Double Loop Electrochemical Potentiokinetic Reactivation Research Articles

Effect of shielding gas composition on intergranular corrosion of stabilized ferritic stainless steel GMA welds

The effect of shielding gas composition on intergranular corrosion of the fusion zone formed by bi-stabilized ferritic stainless steel plates (AISI 441) and ER430Ti and ER430LNb filler metals during gas metal arc welding was investigated. Double loop electrochemical potentiokinetic reactivation tests were conducted to examine the influence of the shielding gas content (Ar + 2%O2, Ar + 8%CO2 and Ar + 25%CO2) on intergranular corrosion of the ferritic stainless steel welds. It was possible to observe an increase in the tendency toward sensitization with the increase in the CO2 content in the shielding gas, especially with the ER430Ti filler metal. Analysis of the formed precipitates by means of optical microscope, scanning electron microscope an energy-dispersive X-ray spectroscopy allowed to notice that high levels of CO2 blended in the shielding gas increase the intergranular precipitates and lead to significant intergranular corrosion, which was induced by the Cr-depletion zone formation, especially for the welds produced with the ER430Ti filler metal.

Effect of carbon content on the low temperature sensitization of 304L SS and its corrosion resistance in simulated ground water

Abstract During vitrification of nuclear high level waste, the molten glass containing the waste oxides is poured into 304L SS canisters at 1323 K (1050 °C), which subsequently, cools through the sensitization temperature, wherein, nucleation of chromium carbides occur. Due to the radioactive decay heat, the canister is exposed to 373 K 573 K (100 °C- 300 °C) over long containment periods, leading to the growth of the carbides, resulting in low temperature sensitization (LTS). The sensitized microstructure is susceptible to intergranular corrosion (IGC) when it comes in contact with ground water. In the present work, AISI Type 304L SS containing 0.02% and 0.03% carbon were subjected to LTS, simulating 10 and 100 years of vitrified nuclear waste containment respectively, and subsequently, the corrosion resistance was evaluated in simulated ground water. The microstructural evaluation was carried out by optical, scanning electron microscope (SEM), transmission electron microscope (TEM) and electron backscatter diffraction (EBSD). Double loop electrochemical potentiokinetic reactivation (DL-EPR) experiments were conducted to quantify the degree of sensitization (DOS). The study revealed that 304L SS containing 0.02% C is resistant to LTS and IGC, while that containing 0.03% C is highly susceptible to LTS and IGC in ground water composition.

The Mechanism on the B Addition to Regulate Phase Precipitation and Improve Intergranular Corrosion Resistance in UNS S31254 Superaustenitic Stainless Steels

The effects of boron on the microstructure and intergranular corrosion behavior of UNS S31254 (S31254) austenitic stainless steels were investigated. The sensitivity to intergranular corrosion in the samples was determined by an altered double loop electrochemical potentiokinetic reactivation method. The microstructure characterization showed that the S31254 steels only precipitated sigma (σ, Fe-Cr-Mo-Ni) phases during the aging process, but boron addition decreased the amount of σ phase precipitation, reduced the size of the σ phases, and promoted the formation of small Laves phases at the grain boundaries. The precipitated σ phases lead to surrounding zones depleted of Cr and Mo, leading to intergranular corrosion. The S31254 alloy with added boron slows down the intergranular corrosion, due to microstructure manipulation at the grain boundaries.

Resistance of Thermally Aged DSS 2304 against Localized Corrosion Attack

In this paper, the effects of thermal aging in the 650–850 °C range on the localized corrosion behaviour of duplex stainless steel (DSS) 2304 was investigated. Pitting corrosion resistance was assessed by pitting potential (Epitt) and critical pitting temperature (CPT) determination, while the degree of sensitisation (DOS) to intergranular corrosion (IGC) was evaluated by double loop electrochemical potentiokinetic reactivation (DL-EPR). The susceptibility to stress corrosion cracking (SCC), evaluated in standard NACE TM-0177 solution at pH 2.7 and 25 °C, with the addition of S2O32− at 10−3 M, resulted in general good agreement with pitting and IGC behaviour. In fact, as-received DSS 2304 aged for 5 min at 650 °C or 750 °C presented a high resistance to localized corrosion. The alloy corrosion behaviour was severely impaired with an aging time of 60 min at 650 °C and of 10 or 60 min at 750 °C, due to the precipitation of finely distributed M23C6-type chromium carbides at ferrite/austenite interphases, which determined the formation of chromium and molybdenum depleted areas. The behaviour of samples aged at 850 °C also depended on the aging time, but, at 60 min, the rediffusion of passivating elements produced a recovery of the alloy resistance to pitting, IGC and SCC.

Corrosion resistance of welded joints of X5CrNi18-10 stainless steel

This paper considers the influence of nitrogen in the shielding gas during TIG welding as well as the influence of a welding current level on the resistance to pitting corrosion and intergranular corrosion in welded joints of X5CrNi18-10 stainless steel. Testing of pitting corrosion was carried out using the potentiodynamic method, while intergranular corrosion was tested with the Double-Loop Electrochemical Potentiokinetic Reactivation method. Pitting corrosion resistance of welded joints (the weld metal and the heat affected zone) is higher in the presence of nitrogen in the shielding gas, while integranular corrosion resistance remains unchanged. Pitting corrosion resistance of the heat affected zone in the welded joint formed in nitrogen-free shielding gas is lower than in the base metal, while the weld metal has similar pitting corrosion resistance as the base metal. With an increase in a welding current level, pitting corrosion resistance and resistance to intergranular corrosion of welded joints decrease. The passivation current density is shown to be a good indicator of pitting corrosion resistance of welded joints. Hardness values of the weld metal and the heat affected zone are slightly lower than the base metal hardness value.

Effects of in-grain misorientation developments in sensitization of 304 L austenitic stainless steels

This paper focussed on sensitization behaviour of cold rolled (CR) and warm rolled (WR) of AISI 304 L austenitic stainless steels. The main aim of this study was to mitigate sensitization issues of austenitic stainless steels. All the specimens were given two different heat treatment of 675 °C 6 h and 675 °C 5 h. The electron backscattered diffraction (EBSD) measurements were revealed developments of in-grain misorientations and types of grain boundary statistics. An EBSD system along with a suitable computer algorithm was used to quantify in-grain misorientations called near boundary gradient zone (NBGZ). The microstructures that contained quantifiable zones of in-grain misorientations exhibited low value of degree of sensitization (DOS) for 5% and 10% rolled specimens. DOS was assessed by double loop electrochemical potentiokinetic reactivation (DL-EPR) test. It was established that low value of DOS for the specimens of 5% and 10% rolled provided diffusion short-cuts or high diffusivity paths for Chromium that improved resistance to sensitization. A slight pre-strain and subsequent annealing failed to increase population of special boundaries. The visible grain fragmentation accelerated carbide precipitation and decreased resistance to sensitization. The sensitized specimens after DL-EPR test subjected to white light interferometry (WLI) measurements that revealed grain boundary depth information. The combined data set of EBSD + WLI provided linkage between grain boundary depth and in-grain misorientations.

Evaluation of the thermal aging of \u03b4-ferrite in austenitic stainless steel welds by electrochemical analysis

Cr-segregation by spinodal decomposition and G-phase precipitation were observed in δ-ferrite of austenitic stainless steel welds thermally aged at 400 °C for up to 20,000 h. A reversion heat treatment (R-HT) at 550 °C for 1 h dissolved the Cr-segregation in the aged welds while some intermetallic precipitates were present. The double-loop electrochemical potentiokinetic reactivation (DL-EPR) analysis showed no significant differences among them. However, after selective etching of the austenite phase, the DL-EPR values of δ-ferrite phase steadily increased with aging time due to the growth of Cr-depleted regions by spinodal decomposition. The electrochemical behavior of δ-ferrite after R-HT condition was similar to that of unaged welds, indicating the intermetallic precipitates did not affect the corrosion resistance in this case. Overall, DL-EPR analysis of δ-ferrite phase provided better correlation with spinodal decomposition.

The effect of grain boundary structure on sensitization behavior in a nickel-based superalloy

The present work discusses the evolution of grain boundary structure during thermomechanical processing and its effect on sensitization behavior in a nickel-based superalloy. Alloy 600 was deformed to varied degrees of strain (4–25%) using hot rolling followed by annealing at 1000 °C for 10 min followed by water quenching. Structure of the grain boundary was analyzed with reference to various parameters, such as grain boundary character distribution, twin-related domains, misorientation, and triple junction distribution. Each thermomechanically processed sample was heat-treated at 650 °C for 24 h before studying its sensitization behavior. The effect of structure of the grain boundary on sensitization was assessed through double loop electrochemical potentiokinetic reactivation test and measured in terms of degree of sensitization (DOS). DOS was found to be in a direct relation with the fraction of random high-angle grain boundaries and their connectivity, while it was inversely related to the fraction of low-Σ coincidence site lattice boundaries and special triple junctions. Residual strain and the fraction of low-angle grain boundaries were found to be weakly related to DOS. We show that a simple parameter can be used to predict the combined effect of all these factors on sensitization behavior.

Investigation on ultra-pure ferritic stainless steel 436L susceptibility to intergranular corrosion using optimised double loop electrochemical potentiokinetic reactivation method

ABSTRACTThe susceptibility of 17Cr ferrite stainless steel to intergranular corrosion (IGC) was investigated using the double loop electrochemical potentiokinetic reactivation (DL-EPR) test and the microstructural characterisation. The results show that the optimised DL-EPR test condition for 17Cr ferrite stainless steel is l M H2SO4 + 0.006 M KSCN solution with a scan rate of 0.1 V min−1 at 25°C The severe IGC occurs in the temperature range of 500–650°C and the nose temperature locates at approximately 600°C. The Ir/Ia value rises up to 16.99% when heat treated at 600°C for 30 min. After aging treatment, M23C6 precipitates are detected using transmission electron microscopy, resulting in the presence of the Cr depletion zone. However, the degree of IGC for 436L is extremely limited due to the Cr element diffusion form the matrix despite prolonging the aging time to 5 h.

Electrochemical Reactivation of AISI 409 Ferritic Stainless Steel Sheet Welded By Laser Process

The susceptibility to sensitization in 409 ferritic stainless steel (FSS) weldments was studied, by means of double loop electrochemical potentiokinetic reactivation test (DL-EPR). After the electrochemical test, the microstructure was analyzed by scanning electron microscopy (SEM) and the presence of elements with dispersive energy of X-rays (EDS), with the X-ray diffraction (XRD) technique they were identified present phases in different welds. Microhardness profiles were employed to evaluate the mechanical properties. With the aim of analyze the effect on the degree of sensitization to welding processes and the base material, a methodological analysis of gas metal arc welding GMAW, gas tungsten arc welding GTAW and LASER welding process was performed. The LASER process generated the highest ratio between the reactivation (Ir) and activation (Ia) current, despite it caused less microstructural change. The higher ratio Ir/Ia is attributed to the lack of penetration in the butt join, showing crevice corrosion.

Effect of residual stress on the stress corrosion cracking in boiling magnesium chloride solution of austenite stainless steel

The effect of residual stress on the stress corrosion cracking (SCC) behaviors of Z3CN20.09M, 304L and 316L austenitic stainless steels (SSs) in boiling magnesium chloride solution were investigated by X‐ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The residual stress of surface and the hardness tested on the cross section of specimens induced by milling operation was decreased with increasing annealing temperature. The crack densities were also decreased with increasing annealing temperature of the specimens, and the SCC did not occur after annealed at 600 °C for 3 h except 316L SS when tested in boiling magnesium chloride solution. Crack initiation mechanism related to the damaged effect on grain boundary during milling operation was analyzed. The double loop electrochemical potentiokinetic reactivation (DL‐EPR) tests indicated that the effect of annealing treatment up to 600 °C on intergranular (IG) corrosion susceptibility of 304L and 316L SSs was minor, while it remarkably increased of the Z3CN20.09M SS.

Evaluation of inter-granular corrosion susceptibility in 316LN austenitic stainless steel weldments

The inter-granular corrosion resistance of 316LN austenitic stainless steel welding joints for an ITER Correction Coil case in the as-welded and post weld heat treatment (PWHT) condition were systematically investigated. Measurements were performed in accordance with the standard ASTM A262 and via double loop electrochemical potentiokinetic reactivation. The pitting corrosion resistance was determined for specimens immersed in sodium chloride solutions. In addition, the effect of microstructural characteristics on the corrosion resistance was analyzed. The results revealed that the dendrite size in the weldment slightly increased after PWHT. Moreover, the corrosion test results showed that the inter-granular corrosion resistance of the PWHT joint is significantly lower than that of the other specimens due to the element segregation in the weldments. However, all the specimens exhibite excellent pitting corrosion resistance, as indicated by the potentiodynamic polarization curves.

Microstructure Evolution and Selective Corrosion Resistance in Underwater Multi-pass 2101 Duplex Stainless Steel Welding Joints

A recently developed promising material, 2101 lean duplex stainless steel, represents an alternative to 304 austenite stainless steel. In this work, multi-pass 2101 weld joints were fabricated using the flux-cored arc welding method in a hyperbaric chamber. The pressure varied from 0 to 0.75 MPa. The evolution of the welding process and microstructure was investigated. γ2 formation in the reheated zones of the WM and HAZ was not uniform. The closer the reheated zone is to the subsequent heat source, the greater the γ2 formation in the reheated zone. Sufficient primary austenite transformation inhibited Cr2N precipitation and the subsequent intragranular γ2 formation in the reheated weld passes of the 0.45 MPa weld metal. The localized corrosion resistance of each zone of the 0.45 MPa DSS joint was measured using non-destructive double-loop electrochemical potentiokinetic reactivation tests. The localized corrosion was induced by γ2 and Cr2N. The root region of the 0.45 MPa weld metal underwent two subsequent welding thermal cycles, which induced increased γ2 formation and lower resistance to corrosion because of the decreased pitting resistance value of γ2. The correlation between microstructure evolution and the distribution of selective corrosion was determined.

Effect of metallurgical evolution during post-weld aging treatment on localised corrosion of resistance spot welding joints of A286 superalloy

ABSTRACTThe combined effect of resistance spot welding and precipitation hardening on the localised corrosion of A286 superalloy is studied. The specimens tested by double loop electrochemical potentiokinetic reactivation were welded in the solution treated condition, and then subjected to different precipitation hardening treatments. For both base metal and weld nugget, the maximum localised corrosion is reached when η phase is clearly observable. The fact that the localised corrosion resistance of weld nugget is different from that shown by base metal may be explained by the segregation of Ni and Ti towards the interdendritic region of weld nugget (studied by using scanning electron microscope/energy dispersive X-ray analysis).

Weld joint design and thermal aging influence on the metallurgical, sensitization and pitting corrosion behavior of AISI 304L stainless steel welds

Experimental investigations were carried out to study the influence of weld joint design and post weld thermal aging treatments on the metallurgical, intergranular and pitting corrosion behavior of AISI 304L stainless steel welded joints. 10 mm thick linear weld joints were fabricated using gas metal arc welding process using single-V and double-V joint design with an average heat input of 503.42 J/mm and 562.78 J/mm respectively. Thermal aging of these welds was carried out at 750 °C for 0.5 h and 4 h, and the cooling media used was furnace, air and water quench (used separately for each specimen), so as to induce variable degree of carbide precipitation in different zones of these welds. Microstructural studies revealed that the extent of carbide precipitation was more in case of welds made using double-V joint design as compared to single-V joint welds. Degree of sensitization (DOS) and pitting performance of these welds evaluated using double loop electrochemical potentiokinetic reactivation (DLEPR) technique and potentiodynamic anodic polarization technique respectively, further showed that single-V joint performed better in terms of corrosion properties owing to lesser precipitation as compared to the double-V joint. This study shows that in comparison to double-V joint, single-V joint design can prove to be an important part of fabrication procedures where service conditions demand better corrosion properties of these welds.

An electrochemical study on the effect of stabilization and sensitization heat treatments on the intergranular corrosion behaviour of AISI 321H austenitic stainless steel

The intergranular corrosion behavior of AISI 321H stainless steel was investigated using oxalic acid etching, double-loop electrochemical potentiokinetic reactivation, anodic polarization, Mott-Schottky, electrochemical impedance spectroscopy, X-ray diffraction, optical microscopy and scanning electron microscopy techniques. The results showed that stabilized samples that are sensitized for less duration have more intact passive films and are more resistant to intergranular corrosion than the others. In addition, stabilization heat treatment cannot prevent intergranular attack when sensitization time is high. Finally, it was observed that by increasing the applied voltage in the transpassive region, the corrosion mechanism changes from uniform type to severe localized attack.

Double-loop electrochemical potentiokinetic reactivation behaviour of continuously cooled SUS329J4L duplex stainless steel

ABSTRACTSigma phase generally precipitates in duplex stainless steel during slow cooling, which usually causes loss in both corrosion resistance and fracture toughness. To develop a non-destructive method to detect the sigma phase in duplex stainless steel for in situ use in the quality management of large structures, the double-loop electrochemical potentiokinetic reactivation behaviour of continuously cooled duplex stainless steel SUS329J4L at different cooling rates was investigated under different conditions. The polarisation condition with the solution of 0.5 M H2SO4 + 0.01 M KSCN + 0.5 M NaCl and the potential scanning rate of 0.83 mV s−1 were determined as suitable and sensitive.This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).

Thermal aging effects on mechanical and electrochemical properties of stainless steel weld overlay cladding

An investigation on mechanical property and electrochemical behavior of the cladding materials aged at 400 °C for different times was carried out by small punch test (SPT) and double loop electrochemical potentiokinetic reactivation (DL-EPR) test. The results showed that thermal aging contributed to spinodal decomposition and G-phase formation in the aged ferrite phase, as well as increase of nano-hardness of ferrite phase. The SP energy by the small punch tests decreased with increase of thermal aging time, which was related to the lattice distortions in G-phase and aged ferrite phase and their interactions with dislocations during deformation process. Thermal aging promoted corrosion and intergranular corrosion susceptibility of the cladding materials. Combined effects of G-phase and spinodal decomposition lowered intergranular corrosion resistance of the cladding material.

Microstructure and intergranular corrosion behavior of HAZ in DP-TIG welded DSS joints

In this study, 10.8-mm-thick workpieces of S32101 duplex stainless steel were welded using a keyhole deep penetration tungsten inert gas welding system. The misorientation angle distribution of grain boundaries in the heat affected zone was analyzed using the electron backscatter diffraction technique. Double-loop electrochemical potentiokinetic reactivation tests were performed to assess the intergranular corrosion resistance, based on the degree of sensitization. The volume fractions of ferrite and austenite and the grain boundary misorientation angle affect the intergranular corrosion of the welded joints. The relationship between the grain boundary misorientation angle of the austenite and the degree of sensitization was established. The volume fraction of austenite and the frequency of Σ3 coincidence site lattice boundary, increases with the increasing heat input. The misorientation angle distribution in the austenite, the frequency of Σ3 coincidence site lattice boundary, and the volume fraction of CrN influence the intergranular corrosion resistance of the heat affected zone.

SENSITIZATION STUDY IN FRICTION STIR WELDS OF INCONEL® 625

The new challenges of the oil and gas industry require noble materials with chemical stability and greater mechanical properties. Alloy 625, popularly known as Inconel® 625, is used as a cladding material for pipelines and other components. Therefore, the study of joining methods that produce excellent welded joints is essential. Thus, in this study, welded sheets of Inconel 625 were produced by Friction Stir Welding (FSW) and afterwards they were subjected to a heat treatment to evaluate the susceptibility to sensitization by the Double Loop Electrochemical Potentiokinetic Reactivation(DL-EPR)technique. In addition, microhardness profiles were performed before and after the isothermal treatment. The microhardness results indicated that the increased hardness is due to the carbides formation after heat treatment. On the other hand, electrochemical tests showed that FSW process can significantly affect the degree of sensitization.