Effect of laser surface melting on intergranular corrosion behaviour of aged austenitic and duplex stainless steels

In the present study, the effect of laser surface melting (LSM) on intergranular corrosion behaviour of aged austenitic stainless steels (UNS S30400, S31603, S32100 and S34700) and aged duplex stainless steels (UNS S31803 and S32950) were investigated. LSM of the aged stainless steels was carried out using a 2.5 kW CW Nd:YAG laser. The microstructure of the aged stainless steels after LSM depends on their compositions. After LSM, the aged austenitic stainless steels mainly contain austenite (γ) with some ferrite (δ) as the minor phase, but the carbide phases are completely eliminated. For the aged duplex stainless steels after LSM, δ becomes the major phase and the δ/γ phase balance is disturbed, whereas the sigma (σ) phase is eliminated. The degree of sensitization (DOS) and corrosion morphology of the aged stainless steels before and after LSM were determined by the double loop electrochemical potentiokinetic reactivation (DL-EPR) using a potentiostat and SEM observation, respectively. Desensitization of the aged stainless steels has been successfully achieved by LSM and their intergranular corrosion resistance is found to be significantly enhanced as reflected by the decrease in DOS due to dissolution of the carbides or σ phase, which reduced Cr depletion or the possibility of solute segregation at the grain or phase boundaries, despite the presence of δ and disturbance of δ/γ phase balance.     

Influence of heat treatments and chemical composition on SCC susceptibility during repairing procedure of overlaying of Inconel 182 by laser surface melting

Abstract

A practical repairing technique using laser surface melting (LSM) was developed to remove the stress corrosion cracking (SCC) in overlaying of Inconel 182. Influence of microstructure of different heat treatments performed during repairing process on intergranular cracking/intergranular stress corrosion cracking (IGC/IGSCC) susceptibility was discussed. The intergranular precipitate was identified as M₂₃ C₆ by TEM. The microstructure with no intergranular precipitate and refiner sub-grain after LSM process shows excellent IGC/IGSCC resistance. The stress relief heat treatment induced severe microstructure of high IGC/IGSCC susceptibility, owing to the semicontinuous intergranular precipitation. The influence of Nb/C ratio on IGC/IGSCC susceptibility of three nickel based superalloys after LSM process was also investigated. For both of the Inconel 182 alloys with different Nb content, the microstructure after LSM process and following sensitisation treatment showed precipitation free grain boundary. The results of corrosion tests also indicated that the material with higher Nb/C ratio showed higher IGC/IGSCC resistance after LSM process and following sensitisation treatment.     

Improving intergranular corrosion resistance of sensitized type 316 austenitic stainless steel by laser surface melting

An attempt was made to modify the surface microstructure of a sensitized austenitic stainless steel, without affecting the bulk properties, using laser surface melting techniques. AISI type 316 stainless steel specimens sensitized at 923 K for 20 hr were laser surface melted using a pulsed ruby laser at 6 J energy. Two successive pulses were given to ensure uniform melting and homogenization. The melted layers were characterized by small angle X- ray diffraction and scanning electron microscopy. Intergranular corrosion tests were carried out on the melted region as per ASTM A262 practice A (etch test) and electrochemical potentiokinetic reactivation test. The results indicated an improvement in the intergranular corrosion resistance after laser surface melting. The results are explained on the basis of homogeneous and nonsensitized microstructure obtained at the surface after laser surface melting. It is concluded that laser surface melting can be used as an in situ method to increase the life of a sensitized component by modifying the surface microstructure.     

Effects of laser surface melting on the pitting resistance of sensitized nitrogen-bearing type 316L stainless steel

Laser surface melting of sensitized nitrogen-bearing type 316L austenitic stainless steel was carried out using a pulsed ruby laser. The sensitization heat treatment was carried out at 923 K for 50, 200, 1000, and 2500 h, and the sensitized microstructure was classified according to ASTM A 262 practice A. The degree of sensitization was assessed by the electrochemical potentiokinetic reactivation (EPR) test. The critical pitting potentials of as-sensitized as well as sensitized-laser melted specimens were determined by potentiodynamic anodic polarization method in a medium containing 0.5 M NaCl and 0.5 M H₂SO₄ at room temperature. Results indicated that upon laser melting the pitting resistance increased significantly. This increase was attributed to the elimination of the sensitized heterogeneous microstructure by laser melting. The microscopic examination of the pitted specimens showed only micropits that developed at the interfaces of oxide/sulfide inclusions of titanium and matrix.     

Intergranular corrosion resistance of microstructure- modified type 316 austenitic stainless steel

Cold working and a double aging treatment was used to produce a microstructure with fine nuclei of carbides distributed throughout the grains to improve the intergranular corrosion (IGC) resistance of austenitic stainless steel. The treatment was carried out on type 316 stainless steel as follows: cold working (20,30, and 40% reductions in thickness), sensitization (923 KJ5 h), and aging each for 1173,1223,1273, and 1323 K/l h, respectively. Specimens in the solution annealed condition (0% cold work) were also given the above treatment. All of the specimens were resensitized at 923 KJ5 h and tested for IGC resistance as per ASTM A262, Practice A(oxalic acid etch test) and Practice E (24 h immersion in boiling Cu-CuSO.₄-H₂SO₄ and the U- bend test). Microhardness measurements were also carried out on all specimens. The results indicated that at an optimum treatment (30% cold work + sensitization + aging) all the specimens showed improved IGC resistance. The 0 and 20% cold worked specimens showed improvement at higher aging temperatures only. Specimens undergoing 40% cold work exhibited a decrease in IGC resistance. Compared to as-cold-worked specimens, an improvement in IGC resistance was obtained with 30% cold working.     

Intergranular corrosion damage evaluation through laser scattering technique

AISI type 316 austenitic stainless steel in sensitized condition was subjected to ASTM A262 practice A test (electrolytic etching in 10% ammonium persulphate at 1 A/cm²) for various durations from 10 s to 90 s. The different degrees of intergranular corrosion attack produced on these tested specimens were evaluated by laser scattering technique using a 1 mW He-Ne laser. The scattering intensity pattern of the laser beam incident on the specimen was acquired using a CCD camera and transferred to a computer for further analysis. The specular intensity of the scattering pattern and its full width at half maximum (FWHM) was calculated. The results indicated that the specular intensity decreased and the FWHM increased as the degree of IGC attack increased. A good correlation was found between the extent of IGC with the laser scattering parameters.     

Precipitation and Cr depletion profiles of Inconel 182 during heat treatments and laser surface melting

Thermodynamic and kinetic modeling were conducted to simulate Cr depletion profiles near grain boundaries in Inconel 182 during heat treatments and laser surface melting (LSM) using Thermo-Calc and DICTRA code. The effect of Nb addition was also considered in the modeling. Based on the good agreement with Cr concentration distributions during the heat treatments measured experimentally, Cr depletion profiles adjacent to grain boundaries during the heat treatments and the LSM process were modeled. The Cr depletion profiles were evaluated using the Cr depletion area below the critical Cr concentration for intergranular cracking/intergranular stress corrosion cracking (IGC/IGSCC) susceptibility (12 mass%). Compared with the result of the Streicher test, the calculated Cr depletion areas showed good agreement with IGC/IGSCC susceptibilities. The sample after stress relief (SR) treatment had the largest Cr depletion area and showed the poorest IGC/IGSCC resistance. Cr depletion showed some recovery during subsequent low temperature sensitization (LTS). The sample after the LSM process had the smallest Cr depletion area and showed the best IGC/IGSCC resistance.     

Peculiarities of Intergranular Corrosion of Silicon-Containing Austenitic Stainless Steels

Experimental data collected for the past two decades and a half are generalized, and the mechanism of intergranular corrosion (IGC) of austenitic silicon-containing (quenched and sensitized) steels in the environments with various redox characteristics is discussed. The synergistic intensification of IGC by the alloying silicon additives and carbon admixtures is experimentally revealed. The cause of the bivariant effect of silicon on the resistance of grain boundaries in sensitized steels at active–passive transition potentials is clarified. The effect of silicon on the electronic structure of a solid solution (including intergranular domains) is supposed to account for some peculiarities in the electrochemical behavior of the steels.     

Double loop electrochemical potentiokinetic reactivation test of Nickel-base Alloy 600 surface-melted by a CO2 laser beam

Ni-base Alloy 600 has been widely used as a steam generator (S/G) tubing material in nuclear power plants because of its good mechanical and corrosion properties at high temperatures. However, degradations of S/G tubes due to intergranular attack (IGA) and intergranular stress corrosion cracking (IGSCC) during normal operation have been frequently reported. In particular, Alloy 600 can be very susceptible to IGA/IGSCC in some sulfur-bearing environments by sensitization. In this paper, the beneficial effects of laser surface melting (LSM) on intergranular corrosion of the sensitized Alloy 600 is presented from the results of the double loop electrochemical potentiokinetic reactivation (DL-EPR) test. The DL-EPR test was performed in de-aerated 0.01 M H₂SO₄+20 ppm KSCN at a scan rate of 0.5 m V/sec at room temperature. The degree of sensitization (DOS) of the sensitized Alloy 600 measured from the DL-EPR test was considerably reduced by LSM. The sensitized Alloy 600 after LSM also exhibited a relatively low DOS, compared with that of the sensitized but not laser treated Alloy 600. From the microscopic observation, it was found that the microstructural changes brought about by the LSM process, especially changes in the precipitation behavior of grain boundary Cr-rich carbides, caused the improvement of resistance to intergranular corrosion of the laser treated Alloy 600. The resistance to IGSCC of the laser treated Alloy 600 in sulfur-bearing environments was also discussed from the results of measured DOS and microstructural examination. This article based on a presentation made in the symposium “The 4th International Conference on Fracture and Strength of Solid”, held at POSTECH, Pohang, Korea, August 16–18 under the auspices of Far East and Ocean Fracture Society (FEOFS),et al.     

A study of intergranular corrosion of austenitic stainless steel by electrochemical potentiodynamic reactivation, electron back-scattering diffraction and cellular automaton

The impact of solution and sensitization treatments on the intergranular corrosion (IGC) of austenitic stainless steel (316) was studied by electrochemical potentiodynamic reactivation (EPR) test, and the results showed the degree of sensitization (DOS) decreased as solution treatment temperature and time went up, but it increased as sensitization temperature prolonged. Factors that affected IGC were investigated by field emission scanning electron microscope (FE-SEM) and electron back-scattering diffraction (EBSD). Furthermore, the precipitation evolution of Cr-rich carbides and the distribution of chromium concentration were simulated by cellular automaton (CA), clearly showing the effects of solution and sensitization treatments on IGC.     

Resistance to sensitization and intergranular corrosion through extreme randomization of grain boundaries

Two grades of austenitic stainless steel, type 304 and 316L, were cold rolled to different reductions by unidirectional and by cross rolling. Subsequent solutionizing of the cold rolled samples produced noticeable textural differences in type 304, but insignificant differences in type 316L. Both the solutionized materials had however the same trend in grain boundary character distribution (GBCD): an increasing fraction of random boundaries with an increasing pre-solutionizing reduction percentage. The degree of sensitization (DOS) was measured by the double loop electrochemical potentiokinetic reactivation (DL-EPR) test in both the alloys. The susceptibility to intergranular corrosion was assessed by the standard weight loss technique (practice B, A262 ASTM) in type 304 alloy. These increased with increase in random boundary concentration, but then dropped significantly beyond a ‘critical’ concentration—a pattern observed in both the grades. Such a pattern may be explained from a balance between nucleation rate of Cr-carbides and grain boundary Cr-flux, though postulating an exact model is premature at this stage. The present study, however, demonstrates a clear possibility of remarkable improvement in DOS and IGC through extreme grain boundary randomization.     

Pitting and Intergranular Corrosion Resistance of AISI Type 301LN Stainless Steels

The pitting and intergranular corrosion (IGC) resistance of AISI type 301LN stainless steels were evaluated using ASTM methods, anodic polarization, and electrochemical impedance techniques. The IGC results indicated that the microstructure of the samples after sensitization heat treatment at 675 °C for 1 h shows step or dual structure for both imported and indigenous materials indicating insignificant Cr₂₃C₆ precipitation. The results of immersion tests in boiling 6% copper sulfate + 16% sulfuric acid + copper solution for 24 h followed by the bend test (ASTM A262 Practice-E method) indicated no crack formation in any of the tested specimens. Pitting corrosion resistance carried out in 6% FeCl₃ solution at different temperatures of 22 ± 2 and 50 ± 2 °C (ASTM G 48) up to the period of 72 h revealed pitting corrosion attack in all the investigated alloys. The potentiodynamic anodic polarization results in 0.5 M NaCl revealed variation in passive current density and pitting potential depending on the alloy chemistry and metallurgical condition. The passive film properties studied by electrochemical impedance spectroscopy (EIS) correlated well with the polarization results. The x-ray diffraction (XRD) results revealed the presence of austenite (γ) and martensite (α′) phases depending on the material condition. The suitability of three indigenously developed AISI type 301LN stainless steels were compared with imported type 301LN stainless steel and the results are highlighted in this article.     

Uniform corrosion and intergranular corrosion behavior of nickel‐free and manganese alloyed high nitrogen stainless steels

The uniform and intergranular corrosion behavior of two kinds of nickel‐free and manganese alloyed high nitrogen stainless steels (HNSSs) were investigated. A type of 316L stainless steel (316L SS) was also included for comparison purpose. Both solution annealed (SA) and sensitization treated (ST) steels were examined. It was found that the SA HNSSs had much weaker resistance to uniform corrosion compared to the SA 316L SS. The addition of molybdenum, to some extent, improved the uniform corrosion resistance of the HNSSs. The sensitization treatment had little influence on the uniform corrosion resistance of all the steels. The HNSSs showed an obvious susceptibility to intergranular corrosion, in particular the ST HNSSs. The intergranular corrosion rates of the sensitized HNSSs were much higher than that of the sensitized 316L SS. The degree of interganular attack for the ST HNSSs was much more serious than that for the 316L SS. The addition of molybdenum obviously improved the resistance of the ST HNSSs to intergranular corrosion. The double loop electrochemical potentiokinetic reactivation tests also proved that the HNSSs were rather susceptible to the sensitization treatment compared to the 316L SS. The relatively weak resistance of the HNSSs to uniform and intergranular corrosion may be due to high manganese promoted anodic dissolution. The improvement of uniform and intergranular corrosion resistance caused by the addition of molybdenum could be attributed to the synergistic effects of molybdenum and nitrogen in the HNSSs on the formation and stability of passive film.     

A hydrogen embrittlement mechanism for sensitized types 304, 316 and 310 austenitic stainless steels in boiling saturated magnesium chloride solutions

We have already proposed a mechanism for intergranular hydrogen embrittlement (IG-HE) for solution annealed austenitic stainless steels (types 304, 316 and 310) in HCl solutions and in boiling saturated magnesium chloride solutions. The proposed IG-HE mechanism was based on martensite transformation, hydrogen-enhanced local plasticity (HELP), grain boundary sliding (GBS). Recently, it was reported that the fracture susceptibility and fracture mode for sensitized steels in boiling saturated magnesium chloride solution under an open-circuit condition were significantly different from those observed for solution annealed steels. In the present paper, the hydrogen embrittlement behavior of sensitized types 304, 316 and 310 in boiling saturated magnesium chloride solutions was explained in more details in terms of an inhibiting effect of chloride ions, martensite transformation, Cr depletion, HELP, the degree of corrosiveness through the comparison with those for the solution annealed steels. Furthermore, a transgranular HE (TG-HE) cracking mode that was not observed for the solution annealed steels was discussed as well as IG-HE. Then a TG-HE mechanism for sensitized austenitic stainless steels was proposed, while the IG-HE mechanism for solution annealed austenitic stainless steels which was discussed in details was applied to IG-HE of sensitized austenitic stainless steels. It was also pointed out that the occurrence of both TG-HE and IG-HE was explained with an identical concept.     

Intergranular corrosion of Ti-stabilized 11 wt% Cr ferritic stainless steel for automotive exhaust systems

Intergranular corrosion (IGC) of type 409L ferritic stainless steel (FSS) was investigated. A free-exposure corrosion and a double loop electrochemical potentiokinetic reactivation (DL-EPR) tests were conducted to examine IGC of the FSS. IGC occurred in the specimens aged at the temperature range of 400–600 °C that has the sensitization nose located around 600 °C. The critical I_r/I_a value was determined to be about 0.03 above which IGC occurred. Based on the analysis of the intergranular precipitates by an energy dispersive spectroscopy (EDS) and a transmission electron microscopy (TEM), IGC was induced by the Cr depletion zone formation due to Cr segregation around intergranular TiC.     

Effect of Solution Annealing on Susceptibility to Intercrystalline Corrosion of Stainless Steel with 20% Cr and 8% Ni

In general, as-received (AR) austenitic stainless steels (ASSs) contain complex carbide precipitates due to manufacturing operations, subsequent annealing treatment, or due to the fabrication processes such as welding. The presence of pre-existing carbides leads to cumulative sensitization and make the steel susceptible to intercrystalline corrosion (ICC)/intergranular corrosion (IGC) which causes premature failure during service. Solution annealing (SA) is one of the ways to deal with such situations. In this present investigation, the AR (hot rolled and mill annealed) chromium-nickel (Cr-Ni) ASS is compared with SA Cr-Ni ASS. The extent of ICC/IGC was evaluated qualitatively and quantitatively by various electrochemical tests including ASTM standard A-262 Practice A and Practice E, double loop electrochemical potentiokinetic reactivation and electrochemical impedance spectroscopy. The degree of sensitization for hot rolled mill annealed AR condition is found to be substantially higher (51.55%) than that of SA condition (26.9%) for thermally aged samples (at 700 °C). The chemical composition across the grain boundary was measured using electron probe micro-analyzer for both (AR and SA) conditions and confirms that the pre-sensitization effect was completely removed after SA treatment.     

Die Bedeutung des Oxalsäure-Tests für die Prüfung der Korrosionsbeständigkeit nichtrostender Stähle

The meaning of the oxalic acid etch test for testing the corrosion resistance of stainless steels In the oxalic acid etch test according to ASTM A 262 practice A, precipitations of phases rich in chromium and molybdenum which can occur in stainless steels, are preferentially dissoved. The behaviour of such phases in the oxalic acid etch test was investigated taking precipitations of carbide M₂₃C₆, s?-phase, χ-phase and Laves-phase in stainless steels AISI 304 L and 316 L as examples. The chemical composition of these was evaluated with a scanning transmission electron microscope (STEM) by EDS. With coarser precipitations, it was possible to support this analytical method by EDS of metallographic cross sections in a scanning electron microscope (SEM). In oxalic acid, critical threshold potentials exist above which the above mentioned phases are preferably attacked, furthermore critical pH values, below which no selective attack of the precipitated carbides and intermetallic phases occurs. The numerical values of the threshold potentials as well as the critical pH values were evaluated. When testing stainless steels in the oxalic acid etch test, the steel specimens are polarized to a highly positive potential in the very trans passive range. In this potential range the corrosion rate of stainless steels increases with increasing chromium content, while in the active and passive range the corrosion rate decreases with increasing chromium content. Other than the nitric-hydrofluoric acid test, the copper-copper sulfate-sulfuric acid test, and the ferric sulfate-sulfuric acid test, the oxalic acid etch test does therefore not indicate any chromium depletion. Hence, an intergranular attack also occurs when precipitations of carbides rich in chromium are present at the grain boundaries of austenitic stainless steels with the carbides being precipitated without any chromium depletion of the areas adjacent to the grain boundaries. Sensitized austenitic stainless steels which are susceptible to intergranular corrosion due to the precipitation of chromium rich carbides and chromium depletion of the areas adjacent to the grain boundaries, can suffer intergranular SCC in high temperature aqueous environments when additionally critical conditions with respect to the mechanical stress level and the oxygen concentration in the environment are given. For the detection of sensitized microstructures, the oxalic acid etch test must be valued critically due to the dependence of the corrosion rate on the chromium content mentioned above, and is obviously by far less suited than the conventional tests for establishing resistance to intergranular corrosion in sulfuric acid-copper sulfate solutions with additions of metallic copper (Strauß test, severe Strauß test).     

Influence of sigma-phase formation on the localized corrosion behavior of a duplex stainless steel

Because of their austenitic-ferritic microstructures, duplex stainless steels offer a good combination of mechanical and corrosion resistance properties. However, heat treatments can lower the mechanical strength of these stainless steels as well as render them susceptible to intergranular corrosion (IGC) and pitting corrosion. In this study, a low-carbon (0.02%) duplex stainless steel is subjected to various heat treatments at 450 to 950 °C for 30 min to 10 h. The heat-treated samples then undergo ASTM IGC and pitting corrosion tests, and the results are correlated with the microstructures obtained after each heat treatment. In the absence of Cr₂₃C₆ precipitation, σ-phase precipitates render this duplex stainless steel susceptible to IGC and pitting corrosion. Even submicroscopic σ-phase precipitates are deleterious for IGC resistance. Longer-duration heat treatments (at 750 to 850 °C) induce chromium diffusion to replenish the chromium-depleted regions around the σ-phase precipitates and improve IGC resistance; pitting resistance, however, is not fully restored. Various mechanisms of σ-phase formation are discussed to show that regions adjacent to σ-phase are depleted of chromium and molybdenum. The effect of chemical composition (pitting resistance equivalent) on the pitting resistance of various stainless steels is also noted.     

Effect of Nitrogen and Sensitization on the Microstructure and Pitting Corrosion Behavior of AISI Type 316LN Stainless Steels

High-nitrogen stainless steels (SS) are receiving increased attention because of the advantages of their strength over the SS with nominal composition. However, they are susceptible to dichromium nitride (Cr₂N) precipitation during thermal exposure between 873 and 1323 K resulting in sensitization and subsequent intergranular corrosion. Round tensile specimens of AISI type 316LN SS, with three different nitrogen content 0.07, 0.14, and 0.22 wt.% in mill-annealed and sensitized (973 K for 24 h) condition were studied for their pitting corrosion behavior. The results of the potentiodynamic anodic polarization studies were correlated with the results obtained using electrochemical impedance spectroscopy (EIS) technique. Critical pitting potential (E _pp) increased with increasing nitrogen content but the same was found to decrease on aging. The parameters indicating passive film stability measured by EIS revealed faster passive film dissolution as indicated by low polarization resistance, in sensitized condition and vice-versa in mill-annealed condition. The EIS results correlated well with the variation in the respective E _pp obtained from the potentiodynamic polarization diagrams.     

The stress corrosion cracking behavior of austenitic stainless steels in boiling magnesium chloride solutions

The change in the mechanism of stress corrosion cracking with test temperature for Type 304, 310 and 316 austenitic stainless steels was investigated in boiling saturated magnesium chloride solutions using a constant load method. Three parameters (time to failure; t_f, steady-state elongation rate; l_ss and transition time at which a linear increase in elongation starts to deviate; t_ss) obtained from the corrosion elongation curve showed clearly three regions; stress-dominated, stress corrosion cracking-dominated and corrosion-dominated regions. In the stress corrosion cracking-dominated region the fracture mode of type 304 and 316 steels was transgranular at higher temperatures of 416 and 428 K, respectively, but was intergranular at a lower temperature of 408 K. Type 310 steel showed no intergranular fracture but only transgranular fracture. The relationship between log l_ss and log t_f for three steels became good straight lines irrespective of applied stress. The slope depended upon fracture mode; −2 for transgranular mode and −1 for intergranular mode. On the basis of the results obtained, it was estimated that intergranular cracking was resulted from hydrogen embrittlement due to strain-induced formation of martensite along the grain boundaries, while transgranular cracking took place by propagating cracks nucleated at slip steps by dissolution.