Loop Electrochemical Potentiokinetic Reactivation Test Research Articles

Experimental investigation of microstructural, mechanical and corrosion properties of 316L and 202 austenitic stainless steel joints using cold metal transfer welding

The objective of the current experimental study is to investigate influence of dissimilar welding between 316 L Austenitic and 202 Austenitic Stainless Steel. This is achieved through the utilization of Cold Metal Transfer (CMT) Welding in conjunction with the GTAW mode. Various welding approaches are employed, including the use of ER316L and ER309L fillers, as well as a no-filler (autogenous) welding technique. Microstructural observation shows martensitic formation in the interface region. The tensile test was conducted by using Universal Testing Machine for the weldment and found that ER316L filler (721 MPa) has higher strength than ER309L (672 MPa) and autogenous weld (528 MPa). Charpy impact test was used to determine toughness of the weldment and found that ER309L Filler weld has toughness (87 J) which is higher than ER316L filler weld (5 4 J) and autogenous weld (23 J). Vicker's hardness test shows that autogenous weld hardness (avg. 193.34 HV) which is highest than ER309L filler (avg.188 HV) and ER316L filler weld (avg. 184.67 HV). The intergranular corrosion test by double loop electrochemical potentiokinetic reactivation (DLEPR) test shows sensitization at highest degree for autogenous weld (18.25 %) than ER316L filler weld (9.22 %) and ER309L filler weld (7.28 %).

An investigation of the effect of sensitization on the metallurgical characteristics of dissimilarly welded austenitic-ferritic stainless steel

Purpose The purpose of this study is to examine the effects of sensitization on the metallurgical characteristics of weld joints made up of austenitic stainless steel (AISI 316L) and ferritic stainless steel (AISI 430), using the gas tungsten arc welding (GTAW) process with ER316L filler wires. Design/methodology/approach A non-consumable tungsten electrode with a diameter of 1.6 mm was used during the GTAW procedure. The filler wire, ER316L, was selected based on the recommendation provided in literature. To explore the interconnections among the structure and properties of these weldments, the techniques including scanning electron microscopy and optical analysis have been used. In addition, the sensitization behaviour of the weldments was investigated using the double loop electrochemical potentio-kinetic reactivation (DLEPR) test. Findings Microstructural analyses revealed the occurrences of coarsened grains with equiaxed columnar grains and migrating grain boundaries in the weld zone. The results of the DLEPR test demonstrated that heat affected zone (HAZ) of AISI 430 was more susceptible to sensitization than HAZ of AISI 316L. Microstructure analysis also revealed the precipitation of large amounts of chromium carbide at the grain boundaries region of AISI 430 welded steel, causing more sensitization and, as a result, more failure or breaking at the side of AISI 430 weld in the dissimilar weldment of AISI 316L–AISI 430. Originality/value The present work has been carried out to determine the appropriate welding conditions for joining AISI 316L and AISI 430, as well as the metallurgical properties of the dissimilar weldment formed between AISI 316L and AISI 430. Owing to the difficulties in measuring the performance of these types of dissimilar joints given their unique mechanical and microstructural characteristics, research on the subject is limited.

Corrosion behavior of wire arc additive manufactured and wrought 309 stainless steel in acidic solution

In this study, 309 stainless steel (309SS) was fabricated by wire arc additive manufacturing (WAAM) process and characterized using optical microscopy and X-ray diffractometry. Corrosion behavior of WAAM 309SS in 1 mol/L H2SO4 solution at room temperature was investigated in comparison to that of wrought 309SS using electrochemical impedance spectroscopy, pontentiodynamic polarization, and immersion test. Mott-Schottky analysis, double loop electrochemical potentiokinetic reactivation (DLEPR) test and galvanic corrosion test using zero resistance ammeter (ZRA) technique were also conducted. The results showed that WAAM 309SS exhibited lower corrosion resistance than AISI 309SS due to microstructural differences engendered by the additive manufacturing process. Galvanic corrosion test showed that galvanic effect between the two stainless steels was minimal despite the microstructural differences while DLEPR test revealed the absence of chromium-depleted zones in both WAAM 309SS and AISI 309SS.

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.

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.

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.

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.

Effect of Austenitization-Cooling on Microstructure and Localized Corrosion Behavior of 13Cr Martensitic Stainless Steel

The present study investigates the microstructural changes and corrosion behavior of 13Cr martensitic stainless steel (MSS), austenitized at 1040 °C followed by cooling to room temperature through oil-quenching, air-cooling and furnace-cooling. Oil-quenching and air-cooling produced a lath martensitic microstructure, while, the oil-quenched condition showed δ-ferrite stringers and quench cracks. The slow furnace-cooling produced a widmanstatten type ferrite with Cr-rich carbides. Air-cooled condition developed a higher hardness than that for the oil-quenched and furnace-cooled conditions due to martensitic structure and fine carbide precipitation along the prior austenitic grain boundaries (PAGB) during cooling from austenitization temperature. Double loop-electrochemical potentiokinetic reactivation (DL-EPR) tests and immersion tests followed by scanning electron microscope examination indicated that furnace-cooled and air-cooled conditions are susceptible to intergranular corrosion (IGC). Slow cooling is highly detrimental to IGC. Air-cooled condition showed preferential attack along the PAGB boundaries in both DL-EPR and immersion tests whereas furnace-cooled condition showed combined heavy uniform corrosion and IGC. No preferential attack is observed in oil-quenched condition, and it exhibited uniform corrosion. The present results show that the austenitization followed by cooling in air is sufficient to cause sensitization in 13Cr MSS. However, controlled (fast) air-cooling (avoiding sensitization during cooling) would be better a choice for austenitization heat treatment of 13Cr MSS.

Effect of Fillers and Autogenous Welding on Dissimilar Welded 316L Austenitic and 430 Ferritic Stainless Steels

This experimental study examines the effects of dissimilar welding of 316L austenitic and 430 ferritic stainless steels, welded with gas tungsten arc welding process with (ER316L and ER309L) and without fillers (autogenous). Microstructural examination was performed by optical and scanning electron microscopy. ER316L and ER309L fillers welds illustrated equiaxed and columnar grains; ER309L weld showed higher Creq/Nieq ratio than that of ER316L weld, produced vermicular ferrite, while autogenous weld shows lathy ferrite and martensite. Chromium carbide precipitate was also observed in autogenous weld. The mechanical properties were measured and higher hardness was obtained in autogenous weld compared to ER316L and ER309L filler weld. Higher tensile and impact strength was measured in ER316L filler weld as compared to ER309L filler and autogenous weld. The corrosion resistance of the joints was carried out by double loop electrochemical potentiokinetic reactivation and potentiodynamic polarization test to assess the sensitization and pitting resistance, respectively. Higher degree of sensitization and pitting were observed in autogenous weld as compared to ER316L and ER309L filler weld due to precipitation formation, but the pitting corrosion was found to be lower for ER309L filler weld as compared to ER316L filler and autogenous weld due to the effect of mode of solidification.

Interlaboratory Testing to Assess Repeatability and Reproducibility of the Double Loop Electrochemical Potentiokinetic Reactivation (DL-EPR) Tests for Type 304H Stainless Steels

The sensitization of stainless steels may decrease their corrosion resistance in industrial applications. Traditional immersion tests exist to determine the degree of sensitization (DOS) of the stainless steels. However, electrochemical methods may be preferred because they are less expensive and faster to perform. The fast and robust double loop electrochemical potentiokinetic reactivation (DL-EPR) test has been introduced to the corrosion community some decades ago but an interlaboratory testing study was necessary to assess the repeatability and reproducibility of the DOS results. This work reports on a recent study where 11 laboratories returned results that show a high degree of confidence in the data obtained by DL-EPR.

Enhancement in intergranular corrosion resistance of the stabilised ultra-pure 430LX ferritic stainless steel by tin addition

ABSTRACTThe intergranular corrosion (IGC) susceptibility of the Ti–Nb-stabilised ultra-pure 430LX FSSs with 0–0.47 wt-% tin addition was investigated using double loop electrochemical potentiokinetic reactivation (DL-EPR) and oxalic acid etch tests. The result of DL-EPR test reveals that IGC resistance increases with the increase in tin content. The specimens with different contents of tin exhibit the same microstructural evolution based on oxalic acid etch test. M23C6 precipitates form along grain boundaries, inducing Cr-depleted zones and finally resulting in the susceptibility to IGC. Tin addition gives rise to the formation of SnO2 on the FSSs surface, thus inhibiting the attack of corrosive ions towards Cr-depleted zones.

Evaluation of the DOS by DL−EPR of UNSM Processed Inconel 718

In this work, influence of ultrasonic nanocrystal surface modification (UNSM) on the degree of sensitization (DOS) in Inconel 718 has been studied and correlated with the resulting microstructure. The UNSM processed samples decreased their grain size from 11.9 µm to 7.75 µm, increasing the surface of grain boundaries, and thus enhancing the area where δ phase and niobium carbides precipitate. The effect of the UNSM process on the DOS of Inconel 718 was studied by the double loop electrochemical potentiokinetic reactivation (DL−EPR) test. The DL−EPR showed that for UNSM processed samples with no thermal treatment, the DOS increased up to 59.6%, while for UNSM treated samples that were post-annealed at 1000 °C for 10 min and water quenched the DOS decreased down to 40.9%. The increase of grain boundaries surface area and triple junctions after the UNSM process enables the formation of twice the amount of δ phase compared to the as-received Inconel 718 bulk sample. The area fraction of the grain boundary covered by δ phase was of 9.87% in the UNSM region while in the bulk the area fraction was 4.09%. In summary, it was found that after UNSM process, the annealing at 1000 °C for 10 min and water quenching promoted the transformation of γ″ to form δ phase on the grain boundaries, which reduces the intergranular corrosion susceptibility.

Effects of heat input and cooling rate during welding on intergranular corrosion behavior of high nitrogen austenitic stainless steel welded joints

Intergranular corrosion of welded joints using multi-strand winded welding wire was investigated using the double loop electrochemical potentiokinetic reactivation (DL-EPR) tests, intergranular corrosion tests and scanning Kelvin probe force microscopy (SKPFM) tests. The results revealed welded joint in high heat input and low cooling rate shows the highest DOS value, and undergoes the most important weight loss during intergranular corrosion tests. While welded joint in low heat input and cooling rate has the highest corrosion resistance. SKPFM also indicated that welded joint in the low cooling rate and heat input presents low potential and is not easy for electrochemical reaction.

Effect of Flux Cored arc Welding Process Parameters on Corrosion Resistance of Austenitic Stainless Steel Claddings

Abstract Cladding process can be used to create corrosion resistant surfaces from low cost materials. Austenitic stainless steel has been extensively used for weld cladding. It has excellent corrosion resistance and good weldability. In this context, experiments were conducted by depositing AWS E316LT1-1/4 stainless steel on to AISI 1020 carbon steel and the effects of flux cored arc welding (FCAW) process parameters on pitting and intergranular corrosion was investigated. Response surface methodology (RSM) based central composite design (CDD) was used to predict and develop the mathematical models for process parameters on corrosion resistance. The responses of interest were obtained by double loop electrochemical potentiokinetic reactivation (DLEPR) and potentiodynamic polarization tests. The process parameters analyzed were the wire feed rate, welding voltage, welding speed and nozzle to plate distance. All RSM models developed were statistically significant and presented good adjustments. The results indicated that the process parameters are important in determining the degree of sensitization and pitting potential, and the interaction between parameters cannot be neglected.

On the Role of Grain Size and Carbon Content on the Sensitization and Desensitization Behavior of 301 Austenitic Stainless Steel

The effect of grain size in the range 72 to 190 μm and carbon content in the range 0.105–0.073 wt.% on the intergranular corrosion of the austenitic stainless steel 301 has been investigated. Grain boundary chromium depletion has been studied directly using energy dispersive X-ray spectroscopy combined with scanning transmission electron microscopy and indirectly using double loop electrochemical potentiokinetic reactivation tests. In addition, chromium depletion has been modelled using the CALPHAD Thermo-Calc software TC-DICTRA. It is shown that the degree of sensitization measured using the double loop electrochemical potentiokinetic reactivation tests can be successfully predicted with the aid of a depletion parameter based on the modelled chromium depletion profiles for heat treatment times covering both the sensitization and de-sensitization or self-healing. Additionally, along with intergranular M23C6 carbides, intragranular M23C6 and Cr2N nitrides that affect the available Cr for grain boundary carbide precipitation were also observed.

Effects of pre-strain on sensitization and interganular corrosion for 304 stainless steel

The degree of sensitization (DOS) and the susceptibility to interganular corrosion (IGC) were investigated for pre-strained 304 stainless steel (SS) following thermally aged at 750 °C for one hour. The maximum pre-strain applied to the steel is 20%.The microstructures of 304 SS subjected to pre-strain and thermal aging were observed. The volume fraction of strain-induced α'-martensite and the vickers hardness were measured, respectively. The double loop electrochemical potentiokinetic reactivation (DL-EPR) tests were used to characterize the DOS as a function of various pre-strain levels. The tests in copper‑copper sulfate‑sulfuric acid (16%) solution were performed to evaluate the susceptibility to IGC. The morphologies of the cracks caused by IGC were observed. The results show, although pre-strained 304SS had been thermally aged at 750 °C for one hour, strain-induced α'-martensite and shear bands in steel are most retained and increase with pre-strain, causing the strengthening effect and leading to the increase in hardness. The DOS and the susceptibility to IGC also increase with pre-strain. The higher the value of the DOS is, the more serious IGC becomes. Obvious intergranular cracks as well as the dissolution of grain boundaries and the grains began to appear in the steel with 5% pre-strain, and the attack had been deteriorated with the increasing pre-strain. When the pre-strain is reached 20%, the longest crack is about 250 μm and its width is about 38 μm, indicating that the severe IGC had occurred. No obvious reversion of martensite to austenite and desensitization occurred for 304 SS withstood the maximum pre-strain up to 20% and thermally aged at 750 °C for one hour.

Corrosion Assessment of ASME Qualified Welding Procedures for Grade 2101 Lean Duplex Stainless Steel

ASME qualified welding procedures do not guarantee suitable corrosion and passivation properties for lean duplex stainless steel welds. An evaluation of two ASME qualified welding procedures to optimize the corrosion performance of tungsten inert gas (TIG) welded grade 2101 duplex stainless steel using ER2209 weld consumable was conducted. The evolution of the microstructure was examined by optical and electron microscopy, ferrite-scope measurements, and scanning Kelvin probe force microscopy. An electrochemical mini-cell was then used to characterize the electrochemical behavior of different weld regions using the techniques such as the double loop electrochemical potentiokinetic reactivation test, standard potentiodynamic polarization tests, and cyclic potentiodynamic polarization. The fusion line was the most critical zone for localized corrosion for both welding procedures, due to the formation of Cr- and Mo-depleted zones, resulting in the highest degree of sensitization. The best performance was attributed to the weld face, due to the presence of higher Cr and Mo contents, highlighting the pitting corrosion resistance. A heat input range of 1.6 kJ/mm to 1.9 kJ/mm and low current density (WPS 1) indicated better corrosion performance of all weld regions. The electrochemical corrosion response was in all cases related to microstructural characteristics of the weld regions. The influence of weld parameters on microstructure development and corrosion performance is discussed.