Electrochemical Potentiokinetic Reactivation Technique Research Articles

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

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

Electrochemical characterization of 13Cr low-carbon martensitic stainless steel - corrosion study with a mini-cell setup

13Cr low-carbon martensitic stainless steels also known as supermartensitic stainless steels (SMSS) have superior properties than conventional martensitic stainless steels. The SMSS have better weldability and corrosion resistance. Nevertheless, corrosion resistance depends on phases transformations which are induced generally by heat treatments. In this work, the electrochemical properties of a SMSS were evaluated as a function of the tempering temperature (400 °C–700 °C). The susceptibility to intergranular corrosion was determined throught the degree of sensitization (DOS) using the Double Loop - Electrochemical Potentiokinetic Reactivation technique (DL-EPR) in a conventional three electrodes corrosion cell. On the other hand, the pitting susceptibility was evaluated by potentiodynamic polarization using a homemade corrosion mini-cell (based on a sessile electrolyte droplet), thus avoiding crevice problems typically seen with conventional arrangements. Imaging the microstructure with SEM, XRD analysis and thermodynamic and kinetic simulations were performed to understand the microstructural transformations and their relationship with corrosion resistance. The most severe sensitization occurs at the temperature where reversed austenite transformation is highest.

On-site weld quality assessment and qualification for stainless steels tanks

Stainless steel tanks are frequently degraded by localized corrosion in bioprocess industries. A case study of 2101 duplex stainless steel tank allowed to apply a portable electrochemical microcell system (PassivityScan) for on-site weld inspection and corrosion monitoring from the manufacturing until 12 months of operation. During the tank manufacturing, the double loop electrochemical potentiokinetic reactivation technique was applied to measure the sensitization degree on the welded regions. The manufactured tank was submitted to the passivation treatment to improve the passivation properties and corrosion resistance. On-site cyclic polarization tests were performed and confirmed the increase of passivation level of treated surface. The passivation level was measured again after 12�months of operation and detected a lower passivation level compared to the initial passivated surface, indicating that the passivation treatment lost its effect after that period. Therefore, an acceptance criterion to passivation level was empirically determined. PassivityScan is an advanced device to qualify the welds and the passivated surfaces, and useful for corrosion monitoring and the reliability maintenance.

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.

Effect of environmental condition on corrosion susceptibility of GTA welded AISI 202 austenitic stainless steel

Austenitic stainless steels with low nickel content or nickel-free content are currently considered structural materials for nuclear fusion reactor applications. Even though the base metal achieves excellent performance, welding may result in premature failures due to morphological and compositional heterogeneities. The presence of pollutants in the environment results in environmental acidification, which influences the corrosion susceptibility of the weldment. This paper it is aimed to examine the influence of the gas tungsten arc welding (GTAW) process on localized corrosion and the degree of sensitization (DOS) of AISI 202 austenitic stainless steel (ASS) butt joints in three distinct regions: base-zone (BZ), heat-affected zone (HAZ), and weld-zone (WZ) in 3.5% NaCl under pH-5 environmental conditions. The localized corrosion behavior of the weldment was studied using open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), and degree of sensitization (DOS) using the double loop electro-chemical potentiokinetic reactivation (DL-EPR) technique. Optical microscopy (OM) is used to understand the microstructural features of weldments and pit morphologies before and after polarisation studies. The present study establishes that the heat-affected zone (HAZ) has low corrosion resistance, high DOS%, and low intergranular corrosion (IGC) resistance compared to the base and weld zones. This could be due to the thermal cycles involved in HAZ during welding, resulting in chromium depletion and the formation of chromium carbide precipitation.

Application of double loop electrochemical potentio-kinetic reactivation for characterizing the intergranular corrosion susceptibility of stainless steels and Ni-based alloys in solar nitrate salts used in CSP systems

In this study, the sensitivity of four corrosion resistant candidate alloys; austenitic stainless steels (AISI 321 and AISI 347) and Ni-based alloys (IN 625 and In 825) to degrade by intergranular corrosion when used with molten salts in conditions of CSP systems were investigated at different temperatures. This was achieved by implementing the Double loop electrochemical potentiokinetic reactivation (DL-EPR) technique to quantitatively measure the degree of sensitisation (DOS) in the selected alloys by aging in air with and without Solar Salts (60 wt% NaNO3 and 40 wt% KNO3) at 550–750 °C for 2 h. Electrochemical etching technique was implemented further to verify the results from DL-EPR. The results show that DL-EPR and microstructural evaluation techniques can be used to assess the synergy of temperature and molten salt electrochemical activities on the tendency for intergranular corrosion attack to occur in CPS systems. IN 625 showed the highest intergranular corrosion resistance among the alloys investigated in both air and molten salt; especially at temperatures higher than salt stability temperature. Samples aged in molten salts showed higher DOS than samples aged in air across all temperatures and linked to Cr-consuming corrosion and chemical reactions that drives molten salt – alloy interaction in CSP plants.

Correlation between microstructure and corrosion behavior in an AISI 316L steel pipeline exposed for 100,700 h at 640°C in a petrochemical plant

AbstractThe microstructure and electrochemical properties of 316L stainless steel were investigated after two conditions: aged at 640°C for 100,700 h and solution annealed at 1050°C for 2 h. While the aged samples were obtained from a pipe of a petrochemical reactor plant that was in service, the solution annealing was carried out in a conventional laboratory furnace. After aging, the precipitates present in decreasing order of quantity were sigma, Laves phase, and M23C6. After solution annealing, the microstructure was full austenitic. These results were in agreement with equilibria phase simulation with Thermo‐Calc software. Intergranular corrosion susceptibility, evaluated by means of the single loop electrochemical potentiokinetic reactivation technique and Practice A of ASTM 262, indicated a preponderant role for the sigma precipitate. The pitting potential (Epit) was evaluated through potentiodynamic polarization curves in 0.6 M NaCl and electrochemical impedance spectroscopy was performed at the corrosion potential to complement the information about the corrosion resistance.

Simulation of Neutron Irradiation Damage in Stainless Steel by Cold Rolling

Simulation of irradiation damage in a stainless-steel type 304L was studied by means of cold rolled samples that were then subjected to different annealing cycles to assess their recovery and compare it with the behavior observed in annealed neutron irradiated samples. Changes in hardness as a function of time and temperature were used to compute the activation energy associated with the recovery in hardness; such value coincided with those reported in literature for irradiated samples. The study was coupled with the evaluation of the degree of sensitization in the cold-rolled and annealed samples by means of the electrochemical potentiokinetic reactivation technique. Annealing the cold-rolled samples for 5 h at 500 °C resulted in the optimum treatment for reducing the hardness as it does not sensitize the steel.

Effect of Nitrogen on the Intergranular Stress Corrosion Cracking Resistance of 316LN Stainless Steel

Here the effect of nitrogen on the intergranular stress corrosion cracking (SCC) resistance of sensitized Type 316LN stainless steel containing different amounts of nitrogen is reported. SCC studies were performed at 70% of yield strength. Double-loop electrochemical potentiokinetic reactivation technique was used to quantify degree of sensitization (DOS) that was correlated with SCC resistance. SCC time to failure increased from 220 h to 285 h with increasing nitrogen content from 0.07 wt% to 0.14 wt%, but decreased drastically to approximately 120 h at 0.22 wt% nitrogen (i.e., beyond N solubility limit), due to excessive precipitation of Cr23C6 and Cr2N and drastic reduction in the coincidence site lattice (CSL) boundary distribution from 48% to approximately 32%. Scanning electron microscope images showed mixed mode of failure.

Sensitization evaluation of the AISI 2205 duplex stainless steel by the IQ value in EBSD technique

The degree of sensitization of the 2205 duplex stainless steel was evaluated by the double loop electrochemical potentiokinetic reactivation technique and EBSD method. The IQ value of ferritic phase was higher than that of austenitic phase for solid solution 2205 duplex stainless steel. However, some local small regions in ferritic phase and grain boundaries showed very low IQ value for sensitized 2205 duplex stainless steel, which was attributed to the precipitation of carbides and the formation of chromium depleted regions. This conclusion was supported by electrochemical results. Therefore, the IQ value could evaluate the degree of sensitization of the duplex stainless steel.

Assessment of Thermal Aging of Austenitic Stainless Steel Weld Metal by Using the Double Loop Electrochemical Potentiokinetic Reactivation Technique

Double loop electrochemical potentiokinetic reactivation (DL-EPR) was applied to evaluate thermal aging of 308L stainless steel weld metal. It was found that the activation and reactivation peaks of DL-EPR curve were induced by dissolution of austenite and δ-ferrite, respectively. Before saturation of hardness, the linear relationship between reactivation ratio and hardness could be used for assessing the thermal aging-induced hardening. In the following thermal aging process, the reactivation ratio is applicable to assess the occurrence of the saturation of thermal aging-induced hardening. The results demonstrated that the DL-EPR test is applicable to assess the evolution of thermal aging.

Impact Toughness, Fatigue Crack Growth and Corrosion Behavior of Thermally Aged UNS S32205 Duplex Stainless Steel

The aim of the present work was to study the impact, fatigue crack growth and corrosion behavior of 2205 duplex stainless steel (DSS) when subjected to one of the most influential thermal aging condition that promotes deleterious secondary precipitation. Formation of Cr- and Mo-rich σ-phase was confirmed by EDX analysis which deteriorated impact toughness causing the material to fail following quasi-cleavage mode of fracture. Susceptibility of DSS 2205 to pitting corrosion assessed though Potentiodynamic polarization technique was enhanced by thermal aging, but sensitization performance evaluated via double-loop electrochemical potentiokinetic reactivation (DLEPR) technique did not show much degradation. However, fatigue crack growth behavior testing divulged drastic enhancement in impediment to crack propagation by the matrix after thermal aging.

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.

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.

Synthesis and Comparative Study on Intergranular Corrosion Characteristics of Stabilized 5A Grade Cast Duplex Stainless Steel

Abstract In this paper, stabilization using zirconium and niobium was investigated to reduce the effect of intergranular corrosion (IGC) in 5A grade cast duplex stainless steel. Computational thermodynamic software was used to determine the various heat treatment temperatures according to phase precipitation. Resistance to IGC was investigated by oxalic acid etch test and double-loop electrochemical potentiokinetic reactivation (DL-EPR) technique. The surface of the specimens was observed using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometry to evaluate the presence of various intermetallic phases prone to IGC. Oxalic acid etch test response has indicated the susceptibility, whereas the DL-EPR test has provided the degree of sensitization. The results revealed that zirconium-stabilized samples are more susceptible to IGC compared to the niobium samples. Earlier formation of niobium compounds led to the reduction of intergranular attack in the niobium-stabilized samples.

Comparative Study on Intergranular Corrosion Characteristics of Cast Duplex Stainless Steel Stabilized with Zirconium

Intergranular corrosion of duplex stainless steel is a major concern in high corrosive environments. Interstitial stabilization of alloy is a possible technique for susceptibility to intergranular corrosion. In this paper, zirconium addition was made to 5A grade cast duplex stainless steel in order to achieve the stabilization. Addition of zirconium forms a mixture of zirconium carbides and nitrides which suppress the chromium depletion and thus increase the intergranular corrosion resistance. The metallurgical characterization was carried out on the specimens by means of optical microscopy, scanning electron microscopy, energy-dispersive spectroscopy, and x-ray diffractometry. Corrosion behavior investigation using oxalic acid etch test and double-loop electrochemical potentiokinetic reactivation technique results underline the superior corrosion-resistant property of zirconium-added cast duplex stainless steel.

Influence of Cold Work on Sensitization Kinetics and Evaluation of Degree of Sensitization in Type 316LN Stainless Steels

The sensitization behavior of Type 316LN stainless steel (SS) with varying nitrogen content (0.07 wt% and 0.22 wt%) in 0% to 25% cold worked (CW) as well as in the as-received condition was evaluated. Time-temperature-sensitization diagrams were established for the cold worked Type 316LN SS (nitrogen content: 0.07 wt% and 0.22 wt%), using ASTM A262 Practice-E test. The time required for sensitization (ts) was found to decrease and critical cooling rates (CCR) increased up to 10% CW in the alloy with 0.07 wt% N. At higher CW, ts increased and CCR decreased. Similarly, ts decreased and CCR increased up to 15% CW in the alloy with 0.22 wt% N and at higher CW, ts increased and CCR decreased. To understand the sensitization behavior of Type 316LN SS with varying nitrogen content during thermal aging at 923 K for various durations, nondestructive electrochemical techniques like double loop electrochemical potentiokinetic reactivation (DL-EPR) technique and electrochemical reactivation technique (ERT) were adopt...

Correlation of Electrochemical Noise Parameters with Degree of Sensitization in AISI Type 316LN Stainless Steel

Degree of sensitization in 316LN stainless steel (SS) specimens sensitized at 898, 923 and 948 K for 500 h was obtained using double loop electrochemical potentiokinetic reactivation (DLEPR) technique as 7.4, 14.5 and 9.3% respectively. The sensitized specimens were pulsed polarized so that only Cr-depleted regions of the sensitized grain boundaries contributed to the electrochemical noise (EN) study. The DOS values evaluated from DLEPR technique were correlated with the EN parameters viz. standard deviation of current, σI and characteristic charge, q and characteristic frequency, fn, obtained from shot noise analysis in order to assess the extent of intergranular corrosion (IGC) attack in 316LN SS. The plot of σI versus time showed highest σI values for the specimen sensitized at 923 K for 500 h, indicating high grain boundary dissolution and hence, severe IGC attack, whereas the specimen sensitized at 898 K for 500 h showed the least σI values indicating lower dissolution and least IGC attack. 316LN SS specimen heat-treated at 948 K for 500 h showed intermediate grain boundary dissolution rate. The charge q, determined from σI versus time plot showed a good correlation (>99%) with the DOS values obtained from DLEPR experiments. The power spectral density values of the current signal in the frequency independent region were found to have excellent correlation with these observations. The above findings were further supported by scanning electron microscopic examination which showed an increase in grain boundary width in the sensitized specimens when the heat-treatment temperature was raised from 898 to 923 K and on further increasing the temperature to 948 K, a marginal decrease in the grain boundary width was observed.

Influence of sensitization on microstructure and passive property of AISI 2205 duplex stainless steel

The results from double loop electrochemical potentiokinetic reactivation technique, potentiostatic critical pitting temperature technique and electrochemical impedance spectroscopy showed that the resistance of the 2205 duplex stainless steel to pitting corrosion decreased with the increasing of sensitization time due to a decreasing in passivation ability. The 2205 duplex stainless steel showed the increase in pitting corrosion in the ferritic phase and in intergranular corrosion between the austenitic and ferritic phases with the increasing of sensitization time. The precipitated phases changed due to the increasing of the sensitization time, which affected degree of sensitization of the 2205 duplex stainless steel.

Influence of sensitization on passive films in AISI 2205 duplex stainless steel

The effect of sensitization at 600 °C on passive films on the surface of 2205 duplex stainless steel was investigated by the double loop electrochemical potentiokinetic reactivation technique, potentiodynamic polarization curves, electrochemical impedance spectroscopy and X-ray Photoelectron Spectroscopy. With the increasing of the passivation potential, the passivation property of the solid solution annealed sample decreased. Moreover, the corrosion resistance of the passive film on sensitized sample was lower than that on solid solution annealed sample, especially at 0.6 VSCE. These results were supported by X-ray Photoelectron Spectroscopy and Mott–Schottky analyses.