Electrochemical Potentiokinetic Reactivation Test Research Articles

Influence of pre-deformation, sensitization and oxidation in high temperature water on corrosion resistance of AISI 304 stainless steel

The effects of pre-deformation on sensitization of AISI 304 stainless steel were investigated by the double loop electrochemical potentiokinetic reactivation test. The effects of pre-deformation and sensitization on high temperature oxidized film formed in high temperature water were analyzed by a XRD and SEM. The electrochemical impedance spectroscopy at room temperature was used to study corrosion resistance of oxidized film. The point defect density of oxidized film was calculated by Mott–Schottky plots. The results showed that the value of the degree of sensitization first decreased and then slight increased with the increasing of engineering strain. Moreover, low temperature promoted to form sensitization induced “secondary” α′-martensite. The sample with 20% engineering strain had higher impedance value than other samples. The result was supported by further Mott–Schottky experiments. Considering increased α′-martensite with the increasing of strain, the results of the impedance were more consistent with values of the degree of sensitization.

Effect of Heat Treatment and Surface Condition on Inter-Granular Corrosion of Alloy 800

Abstract The alloy 800 is being used for steam generator tubings for pressurized heavy-water reactors (PHWRs). This material is susceptible to degradation because of severe operating conditions, like high temperature, stress, and corrosive environment. These degradation mechanisms include primary water stress corrosion cracking (PWSCC), secondary side or outer diameter stress corrosion cracking (ODSCC), inter-granular corrosion (IGC), fretting, wear, denting, high cycle fatigue, corrosion fatigue, etc. The present study was conducted to analyze the effect of solution annealing temperatures, sensitization treatments, and surface conditions on the corrosion rate. The alloy 800 tubular samples were investigated by means of a conventional corrosion test (according to ASTM G28-02) and an electrochemical potentiokinetic reactivation (EPR) test. Susceptibility to inter-granular corrosion under various experimental parameters was examined by using both the test methods and the results are compared. The observed trends in corrosion rate obtained by using conventional method and EPR test were found to be similar. These results were used to obtain most optimum heat-treatment parameters and surface condition, which will yield best corrosion resistance.

Effect of Sensitization Treatment on Corrosion Properties in Austenitic Stainless Steel 304

Chromium depletion in AISI304 stainless steel can lead to intergranular corrosion due to temperature conditions and degree of deformation, and consequently to materials failure. The aim of this work is to study the effect of sensitization treatment on the corrosion resistance in AISI304 stainless steel after solution heat treatment at 1,000°C. Sensitization treatments were carried out at 670°C for 1 h, 2h, 5h and 10h in the electrical furnace. The intergranular corrosion resistance of the sensitized specimens was evaluated by double loop electrochemical potentiokinetic reactivation test. The microstructure of the specimens before and after the corrosion tests was observed through optical and scanning electron microscope. Solution treatment shows the dual structure, however, aging for 5 hours depicts the ditch structure. The ratio of current density increased as a function of aging time resulted in increase in the degree of sensitization.

Microstructure, Mechanical and Intergranular Corrosion Behavior of Dissimilar DSS 2205 and ASS 316L Shielded Metal Arc Welds

There are many industrial situations particularly in petro-chemical, marine, power plant and other such industries where the use of dissimilar metal weldments is necessary, mainly due to economic benefits and also sometimes to improve the performance of the component. Both austenitic stainless steels and duplex stainless steels have received much attention in recent days due to their superior anti-corrosive and mechanical properties. Further, the use of shielded metal arc welding (SMAW) process is inevitable in engineering industries. In the present work, microstructure, mechanical and intergranular corrosion behavior of dissimilar 2205 duplex stainless steel and 316L austenitic stainless steel fabricated by SMAW process using E2209 electrode by taking two different heat input (0.45–0.60 kJ/mm) was investigated. The microstructures were characterized by using optical microscopy and scanning electron microscopy (SEM), while the localized chemical information was obtained by an energy dispersive spectrometer attached to the SEM. Double loop electrochemical potentiokinetic reactivation test was performed to quantitatively assessing the intergranular corrosion based on degree of sensitization. The effect of weld dilution on mechanical properties (i.e. tensile/hardness properties) was also studied. The ferrite content was experimentally measured by using ferritoscope and it was observed that the weld joint achieved the required ferrite content for both the heat inputs. Higher ferrite content (results of faster cooling rate) increased the hardness and tensile strength of low heat input as compared to high heat input. While, high heat input improved the corrosion resistance due to formation of higher austenitic phases. Higher impact energy was observed in E2209 weld metal than that of the base metals. No welding defects were observed and recommended for industrial use.

Microstructure and corrosion resistance of radial friction welded supermartensitic stainless steels

In this work, supermartensitic stainless steel pipes were radial friction (RF) welded and their corrosion behaviours were studied based on potentiodynamic polarisation and double-loop electrochemical potentiokinetic reactivation tests. Measurements were performed on samples taken from the base metal (BM), weld interface and consumable ring (CR) of the RF weldment. The corrosion properties are discussed in terms of their resulting metallurgical microstructure. The precipitation of Cr carbides that takes place during the tempering treatment induces a substantial Cr depletion value (Ir/Ia = 54.22%). On the other hand, CR and weld interface regions, which had their microstructure transformed and their Cr carbide precipitates redissolved by the RF welding thermomechanical cycle, present a low level of Cr depletion (Ir/Ia < 1%). The AC microstructure, which is composed of a mixture of virgin martensite and stable retained austenite, presents an increase in pitting corrosion resistance compared to the tempered structure of the BM region. It was also observed that the δ-ferrite decreases the pitting resistance of the weld interface region.

Failure Analysis of AISI 321 Austenitic Stainless Steel Water Piping in a Power Plant

The objective of this work was to analyze a reported pitting damage mechanism in water piping of a power plant. The investigated material was a longitudinal-seam welded pipe which was made of AISI 321 austenitic stainless steel. Pitting occurred on the internal surfaces of the pipe and adjacent to the girth welds. Optical emission spectrometry, metallography, and electrochemical testing were the techniques employed to analyze the failure. The investigation showed that the flowing water contained chloride ions in which AISI 321 austenitic stainless steel is not immune against pitting corrosion. In addition, the polarization data showed the susceptibility of the material to pitting corrosion in the service conditions. Furthermore, the result of double loop electrochemical potentiokinetic reactivation tests showed that the material was sensitized during welding. It was found that the damage was due to pitting corrosion in chloride-containing water. Thus, it was recommended to replace the pipe with material having higher pitting resistance equivalent number to be more resistant against pitting corrosion.

Effect of Shot Peening on the Intergranular Corrosion Susceptibility of a Novel Super304H Austenitic Stainless Steel

The surface phase constituent of Super304H austenitic stainless steel, after shot peening and sensitization treatment at 600, 650, and 700 °C for 2 h, was characterized using x-ray diffraction method. The degree of sensitization (DOS) was investigated by means of double-loop electrochemical potentiokinetic reactivation (DL-EPR) test, and the morphology after DL-EPR test was observed by scanning electron microscopy (SEM). The results showed that nano-sized grains and strain-induced martensite together with compressive residual stress formed on the surface of Super304H steel after shot peening. Surface compressive residual stresses relaxed greatly after being sensitized at 600-700 °C for 2 h, and no systematic correlation was observed between the compressive residual stresses developed and the intergranular corrosion susceptibility (IGCS). Because of the occurrence of strain-induced martensite in the shot-peened specimens, their IGCS is much higher than that of the as-received specimen when being sensitized at 600-650 °C for 2 h. Besides, the DOS increased with the increasing of shot peening time and the content of strain-induced martensite. On the contrary, the IGCS of Super304H stainless steels subjected to shot peening was eliminated when being sensitized at 700 °C for 2 h because of the reverse transformation of strain-induced martensite and faster diffusion rate of Cr at higher temperature in ultrafine-grained austenite which had helped healing the chromium depletion zone in a very short time. In a word, shot peening promoted desensitization of Super304H steel in a time shorter than 2 h at higher temperature up to 700 °C.

Effect of Precipitation on Intergranular Corrosion Resistance of 430 Ferritic Stainless Steel

With Nb-Ti-stabilized 430 ferritic stainless steel (NTS430FSS) and SUS 430 ferritic stainless steel (SUS430FSS) as experimental materials, the influence of precipitation on intergranular corrosion resistance was investigated. A series of aging treatment were carried out. The free-exposure corrosion test and double loop electrochemical potentiokinetic reactivation (DL-EPR) test with a scan rate of 1.67 mV/s at 26 °C were applied to evaluate the intergranular corrosion (IGC) resistance. Metallographic observation, scanning electron microscope (SEM), transmission electron microscope (TEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis were conducted. The results show that IGC occurred in SUS430FSS aged above 700 °C, while it occurred in NTS430FSS as the temperature was improved to 1 050 °C. The critical degree of sensitization Ir/Ia reaches 0.305 in SUS430FSS, which is higher than that of NTS430FSS, i.e. 0.010, aged at 950 °C for 2 h. The TEM, EDS and XRD results show that a large amount of Cr23C6 precipitates with size of 60 nm×22 nm are located at the SUS430FSS grain boundaries as chains. With the addition of Nb and Ti and reduction of C, the amount of precipitates reduces significantly in NTS430FSS. A majority of Cr23C6 were replaced by TiC and NbC. Only a small amount of spherical TiC (R=186 nm) and square TiN (312 nm×192 nm) with Nb and Cr adsorbed are left along grain boundaries. Due to the dual stabilization of Nb and Ti, the precipitation of Cr23C6 is restrained, the chromium depleted region is avoided and accordingly the resistance to the intergranular corrosion is improved.

Corrosion Behavior of Alloy 625 in Simulated Nuclear High-Level Waste Medium

The present investigation aims to study the effect of various ions present in nuclear high-level waste (HLW) (acidic) medium on the corrosion resistance of Alloy 625, with solution-annealed and sensitized microstructure. The heat-affected zones are prone to sensitization during welding of components and subsequent exposure to acidic waste during service could result in intergranular corrosion in these regions and hence it was attempted to study the corrosion behavior of the alloy under sensitized conditions. Double loop electrochemical potentiokinetic reactivation test was carried out to obtain the extent of chromium depletion. Potentiodynamic anodic polarization and electrochemical noise investigations were carried out on Alloy 625 in 3 M nitric acid and simulated nuclear HLW medium (prepared in 3 M nitric acid) at 298 K and 323 K. The study showed that the alloy possess good corrosion resistance in 3 M nitric acid and simulated HLW medium. However, a marginal decrease in the corrosion resistance occurred in simulated HLW when compared to the plain acid, as observed from an increase in passivation current density, decrease in transpassive potentials, and decrease in electrochemical noise resistance. Increase in temperature of the medium and change in microstructure from solution-annealed to sensitized state further decreased the corrosion resistance of Alloy 625. Electrochemical noise time records obtained at open circuit conditions showed a stable passive film for 22 h of immersion of the alloy in 3 M nitric acid and simulated HLW. However, the amplitude of current fluctuations was higher for the sensitized microstructure when compared to the solution-annealed microstructure.

Effects of Induction Heat Bending Process on Microstructure and Corrosion Properties of ASME SA312 Gr.TP304 Stainless Steel Pipes

The usage of bending products recently have increased since many industries such as automobile, aerospace, shipbuilding, and chemical plants need the application of pipings. Bending process is one of the inevitable steps to fabricate the facilities. Induction heat bending is composed of compressive bending process by local heating and cooling. This work focused on the effect of induction heat bending process on the properties of ASME SA312 Gr. TP304 stainless steel pipes. Tests were performed for base metal and bended area including extrados, intrados, crown up, and down parts. Microstructure was analyzed using an optical microscope and SEM. In order to determine intergranular corrosion resistance, Double Loop Electrochemical Potentiokinetic Reactivation (DL-EPR) test and ASTM A262 practice A and C tests were done. Every specimen revealed non-metallic inclusion free under the criteria of 1.5i of the standard and the induction heat bending process did not affect the non-metallic inclusion in the alloys. Also, all the bended specimens had finer grain sizethan ASTM grain size number 5 corresponding to the grain sizes of the base metal and thus the grain size of the pipe bended by induction heat bending process is acceptable. Hardness of transition start, bend, and transition end areas of ASME SA312 TP304 stainless steel was a little higher than that of base metal. Intergranular corrosion behavior was determined by ASTM A262 practice A and C and DL-EPR test, and respectively step structure, corrosion rate under 0.3 mm/y, and Degree of Sensitization (DOS) of 0.001 ~ 0.075 % were obtained. That is, the induction heat bending process didn’t affect the intergranular corrosion behavior of ASME SA312 TP304 stainless steel.

Intergranular Corrosion of Low Cr Ferritic Stainless Steel 429 Evaluated by the Optimized Double Loop Electrochemical Potentiokinetic Reactivation Test

Intergranular corrosion (IGC) of Nb-Ti stabilized ferritic stainless steel (FSS) 429 was investigated using the double loop electrochemical potentiokinetic reactivation (DL-EPR) test combined with the microstructure observation. The results indicated that the optimized DL-EPR test condition for FSS 429 was the solution of 0.5 M H2SO4+ 0.0001 M KSCN with a scanning rate of 100 mV/min at 30°C. Based on this condition, the specimens aging at 400–700°C for different duration were tested and a time-temperature-sensitization (TTS) curve for FSS 429 was obtained, which reveals the sensitization nose was located around 550°C. The criticalIr/Iavalue was determined to be about 3% above which IGC occurred. After aging treatment, Cr depletion zone was detected using energy dispersive spectroscopy (EDS), most possibly due to Cr segregation around intergranular TiC and NbC.

Intergranular corrosion behavior associated with delta-ferrite transformation of Ti-modified Super304H austenitic stainless steel

A double loop electrochemical potentiokinetic reactivation (DL-EPR) test was conducted to investigate the relationship between the evolution of delta-ferrite and the intergranular corrosion (IGC) of Ti-modified Super304H, which was aged at 650°C for 4–500h. Scanning electron microscopy and transmission electron microscopy were adopted to analyze the evolution of delta-ferrite. The results indicated that a higher fraction of delta-ferrite with poor stability increased the IGC sensitisation of Ti-modified Super304H. Moreover, the self-healing of the sensitisation of Ti-modified Super304H occurred after 48h due to the diffusion of chromium atoms mainly from the adjacent primary austenite rather than the delta-ferrite.

Influence of Sinter-Cooling Rate on the Corrosion Behavior of High-Nitrogen, Low-Nickel Powder Metallurgy Austenitic Stainless Steel

A high-nitrogen, low-nickel powder metallurgy austenitic stainless steel was studied and compared with a conventional powder metallurgy austenitic stainless steel. Three different sintering processes were applied by controlling the sinter-cooling rate (furnace, gas, and water). Nitrogen was chosen as the sintering atmosphere. A microstructural study was performed and the corrosion behavior was evaluated in terms of anodic polarization measurements and an electrochemical potentiokinetic reactivation test. It was found that the water-cooling process is advisable for good corrosion behavior. The results were correlated with the microstructural features.

Grain boundary character distribution and sensitisation behaviour of grain boundary engineered stable austenitic stainless steel (AISI 316L)

Thermomechanical processing involving a wide range of strain (5–80%) followed by annealing was applied to a type 316L austenitic stainless steel to encourage grain boundary engineered (GBE) structure. As a result of GBE process, the total length fraction of low Σ coincidence site lattice boundaries increased noticeably in conjunction with different levels of grain growth. The GBE structures resulted in significant decreases in the degree of sensitisation following exposure at 948 K for 20 h and assessment through double loop electrochemical potentiokinetic reactivation tests. Strain of low level (5%) was more effective than medium to high strain in inspiring GBE. The role of grain size on the sensitisation process has also been discussed. Additionally, abnormal grain growth contributes to the optimisation of GBE structure.

Weld decay failure of a UNS S31603 stainless steel storage tank

A brine storage tank was reported to have visible and severe localized corrosion 1year after installation. The material used for this tank was UNS S31603 stainless steel. The corrosion was mainly noticeable at various points/areas along the welded joint, and there was also some pitting on the base metal at the bottom of the tank. Corrosion was also found at the welded joint at the hatch wall. Indications of severe corrosion that resulted in deep metal loss were found at some points along the welded joint. The results of an oxalic acid etch test in the heat-affected zone (HAZ) showed the interdendritic ditches, which are considered to be prone to intergranular corrosion/weld decay. The results of a double-loop electrochemical potentiokinetic reactivation (DL-EPR) test and 3D multiscale images were in agreement with the results of an oxalic acid etch test that indicated the intergranular corrosion susceptibility of the HAZ. The base metal, in contrast, did not show any intergranular corrosion.

Influence of sinter-cooling rate on intergranular corrosion of powder metallurgy superaustenitic stainless steel

Powder metallurgy superaustenitic stainless steel sintered in N2–H2 atmosphere (95%–5%) was obtained. Three different processes were applied by controlling the sinter-cooling rate (furnace, gas and water). Intergranular corrosion resistance was evaluated. Furthermore, microstructure, density, porosity, macro- and microhardness were investigated. Microstructures were characterised by optical microscopy and scanning electron microscopy with energy dispersive analysis of X-rays. Intergranular corrosion behaviour was evaluated by immersion and by single and double loop electrochemical potentio-kinetic reactivation tests. The superaustenitic steel after fast cooling by water quenching showed the best intergranular corrosion behaviour, the highest microhardness values and the most homogeneous microstructure.

Sensitization of as Rolled and Stable Annealed Alloy 625

Alloy 625 is used as cladding material in the oil and gas industry. Different annealing conditions are used, these are stable annealed and as rolled. Both conditions have been isothermally sensitized in a laboratory resistance furnace. The annealing temperature was up to 1050 °C and the annealing time up to 10 h. Susceptibility to intergranular corrosion has been investigated by means of a conventional immersion tests according to ASTM G28A and the Double Loop—Electrochemical Potentiokinetic Reactivation Test (DL-EPR method). Scanning electron microscopy (SEM) was used to evaluate microstructure and type of attack after testing. The chemical composition of precipitated phases was characterized by SEM and Transmission Electron Microscopy (TEM). High-resolution investigations with concentration profiles proved high molybdenum precipitates to determine the degree of sensitization.

Electrochemical noise resistance evaluation of 304L SS in nitric acid and simulated nuclear high level waste

The paper focuses on corrosion monitoring of 304L stainless steel (SS) (solution annealed, sensitised) in nitric acid (4, 8 and 12M) at 298 and 323 K electrolyte temperatures, and simulated high level waste (HLW, 323 K), using electrochemical noise (EN) measurements. Double loop electrochemical potentiokinetic reactivation test was conducted to assess the degree of sensitisation. Electrochemical noise records revealed passivation process during the monitoring period, under all conditions studied, except for the sensitised specimen in 4M nitric acid (323 K), which showed localised attack. The results showed an inverse relation between EN resistance and corrosion activity. An increase in nitric acid concentration resulted in an increase in corrosion activity, while increasing the electrolyte temperature increased the corrosion activity marginally. A profound increase in corrosion activity occurred for the sensitised specimen in nitric acid when compared to the solution annealed specimen, but the increase was marginal in simulated HLW. The results are detailed in the paper.

The Effect of Low Temperature Sensitization on Corrosion Resistance of Ultrafine-Grained Type 321 Stainless Steels

The ultrafine grain 321 stainless steel was produced by cold rolling. The microstructure, after 87% cold rolling and annealing at 380 and 450 °C, has been characterized using transmission electron microscopy and x-ray diffraction. The degree of sensitization (DOS) was investigated by means of double loop electrochemical potentiokinetic reactivation test. The results showed that the DOS was higher in samples annealed at 380 °C than that in samples annealed at 450 °C. The strain-induced martensite promoted sensitization at 380 °C and the reversion-transformed austenite-restrained sensitization or promoted healing of chromium-depleted zone at 450 °C. Although the activation and reactivation current densities in samples sensitized at 450 °C were higher than those in samples sensitized at 380 °C, higher corrosion resistance of the former was attributed to lower DOS, which was proven further by electrochemical impedance spectroscopy experiment.

Effect of Cold Rolling on Sensitization of 304 Stainless Steel Weldments

The sensitization of stainless steel type AISI304 (304SS) is evaluated as a function of pre-welding cold work and welding heat input. 304SS is cold rolled to various percentages of thickness reduction of up to 50% at ambient temperature before being TIG welded. The susceptibility of 304SS to sensitization (weld decay) is evaluated using ASTM A262, practice A and electrochemical potentiokinetic reactivation tests. The results indicate that the influence of pre-welding cold work on the degree of sensitization (DOS) is distinct and appreciable within the range of 10% to 35% CR. In addition, within this range the optimum level of cold work after which the DOS starts to decrease is found to be inversely related to the amount of welding heat input. At higher levels of cold work of ≥ 35% CR, it is found that neither the cold work nor the heat input significantly influences the amount of DOS. Microstructural investigation showed that the predominant intergranular corrosion (IGC) occurs at austenite grain boundaries and less likely at ferrite / austenite interfaces of the weld metal. Meanwhile, transgranular attack at austenite bulk matrix along the defects is rarely observed.