Electrochemical Potentiodynamic Reactivation Test Research Articles

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.

Microstructure, mechanical properties and sensitization of ultra-low nickel Cr–Mn austenitic stainless steels

The present study deals with the effect of shielded metal arc welding (SMAW) and gas metal arc welding (GMAW) processes with different welding speeds on microstructural and mechanical properties including intergranular corrosion susceptibility of ultra-low nickel chrome-manganese austenitic stainless steel (ASS). The microstructural analysis was performed by an optical microscope. The Fisher Ferritscope was used for determining ferrite content in the weldment. The microstructural analysis shows that the size of dendrites of δ ferrite and the width of heat affected zone (HAZ) decreases when welding speed increases. It was found more δ-ferrite in SMAW weld compared to GMAW weld. It was also found that when welding speed increases, tensile strength, impact strength and hardness values decrease for both welding processes. From the U-bending test, it was found that the welded joints are strong enough to withstand bend at 1800 without any cracks for both welding processes. It was also found that Cr-Mn ASS weld becomes susceptible to intergranular corrosion on welding. For both welding processes, the degree of sensitization value decreases with an increase in welding speed for both the welding processes as evaluated by the double loop electrochemical potentiodynamic reactivation (DLEPR) test. Also, the weld joint made using the GMAW process is more susceptible to intergranular corrosion than that made using the SMAW process.

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.

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.

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 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 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.

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.

Quantitative evaluation of global and local chromium contents with the EPR test on ferritic and martensitic stainless steels

The cause of localised corrosion phenomena in stainless steels is often related to the element chromium, which is essential for the formation and resistance of the passive layer. A lower than nominally stated content of chromium occurs when the element is highly concentrated and locally bound in chromium-rich phases or precipitates, mostly carbides, and thus locally or globally absent for passivation. The method of electrochemical potentiodynamic reactivation (EPR) is uniquely suited to demonstrate the effect of chromium depletion on the passivation. Here, the steel surface is initially active in sulfuric acid and passivation is achieved by external dynamic polarization. After passivation, the polarization direction is reversed and in chromium-depleted areas a reactivation takes place, which is detected by an increase in current. The detection limit for a reduced chromium content depends largely on the strength of the sulfuric acid used in the EPR test and the polarization speed. The relationship between the sulfuric acid concentration and the electrochemical parameters from the EPR test is shown here using the example of ferritic Fe-Cr alloys with graded Cr contents in the range of approx. 6 to 18 %. From the passivation and reactivation current densities of the EPR data, models for the detection of global and local chromium contents (detection limits) are derived. The developed models are applied to the martensitic stainless steel X20Cr13 in different heat treatment conditions and the influence on the distribution of chromium in the microstructure, which is responsible for the passivation and thus the corrosion resistance, is shown.

Sensitization behaviour of the nitrogen alloyed austenitic stainless steel X8CrMnMoN18-19-2

Austenitic stainless steels have been used in many different industrial branches. Nitrogen alloyed austenite´s with up to 20 wt. % manganese and 0.8 wt. % nitrogen have been available on the market as nickel-free grade for some years. In these steels, the austenitic matrix is stabilized without the addition of nickel, while the corrosive resistance and mechanical properties are significantly improved by addition of Mn and N. As with all stainless steels, the chemical composition and heat treatment have a decisive influence on structure, mechanical properties and corrosion resistance. Solution annealing, quenching and work hardening are applied to combine excellent mechanical properties (Rm from 900 MPa to 2,000 MPa, A5 > 50 %, Av > 350 J) with high corrosion resistance. Solution annealing is applied to eliminate undesired precipitation phases (Cr2N, M23C6 and sigma phase), distribute the alloy elements homogeneously in the austenite and assure the high corrosion resistance. If the distribution of the alloy elements (Cr, Mo and N) is impaired by suboptimal heat treatment, processing or high temperature conditions, the corrosion resistance can be reduced. Precise knowledge of the sensitization behaviour of these high nitrogen alloyed steels is necessary. Therefore, the sensitization behaviour of the nitrogen alloyed austenitic steel X8CrMnMoN18-19-2 (1.4456) was investigated by thermal ageing in the temperature rage of 500 °C to 900 °C for up to 100 h. Thermodynamic calculations were used to predict the formation of different phases. The microstructure of the various sensitization states was evaluated by scanning electron microscopy (SEM) and compared with the corrosion resistance characterized by the electrochemical potentiodynamic reactivation (EPR) test and the KorroPad method. The reduction of corrosion resistance is correlated with the occurrence of M2N nitrides in the microstructure and two sensitization diagrams were created based on the experimental results.

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

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

Influence of grinding parameters on the corrosion behavior of austenitic stainless steel

AbstractSamples of the austenitic stainless steel grade X5CrNi18‐10 (1.4301, AISI 304) were ground industrially with various grinding parameters to study their influence on corrosion resistance. The ability of the mechanically ground surfaces to form a stable passive layer was evaluated by KorroPad test and a modified electrochemical potentiodynamic reactivation test based on a single loop (EPR‐SL). Furthermore, the surfaces were characterized by surface analytical methods. The main influence was determined regarding abrasive belt type. Surfaces mechanically ground with granulate abrasive belts constantly had a lower corrosion resistance than surfaces ground with single‐coated grain. The granulate abrasive belts generated more sensitized surface areas and left formations of welded sample material on the mechanically ground surfaces. A post‐treatment with a nonwoven abrasive proved to be an effective finishing process by which the surface defects and sensitized material got removed and the surfaces regained the expected corrosion resistance.

Characterization of the Native Passive Film on Ferrite and Austenite Phases of Sensitized 2205 Duplex Stainless Steel

The influence of isothermal sensitization of 2205 duplex stainless steel (DSS) in 850°C was quantified using double-loop electrochemical potentiodynamic reactivation test. We assessed the microstructure, Volta potential distribution, and semiconductive properties of the native passive film on individual austenite and ferrite phases of solution annealed and sensitized DSS, using optical and scanning electron microscopy, scanning Kelvin probe force microscopy, and scanning tunneling spectroscopy. Beside conventionally used line profile scans, we used a multi-modal Gaussian distribution method for analyzing the Volta potential distribution over the entire scanned area. The Volta potential distribution evaluations indicated that the driving force for preferential corrosion at the ferrite/austenite interface in DSS increases after sensitization. Scanning tunneling spectroscopy revealed that the formation of intermetallic phases and depleted zones in DSS after sensitization, decrease the surface energy bandgap in both austenite and ferrite phases, whereas the density of electronic states in these phases increases, compared to those of solution annealed DSS. The multi-analytical approach that is used in this research provides complementary evidence for the detrimental effects of sensitization on corrosion resistance of DSS.

Pitting Corrosion on a Water Pipe of the Grade 1.4301

Abstract Pipes made of stainless steel in the drinking water supply system showed pitting corrosion in the heat-affected zone (HAZ) of the welded joints. This was very surprising considering the chemical composition of the fresh water and the fact that no microbiologically influenced corrosion (MIC) had been found. Testing for pitting corrosion showed no increased sensitivity of the HAZ. The Electrochemical Potentiodynamic Reactivation test (EPR test), however, was positive for the tube interior and only within the HAZ. Metallographic examinations were carried out to clarify if sensitization or intergranular attack, a type of corrosion, were the cause. The holes showed an irregular propagation which can be explained by segregation lines in the steel. It is noticeable that the holes preferably propagate in the mid-thickness region of the steel sheet. The metallographic samples showed no preferred corrosive attack along the grain boundaries (intergranular attack) and also no sensitization of the steel.

Effect of grain size on self -healing behaviour of sensitized S304HCu stainless steel

S304HCu stainless steel is extensively used for superheater and reheater applications in power plants due to its superior creep strength and oxidation resistance which are derived from the coherently dispersed precipitates formed in austenite matrix. In addition to the beneficial precipitates, chromium carbide precipitates can also form. Formation of chromium carbide precipitates during service will lead to sensitization. In this investigation, S304HCu samples were solutionized at 1100 °C and 1200 °C for 1 h, and further aged at 750 °C for upto 400 h to introduce sensitization and subsequent healing. The influence of ageing time and grain size on the degree of sensitization (DOS) was evaluated by using oxalic acid and double loop electrochemical potentiodynamic reactivation (DLEPR) tests. DOS initially increased up to 10 h of ageing and subsequently was reduced due to healing and attained a steady value at 50 h. Faster and higher DOS was observed in coarse grain structure than in the fine grain structure. However, healing was largely delayed in coarse grained structure. The difference in solute and excess vacancy concentration and shifting of the nose in TTP curve were attributed to the difference in the kinetics of sensitization and healing. The extent of the attack on the grain boundary was analyzed using AFM.