Double Loop Electrochemical Potentiokinetic Reactivation Research Articles

Electrochemical Analysis on Intergranular Corrosion of Austenitic Stainless Steel Weld in Molten Nitrate Salt

An investigation of intergranular corrosion (IGC) sensitization in molten nitrate salts of austenitic stainless steel welds of AISI 304, AISI 304H, and AISI321 produced by GTAW with ER 308L and ER 347 fillers was performed. The degree of sensitization (DOS) to IGC was assessed using a double loop electrochemical potentiokinetic reactivation and pitting potential. It was found that DOS levels in weld zones were quite low, not exceeding 15%, while those in HAZs were up to 60% after exposure at 600 °C for 300 h. The low DOS levels were due to low carbide precipitation. However, another cause of DOS was the delta-ferrite to sigma transformation in weld zones. Linear sweep voltammetry was used to quantify the sigma phase.

Physical properties and microstructural corrosion propertiesof Laser-bent 304 Stainless Steel Sheets

Laser treatment by laser bending is a technique of treating sheet metal by thermal residual stresses generated by laser assisted heating without any externally applied mechanical forces. The advantages of laser assisted bending over conventional bending include flexibility of non-contact processing, amenability to materials with diverse shape/geometry, and high precision/productivity. Up to now, most of previous work on laser bending was concentrated on study of bending angles/shapes with laser operating conditions, and modelling of the thermal process, however, no work has been reported on corrosion behavior of the laser-bended components. Since the laser bending is a thermal process, it is believed that the thermal process could not only alter microstructure, but also affect corrosion performances, such as intergranular corrosion, due to possible sensitisation caused by repeating laser scans. The aim of the present work is to investigate corrosion performance of laser-bent 304 austenitic-steel sheet with thicknesses of 2 mm and 1.5 mm, respectively, using a 2 kW CO2 laser. After laser treatment, the laser-bent samples were characterised in terms of microstructural change within bent zones consisting of melt-zone and heat-affected zone, using optical microscopy and scanning electron microscopy (SEM). Hardness profiles were obtained across the bent zones. Corrosion performances of the laser-bent evaluated by means of electrolytic etching in oxalic acid and double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests. Laser bending is a thermal process, it is believed that the thermal process could not only alter microstructure, but also affect corrosion performances, due to possible sensitisation caused by repeating laser scans.

Effect of Tempering Temperature on Microstructure and Intergranular Corrosion Property of 2205 Duplex Stainless Steel

The effect of tempering temperature on the microstructure and intergranular corrosion property of 2205 duplex stainless steel (DSS2205) was determined using a boiling acid intergranular corrosion test (boiling 65% nitric acid and 50% sulfuric acid–ferric sulfate), an optical microscope, a transmission electron microscope, and a double-loop electrochemical potentiokinetic reactivation (DL-EPR) test. The results show that the ferrite content of the DSS2205 is about 50% in a specimen close to a solid solution state (1050 °C for 1 h, then water-quenched) when tempered at 675 °C–725 °C for 1 h. As the tempering temperature rises to 750 °C–800 °C for 1 h, the ferrite content drops gradually from 49% to 35%. M23C6, FeCr (σ phase), and Cr2N phases are precipitated when the specimen is tempered at 675°C–800 °C for 1 h. When the tempering temperature rises to 750 °C–800 °C for 1 h, the content and size of σ phase increase significantly. In the boiling acid intergranular corrosion test, when the specimen is tempered at 675 °C–725 °C for 1 h, the corrosion rate is higher than when it is tempered at 750 °C–800 °C for 1 h. In the DL-EPR test, when the specimen is tempered at 675 °C–800 °C for 1 h, the intergranular corrosion sensitivity rises gradually. External polarization is added during the DL-EPR test, and the test principle is different from that of the boiling acid intergranular corrosion test, resulting in a different sensitivity to intergranular corrosion compared to boiling acid intergranular corrosion.

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

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

Effect of Ultrasonic Shot Peening on Microstructure and Corrosion Properties of GTA-Welded 304L Stainless Steel

Austenitic stainless steels used in structural applications suffer from stress corrosion cracking due to residual stresses during welding. Much research is being conducted to prevent the stress corrosion cracking of austenitic steels by inducing compressive residual stresses. One method is ultrasonic shot peening (USP), which is used to apply compressive stress by modifying the mechanical properties of the material’s surface. In this study, 304L stainless steel was butt-welded by gas tungsten arc welding (GTAW) and subsequently subjected to compressive residual stress to a depth of 1 mm from the surface by a USP treatment. The influence of USP on microstructural changes in the base metal, the HAZ and weldment, and the corrosion properties was analyzed. A microstructural analysis was conducted using SEM-EDS, XRD, and EBSD methods alongside residual stress measurements. The surface and cross-sectional corrosion behavior was evaluated and analyzed using a potentiodynamic polarization test, electrochemical impedance spectroscopy (EIS) measurements, a double-loop electrochemical potentiokinetic reactivation (DL-EPR) test, and an ASTM A262 Pr. C test. The surface was deformed and roughened by the USP. The deformed areas formed crevices, and the inside of the crevices contained some cracks. The crevices and internal cracks caused pitting, which reduced the resistance of the passivation film. The cross-section was subjected to compressive residual stress to a depth of 1 mm from the surface, and the outermost area of the cross-section had fine grain refinement, forming a solid passivation film that improved the corrosion resistance.

Influence of ageing treatment time on microstructural evolution, sensitization and self-healing of Inconel 600

Samples of Inconel 600 were thermally aged at 739 °C from 1 up to 300 h followed by water quenching. The changes in microstructure were characterized and its effect on sensitization was evaluated by subjecting the heat-treated samples to double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests. The evolution of the microstructure was analysed in terms of grain size, carbide formation and chromium diffusion by optical and scanning electron microscopy. The results showed that the formation of chromium carbides is rapid at this temperature, increasing the density of carbides at the grain boundaries in the early stages of the ageing treatment. The formation of Cr7C3 and Cr23C6 along the grain boundaries and within the austenitic matrix modified the corrosion behaviour as seen in the measurements of the degree of sensitization (DOS). A decrement in DOS was seen with ageing time up to 200 h, due to the redistribution of chromium, induced by the equilibrium between chromium at grain boundaries and within the matrix. Electron backscattering diffraction suggested that self-healing of the alloy may be due to the mechanism of near-boundary gradient zones, where the increased number of dislocations leads to precipitation of carbides by promoting diffusion and thereby decreasing the Cr-depleted zones.

Intergranular Corrosion Analysis of Austenitic Stainless Steels in Molten Nitrate Salt Using Electrochemical Characterization

This study investigates the influence of molten nitrate salt exposure on the intergranular corrosion (IGC) behavior of three grades of austenitic stainless steel (namely, AISI 304, AISI 304H, and AISI 321H). Two electrochemical techniques, double loop electrochemical potentiokinetic reactivation and potentiodynamic polarization methods, are applied after stainless steel is exposed to 600 °C molten nitrate salt, 60% NaNO3, and 40% KNO3 for varying immersion durations. Corrosion morphology is examined using optical microscopy and scanning electron microscopy images to assess susceptibility to IGC. IGC is prompted by the presence of chromium carbides at grain boundaries, which leads to chromium depletion around these carbides. The findings of the experiments reveal distinct IGC behavior among stainless steel grades. For AISI 304, the degree of sensitization (DOS) increases as exposure time progresses. However, AISI 304H and AISI 321H stainless steel exhibit diminishing DOS after 100 and 10 h of exposure, respectively. This trend is attributed to desensitization or the healing effect when stainless steel is exposed to molten salt for a prolonged time. The depletion and recovery of Cr near grain boundaries are confirmed by the inverse relationship to DOS of pitting potential.

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

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

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 %).

Tensile Effect on Sensitization and Corrosion Resistance of Stainless Steels

The purpose of this research is to investigate cold work effect on corrosion behavior of stainless steels of type AISI 304, AISI 316L and AISI 2101. The specimen were processed through cold work simulated by the tensile test at various percentages elongation and then tempered at 600°C for different time intervals. Two electrochemical techniques and hardness test were utilized in this investigation. The first technique was the double loop electrochemical potentiokinetic reactivation (DL-EPR) that yields the values of degrees of sensitization (DOS). The second one was the cyclic potentiodynamic polarization (CPP) to study the ability of the metals to build or repair the damaged films under localized corrosion. The DOS values from the DL-EPR tests of the samples under the same heating conditions suggested that specimens drawn at higher percentage elongation tend to have more chromium depleted areas. Also, the test specimens with more exposure time to heat were more prone to chromium deficiencies. As for the ability of the metal to create or repair damaged film after corrosion has occurred, it was found that AISI 316L had hihger corrosion resistance than AISI 304. However, AISI 2101 stainless steel had highest corrosion resistance. In this study, it was also found that AISI 2101 did not exhibit pitting corrosion, but the crevice corrosion. This could be due the fact that AISI 2101 which is a duplex stainless steel has high corrosion resistance and could trigger the crevice corrosion take place before the pitting initiation.

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.

The high temperature degradation of ferritic stainless steel in solid carbon atmospheres

Stainless steel is widely used for many components and parts in coal-fired thermal power plants. AISI 430 ferritic stainless steel (FSS) is one common grade to combat the degradation at high temperatures in coal combustion atmospheres containing flue gas, coal ash, and soot (impure solid carbon particles). However, the effect of the solid carbon particles on the degradation of FSS needs to be clarified. Graphite powder was used to simulate solid carbon atmospheres for investigating the degradation of AISI 430 at high temperatures of 1150℃ to 1350℃ in coal-fired boilers. After the carbothermic reduction, the mass gain of a pre-oxidized sample at 750℃ was approximately 0.0793 mg⸳cm-2 and increased when increasing the reduction temperature. The peak of Fe2O3 and Cr7C3 were detected by X˗ray diffraction (XRD) after the oxidation and reduction test, respectively. Besides, the degree of sensitivity (%DoS) of the samples was measured by double loop electrochemical potentiokinetic reactivation (DL-EPR) technique and increased around 30 times after heating the pre-oxidized sample to 1150℃.

On the Intergranular Corrosion Susceptibility of 304 Stainless Steel with Ultrafine Grains and Comparison with Micrometer Austenitic Grains Counterpart

The intergranular corrosion (IGC) susceptibility of ultrafine grains (∼430 nm) and micrometer grains (∼3.1 µm to 9.8 μm) 304 stainless steel obtained by cryogenic rolling and reversion annealing treatments were studied. Transmission electron microscopy results showed that after sensitized treatment at 650°C for 2 h, the micrometer grains were sensitized with many M23C6 precipitates at the grain boundaries, while no precipitates were in the ultrafine grains. The immersion corrosion tests in H2SO4-CuSO4 solution showed that ultrafine grains exhibited weaker IGC attacks than micrometer grains. The double-loop electrochemical potentiokinetic reactivation tests demonstrated the degree of sensitization decreased from 26.61% to 1.52% with the grain ultra-refinement from micrometer to ultrafine. Corrosion studies indicated that the ultrafine grains exhibited lower IGC susceptibility compared with micrometer grains. According to the findings, the large number of grain boundaries generated by grain ultra-refinement inhibited M23C6 precipitates at the grain boundaries during the sensitized process, thereby reducing the susceptibility of ultrafine grains to IGC.

Sensitization of 316L Stainless Steel made by Laser Powder Bed Fusion Additive Manufacturing

Additively manufactured (AM) 316L stainless steel (SS) manufactured by laser powder bed fusion (L-PBF) and wrought 316L SS were subjected to sensitization heat treatments at 700°C up to 100 h. Using two evaluation methods, double-loop electrochemical potentiokinetic reactivation (DL-EPR) and ditching tests, degree of sensitization (DOS) and intergranular corrosion (IGC) susceptibility was evaluated. It was found that the wrought samples showed slightly lower IGC susceptibility compared to their AM counterpart. DOS and IGC attacks increased with sensitization time for all samples. Dislocation cellular structures were found to have little to no impact on DOS and IGC for the AM samples. Sensitized at 100 h, the AM sample showed significant Cr depletion along high-angle grain boundaries (12.35 wt% on average) and exhibited Cr carbide precipitation. Mo-rich particles along grain boundaries were also observed. The DL-EPR test attacks the surface oxide film and grain boundaries while the ditching test attacks the melt pool boundaries and grain boundaries (IGC and pitting). Changes to the DL-EPR and ditching standards for AM application have been proposed in this work.

Microstructure and electrochemical evaluation of ER-308L weld overlays on AISI 321 stainless steel for repair applications

Weld overlaying is a promising and economical solution for repairing stainless steel structures. The present work deals with the deposition of ER-308L weld overlays on AISI 321 plate with Robotic gas metal arc welding setup. Microstructural examination depicts the presence of dendritic structures (equiaxed and columnar) and ferritic stringers. Energy-dispersive X-ray analysis confirmed the absence of precipitates and phases susceptible to corrosion in the as-deposited weld overlays. Specimens from ER-308L (weld overlay) and ER-308L&321-SS (interface) regions were subjected to electrochemical analysis using the Potentiodynamic polarization and double-loop electrochemical potentiokinetic reactivation measurements. Both overlay and interface sections exhibited comparable corrosion rate – 3.30 and 3.62 mils penetration per year, respectively with different passivation behaviour. The improved passivation behaviour of the ER-308L weld overlay authenticates the presence of δ-ferrite stringers. Double-loop electrochemical potentiokinetic reactivation tests highlighted the absence of sensitization on both weld overlay and interface specimens. This study presents the initial assessment results for repairing structural steel components subjected to corrosion damage.

High temperature degradation of AISI 304L stainless steel in atmospheres containing CO of biomass to liquid plants at 800 °C

Austenitic stainless steel (ASS) AISI 304L is required for the tube parts in biomass power plants due to high-temperature corrosion resistance properties. However, the high percentage of 35–40 % carbon monoxide and 25–35 % carbon dioxide in biomass atmospheres lead to deterioration in the ASS steels at high temperatures, resulting in chromium carbide formation and intergranular corrosion (IGC). This present work explored the degradation of the stainless steel AISI 304L in a simulated 15 %CO-N2 atmosphere at 800 °C at different testing times between 15 and 60 min. Besides, the ASS samples were pre-oxidized at 800 °C for six hours in air atmospheres before the test. Field emission scanning electron microscope (FE-SEM) and energy dispersive spectroscopy (EDS) were used for carbide characterization. The mass gain of samples was seemingly not changed after prolonging the reduction times. A double loop electrochemical potentiokinetic reactivation (dl-EPR) was used to observe the degree of sensitivity (%DOS) of AISI 304L samples. After the reduction test, the %DOS of samples exceeded the intergranular corrosion severity of 1 %, and the highest %DOS was 7.73 % after the reduction time of 45 min.

The Failure Mechanism of the 316 SS Heat Exchanger Tube in the Geothermal Water Environment.

In this work, the intrinsic reason for the premature failure of a 316 stainless steel heat exchanger tube in geothermal water environment is disclosed. The chemical composition of the tube was tested, and the microstructure was examined for material inspection. Fracture morphology and secondary cracks were analyzed, and electron backscattered diffraction was applied to explore the crack propagation mode. The corrosion morphology was observed. The electrochemical behavior was studied with cyclic polarization and double-loop electrochemical potentiokinetic reactivation. It is found that the main failure cause was stress corrosion cracking (SCC). Attacked by chloride ions, the tube is susceptible to SCC under the residual stress as a result of the substandard Mo and Ni content. The SCC mechanism is localized anodic dissolution, and the propagation mode is a mixture of transgranular SCC and intergranular SCC.

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.

DL-EPR vs. LSV-KOH: from simple detection of deleterious phases to analysis of morphology and high accuracy on determination of sigma phase in UNS S32750 stainless steel

ABSTRACT An optimised electrochemical technique, linear sweep voltammetry (LSV) in KOH solution (LSV-KOH) was confronted with the double loop electrochemical Potentiokinetic reactivation (DL-EPR) to verify which is the best technique to quantify deleterious phases (DP) in a UNS S32750 super duplex stainless steel. DL-EPR presented an error in determination of deleterious phases in the range of 6.9–100%. LSV-KOH show a better accuracy in quantification of deleterious phases volume fractions, the error range for LSV-KOH was 2.4–26.2%. When the morphology of sigma changes from lamellar to divorced, was observed modifications on LSV-KOH voltammograms. In specimens where lamellar morphology is present, the voltammograms show two peaks curve. Meanwhile, in specimens where occurred also divorced precipitation of sigma the voltammograms changes to a 3 peaks curve. This feature is due to the dissolution of sigma phase boundaries which has different compositions according to the form of sigma precipitation.

Ferritic-induced high-alloyed stainless steel produced by laser powder bed fusion (L-PBF) of 2205 duplex stainless steel: Role of microstructure, corrosion, and wear resistance

Stainless steels are dominant in application where combination of corrosion and mechanical resistances are requested. They have gained scientific attention in additive manufacturing (AM) field, and stainless steel products produced by AM find use in a wide array of applications. This study aims to evaluate the microstructure, the wear and corrosion resistances of a high-alloyed ferritic-induced stainless steel (FSS) obtained by laser powder bed fusion (L-PBF), from a duplex stainless steel (DSS) precursor, SAF 2205, used to produce gas-atomized powders. No heat treatment was applied to the ferritic-induced L-PBF stainless steel to restore the duplex structure. The L-PBF FSS was dense and high alloyed with Cr, Mo, and N (pitting resistance equivalent number, PREn, ~33), presenting a negligible corrosion current density in the order of 10−7 A.cm−2, with an enlarged passivation window ~ 1.2 V with respect to the corrosion potential, restraining the current density at the passivation plateau below 10−4 A.cm−2. Also, double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests indicated unsensitized L-PBF FSS, with negligible degree of sensitization caused by the cyclic heat input during building up of the specimen. Besides be corrosion-resistant, the L-PBF FSS was hard (290 HV0.5) and wear-resistant against sliding wear, with a reduced specific wear rate of 5 × 10−4 mm3·N−1·m−1. The corrosion resistance of the L-PBF specimen (almost fully ferritic microstructure) was comparable to its hot-rolled precursor that displayed a duplex microstructure, albeit slight inferior. The L-PBF FSS was harder and more wear resistant than the DSS precursor. These results point out to the formation of a new high-alloyed ferritic-induced stainless steel, which are corrosion-resistant in chloride-rich medium and wear resistant in sliding conditions.