular corrosion (IGC) behavior of a novel Cr-Mn-N austenitic stainless steel (named as MPMD133) sensitized in the temperature range from 600 oC to 1000oC for 3 hours was investigated by the double loop electrochemical potentiokinetic reactivation (DL-EPR) test. The effects of the sensitization temperature on the susceptibility to IGC of MPMD133 were examined in the 0.5M H2SO4 +0.01M KSCN electrolyte solution. The results show that the degree of IGC increases as the sensitization temperature increasing till 900oC. Meanwhile it decreases rapidly when the sensitization temperature is elevated up to 1000oC, which is confirmed by the subsequent optical micrographs observation. The XRD analysis reveals that the precipitation of Cr23C6 leads to chromium depletion along the grain boundaries, consequently results in the susceptibility to IGC.

Effect of Solution Annealing on Susceptibility to Intercrystalline Corrosion of Stainless Steel with 20% Cr and 8% Ni

In general, as-received (AR) austenitic stainless steels (ASSs) contain complex carbide precipitates due to manufacturing operations, subsequent annealing treatment, or due to the fabrication processes such as welding. The presence of pre-existing carbides leads to cumulative sensitization and make the steel susceptible to intercrystalline corrosion (ICC)/intergranular corrosion (IGC) which causes premature failure during service. Solution annealing (SA) is one of the ways to deal with such situations. In this present investigation, the AR (hot rolled and mill annealed) chromium-nickel (Cr-Ni) ASS is compared with SA Cr-Ni ASS. The extent of ICC/IGC was evaluated qualitatively and quantitatively by various electrochemical tests including ASTM standard A-262 Practice A and Practice E, double loop electrochemical potentiokinetic reactivation and electrochemical impedance spectroscopy. The degree of sensitization for hot rolled mill annealed AR condition is found to be substantially higher (51.55%) than that of SA condition (26.9%) for thermally aged samples (at 700 °C). The chemical composition across the grain boundary was measured using electron probe micro-analyzer for both (AR and SA) conditions and confirms that the pre-sensitization effect was completely removed after SA treatment.

Evaluation of the intergranular corrosion susceptibility of UNS S31803 duplex stainless steel with thermoelectric power measurements

In this study, the use of thermoelectric power (TEP) measurements as a nondestructive method for the determination of the degree of sensitization (DOS) in UNS S31803 duplex stainless steel (DSS) is presented. Samples were aged for times up to 240h and subjected to the double loop electrochemical potentiokinetic reactivation (DL-EPR) test to calculate the DOS induced. The correlation between TEP and the DOS evaluated by the DL-EPR method indicates that the TEP coefficient is sensitive to gradual microstructural changes produced by thermal aging and may be used to monitor the susceptibility to intergranular corrosion in UNS S31803 DSS.

Prevention of weld-decay in austenitic stainless steel by using surface mechanical attrition treatment

Surface mechanical attrition treatment (SMAT) was applied to the samples of a type AISI 304 stainless steel in order to induce grain refinement as well as formation of twins. Transmission electron microscopy and X-ray diffraction analysis results showed that the average grain size at the surface of the SMATed sample was about 10 nm. The untreated and SMATed samples were then welded using a one-pass gas tungsten arc procedure. The heat-affected zone (HAZ) of the samples was examined by optical microscopy and corrosion tests. Results of the double loop electrochemical potentiokinetic reactivation tests showed that the degree of sensitization in the HAZ for the SMATed sample was very low as compared to that of the untreated one. The pre-SMATed sample was resistant to intergranular corrosion. This is mainly due to the formation of high density of twins which are not prone to carbide precipitation because of their regular and coherent atomic structure and extreme low grain boundary energy as compared with those of other grain boundaries.

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.

Microstructural and electrochemical characterisation of heat-treated 347 stainless steel with different phases

Type 347 stainless steel was solution annealed at three different temperatures (1100, 1250 and 1300°C) followed by ageing at 650 and 750°C. This was done to study the phase transformations that may occur in stainless steel 347 during welding and/or subsequent ageing. The microstructural examination showed chromium carbide and σ phase formation at the grain boundaries after ageing. The electrochemical behaviour was evaluated by double-loop electrochemical potentiokinetic reactivation test and anodic polarisation in HCl solution. Chromium carbide precipitation caused large increase in the degree of sensitisation, while σ phase had the major effect in decreasing the pitting resistance.

Role of grain boundary nature and residual strain in controlling sensitisation of type 304 stainless steel

The effects of residual strain and grain boundary character distribution on sensitisation of type 304 stainless steel at 525°C were evaluated using electrochemical potentiokinetic reactivation (EPR) technique. The results indicated that a very low level of residual strain and a high fraction of annealing twins significantly improved the resistance to sensitisation. Image analysis indicated that the fraction of area attacked during the EPR test correlated well with the EPR data. The volume loss, calculated using atomic force microscopic examinations, during the EPR tests also correlated well with the EPR results.

Evaluation of the effect of grain size on chromium carbide precipitation and intergranular corrosion of 316L stainless steel

The effect of grain size on chromium carbide precipitation and sensitization of 316L stainless steel was investigated based on inspection of microstructures and electrochemical potentiokinetic reactivation test. Various grain size samples were produced by heat treating the base material at 1100°C for different durations. The result showed that chromium carbide precipitations were much delayed in larger grains. The degree of sensitization decreases with increasing grain size. The diffusion bonded joint has good resistance to intergranular corrosion due to the coarsened grains and the increased percentage of low energy coincident site lattice, while the grain coarsening is the main contribution.

Instantaneous impedance evaluation of dissolution of AISI 304 stainless steel during intergranular corrosion

PurposeThe purpose of this paper is to focus on diversification between electrical parameters determined on the basis of instantaneous impedance measurements within the activation and reactivation scan of dissolution of sensitized AISI 304 stainless steel during a proceeding intergranular corrosion process.Design/methodology/approachThe investigations were carried out by means of dynamic electrochemical impedance spectroscopy (DEIS). DEIS measurements were conducted “on‐line” while the samples were polarized in agreement with a measurement procedure presented in the ASTM G108‐94 standard, in order to guarantee conditions equivalent with the DL‐EPR tests performed on AISI 304 stainless steel.FindingsPerformed researches revealed the advantages of the DEIS technique over standard double‐loop electrochemical potentiokinetic reactivation (DL‐EPR) tests in the field of intergranular corrosion investigations. Application of the DEIS technique made it possible to trace instantaneous changes in the examined system's impedance versus potential during the intergranular corrosion process. The form of recorded DEIS spectra and obtained distribution of measurement frequencies within the reactivation potential range were equivalent to those obtained for pure iron dissolution in sulfuric acid medium. As a result, instantaneous changes of electrical double layer capacitance and charge transfer resistance as a function of potential have been obtained in the range of activation and reactivation scans.Originality/valueThe paper provides information regarding diversification between the electrical double layer capacitance and the charge transfer resistance determined for sensitized AISI 304 stainless steel with respect to polarization conditions during the standard DL‐EPR test, which were obtained in order to evaluate the susceptibility to intergranular corrosion.

Corrosion Behavior of Alloy 600 in Simulated Nuclear High Level Waste Medium

Nickel-based alloys are being considered as candidate materials for the storage of high level waste. In the present investigation, Alloy 600 was assessed by potentiodynamic anodic polarization technique for its corrosion behavior in the as-received, solution annealed, and sensitized condition in 3 M HNO3 and 3 M HNO3 containing simulated high level waste. From the results of the investigation, it was found that the solution annealed specimen possesses superior corrosion resistance compared to the as-received and sensitized specimen. Double loop electrochemical potentiokinetic reactivation test was carried out to study the degree of sensitization. The effect of different concentrations of chloride ions in 3 M HNO3 at 25 °C indicated tendency for pitting as the concentration of chloride ions was increased. Microstructural examination was carried out by optical microscope and scanning electron microscope after electrolytic etching. X-ray photoelectron spectroscopy study was carried out to investigate the passive film formed in 3 M HNO3 and 3 M HNO3 simulated high level waste.

공정 디자인에 따른 클래드강 맞대기 용접부의 기계적 특성에 관한 연구

In this study, some considerations have been suggested in developing on-site techniques to evaluate the sensitization of stainless steels. Electrochemical potentiokinetic reactivation (EPR) technique is known to be a candidate tool for field applications since it enables quantitative assessment in reasonable test time, compared to oxalic etching (ditch) technique. The on-site application of the test method imposes additional restrictions on the selection of the test method (for example, minimum surface preparation requirement, insensitivity to testing temperature, etc.). The EPR and etching techniques have been compared in order to sensitization of stainless steel structures. It has been widely reported that the maximum sensitivity in the welded structure of stainless steel is shown at heat-affected zone (HAZ) than weldments with cast structure. In this work, sectioned weldments and external surfaces were investigated to reveal the degree of sensitization by the etching and the results were compared with those of EPR test. The EPR test showed little sensitivity to surface roughness and test temperature.

Intergranular precipitation and corrosion in weld of low Cr ferritic stainless steel

In the present research, intergranular precipitation and corrosion in the weld of low Cr ferritic stainless steel was investigated. Free-exposure and double loop electrochemical potentiokinetic reactivation corrosion test results revealed that IGC occurred in both the weld metal and heat-affected zone of the welds aged for more than 1 h at 500 °C. Intergranular Cr segregation was observed around the TiC precipitation of the weld in which IGC developed during the corrosion tests. This observation is subsequently discussed in the experimental results.

On the role of residual strain in controlling sensitisation of twin-boundary engineered type 304 stainless steel

Sensitisation in twin boundary engineered type 304 austenitic stainless steel (SS) was investigated using electrochemical potentiokinetic reactivation technique. As-received (AR) specimens of type 304 SS were given two solution-annealing treatments at 1000°C for 0.5 and 2h. Solution-annealed specimens were given a small strain of 5% using cold-rolling followed by strain–annealing (strain–annealing) at 927°C for 24, 48, and 72h. Electron backscatter diffraction analyses have shown increase in fraction of special boundaries (Σ⩽29) in two solution-annealed specimens. Twin-boundary engineered specimens were given two potential sensitisation treatments at 575 and 675°C for 1h. The increase in twin fraction and decrease in grain average misorientation (GAM) always resulted in reducing the extent of sensitisation. An exponential correlation between GAM and twin-fraction was established and increase in GAM always associated with increase in twin-fraction.

Influence of tensile pre-strain and sensitization on passive films in AISI 304 austenitic stainless steel

The degree of sensitization for tensile pre-strain (0%, 10%, 20%, 30%, 40%) of AISI 304 stainless steels and sensitization at 575°C was investigated by the double loop electrochemical potentiokinetic reactivation technique and scanning electron microscopes (SEMs). The properties of passive films of all the sensitized specimens in borate buffer solution (pH=9.2) with 5000ppm Cl− were investigated by the Mott–Schottky analysis. The degree of sensitization results showed that two reactivation current peak values were obtained for the sensitized specimens after 30% and 40% pre-strain. The degree of sensitization was decreased by pre-strain, except for specimen with 10% pre-strain. The results were further confirmed by the Mott–Schottky analysis.

Effect of Carbon Content on Eddy Current Response to Sensitization and Intergranular Corrosion In Simulated Heat-Affected Zone of Austenitic Stainless Steel

AISI Type 316 stainless steel (C = 0.055 wt. %) and type 316L stainless steel (C = 0.016 wt. %) were subjected to various heat treatments between 873 and 1073 K for various durations of time. The aged specimens were then subjected to ASTM A262 Practice E test in a boiling 16 % H2SO4 solution containing 100 g/l of CuSO4.6H2O for 24 h and followed by a bend test. Based on the appearance at the bend portion, the specimens were classified into four categories viz. unaffected, fissured, cracked and broken. Electrochemical potentiokinetic reactivation (EPR) technique was used to determine the degree of sensitization (DOS) for the various categories of specimens. Eddy current (EC) tests were carried out to determine the eddy current amplitudes for the various categories of specimens in both the as-aged and Strauss-test exposed condition. The results of the EPR tests and the EC measurements were then classified into these four categories of specimens, in the as-aged condition and Strauss-tested condition (prior to bending). The results indicated that it was possible to predict DOS in the as-aged conditions using EC technique, though the overall change in the EC amplitude was small. The EC response from the Strauss test-exposed specimens was much higher than that observed in the as-aged specimens. The results indicated that EC testing was sensitive enough to detect both sensitization and IGC. The differences in the response of the two sensitized steels to eddy currents indicated that their carbon content played a major role in altering the chemical composition and the geometrical features of the depleted regions.

Intergranular Corrosion of Weld Metal of Super Type 304H Steel During 650°C Aging

The intergranular corrosion of the weld metal of super Type 304H (UNS S30409) stainless steel during aging at 650°C was investigated using electrochemical potentiokinetic reactivation (EPR). Experimental results showed that the as-welded weld joint possesses a good corrosion-resistant property, but the formation of chromium-depleted zones caused by precipitation of Cr23C6 between cellular dendrites increased the reactivation ratio (Ra) when the weld metal was aged at 650°C for a prolonged time, indicating the acceleration of intergranular corrosion with increased aging time. Nevertheless, it was found that Cr23C6 showed the morphology with isolated particles after being aged for 500 h and Ra became stable.

Effect of prior cold-work on radiation-induced segregation in proton-irradiated austenitic stainless steel

The effect of prior cold-work on radiation-induced segregation in proton-irradiated type 304 SS (1.00dpa) was investigated using electrochemical potentiokinetic reactivation (EPR) test followed by atomic force microscopic examination. Attacked linear features were noticed after EPR testing of the irradiated specimen and it was attributed to chromium depletion and/or decoration of strain-regions by point-defects on slip planes. Attack on locations near grain boundaries was also noticed after EPR tests in the irradiated specimen. Electron backscatter diffraction analysis of the irradiated specimen showed increase in Σ1 fraction within the matrix and formation of strain-regions near grain boundaries.

Effect of Heat Treatment on Corrosion Behavior of Alloy 690 and Alloy 693 in Simulated Nuclear High-Level Waste Medium

Nickel-based alloys are being considered as candidate materials for the storage of high-level waste generated from the reprocessing of spent nuclear fuel. In the present investigation, Alloy 690 (UNS N06690) and Alloy 693 (UNS N06693) were assessed using the potentiodynamic anodic polarization technique for the corrosion behavior in the solution-annealed and sensitized conditions, in 3 M nitric acid (HNO3) and simulated high-level waste (HLW) at 25°C and 50°C. From the results of the investigation, it was found that the polarization curves for the solution-annealed specimens were characterized by a long passive range and low passivation current density, at 25°C. Increasing solution temperature to 50°C led to a corresponding increase in corrosion potential as well as passivation current density. The solution-annealed specimen showed improved corrosion resistance compared to the sensitized specimen in both media, for both alloys. Double-loop electrochemical potentiokinetic reactivation (DL-EPR) test was carried out for both solution-annealed and sensitized Alloy 690 and Alloy 693 in 0.5 M sulfuric acid (H2SO4) containing 0.0001 M potassium thiocyanate (KSCN), to measure the degree of sensitization (DOS). Microstructural examination was carried out by optical microscopy and scanning electron microscopy (SEM) after electrolytic etching. The results of the present investigation are discussed in the paper.