Double Loop Electrochemical Potentiodynamic Reactivation Research Articles

Improved Ability to Resist Corrosion of Selective-Laser-Melted Stainless Steel Based on Microstructure and Passivation Film Characteristics

The local corrosion resistance of forging and selective laser melting (SLM) 304 steels was explored by intergranular corrosion analysis, double-loop electrochemical potentiodynamic reactivation, dynamic polarization experimentation, structural analysis, and passivation film characteristics analysis. The ability to resist sensitization of SLM 304 steel is greater than that of forging 304 steel at a temperature of 650 °C for 9 h. Moreover, the pit corrosion resistance of forging and SLM 304 steels is weakened by sensitization, while the pit corrosion resistance of SLM 304 steel is much greater than that of forging steel. Therefore, SLM technology can improve the ability to resist sensitization and pit corrosion of 304 steel. Analysis showed that the ability to resist corrosion of the passivation film of SLM 304 steel is greater than that of forging steel. In addition, corrosion pits are easier to generate at the interface of forging steel and SLM 304 steel. The grain boundary corrosion of SLM 304 steel intensified while the corrosion of the melt pool boundaries weakened after the sensitization treatment, resulting in a decrease in pit corrosion resistance. The coupling effect of these different structures and passivation films decides the pit and sensitization resistance of forging and SLM 304 steels. Clarifying the corrosion mechanism of forging and SLM steels is of great significance for scientific research and the widespread use of SLM technology.

Sensitization characteristics for type 445 ultra-pure ferritic stainless steel with an electrochemical reactivation method

The sensitization characteristics of type 445 ultra-pure ferritic stainless steel (FSS) under different aging conditions were investigated by the optimized double loop electrochemical potentiodynamic reactivation (DL-EPR) test and microstructure observation. The solution 2 mol L−1 H2SO4 + 0.01 mol L−1 KSCN +0.5 mol L−1 NaCl and a scan rate of 100 mV min−1 are suitable for the DL-EPR test of 445 FSS. With the change of aging temperature from 500 to 900 °C, the degree of sensitization (DOS) slightly increases at first and then decreases, which reaches a maximum value at about 700 °C. Under the aging condition at 700 °C for 10 min to 2 h, the DOS value gradually increases, but it is less than 3% due to the low intergranular corrosion susceptibility. The aging treatment at 700 °C results in the precipitation of Laves phase and (Nb, Ti)(C, N), but no Cr-carbides. The sensitization may be related to the segregation of Cr atoms around (Nb, Ti)-precipitates at the grain boundaries.

Investigation of microstructural and corrosion behavior of 316LSi structure developed by wire arc additive manufacturing

SS316LSi austenitic stainless steel thin wall was deposited using wire arc additive manufacturing technology, which is known for its high deposition rate and effective material utilization. The optical micrograph revealed the formation of vermicular δ-ferrite, dispersed in the austenite matrix in the deposited wall. The X-ray diffraction patterns confirmed the formation of γ-austenite, δ-ferrite, and chromium carbide (Cr23C6) phases. The SEM and energy dispersive spectroscopy study exhibited the formation of intergranular precipitates. In the magnetic study, ferrite number was found from 3 to 7 in a different portion of the wall which revealed the formation of the ferromagnetic δ-ferrite phase. The Cr and Ni equivalent ratio, 1.66, ferrite number, and presence of vermicular δ-ferrite in the microstructure showed that ferrite austenite can be the mode of solidification in the alloy. The preferred grain growth direction was determined by electron backscattered diffraction analysis to be < 001 > orientation along the build-up direction. In the potentiodynamic polarization and double loop electrochemical potentiodynamic reactivation study, the corrosion rate and intergranular corrosion were correlated to the amount of δ-ferrite and Cr23C6 respectively. The degree of sensitization confirmed that the amount of Cr23C6 formed is least in the lowermost and highest in the central portion of the deposited wall.

Investigation on intergranular corrosion susceptibility of a nitrogen-containing austenitic stainless steel QN2109 by optimized DL-EPR tests

PurposeThe purpose of this study is to investigate the intergranular corrosion (IGC) susceptibility of a nitrogen-containing austenitic stainless steel QN2109. The intergranular corrosion (IGC) susceptibility of a nitrogen-containing austenitic stainless steel QN2109 was investigated.Design/methodology/approachThe double-loop electrochemical potentiodynamic reactivation (DL-EPR) tests were carried out. Scanning electron microscopy and atomic force microscopy were used to characterize the microstructure.FindingsThe optimized test condition for QN2109 was 1 M H2SO4 + 0.01 M NH4SCN at 40°C. The nose temperature of the temperature–time–sensitization (TTS) curve of QN2109 plot was approximately 750°C. Moreover, the IGC susceptibility started to appear at approximately 120 min. The Cr-depletion zone of QN2109 was generated by the formation of M23C6 rather than by the addition of nitrogen. The depth–width ratio of the grain boundaries after the DL-EPR tests decreased as the aging temperature increased. The degree of Cr depletion and size of the Cr-depletion zone at the grain boundary were reflected by the degree of sensitization and depth–width ratio, respectively.Originality/valueThe optimized test condition for DL-EPR tests of a nitrogen-containing austenitic stainless steel QN2109 was investigated. The TTS curve of QN2109 was first plotted to avoid IGC failure. The morphology of the Cr-depletion zone was reflected by the depth–width ratio.

Investigation of intergranular stress corrosion cracking in a failed 347H stainless steel furnace tube

A 347H stainless steel tube of a furnace of a hydroprocessing unit was cracked during operation shortly after the last shutdown. Both internal and external surfaces of the tube were exposed to sulfur compounds. Material characterization methods including tensile test, double loop electrochemical potentiodynamic reactivation test and fractography were used to investigate the failure causes. Based on stereo investigations, the external surface of the tube was identified as the crack initiation location. The microstructure evaluation and DL-EPR test showed that sensitization with a high DOS value of 18 occurred in the tube material. The sensitization has resulted in a dramatic reduction in the elongation parameter. Fractography examination exhibited a brittle fracture with presence of sulfide corrosion products on the fracture surface. Finally, due to the combination of the obtained results and additional observations including corrosion within grain boundaries and intergranular cracking, polythionic stress corrosion cracking was determined as the failure mechanism.

Electrochemical Studies of the Effect of Preannealing Temperature and Aging Time on the Degree of Sensitization in 2205 Duplex Stainless Steel

The degree of sensitization (DOS) of an aged 2205 duplex stainless steel preannealed at three solution temperatures was determined using the double loop electrochemical potentiodynamic reactivation method. Solution annealing at 1,250°C prior to 10 min aging at 850°C resulted in a much lower DOS than that of samples preannealed at 1,050°C and at 1,150°C. However, compared to other samples, the increase in DOS of the sample preannealed at 1,250°C was more significant after longer aging at 850°C. A promising correlation between the DOS and the volume fraction of the sigma phase was observed.

Thermal Aging Effects on Mechanical and Intergranular Corrosion Resistance of Super-Austenitic Stainless Steel exposed at 600˚C

Super-austenitic stainless steels (SASS) are alloys with better performance in industrial applications in comparison with standard degrees of austenitic stainless steels (ASS), mainly because of elevated Ni and Mo contents and residual control of certain elements because the improvement of refine techniques. These facts, promotes an improvement of mechanical properties and minimize detrimental effects of intergranular corrosion. Besides, some industrial applications involve high temperatures services, because these alloys families have an excellent creep and oxidation resistances performances. In this work, the objective is evaluate the microstructural and mechanical properties in high Mo SASS exposed at 600˚C in specimens aged until 2000 hours. Thus, hardness and toughness values and Double Loop Electrochemical Potentiodynamic Reactivation (DL-EPR) measurements were analyzed in several thermal aging in comparison to solution treatment condition. Complementary, analysis by Light Optical (LOM), of specimens tested by DL-EPR, and Scanning Electron Microscopies (SEM) in fractured surface from Charpy specimens were also analysed by SEM by backscattered electron mode (BSE). The results show that the aging provokes intergranular embrittlement by inhomogeneous distribution in grain boundaries of Mo rich phase precipitation.

Effect of Sensitization Time on Intergranular Corrosion of 347H Stainless Steel

In this paper, 347H stainless steel of three situation of solution, primary and secondary stabilizing with 0, 12, 24, 48 and 72 h accelerated sensitization heat treatment. The change of sensitization degree with time was studied by metallographic test, double-loop electrochemical potentiodynamic reactivation (DL-EPR) test, intergranular corrosion test and microscopic morphology observation. The result shows that the sensitization degree of the solution material increases rapidly and reaches the maximum value after 12h sensitization heat treatment. After that, it still belonged to severe sensitization situation, but the index gradually decreased. After stabilizing heat treatment, the sensitization degree of the material is lower than the situation of solution. After heat treatment for 48h, the material located on “possible sensitization” range, and the sensitization degree of the secondary stabilized material was always lower than is of the primary. It indicates that the sensitization of materials can not be completely inhibited by stabilizing heat treatment, and other anti-corrosion measures should be considered.

Effect of post-heat treatment on the mechanical and corrosion behaviour of SS316L fabricated by wire arc additive manufacturing

The current work addresses the effect of post-processing on the mechanical and corrosion properties of SS316L fabricated by gas metal arc welding (GMAW) based wire arc additive manufacturing (WAAM) technology. During the fabrication of thin-walled structure repeated remelting occurs, and solidification causes a change in the microstructure along the building direction. The as-fabricated samples were subjected to post heat-treatment involving solution treatment and sensitization. The mechanical properties of the as-deposited and heat-treated samples were compared. Kernel average misorientation (KAM) values reveal the reduction in dislocation density due to solutionizing. Cyclic potentiodynamic polarization (Tafel scan) and double-loop electrochemical potentiodynamic reactivation (DLEPR) studies confirm that post-heat treatment can enhance the corrosion resistance. The corrosion rate ranged between 0.5 and 3.0 mils per year (mpy) and suggests that SS316L as a inexpensive material with compact passive film formation. The combination of post-heat treatment and sensitization studies could be a promising post-processing technique to certify the WAAM processed stainless-steel components for critical applications.

Effect of recrystallization on degree of sensitization in nickel free austenitic stainless steel

PurposeEffect of grain size on degree of sensitization (DOS) was been evaluated in Nickel free steel. Manganese and nitrogen contained alloy is a Ni-free austenitic stainless steels (ASS) having type 202 grade. The main purpose of this investigation is to find the effect of recrystallization on the DOS of stainless steel after the thermo-mechanical processing (cold work and thermal aging).Design/methodology/approachIn the present investigation, the deformation of 202 grade analyzed using X-ray diffraction (XRD) and microstructural testing. Optical microstructure of Ni-free ASS has been done for cold worked samples with thermally aged at 900°C_6 h. Double loop electrochemical potentiodynamic reactivation test used for findings of degree of sensitization.FindingsNi-free ASS appears to be deformed more rapidly due to its higher stacking fault energy which gave results in rapid transformation from strain induced martensite to austenite in form of recrystallized grains, i.e. it concluded that as cold work percentage increases more rapidly recrystallization occurs. XRD results also indicate that more fraction of martensite formed as percentage of CW increases but as thermal aging reverted those all martensite to austenite. So investigation gives the conclusion which suggests that with high deformation at higher temperature and duration gives very less DOS.Originality/valueVarious literatures available for 300 series steel related to the effect of cold work on mechanical properties and sensitization mechanism. However, no one has investigated the effect of recrystallization through thermomechanical processing on the sensitization of nickel-free steel.

Experiment Investigation for Design on the Sensitization-Dependent Passivation Characteristics of AL-6XN in 3.5% NaCl Solution.

The sensitization and passivation characteristics of AL-6XN heat-treated for up to 24 h at 800 °C were evaluated through double loop electrochemical potentiodynamic reactivation (2 M H₂SO₄ + 0.01 M KSCN +2 M NaCl solution) and cyclic potentiodynamic polarization (3.5% NaCl solution) test methods. Furthermore, the dependence between the characteristic values (Epit, Erep, and degree of sensitization) was discussed. The σ phase was precipitated at the intergranular and intragranular in AL-6XN at 800 °C. When the heat treatment time exceeded 1 hour, AL-6XN occurred rapid sensitization due to the precipitation of σ phase. In a 3.5% NaCl solution, the passivation characteristics of AL-6XN with sensitivity were very poor and depended much on the degree of sensitization.

A detailed electrochemical analysis of sensitized AISI 410 martensitic stainless steel

In the present work, the effect of sensitization heat treatment on the intergranular corrosion (IGC) behavior of AISI 410 martensitic stainless steel was investigated using potentiodynamic electrochemical impedance spectroscopy (PDEIS) in conjunction with conventional methods including acid etching test, anodic potentiodynamic polarization, double-loop electrochemical potentiodynamic reactivation (DLEPR) and electrochemical noise (EN). Microstructural studies by scanning electron microscopy revealed that increasing tempering time up to 2h would be followed by more severe localized attack, but desensitization state attributed to specimens undergone prolonged tempering cannot be interpreted based on the standard test method. Susceptibility to IGC via DLEPR experiments indicates differences in the degree of sensitization (DOS) of specimens with varying tempering time, revealing a healing effect arising from prolonged tempering. The EN results elucidated the alloy tendency for active dissolution, but no obvious correlation can be established between the EN results and IGC behavior. The PDEIS data clarified that the charge transfer resistance values related to the AC responses in the middle of transpassive potential region can be utilized to investigate and compare the IGC susceptibility of the alloy. Moreover, the surface morphology of corroded samples under such experimental conditions can be used to qualitatively compare the severity of intergranular attack.

A survey on the passivity of tempered AISI 420 martensitic stainless steel

This research aims to investigate the impact of tempering and passivation times on the electrochemical and structural properties of passive films forming on the surface of martensitic stainless steel using double-loop electrochemical potentiodynamic reactivation, anodic potentiodynamic polarization, Mott-Schottky, Vickers microhardness and X-ray diffraction techniques. The results indicated that less internally-stressed samples possess denser and less defective passive films on their surfaces, which can be attributed to the presence of higher concentration of metal ions at the metal/solution interface during anodization process. Moreover, prolonged passivation times led to incorporation of more impurities into the growing passive film, deteriorating its electrochemical properties.

Comparative Study Between Sensitization Degree of the 0.4% Mo Austenitic Stainless Steel and UNS S31803 Duplex Stainless Steel

This study aims at assessing the degree of susceptibility to intergranular corrosion of the UNS S31803 duplex stainless steel and the 0.4% Mo austenitic stainless steel through the double-loop electrochemical potentiodynamic reactivation (DL-EPR) test. Both steels were subjected to isothermal treatments at 650ºC and 900ºC in order to compare the effect of temperature on the respective degrees of sensitization. It was observed that the 0.4% Mo austenitic stainless steel presented a higher degree of sensitization and, consequently, a greater susceptibility to the intergranular corrosion process when compared to the UNS S31803 duplex stainless steel in the two temperatures studied. It was also observed that, for both steels, the heat treatment at 900ºC resulted in a higher degree of sensitization when compared to the heat treatment at 650ºC.

Establishing the passive film stability formed at different depths from the surface of machined type 304 L SS

Using a novel approach, passive film stability was established at increasing depths from surface for machined and heat-treated (675 °C/1 h) type 304 L SS. Severely plastically deformed (SPD) top surface had nano/ultrafine grains till ∼8 μm depth. Double loop-electrochemical potentiodynamic reactivation test and potentiostatic test showed passive film over SPD layer was unstable. Film stability improved till ∼100 μm depth, beyond which it matched with that of mid-thickness (protective). Defect density (Mott-Schottky analysis) in film over SPD surface was 1.7–1.8 times higher than that over mid-thickness. Point defect model was used to explain the role of machining on passive film stability.

Eddy current technique as a nondestructive method for evaluating the degree of sensitization of 304 stainless steel

The application of eddy current (EC) technique in detection of sensitization before the occurrence of intergranular corrosion was investigated. For this purpose, EC experiments were performed on 304 stainless-steels sensitized at 650 °C and 750 °C for 1–20 hours. The double-loop electrochemical potentiodynamic reactivation (DL-EPR) and microstructural investigations were also carried out to evaluate the degree of sensitization (DOS). The DL-EPR results showed that the maximum sensitivity occurred within 4 h at 650 °C. The EC results indicated a reduction in voltage and impedance with increasing DOS. Finally, empirical relationships between DL-EPR and EC test results were developed.

Effects of High-Temperature Aging on Precipitation and Corrosion Behavior of a Ni-Cr-Mo-Based Hastelloy C276 Superalloy

The effects of short-term aging on the precipitation and corrosion behavior of a Ni-Cr-Mo-based superalloy Hastelloy C276 were studied in this work. Aging treatments were performed at 800 °C for 1 h, 2 h, 4 h, 8 h, 12 h and 15 h. It is found that M6C carbide, which is rich in Mo and W, is prone to precipitate at grain boundary when the aging time exceeds 4 h, associated with the formation of a Mo and W depletion zone adjacent to the grain boundary. The potentiodynamic polarization curves in 3 wt.% NaCl solution show no obvious differences among the aged samples. But the surface morphologies of samples after double-loop electrochemical potentiodynamic reactivation method (DL-EPR) tests in HCl solution present typical intergranular corrosion attack. The calculated degrees of sensitization based on the DL-EPR curves increase dramatically with the increase in aging time, which is consistent with the precipitation trends. It is recommended that hot working of Hastelloy C276 superalloy should avoid high temperature exposure (e.g., at 800 °C more than 4 h) or do heat treatment again after high-temperature processing.

Evolution of Metallurgical Phases and Its Co-relation with Mechanical Properties and Corrosion Resistance of 22Cr–5Ni–3Mo and 16Cr–10Ni–2Mo Dissimilar Weldments

22Cr–5Ni–3Mo is highly sustainable in aggressive environments and also better substitute of 16Cr–10Ni–2Mo austenitic grade, which illustrates the unavoidability of dissimilar joint in many corrosive environments for cost effectiveness. The information about welding parameters, microstructure, mechanical properties, texture, and corrosion resistance behavior in highly harsh environment of dissimilar weld by common fusion welding is limited. The present study addresses dissimilar welds between 22Cr–5Ni–3Mo and 16Cr–10Ni–2Mo stainless steels, employing SMAW process by using two different weld parameters (based on current) and investigates the microstructural evolution and correlated with mechanical and corrosion resistance of the weldment. The microstructural studies were done by using an optical microscope and scanning electron microscopy. Mechanical properties are studied in terms of tensile strength and hardness. The electrochemical assessments were carried out including modified double-loop electrochemical potentiodynamic reactivation test and modified tensile test. EBSD analysis was carried out to know the grain size variation quantitatively in the weld metal and/or heat affected zone. Analyzed results showed the well-balanced ferrite/austenite amount for both high and low heat input. 16Cr–10Ni–2Mo base metal showed twin boundaries, whereas 22Cr–5Ni–3Mo base metal showed banded structure of ferrite and austenite. EBSD analysis revealed that the low heat input weld showed less grain growth as compared to high heat input with austenite orientation, because of lower misorientation. With increasing heat input, ferrite content of the weld region decreases resulting in lower hardness and tensile strength. Modified tensile test in chloride environment affected the plasticity of the materials. High heat input helps to increase the intergranular corrosion resistance by increasing austenite phase formation.

Influence of Sensitization on Mechanical Properties of AISI 304 Stainless Steel under High-Temperature.

This study investigated influence of sensitization on the mechanical properties of AISI 304 stainless steel. The small and tensile specimens were isothermally aged at 650 °C for up to 1000 hours. Microstructure and precipitates of these specimens were characterized by scanning electrode microscope with energy dispersive X-ray spectroscopy. The degree of sensitization of aged specimens was measured by double loop electrochemical potentiodynamic reactivation method. The tensile test was conducted to evaluate mechanical properties of these specimens. With the progress of aging, the degree of sensitization value drastically increased to more than 50% as Cr-rich carbides were precipitated in the austenite grain boundaries. Also, the mechanical properties were degraded, and fracture mode drastically changed from ductile to brittle. As a result, the surface damage and degradation of mechanical properties were correlated with the degree of sensitization value.

Characterization of Passive Films Formed on As-received and Sensitized AISI 304 Stainless Steel

The current research of corrosion resistance of stainless steels mainly focuses on characterization of the passive films by point defect mode and mixed-conduction model. The corrosion resistance of the passive films formed on as-received and sensitized AISI304 stainless steel in borate buffer solution were evaluated in this paper. The degree of sensitization and corrosion resistance of AISI304 stainless steels was evaluated by double loop electrochemical potentiodynamic reactivation and electrochemical impedance spectroscopy. The passive films formed on the stainless steels were studied by XPS technique. It was found that as-received specimen had higher pitting corrosion potential and corrosion resistance than sensitized one. The Mott-Schottky results showed that sensitized stainless steel had more defects in the passive film than as-received one. The compositions of the passive films were mainly Cr and Fe oxides according to XPS results.