Double-loop Electrochemical Potentiokinetic Reactivation Tests Research Articles

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.

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.

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.

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.

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.

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.

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.

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.

Detecting Intergranular Phases in UNS N07725: Part I. Adapting the Double-Loop Electrochemical Potentiokinetic Reactivation Test

Precipitation hardened nickel-based Alloy 725 (UNS N07725) has been proven susceptible to hydrogen embrittlement in oil and gas production. Moreover, current standards cannot differentiate unequivocally between acceptable and affected microstructures, making the development of an appropriate quality control test a priority for the oil and gas industry. Recently, Alloy 725 failures were associated with the precipitation of nanoscale Cr- and Mo-rich phases at grain boundaries (GBs). Since these intergranular precipitates could lead to Cr- and Mo-depletion along the decorated GBs (i.e., sensitization), the double-loop electrochemical potentiokinetic reactivation (DL-EPR) test was explored as an alternative approach to detect affected microstructures. Herein, the DL-EPR technique was optimized for Alloy 725 by testing three types of samples with different degrees of sensitization, including a full GB-decorated microstructure that was shown prone to hydrogen embrittlement in oil and gas service. The optimized DL-EPR test conditions were 2 M HCl + 1 M H2SO4 + 10−4 M KSCN aqueous solution at 30 °C with a 1.667 mV s−1 scan rate and a vertex potential of Ecorr + 700 mV. Results were reproducible and consistent with the degree of GB decoration determined using metallographic methods. The test procedure developed herein could lead to the standardization of the DL-EPR method for Alloy 725.

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 Heat Treatment on the Microstructure Evolution and Sensitization Behavior of High-Silicon Stainless Steel

The microstructure and sensitization behavior of high-silicon stainless steel containing 4.0 wt.% Si was studied after stabilization heat treatment followed by isothermal heat treatment. Isothermal heat treatment for 6 h was carried out on the alloy at two different temperatures, viz. 500 and 750 °C, after stabilization heat treatment at 950 °C. Phase fraction of the high-silicon stainless steel remained constant after isothermal heat treatment. Fine (Ti,C) precipitates of angular and spherical morphology were observed throughout the microstructure, in all the heat-treated sample conditions. Their number density was measured and related to the processing conditions that they were exposed to. Additionally, the presence of Cr and C was confirmed by compositional analysis, and they appeared at the grain boundary in the sample isothermally heat-treated at 750 °C for 6 h. Sensitization study of the heat-treated samples was carried out with the help of double-loop electrochemical potentiokinetic reactivation test in specially prepared acid etchant, and it was observed that degree of sensitization depended strongly on the formation of Cr-depleted zone along the grain boundary. Based on our study, we conclude that stabilization heat treatment significantly reduces the sensitization behavior, even for steel containing significant fraction of Si.

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.

Effect of Plastic Deformation on Passivation Characteristics of Type 304 Stainless Steel

Electrochemical response of Type 304 stainless steel, with different levels of plastic deformation, was investigated in solutions containing different amounts of H2SO4 and KSCN at room temperature. Potentiodynamic polarization and double-loop electrochemical potentiokinetic reactivation (DL-EPR) techniques were used to characterize different levels of plastic deformation. The results have indicated that DL-EPR values and passivation current density increased with increase in plastic deformation. The DL-EPR values were correlated well with hardness and fraction of strain-induced martensite. Attack during DL-EPR tests occurred on strain regions within the matrix such as dislocations in austenite and martensite and/or interface between austenite and martensite. No well-defined correlation was observed between the levels of plastic deformation and potentiodynamic polarization behavior.

On the unusual intergranular corrosion resistance of 316L stainless steel additively manufactured by selective laser melting

The intergranular corrosion (IGC) resistance of 316L stainless steel (316L SS) produced by selective laser melting (SLM) was investigated using microscopy analysis and electrochemical measurements. The IGC resistance of SLM-produced 316L SS, determined using a double-loop electrochemical potentiokinetic reactivation test, was found to be substantially higher than that of conventional 316L SS. This unusual behaviour was explained by the fact that no Cr-rich precipitates were detected for SLM-produced specimens after long-term sensitisation heat-treatment and those SLM-produced specimens exhibited a high frequency of twin boundaries and low-angle grain boundaries along with fine grains, leading to the avoidance of localised Cr depletion.