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.

Study on Grain Boundary Sensitization in Heat Affected Zone of Austenitic Stainless Steel and its Evaluation Method

In this paper, the simulated welding structure at different areas of heat affected zone (HAZ) of austenitic stainless steel are prepared by heating furnace, and the deference of intergranular corrosion resistance of all samples were determined by comparative methods of oxalic acid etching, hot-acid test and electrochemical potentiodynamic reactivation tests (EPR), and the effectiveness of all those methods were analysis and determination at the same time. The result shows that the susceptibility degree at different areas of HAZ increased with the heating temperature increase, but all structures can pass hot-acid test and EPR test, indicating that it could maintain good performance of intergranular corrosion resistance. The hot-acid test results have good consistency with EPR test , but inconsistent with oxalic acid etching test. The results show that the evaluation of oxalic acid etching is too much strict to be used as a judgment method, so that not suitable of use for inspection.

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.

Effects of Stabilizing Heat Treatment on Properties of Simulated Microstructures in the Heat-Affected Zone of 347H Stainless Steel

In this paper, two kinds of heat affected zone (HAZ) simulation structures of 347H stainless steel, which are coarse grain zone (CGZ) and unmixed zone (UZ), were prepared by thermal simulator. The material properties of toughness, reheat crack susceptibility and intergranular corrosion susceptibility of the two kinds of HAZ simulation structures were studied by impact test, high temperature tensile test, electrochemical potentiodynamic reactivation (EPR) test and micro morphology test. The result shows that CGHAZ had better toughness. But after the stabilizing heat treatment, it was weakened while that of the UZ was enhanced. The reheat crack susceptibility of the CGZ and UZ both increases after stabilization heat treatment, and the tendency of the UZ are more obvious. Stabilizing heat treatment has a certain effect on the prevention of sensitization process, which can improve the intergranular corrosion resistance of the material. Stabilizing heat treatment is double-edged to 347H HAZ, and it needs combined with the specific situation to used.

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 Twice Stablizing Heat Treatment on the Properties of 347H Stainless Steel

In this study, the mechanical properties and intergranular corrosion properties of 347H austenitic stainless steel were studied by tensile test, impact test, double-ring electrochemical potentiodynamic reactivation test (DL-EPR) and microstructure observation in three states of solution, primary and twice stabilized state. Results showed that the key mechanical properties of 347H stainless steel under three different conditions had little change, and the mechanical properties at room temperature were not affected by the stabilizing heat treatment. After 12h of sensitization, the solution material showed obvious sensitization behavior, and the Ir/Ia index exceeded 0.3, indicating that the material entered the range of complete sensitization. Both primary and twice stabilizing heat treatment can significantly reduce the occurrence time of sensitization and prevent the sensitization process. However, the stabilizing heat treatment cannot completely prevent the material sensitization, and it must be combined with other methods .

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.

Effect of heat treatment on the chromium‐depleted zones of a high carbon martensitic stainless steel

AbstractHeat treatments are commonly applied in the martensitic stainless steels to achieve high wear resistance and hardness; however, they can lead to a decrease in corrosion resistance due to the precipitation of chromium‐rich carbides. The present work investigates the effect of austenitizing and tempering parameters on the microstructure, hardness, and passivation and reactivation currents of AISI 440C. A double‐loop electrochemical potentiodynamic reactivation test is performed to evaluate the dissolution and precipitation of chromium‐rich carbides. The volume fraction of phases after heat treatment is estimated by X‐ray diffraction. Results show that an increase in the austenitizing temperature leads to an increase in the carbide dissolution, and also in the amount of retained austenite. As a consequence, low passivation and reactivation currents are obtained. When austenite is the main phase, the increase of austenitizing time contributed to an increase in the degree of sensitization due to austenite grain boundary carbon enrichment. However, the presence of austenite after tempering kept part of the carbon in a solid solution resulting in low passivation and reactivation currents.

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.

Application limits and sensitisation behaviour of the manganese‐ and nitrogen‐alloyed austenitic stainless steel P2000 (X13CrMnMoN18‐14‐3)

AbstractNickel‐free high‐nitrogen‐alloyed stainless steels like the P2000 (X13CrMnMoN18‐14‐3) were developed to enhance the strength and corrosion resistance of austenitic stainless steels like 304 and 316 while keeping the typical high ductility. The mechanical and corrosive properties of P2000 were investigated and compared with 304 and 316 to highlight the application opportunities of this new alloy. The microstructure of the solution‐annealed condition was characterised by electron backscatter diffraction and the mechanical properties were studied by uniaxial tensile tests, Charpy impact tests and hardness measurements. The passivation behaviour was analysed using the electrochemical potentiodynamic reactivation, whereas the pitting corrosion resistance was compared by pitting potentials and pitting temperatures. However, secondary thermal influences or suboptimal heat treatment can impair the corrosion resistance due to the precipitation of secondary phases and the resulting sensitisation. Thermodynamic calculations and artificial ageing treatment in the range of 500–900°C for up to 100 h were used to determine critical time–temperature parameters for sensitisation. The microstructure of the various aged states was evaluated by scanning electron microscopy and compared with the degrading corrosion resistance characterised by the KorroPad method.

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.

Corrosion Susceptibility and Allergy Potential of Austenitic Stainless Steels.

Although called stainless steels, austenitic steels are sensitive to localized corrosion, namely pitting, crevice, and intergranular form. Seventeen grades of steel were tested for localized corrosion. Steels were also tested in general corrosion and in galvanic couplings (steels–precious alloys) used in watchmaking applications. The evaluations have been carried out in accordance with the ASTM standards which specifically concern the forms of corrosion namely, general (B117-97, salt fog test), pitting (G48-11, FeCl3), crevice (F746-87) and intergranular (A262-15, Strauss chemical test and G108-94, Electrochemical potentiodynamic reactivation test). All tests revealed sensitivity to corrosion. We have noticed that the transverse face is clearly more sensitive than the longitudinal face, in the direction of rolling process. The same conclusion has been drawn from the tests of nickel release. It should be pointed out that, despite the fact that the grade of steel is in conformity with the classification standards, the behavior is very different from one manufacturer to another, due to parameters dependent on the production process, such as casting volume, alloying additions, and deoxidizing agents. The quantities of nickel released are related to the operations involved in the manufacturing process. Heat treatments reduce the quantities of nickel released. The surface state has little influence on the release. The hardening procedures increase the quantities of nickel released. The quantities of released nickel are influenced by the inclusionary state and the existence of the secondary phases in the steel structure. Another aspect is related to the strong dispersion of results concerning nickel release and corrosion behavior of raw materials.