Determination of susceptibility to intergranular corrosion and electrochemical reactivation behaviour of AISI 316L type stainless steel

In this study, double loop electrochemical potentiokinetic reactivation (DLEPR) test was applied to determine the degree of sensitization in 316L type stainless steel, where obtained results were correlated with revealed microstructures after oxalic acid test and weight loss measurements of Streicher and Huey acid tests. Best agreement was provided with test parameters which are 1 M H₂SO₄ and 0.005 M KSCN at 0.833 mV/s scan rate at 30 °C. Specimens were classified structurally as absence of chromium carbides - step, no single grain completely surrounded by carbides - dual and one or more grain completely surrounded by carbides - ditch, in the as-etched structure, if the I_r:I_a (×100) ratios were obtained to be between 0 and 0.2, 0.2 and 5.0 and 5.0 and higher, respectively. It was also found that at high KSCN concentrations, reactivation current profile skewed to higher potentials where this was attributed the formation of metastable pits, during the anodic scan of the test procedure.     

Assessment of intergranular corrosion in AISI Type 316L stainless steel weldments

Abstract

This paper deals with the applicability of various techniques for the assessment and quantification of sensitisation in AISI Type 316L welds. Welded joints of AISI Type 316L stainless steel were aged at 973 K for periods of up to 200 h. The base and weld metal components of the aged joints were then assessed for susceptibility to sensitisation and intergranular corrosion (IGC) by using various tests specified by ASTM A262, Practices A and E, and ASTM G108 (the electrochemical potentiokinetic reactivation (EPR) test). The possibility of using eddy current testing (ECT) to detect sensitisation and IGC was also assessed. The use of ASTM A262 Practice A and E tests indicated sensitisation in base metal aged for 20 h and above. Aged weld metals showed no failure in these tests. Tensile tests on the weld joints before and after exposure to Cu–CuSO₄-H₂SO₄ solution did not indicate any differences in the tensile properties. Double loop EPR tests indicated a significant increase in the ratios of charge and peak current densities on reactivation to activation after aging the welded joint for 20 h and above. However, weld metal showed no change in the ratios of the above two parameters. Following doubts about the suitability of the EPR test for on line corrosion monitoring, the ECT technique was investigated in order to assess its suitability for the detection and quantification of sensitisation. The ratio of eddy current amplitudes after and before exposure to Cu–CuSO₄-H₂SO₄ solution was used as an assessment criterion. A significant increase in this ratio was observed on aging the base metal for more than 20 h. No significant change was observed in the ratios of eddy current amplitudes for weld metal. The ECT results correlated very well with the findings of the EPR and ASTM Practice E tests. This indicates that ECT holds promise as an on line monitoring tool for sensitisation and IGC.     

Surface spreading of intergranular corrosion on stainless steels

The spreading of intergranular corrosion was investigated using micrometer scale simulations and experimental verifications on sensitized stainless steel [UNS S30400]. The degree of sensitization, presence of a pit, and applied potential all affected spreading. The inputs used in the simulation were obtained from Fe-XCr(X = 10, 12, 14, 16 wt.%)-Mo-Ni alloys representing various grain boundary Cr depletion levels. Corroding grain boundaries and pits triggered corrosion of nearby sensitized boundaries due to Ohmic potential drop. Large connected clusters of corroding grain boundaries formed at high fractions of Cr-depleted grain boundaries. The metallurgical, electrochemical and geometric conditions for this behavior could be forecasted.     

Phase instability and corrosion behaviour of AISI 316 P/M stainless steel

The corrosion behavior of metal powder compacts of different densities made from AISI 316 stainless steel prealloyed powder, heat-treated at different temperatures and times was investigated. Microcharacterisation of polished and fracture surfaces was performed by scanning electron microscopy. Energy dispersive X-ray analysis was used to identify precipitated phases. The anodic and cathodic polarization curves in 1 N sulfuric acid were compared with those of a wrought alloy of similar composition. A distinct difference in the electrochemical behaviour of the powdermetallurgical and the wrought alloy was found. This was attributed to enhanced precipitation of intermetallic phases in the P/M-alloy due to grain-size and density factors.     

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.     

Corrosion behaviour of a dissimilar joint TIG weld between austenitic AISI 316L and ferritic AISI 444 stainless steels

The AISI 444 stainless steel (SS) has become an option to substitute the AISI 316L SS because of its low cost and satisfactory corrosion resistance. However, the use of AISI 444 alloy tubes in heat exchangers causes the welding of a dissimilar joint. The aim of this study was evaluate the corrosion resistance of the tube-to-tubesheet welded by a TIG process composed of AISI 316L and AISI 444. Preparation of samples was executed through replication of tube-to-tubesheet joints. In order to test the corrosion resistance of the welded joint, the following tests were applied: sensitisation, mass loss from room temperature up to 90 °C and electrochemical corrosion tests in 0.5 mol/L HCl and 0.5 mol/L H₂SO₄ electrolytes. The results have shown that the dissimilar joint suffers galvanic corrosion with increased degradation of the heat-affected zone of the AISI 444 tube. Nevertheless, the mechanisms of localised corrosion (pit and intergranular) were more active in the AISI 316L alloy. It is concluded that the dissimilar joint showed better corrosion resistance than the welded joint composed solely of AISI 316L at temperatures up to 70 °C, as the conditions observed in this work.     

New concept of the mechanism of intergranular corrosion of stainless steels

The electronic structure of chromium-nickel austenite stainless steels of type X20H20 and X10H20, in particular, added with 4 to 6% Si, is studied using nuclear γ-resonant (M?ssbauer) spectroscopy. The structure varies significantly depending on the chromium and silicon content. Low-chromium steels are used for the modeling of chromium-depleted boundary zones that neighbor chromium-rich excess phases. Eventually, a mechanism of the intergranular corrosion of stainless steels that takes into account the changes in the electronic structure of the chromium-depleted boundary zones is first suggested. These changes in the chemisorption interaction of the iron carcass ions with corrosive components of the medium, cannot but affect the zones’ stability. It is this interaction that synergistically intensifius the intergranular corrosion of austenite stainless steels containing combinations of silicon dopant and carbon impurity, or the phosphorus and carbon impurities. Original Russian Text ? O.V. Kasparova, Yu.V. Baldokhin, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 3, pp. 256–261.     

Electrochemical methods for testing the intergranular corrosion susceptibility of stainless steels

In addition to the standardized intergranular corrosion testing methods to detect sensitization in austenitic stainless steels, electrochemical test methods, i.e. the potentiokinetic reactivation test (EPR) and the anodic polarization and constant potential etching methods in perchloric acid solution containing sodium chloride, have been developed. The results of these electrochemical measurements are compared with those obtained from copper sulfate-sulfuric acid tests.     

Effect of metallurgical variables on the stress corrosion crack growth behaviour of AISI type 316LN stainless steel

Mill-annealed AISI type 316LN stainless steels, received from two different sources (one indigenous (SS-2) and the other foreign (SS-1)), were tested for stress corrosion cracking (SCC) resistance in a boiling acidified environment of NaCl. SCC results indicated a remarkably lower value of plateau crack growth rate (PCGR) and higher values of K_ISCC and J_ISCC for SS-2, which was attributed to the lower effective grain boundary energy resulting from a higher amount of copper in it. Cold working reduced K_ISCC and PCGR; while thermal aging and welding decreased K_ISCC and increased PCGR vis-à-vis the annealed material.     

The semiconducting properties of passive films formed on AISI 316 L and AISI 321 stainless steels: A test of the point defect model (PDM)

The semiconductor properties of passive films formed on AISI 316L in 1 M H₂SO₄ in three temperatures and AISI 321 in 0.5 M H₂SO₄ were studied by employing Mott–Schottky analysis in conjunction with the point defect model (PDM). Based on the Mott–Schottky analysis in conjunction with PDM, it was shown that the calculated donor density decreases exponentially with increasing passive film formation potential. Also, the results indicated that donor densities increased with temperature. By assuming that the donors are oxygen ion vacancies and/or cation interstitials, the diffusion coefficients of the donors for two stainless steels are calculated.     

Influence of chromizing treatment on the corrosion behavior of AISI 316 stainless steel in supercritical water oxidation

SCWO, sometimes referred to as hydrothermal waste processing, uses the solvating traits of water in its supercritical condition to effectively destroy liquid organic wastes. One major problem in the supercritical water oxidation process is corrosion, because all metallic tubes in the process are exposed to high temperature and high pressure as well as severe corrosive species such as Cl⁻, F⁻, S²⁻, and O²⁻. The presence of Cl⁻ when the pH of a solution is very low and the solution has excess oxygen causes active corrosion and metal loss by metal-chloride and/or oxychloride formation. This study performed a chromizing treatment on 316 stainless steel and immersion tests in supercritical water. Weight change of chromized steels and untreated steels was measured, and the chemical state and composition of oxide films on 316 stainless steel were investigated. On the basis of SCWO tests using distilled water, the oxide layer was found to be very thin and homogeneous and weight gain was observed regardless of testing temperature, while the chromizing treatment slightly reduced weight gain. In the case of SCWO tests using salt water, weight loss was observed regardless of testing temperature and its corrosion mode was pitting by chloride ion, while chromizing treatment greatly decreased the corrosion rate.     

Method of testing stabilized stainless steels for resistance to intergranular corrosion

Relationships between the effective chromium and carbon contents and the parameter of resistance (k) are discussed in this paper in relation to their applicability to stabilized steels. The efficiency coefficients of stabilization in Ti-stabilized steels of four types, determined according to the completeness of chemical composition, were the basis for indicating the parameters of the resistance of these steels to intergranular corrosion. The comparison with low carbon steels, two of optimized chemical composition, one stabilized with niobium, reveals sufficient resistance to, and guarantee of this resistance to the intergranular corrosion for the 08Cr18Ni10Ti grade. The method of evaluating the resistance to intergranular attack based on calculating the minimum effective chromium content, the maximum effective carbon content and the resistance parameter k that issues therefrom, is suitable for practical application directly during the making of stainless steels.     

Effect of Mo and Mn additions on the corrosion behaviour of AISI 304 and 316 stainless steels in H2SO4

The work addresses the influence of Mn and Mo additions on corrosion resistance of AISI 304 and 316 stainless steels in 30 wt.% H₂SO₄ at 25 and 50 °C. Corrosion mechanism was determined by gravimetric tests, DC polarization measurements and electrochemical impedance spectroscopy (EIS). The morphology and nature of the reaction products formed on the material surface were analysed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Reduction of temperature from 50 to 25 °C drastically decreased the corrosion rate of AISI 304 and 316 stainless steels in sulphuric acid solution. Mn additions did not affect significantly the general corrosion resistance due to its low ability to form insoluble compounds in acid medium. Meanwhile, the formation of molybdenum insoluble oxides enhanced the corrosion performance.     

Stress corrosion cracking of weldments of AISI Type 316 stainless steel

The weldments of AISI Type 316 stainless steel prepared by the tungsten inert gas (TIG) and manual metal arc (MMA) welding processes, were tested in a boiling solution of 5 M NaCl + 0.15 M Na₂SO₄ (pH = 1.3) at an initial stress level of 200 MPa under the open circuit potential (OCP) and the impressed potential (IP) conditions using the constant load technique. Anodic polarisation above the critical cracking potential (CCP) which was determined by the OCP test, accelerated the stress corrosion cracking of both the weldments whereas slight cathodic polarisation below the CCP prevented it. The cracks initiated through pits in both the types of weldments under open circuit as well as impressed potential conditions. The TIG weldments cracked in the heat affected zone whereas the MMA weldments failed in the fusion zone at all potentials. At open circuit potential, the MMA weldments failed by the stress assisted dissolution of delta ferrite, while the TIG weldments failed by the TGSCC of austenite. On anodic polarisation, the MMA weldment failed by the dissolution of delta ferrite and intergranular SCC of austenite, while in the TIG weldments the cracks initiated and propagated in the intergranular mode in the austenite.     

The influence of sulphate-reducing bacteria biofilm on the corrosion of stainless steel AISI 316

This work investigates microbially-influenced corrosion (MIC) of stainless steel AISI 316 by two sulphate-reducing bacteria, Desulfovibrio desulfuricans and a local marine isolate. The biofilm and pit morphology that developed with time were analyzed using atomic force microscopy (AFM). Electrochemical impedance spectroscopy (EIS) results were interpreted with an equivalent circuit to model the physicoelectric characteristics of the electrode/biofilm/solution interface. D. desulfuricans formed one biofilm layer on the metal surface, while the marine isolate formed two layers: a biofilm layer and a ferrous sulfide deposit layer. AFM images corroborated results from the EIS modeling which showed biofilm attachment and subsequent detachment over time.     

Effect of chromium content on intergranular corrosion and precipitation of Ti-stabilized ferritic stainless steels

Intergranular corrosion (IGC) and precipitation of ferritic stainless steels (FSS) were investigated with change in Cr content from 11 wt.% to 17 wt.%. The increase in Cr content improved IGC resistance as temperature and time for the sensitization became higher and longer, respectively, but it did not prevent IGC. The analysis on the intergranular precipitates revealed that Cr segregation around fine intergranular TiC in developed all FSS regardless of Cr content. This Cr segregation is proposed to explain the Cr depletion for the cause of IGC in Ti-stabilized Cr FSS.     

Assessment of intergranular corrosion of heat treated austenitic stainless steel (AISI 316L Grade) by electron microscopy and electrochemical tests

The resistance of austenitic stainless steel to intergranular corrosion (IGC) varies during the process of aging at temperatures between 500 and 700°C. This follows the well-known phenomena of precipitating of M₂₃C₆ chromium carbides and intermetallic phases (η, σ, χ). Consequently, this leads to significant Cr-depletion zones at grain boundaries responsible for material sensitization to IGC. The assessment of the sensitivity to IGC from the Strauss or equivalent tests requires cutting a sample off the material, which can be harmful to the integrity of the structure in service. Such a sampling is in essence only qualitative and insufficiently sensitive to the low widths of Cr-depletion at the beginning of precipitation. The DL-EPR method (Double-Loop Electrochemical Potentiodynamic Reactivation test) is known to be a non-destructive and quantitative test method of detecting relatively mild degrees of sensitization in austenitic stainless steel. The current ratios Ir/Ia > 1% (sensitization criteria) and, as a consequence, the electric charge ratios Qr/Qa > 1% of the degree of sensitization (DOS) to intergranular corrosion can be considered as good parameter values to differentiate materials with only difference in the DOS and to detect the fine precipitation responsible for the depletion in the elements of an alloy. This criteria is also valid for the detection of desensitization during againg for longer periods of time. The text was submitted by the authors in English.     

The preparation, characterization and corrosion behaviour of ion-implanted and ceramic-coated AISI 321 steel samples

Ion Beam Techniques were used to modify near-surface layers of austenitic stainless steel AISI 321 samples in order to improve their corrosion resistance. Mg-, Y- (implantation energy: 40 keV) and Al-implantation (implantation energy: 40, 60, 80 and 200 keV), as well as Dynamic Ion Mixing (DIM) (SiC deposition and mixing with 160 keV Xe ⁺ ions) and Sputtering (Si₃N₄ deposition with 15 keV Ar⁺ ions) were applied for this purpose. The characterization of the samples was performed using Nuclear Reaction Analysis (NRA) and ⁴He-Rutherford Backscattering Spectroscopy (RBS). The corrosion behaviour was investigated in 1N H₂SO₄ using cyclovoltammetry and potentiodynamic polarization. The values of the critical and passivation current density as well as of the corrosion, passivation and repassivation potentials, showed an increase of the corrosion resistance of the treated steel samples. This improvement is connected with the properties of the modified region (thickness, adhesion, formation of oxide films). The surface morphology and microstructure of the specimens before and after the corrosion experiments were investigated by Scanning Electron Microscopy (SEM).     

Effect of hot corrosion on the creep properties of types 321 and 347 stainless steels

Problems caused by hot corrosion and creep damage on superheater and reheater tubes of power plants using heavy oil as fuel inhibit the continuous service of the boilers and shorten their design lives. The acceleration of hot corrosion attack of boilers is caused by the presence of fuel ash deposits containing V, Na, and S, in the form of Na₂SO₄ and V₂O₅, which form low melting point phases. In addition to this, the tubes are exposed to the action of both high stresses and high temperatures, producing a continuous plastic deformation of the tube walls called creep damage. Creep rupture tests were carried out in the temperature range 620 to 670 °C in static air in the presence of corrosive environments using 321H and 347H type stainless steels used in superheater and reheater tubes under hot corrosion and creep environments. The corrosive environment includes synthetic Na₂SO₄, V₂O₅, and the mixture 80% V₂O₅-20%Na₂SO₄. Also, the role of the different elements present in the environments on corrosion was investigated using electronic microscopy and x-ray diffraction techniques.