Double Loop Electrochemical Potentiokinetic Reactivation Research Articles

Effect of Some Environmental and Stainless Steel Metallurgical Variables on the Values of Degree of Sensitization Measured by the Double Loop-Electrochemical Potentiokinetic Reactivation Test

The double loop-electrochemical potentiokinetic reactivation (DL-EPR) method estimates the degree of sensitization in stainless steels with ir/ia, the ratio of peak current densities during reactivation (reverse) and activation (forward) scans. Beyond sensitization level, other metallurgical variables, like inclusion content and cold work, or testing environment variables, like solution deaeration and solution aging, could potentially affect ir/ia. Austenitic stainless steel Types AISI 304 (UNS S30400) and AISI 303 (UNS S30300) were tested to assess the effect of those secondary variables on ir/ia. AISI 303 stainless steel had similar chromium and nickel contents to AISI 304, but higher contents of sulfur, resulting in a higher volume fraction of inclusions. ir/ia increased with inclusion content and decreased with cold work in AISI 304 stainless steel. In all cases, the DL-EPR test could discriminate solubilized from thermally aged specimens because confidence intervals of respective samples never overlapped. The testing solution was 0.5 mol/L H2SO4 + 0.01 mol/L KSCN, (sulfuric acid and potassium thiocyanate solution) and a common practice is to freshly prepare it just before the test. Measurements are usually performed under deaerated conditions. However, deployment of the technique, especially in the field, could be facilitated if naturally aerated solutions prepared in advance in a laboratory can be used. The oxygen content in solution and solution aging did not have a statistically significant effect on ir/ia.

Hydrogen stress cracking of Alloy 716 under cathodic protection—root cause analysis, and an evaluation of quality control methods

A subsea fastener made of Alloy 716 (UNS N07716) experienced a brittle fracture in service. The investigations described herein suggested that the failure was caused by hydrogen stress cracking (HSC), thus, adding to the number of reported HSC incidents on precipitation hardened nickel-based alloys over the last two decades. Incremental step loading testing on notched tensile specimens showed a net section threshold stress of <44% of the fastener materials actual yield strength. X-ray diffraction revealed that the grain boundary (GB) precipitates were predominantly the σ–related precipitate F-phase, which were suggested to have a detrimental effect on HSC resistance—even in low amounts. The low amounts of GB precipitates were indicated by a low degree of sensitization in the double-loop electrochemical potentiokinetic reactivation test. Two additional heats from the same alloy were investigated to elucidate the effect of different GB precipitates on HSC resistance and to evaluate quality control methods that are discussed in the research community.

Study of strain induced martensite and its reversal on sensitization behaviour of metastable austenitic stainless steel

PurposeThe purpose of the present study is to investigate the effects of strain-induced martensite (SIM) and its reversal on metastable austenitic stainless steel (MASSs) through the analysis of metallurgical and sensitisation behaviour.Design/methodology/approachIn the present investigation, the samples of Cr-Mn ASS (also known as MASSs) including 15%, 30% and 50% cold worked, solution annealed samples with and without thermal ageing (at 700°C for 3 h) were analysed with the help of X-ray diffraction analysis, microstructure examination and electrochemical behaviour. The scanning electron microscope (SEMJOEL 6380 A) was used to examine the microstructure of the sample, and the double-loop electrochemical potentiokinetic reactivation test was used to determine the degree of sensitisation (DOS) in the samples. The cold worked solution annealed samples without thermal ageing are named as CR15, CR30 and CR50, respectively, and the samples with thermal ageing are named as CR15_TA, CR30_TA and CR50_TA, respectively.FindingsIn CR15, CR30 and CR50 samples, the DOS increased with increase in the extent of cold working, which was attributable to passivation deterioration. Because of the high degree of passivation at the grain boundaries, the DOS of CR15_TA and CR30_TA were practically identical. The DOS in the CR50_TA sample, on the other hand, was lowered due to SIM recovery in the austenite.Originality/valueThe present study sheds light on how to choose the right cold working percentage to avoid sensitisation in MASSs during the fabrication of metal forming components.

New Findings on Intergranular Corrosion Mechanism of 445J2 Ultra-Pure Ferritic Stainless Steel

A criterion for characterizing the intergranular corrosion sensitivity of 445J2 ultra-pure ferritic stainless steel was established using microstructure analysis and electrochemical impedance spectroscopy (EIS). The differences in the characterization results for the intergranular corrosion sensitivity of 445J2 stainless steel by EIS and the double loop-electrochemical potentiokinetic reactivation test were analyzed. The Mo-rich intermetallic compound did not cause depleted chromium zones in the sensitization temperature range of 500°C to 700°C but caused a surrounding elastic stress field. It was determined that the intergranular corrosion in 445J2 is due to the elastic stress field caused by the precipitated phase.

On-site weld quality assessment and qualification for stainless steels tanks

Stainless steel tanks are frequently degraded by localized corrosion in bioprocess industries. A case study of 2101 duplex stainless steel tank allowed to apply a portable electrochemical microcell system (PassivityScan) for on-site weld inspection and corrosion monitoring from the manufacturing until 12 months of operation. During the tank manufacturing, the double loop electrochemical potentiokinetic reactivation technique was applied to measure the sensitization degree on the welded regions. The manufactured tank was submitted to the passivation treatment to improve the passivation properties and corrosion resistance. On-site cyclic polarization tests were performed and confirmed the increase of passivation level of treated surface. The passivation level was measured again after 12�months of operation and detected a lower passivation level compared to the initial passivated surface, indicating that the passivation treatment lost its effect after that period. Therefore, an acceptance criterion to passivation level was empirically determined. PassivityScan is an advanced device to qualify the welds and the passivated surfaces, and useful for corrosion monitoring and the reliability maintenance.

Effect of strain-induced martensite reverse transformation on microstructure evolution and electrochemical behaviour of 2304 lean duplex stainless steel

ABSTRACT The effect of strain-induced martensitic transformation (SIMT) and its reversion (SIMRT) on the microstructure evolution and localised corrosion of 2304 lean duplex stainless steel (LDSS) was investigated. The cold-rolled LDSS was subjected to reverse annealing at 600°C to 900°C for 1800 s. To evaluate the microstructure, scanning (SEM) and transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) were used as the main techniques. Subsequent electrochemical characterisation included cyclic polarisation and double loop-electrochemical potentiokinetic reactivation test. SIM formation was noticed after cold rolling, and a partial reverse transformation was seen after annealing at 600°C. At 600°C, SIMRT led to decrease in pitting resistance and increase in the degree of sensitisation (DOS). Increasing the annealing temperature favoured localised and intergranular corrosion resistance. The obtained results indicate that reversed austenitic and recrystallised grains have a beneficial effect on desensitisation and corrosion resistance of 2304 LDSS.

Effect of thermomechanical processing via rotary swaging on grain boundary character distribution and intergranular corrosion in 304 austenitic stainless steel

In the present work, the thermomechanical processing consisting of rotary swaging deformation and annealing treatment was performed to optimize the grain boundary character distribution (GBCD) of 304 austenitic stainless steel. To systematically investigate the effect of GBCD evolution on intergranular corrosion (IGC), double loop electrochemical potentiokinetic reactivation and electrolytic oxalic acid etch tests were employed. The experimental results show that the fraction of low-Σ coincident site lattice (CSL) boundaries increased from 58.1% to 74.0% for the specimen swaged to 0.06 true strain and then annealed at 1050 °C for 5 min duration. By characterizing the evolution of GBCD as a function of strain level in terms of low-Σ CSL boundaries fraction, average twin-related domain (TRD) size, average number of grain per TRD and fractal dimension of the maximum random boundary connectivity, the grain boundary engineering (GBE) microstructure was realized by the occurrence of prolific multiple twinning events during strain-induced boundary migration while static recrystallization has a detrimental effect on optimizing GBCD for the prolific of new strain-free grains with random boundaries. The IGC resistance of the GBE-treated 304 austenitic stainless steel is enhanced by inhibiting the nucleation and propagation of IGC cracks, resulting from the increase in the fraction of low-Σ CSL boundaries, especially Σ3 boundaries and the disruption of random boundary network connectivity.

Influence of long term Sodium Exposure on the Corrosion and Tensile Properties of AISI Type 316LN stainless steel and modified 9Cr-1Mo steel

AISI Type 316LN stainless steel (SS) and modified 9Cr-1Mo (P91) steel are used in the intermediate heat exchanger and steam generator (SG), respectively of prototype fast breeder reactor (PFBR). This paper presents the effect of long-term sodium exposure (50000 h) on the corrosion and tensile properties of 316LN SS and P91 steel. The results showed that the prolonged sodium exposure of 316LN SS can lead to selective leaching of alloying elements (depletion of Cr, Ni, and Mo), enrichment of Fe, formation of ferrite layer and carburized zones with increased hardness at the surface (272 HV), and a reduction in ductility by nearly 40 %. Double-loop electrochemical potentiokinetic reactivation (DLEPR) results of sodium exposed 316LN SS showed a 25% higher degree of sensitization (DOS) as compared to thermally aged one. The microstructural examination of sodium exposed 316LN SS revealed a ditch structure near the surface and carbide precipitates along the grain boundaries indicating the diffusion of carbon through grain boundaries and surface carburization. XRD studies of sodium exposed 316LN SS revealed predominant phases of ferrite and Cr23C6 phases, which further corroborated the results of DLEPR, microstructure, and hardness data. The sodium exposed P91 steel showed no significant microstructural, microchemical changes and tensile properties after sodium exposure. The carbon depth profiles carried out using SIMS in sodium exposed 316LN SS and P91 steel showed a surface carbon concentration of 0.85 and 0.32 wt.%, respectively. Fractography of sodium exposed 316LN SS showed intergranular fractures whereas the fractographs of P91steel showed fibrous dimple and cleavage type of fractures indicating the growth of carbides. The present results confirm that P91 steel can sustain the reactor life in flowing sodium containing 25 ppm carbon and less than 2 ppm oxygen.

Air carbon arc gouging of austenitic stainless steels prior to arc welding – opportunities and risks

This paper presents the effect of air carbon arc gouging (ACAG) and submerged arc welding (SAW) on the risk of intergranular corrosion and on the mechanical properties of AISI 304 stainless steel plate, welded with AWS-ER308L wire, and AISI 347H stainless steel plate, welded with AWS-ER347 wire. A high risk of carbon uptake was found for as-gouged surfaces. Different mechanical surface cleaning procedures were evaluated. Brushing was ineffective to remove the carbon-contaminated layer, while grinding was more efficient, as assessed by double-loop electrochemical potentiokinetic reactivation (DL-EPR), but still indicating few remnants carbon-contaminated areas. DL-EPR and oxalic etching tests of gouged and welded 304/ER308L stainless steel indicated the risk of sensitization and suspect etch microstructures regarding intergranular corrosion, while low sensitization risk was found for 347H/ER347 stainless steel welds, highlighting the role of low alloying elements such as Nb, recognized to reduce the risk of intergranular corrosion. Thus, the 304/308L SS weld joints were sensitized, even if no gouging was employed. This indicates a higher susceptibility of sensitization and intergranular attack caused by the thermal cycle induced during the SAW of AISI 304 SS using AWS-ER308L SS wire. Acceptable mechanical behavior of welding onto gouged surfaces was found for both weld joints (i.e., Charpy energy > 90 J and Ultimate Tensile Strength ≥ 600 MPa), revealing that the major risk is the sensitization in the case of 304/ER308L SS welds.

Mitigating the susceptibility to intergranular corrosion of alloy 625 by friction-stir welding

In this work, friction-stir welding (FSW) was employed to alloy 625 grade I (soft annealed) sheets. Therefore, solid-state based welding was undertaken with a tool rotational speed of 200 rpm and a welding speed of 1 mm/s. Microstructural features were analyzed by light optical and scanning electron microscopy (LOM and SEM). Moreover, microhardness measurements were performed. The susceptibility to intergranular corrosion was verified by the double-loop electrochemical potentiokinetic reactivation (DL-EPR) test. Complementary, intergranular corrosion was also evaluated by the ASTM G28 Method A. FSW promoted grain refinement, increased microhardness, and reduction in the degree of sensitization. Finally, the mean corrosion rate observed in the ASTM G28 Method A test was 0.4406 mm/year, which suggests a good weld quality.

Intergranular corrosion behaviour of FeCoCrNi high-entropy alloy fabricated by selective laser melting

Recently, high-entropy alloys (HEAs) show a wide application prospect in the marine field to achieve excellent corrosion resistance. In this paper, the intergranular corrosion (IGC) behaviour of FeCoCrNi high-entropy alloy fabricated by selective laser melting (SLM) at different laser powers (140, 170, 200, 230, and 260 W) was studied by microstructure analysis and double-loop electrochemical potentiokinetic reactivation. All specimens show high IGC resistance, and the degree of sensitisation of SLMed FeCoCrNi HEAs increased with the increase of laser power. Interestingly, no chromium carbides precipitation was found at the grain boundary, which indicates the intergranular corrosion mechanism of SLMed FeCoCrNi HEAs is totally different from that of most alloys caused by Cr depletion. It is found that the superior IGC resistance in SLMed FeCoCrNi HEAs produced by low laser power can be attributed to larger grain size, a higher fraction of [Formula: see text][Formula: see text] ([Formula: see text] 3), and low-[Formula: see text] coincidence site lattice boundaries. This work provides a strong insight into the application of additive-manufactured HEAs in harsh corrosion environments.

Some aspects of pitting corrosion susceptibility of As‐welded and thermally aged nitrogen‐alloyed 316SS weld metal

AbstractType 316N (0.07% N) stainless‐steel welds prepared by shielded metal arc welding were evaluated for their pitting corrosion susceptibility in an acidified chloride environment and a neutral chloride environment in as‐welded and thermally aged conditions at 823 and 923 K for 4, 10, 24, and 100 h. After 100 h thermal aging at 823 K, 46% of δ ferrite was transformed into carbide and 81% δ ferrite was transformed into carbide and sigma phases at 923 K for 100 h. On thermal aging at 823 K (0–10 h) and 923 K (10–100 h), the changes in the degree of alloy depletion (DOA) and pitting potential were marginal due to the presence of nitrogen. Thermally aged samples at 823 K beyond 24 h showed a significant decrease in pitting potential in acidic and neutral chloride media, whereas at 923 K, the pitting potential decreased significantly up to 10 h. The results of DOA and passive current density from the double‐loop electrochemical potentiokinetic reactivation (DLEPR) test and the electrochemical reactivation test (ERT) correlated well with the observed secondary phases. The role of alloy depletion in terms of the parameters DLEPR, ERT, microstructure, and nitrogen alloying on pitting corrosion is discussed.

DL‐EPR and AFM study of sensitization of a 17%Cr multiphase stainless steel

AbstractThe corrosion behavior of a 17Cr multiphase martensitic stainless steel quenched (1000°C, water cooling) and tempered at different temperatures was determined using the double‐loop electrochemical potentiokinetic reactivation (DL‐EPR) test. The specimen tempered at 550°C showed the highest degree of sensitization. For the specimen treated at 600°C, a healing process was observed, followed by a second sensitization at 625°C coincident with an increase in the austenite volume fraction. This phase was quantified using the magnetic method and an increase in this phase was observed with the tempering temperature. The DL‐EPR results were correlated with surface aspects obtained by atomic force microscopy.

Improvement in corrosion resistance of AISI 316L stainless steel weld cladding using GTA remelting technique

The present work was focused on studying the influence of gas tungsten arc (GTA) remelting on metallurgical and intergranular corrosion behavior of AISI 316L stainless steel (SS) cladding fabricated using gas metal arc welding (GMAW) process. The microstructure of the clads was examined using optical microscopy. Vickers microhardness testing method was used to measure the microhardness of the clad in the as-deposited (using GMAW process) and GTA-remelted conditions. Intergranular corrosion (IGC) behavior of the clads was evaluated using the double loop electrochemical potentiokinetic reactivation (DLEPR) technique. The results showed that when AISI 316L SS clad was subjected to GTA remelting, there was a noticeable modification of its microstructure, as a consequence of which, an increase in microhardness was observed in the GTA-remelted clad. Furthermore, DLEPR tests showed that the GTA-remelted clad layer possessed a relatively lower degree of sensitization (DOS), and thus higher corrosion resistance compared to the GMA-deposited clad layer. In general, this work shows that GTA remelting can prove to be a cost-effective alternative for improving the surface properties of austenitic SS cladded components used for critical applications.

Effect of environmental condition on corrosion susceptibility of GTA welded AISI 202 austenitic stainless steel

Austenitic stainless steels with low nickel content or nickel-free content are currently considered structural materials for nuclear fusion reactor applications. Even though the base metal achieves excellent performance, welding may result in premature failures due to morphological and compositional heterogeneities. The presence of pollutants in the environment results in environmental acidification, which influences the corrosion susceptibility of the weldment. This paper it is aimed to examine the influence of the gas tungsten arc welding (GTAW) process on localized corrosion and the degree of sensitization (DOS) of AISI 202 austenitic stainless steel (ASS) butt joints in three distinct regions: base-zone (BZ), heat-affected zone (HAZ), and weld-zone (WZ) in 3.5% NaCl under pH-5 environmental conditions. The localized corrosion behavior of the weldment was studied using open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), and degree of sensitization (DOS) using the double loop electro-chemical potentiokinetic reactivation (DL-EPR) technique. Optical microscopy (OM) is used to understand the microstructural features of weldments and pit morphologies before and after polarisation studies. The present study establishes that the heat-affected zone (HAZ) has low corrosion resistance, high DOS%, and low intergranular corrosion (IGC) resistance compared to the base and weld zones. This could be due to the thermal cycles involved in HAZ during welding, resulting in chromium depletion and the formation of chromium carbide precipitation.

The Effect of Reversion of Strain-Induced Martensite (SIM) and Grain Refinement on the Intergranular Corrosion of High Mn Steel

The surface phase constituent of high-manganese austenitic stainless steel after cold work (15%, 30%, and 50%) and thermal aging at 900°C for 30 min and 6 h, was characterized using x-ray diffraction. The microstructural analysis was conducted using an optical microscope, a scanning electron microscope, and the electron backscatter diffraction technique. The double-loop electrochemical potentiokinetic reactivation test was used to measure the intergranular corrosion resistance (degrees of sensitization). The results showed that fine-recovered grains of austenite and strain-induced martensite together formed the surface of high-manganese steel after cold work. Because of the formation of strain-induced martensite in the cold-worked samples, their intergranular corrosion was much higher than that of the as-received sample. Also, the degree of sensitization of 15% cold work was higher (i.e., more intergranular corrosion) compared to the degrees of sensitization of 30% and 50% cold work. On the contrary, the intergranular corrosion of high-manganese austenitic stainless steel sample subjected to cold work was eliminated during thermal aging at 900°C for 6 h because of the reversion of strain-induced martensite and fine-reverted austenite grains. Owing to this grain refinement of austenite, faster diffusion rate of Cr at higher temperature and cold work helped healing of Cr-depletion zone in a shorter time. In other words, because the results showed that on 50% cold work and thermal aging at 900°C for 6 h, the high-manganese austenitic stainless steel does not become susceptible to intergranular corrosion. Hence, it could be beneficial to investigate the intergranular corrosion of high-manganese austenitic stainless steel.

Application of double loop electrochemical potentio-kinetic reactivation for characterizing the intergranular corrosion susceptibility of stainless steels and Ni-based alloys in solar nitrate salts used in CSP systems

In this study, the sensitivity of four corrosion resistant candidate alloys; austenitic stainless steels (AISI 321 and AISI 347) and Ni-based alloys (IN 625 and In 825) to degrade by intergranular corrosion when used with molten salts in conditions of CSP systems were investigated at different temperatures. This was achieved by implementing the Double loop electrochemical potentiokinetic reactivation (DL-EPR) technique to quantitatively measure the degree of sensitisation (DOS) in the selected alloys by aging in air with and without Solar Salts (60 wt% NaNO3 and 40 wt% KNO3) at 550–750 °C for 2 h. Electrochemical etching technique was implemented further to verify the results from DL-EPR. The results show that DL-EPR and microstructural evaluation techniques can be used to assess the synergy of temperature and molten salt electrochemical activities on the tendency for intergranular corrosion attack to occur in CPS systems. IN 625 showed the highest intergranular corrosion resistance among the alloys investigated in both air and molten salt; especially at temperatures higher than salt stability temperature. Samples aged in molten salts showed higher DOS than samples aged in air across all temperatures and linked to Cr-consuming corrosion and chemical reactions that drives molten salt – alloy interaction in CSP plants.

Effect of filler and autogenous welding on microstructure, mechanical and corrosion properties of low nickel Cr-Mn ASS

ABSTRACT This experimental study investigates the effects of welding low nickel Cr-Mn austenitic stainless steel with gas tungsten arc welding process with and without filler (autogenous) weld by varying the welding currents (120A, 130A and 140A). The microstructural examination of weld has been carried out with optical microscope and mechanical properties with Vickers microhardness and tensile tester. The corrosion behaviour of the welded joints are analysed by double loop electrochemical potentio-kinetic reactivation test to obtain degree of sensitisation. The microstructural examination of welded joints revealed that, as the welding current increases, the inter-dendritic spacing as well as the dendrite size increases. The tensile strength and hardness decrease with increasing welding current for 308 L filler and autogenous weld. The degree of sensitisation increases with increasing welding current for both the welds. The results reveal that the autogenous welds have higher tensile strength, higher hardness and lower degree of sensitisation as compared to welds with 308 L filler.

Reductions of Intergranular Corrosion Resistance and Wear Resistance in a Ni-Cr-Mo-Based Superalloy by Aging-Treatment-Induced Precipitation

Ni-Cr-Mo-based superalloy is widely used as a key component in many critical environments. To ensure that the manufacturing process does not impact the long-term service performance of these components, the aging precipitation behavior at different temperatures and its effect on intergranular corrosion (IGC) resistance and wear resistance of a Ni-Cr-Mo-based C276 superalloy were investigated. The equilibrium phase diagram was calculated first using thermodynamic software to confirm the potential phases. Carbides of M6C were found to be formed at grain boundaries after aging at 800–850 °C for short-term treatment. The other two phases (μ phase and P phase) indicated in the phase diagram were not observed for the samples after aging treatment up to 15 h. Furthermore, double loop electrochemical potentiokinetic reactivation (DL-EPR) tests were conducted to examine the IGC resistance. The degree of sensitization increased with the aging time and severe corrosion was found to occur at grain boundaries. For the first time, the influence of aging treatment on the wear behavior of this superalloy has been specifically studied. Concerning the hot processing of Ni-Cr-Mo-based C276 superalloy, these results indicate the importance of avoiding high-temperature heat treatment for long periods.

Effect of number of welding passes on the microstructure, mechanical and intergranular corrosion properties of 409M ferritic stainless steel

PurposeThis paper aims to study the effect of number of welding passes on microstructure, mechanical and corrosion properties of 409 M ferritic stainless steel. Shielded metal arc welding (SMAW) process is used to weld two metal sheets of 409 M having 3 mm thickness as bead-on-plate with single, double and triple passes. Microstructures were observed at transverse section with the help of optical microscope and with increasing number of passes grain growth, and the width of heat-affected zone (HAZ) increases. The results of tensile tests revealed that as number of passes increases, there is reduction in tensile strength and ductility. Double loop electrochemical potentiokinetic reactivation (DL-EPR) test revealed that as number of passes increases, the degree of sensitization increases. This is due to the deposition of chromium carbides at the grain boundaries and the associated depletion of chromium.Design/methodology/approachThree welded plates of single, double and triple pass were welded by SMAW process. From three welded plates (single, double and triple passes), samples for microstructural examination were cut in transverse direction (perpendicular to welding direction) with the help of wire-cut electrical discharge machine (EDM). The welded plates were sliced using wire-cut EDM along transverse direction for preparing optical microscopy, tensile testing, microhardness and DL-EPR testing specimens.FindingsFrom the microstructure, it was observed that the large grain growth, which is dendritic, and the structure become finer to increase in number of welding passes. As number of passes increases, the width of HAZ increases because of the higher temperature at the welded zone. The tensile strength decreases to increase the number of welding passes because of grain coarsening and chromium carbide precipitation in sensitized zone and wider HAZ. The maximum microhardness value was observed for single-pass weld as compared to double- and triple-pass welds because of the fast cooling rate. The degree of sensitization increases to increase the number of passes because of chromium carbide deposition at the grain boundaries.Originality/valueThe authors declare that the manuscript is original and not published elsewhere, and there is no conflict of interest to publish this manuscript.