Investigation of intergranular corrosion of 316L stainless steel diffusion bonded joint by electrochemical potentiokinetic reactivation

Electrochemical potentiokinetic reactivation technique (EPR) was employed to assess degree of sensitization in 316L stainless steel diffusion bonded joint (DBJ). The result showed the degree of sensitization of DBJ was much smaller than that of base material (BM). No chromium carbides precipitated at grain boundaries in DBJ after 100 h treatment at 650 °C, while chromium carbides could be seen clearly in the BM after 8 h treatment, indicating that DBJ has better intergranular corrosion resistance than BM. Diffusion bonding technique will not increase intergranular corrosion susceptibility of 316L DBJ. Reactivation potential has the biggest effect on sensitization.     

Use of EPR test to study the degree of sensitization in resistance spot welding joints of AISI 304 austenitic stainless steel

The degree of sensitization (DOS) in resistance spot welding (RSW) joints is considered as the combined effect of intergranular corrosion (IGC) and transgranular corrosion (TGC) in the heat affected zone (HAZ) and of interdendritic corrosion (IDC) in the weld nugget (WN). The DOS is evaluated from electrochemical potentiokinetic reactivation (EPR) tests. The application of EPR test to RSW joints is optimized and an electrochemical minicell is used to study the effect of heat input on IDC in the WN.     

Potentiodynamic polarization behaviour of AISI type 316 stainless steel in NaCl solution

Potentiodynamic polarization behaviour of AISI type 316 SS in NaCl solution was investigated in terms of the potential scan rate effect. The critical pitting potential, E_pit, of the stainless steel appeared to be strongly dependent on the potential scan rate. A cumulative anodic electric charge density of the steel was defined as the total charge density from the open circuit potential, E_ocp, and calculated using the potentiodynamic polarization curves. It was found that, plotted as a function of the polarization time, the values of the cumulative charge density consisted of two lines with different slopes. It was confirmed that the deflection of the cumulative charge density vs. time plots corresponded to E_pit and the values of the cumulative charge density at the deflection were little dependent on the applied scan rate. The cumulative charge density at the deflection was defined as a critical electric charge density for the stable pitting. Also, it was suggested that this electric charge density should be associated with the critical condition for the stable pitting and the critical electric charge for stable pitting should be a representative parameter for the pitting resistance of a material.     

Assessment of intergranular corrosion (IGC) in 316(N) stainless steel using electrochemical noise (EN) technique

In the present work, an attempt was made to demonstrate the use of electrochemical noise (EN) technique in assessment of intergranular corrosion (IGC) in 316(N) stainless steel (SS). Degree of sensitization (DOS) in the specimens aged at 923 K for 24, 40, 50 and 100 h was determined using double loop electrochemical potentiodynamic reactivation (DLEPR) technique. Immediately after applying a prior preconditioning treatment, current and potential noise measurements were made. The DOS was determined from standard deviation of current noise (σ_I) versus time plot and it was found to bear a good correlation with the values obtained by DLEPR. Shot noise analysis of the EN data confirmed the above results.     

Improving the intergranular corrosion resistance of 304 stainless steel by grain boundary network control

The grain boundary network (GBN) was controlled by grain boundary engineering (GBE) in a 304 stainless steel. The total length proportion of Σ3ⁿ coincidence site lattice (CSL) boundaries was increased to more than 70% associating with the formation of large size highly twinned grain-cluster microstructure. Only coherent twin boundaries (Σ3_c) were found to be resistant to intergranular corrosion (IGC) and only such boundaries could be termed “special” ones. The improvement of resistance to IGC of the GBE specimen can be attributed to the large size grain-clusters associated with high proportion of the Σ3ⁿ boundaries and the interconnected Σ3ⁿ-type triple junctions.     

Combined effect of resistance spot welding and post-welding sensitization on the degree of sensitization of AISI 304 stainless steel

This work studies the combined effect of resistance spot welding (RSW) and post-welding sensitization on the degree of sensitization (DOS) of AISI 304 stainless steel (SS) by using EPR and DLEPR tests. The combined effect of RSW and post-welding sensitization at 750 °C gives rise to an overall DOS that: (i) is lower than that of the state without prior welding; (ii) that decreases initially with increasing sensitization time but then increases. This behaviour is due to the fact that the interdendritic corrosion (IDC) located in the weld metal decreases with increasing sensitization time.     

The role of alkaline-earth additives on the molten carbonate corrosion of 316L stainless steel

The corrosion behavior of 316L stainless steel in eutectic Li/Na molten carbonate containing various amounts of Mg, Ca and Ba ions has been evaluated by electrochemical techniques in combination with oxygen solubility determinations. Open circuit potential and corrosion rates have been correlated to the oxygen solubility properties of the carbonate melt for an understanding of the kinetic aspects of the corrosion process. It has been found that minor additions of Mg and Ca ions (1.5% molar fraction) distinctly promote a higher carbonate oxygen solubility, whereas Ba has only a marginal effect on it. In general, the electrochemical investigations showed that at 1.5% molar fraction addition there exist a strong correlation between steel corrosion rate and oxygen solubility indicating that (i) corrosion process takes place under a diffusion-limited cathodic reduction of dissolved O₂ and (ii) corrosion rate is significantly increased with respect to the no-added Li/Na carbonate by the introduction of Mg and Ca ions that therefore serve as a sort of oxidizing agents. However, with larger Mg and Ca additions (up to 10% mol) the corrosion process was found to pass progressively under an anodic control despite decreasing oxygen solubility values. This effect is ascribed to the growth of an alkaline-earth doped lithium ferrite layer with enhanced barrier-like properties. In contrast, further additions of Ba cation in carbonate did not change the corrosion mechanism for its inability to react with the growing corrosion scale. Only minor reductions of corrosion rate are detected in agreement with the lower tendency of the melt containing high molar fractions of Ba to solubilize the oxygen gas.     

Hydrogen-assisted fatigue crack growth of AISI 316L stainless steel weld

The fatigue crack growth behaviors of AISI 316L stainless steel (SS) welds in air and gaseous hydrogen were evaluated, and further compared with the base plate. In air, the fatigue crack growth rate (FCGR) of the weld after heat-treatment at 1050 ^oC/1 h was similar to that of the base metal. Furthermore, all specimens became susceptible to hydrogen-accelerated crack growth. Mainly quasi-cleavage fracture related with the strain-induced martensite accounted for the accelerated crack growth in hydrogen. A smaller amount of martensite in the weld was responsible for the decreased susceptibility to hydrogen-enhanced fatigue crack growth relative to the base metal.     

A new approach for DL-EPR testing of thermo-mechanically processed austenitic stainless steel

Standard test methods such as the electrochemical potentiokinetic reactivation test (EPR) and double-loop EPR test (DL-EPR) are commonly used to characterise sensitisation behaviour in austenitic stainless steels and nickel-based alloys. In this study, the DL-EPR test is augmented by large-area image analysis (IA) to characterise and quantify the networks of attacked grain boundaries. A new analysis approach that is based on a grain boundary cluster parameter is proposed to describe the network of corrosion susceptible grain boundaries, which may be estimated from electron backscatter diffraction (EBSD) data. This method may provide a better assessment of the relative DOS of different heats of austenitic stainless steels.     

An experimental study of crevice corrosion behaviour of 316L stainless steel in artificial seawater

The effects of applied torque on corrosion behaviour of 316L stainless steel with crevices were investigated using the cyclic potentiodynamic polarization method. Three kinds of crevices (316L-to-polytetrafluoroethylene, 316L-to-fluoroelastomeric and 316L-to-316L) were tested in artificial seawater at 50 °C. Corroded surface morphology was also investigated using scanning electron microscopy. Results indicate similar trends in crevice corrosion susceptibility with increasing applied torque. Among the three crevices, the 316L stainless steel specimen, coupled to the 316L stainless steel crevice former, is the most susceptible to crevice corrosion.     

Grain boundary structure and intergranular stress corrosion crack initiation in high temperature water of a thermally sensitised austenitic stainless steel,observed in situ

The development of an intergranular stress corrosion crack initiation site in thermally sensitised type 304 austenitic stainless steel has been observed in situ in high temperature oxygenated water using digital image correlation of time-resolved optical observations. The grain boundary normal stresses were calculated using the Schmid-Modified Grain Boundary Stress (SMGBS) model of Was et al., applying three-dimensional data for the grain boundary planes and grain orientations. The initiation site coincided with the most highly stressed sensitised boundary, demonstrating the importance of the combined contributions to crack initiation of grain boundary structure and plastic strain incompatibility.     

Effect of the micro-plasma arc welding technique on the microstructure and pitting corrosion of AISI 316L stainless steels in heavy LiBr brines

The effects of the micro-plasma arc welding technique on the microstructure and pitting corrosion of different zones of an AISI 316L stainless steel were studied using different microscopy and electrochemical techniques. Galvanodynamic measurements and laser scanning confocal microscope were used to evaluate the corrosion evolution in situ. Results show, in general, the worst corrosion behaviour for the heat affected zone. Furthermore, there is a relation between the effects of the micro-plasma arc welding process on the materials microstructure and their pitting corrosion resistance. The weld zone was always in the cathodic position of the possible galvanic pairs.     

Intergranular corrosion of welded joints of austenitic stainless steels studied by using an electrochemical minicell

An intergranular corrosion study of welded joints of austenitic stainless steels (AISI 304 and 316L) has been addressed. A specific small-scale electrochemical cell (minicell) has been used. Four different weldment zones have been studied. The electrochemical methods applied were the electrochemical potentiokinetic reactivation test and electrochemical potentiokinetic reactivation double loop test. These techniques showed that the HAZ was the most critical zone to intergranular corrosion for both materials. The weld metal was susceptible to interdendritic corrosion and the fusion line showed a mixture of intergranular and interdendritic corrosion. The effect of pre- and post-welding heat treatments for AISI 316L was analyzed. The HAZ was again the most critical zone in every heat treatment condition. The results were correlated to the microstructural features of the materials.     

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.     

Interpretation of Electrochemical Potentiokinetic Reactivation data in the presence of sulphide/oxysulphide inclusions in 316LN stainless steel

This paper presents the Electrochemical Potentiokinetic Reactivation (EPR) parameters obtained for non-sensitised 316LN stainless steel containing sulphide/oxysulphide inclusions. Even in the absence of sensitization, reactivation peaks were obtained in single loop and double loop EPR experiments for longitudinal and cross transverse sections. Using Laser Raman Spectroscope, Raman maps of Cr (VI) oxide were obtained around the inclusions. It was inferred that the reactivation peak is due to dissolution of sulphide inclusions and chromium depleted zones arising around chromium segregated flow lines. The need to interpret EPR parameters carefully in the presence of sulphide inclusions is emphasised.     

Electrochemical reactivation methods applied to PM austenitic stainless steels sintered in nitrogen-hydrogen atmosphere

Exposure methods (ASTM A262 Practice A and Modified Strauss Test) and electrochemical reactivation methods such as EPR and EPRDL was applied to PM 304L and PM 316L stainless and comparisons with wrought stainless steels were made. Sintering in vacuum and nitrogen-hydrogen atmosphere was chosen. The latter was the focus of the work and the former was used on comparative basis. It was seen how nitrogen in PM stainless steels plays a relevant role both when is dissolved and when is in intermetallic precipitates. A set of heat treatments were applied: annealing and sensitizing at 375, 675 and 875 °C. The results indicate that the electrochemical reactivation methods are suitable to evaluate the degree of sensitization of PM austenitic stainless steels.     

Corrosion behaviour of AISI type 304L stainless steel in nitric acid media containing oxidizing species

The corrosion behaviour of AISI type 304L stainless steel (SS) in different concentration of 0.01 M, 1 M and 5 M HNO₃ in presence of oxidizing ions at different temperatures has been evaluated. The main objective of this study is to assess the corrosion resistance of type 304L SS in non-radioactive conditions encountered during storage of liquid nuclear waste. Electrochemical impedance spectroscopy (EIS) and laser Raman spectroscopy (LRS) has clearly brought out the deleterious effect of oxidizing species on the passive film leading to increased corrosion along with increase in HNO₃ concentration and higher temperature.     

Determination of the sensitized zone extension in welded AISI 304 stainless steel using non-destructive electrochemical techniques

Extension of sensitized zone (SZ) in welded AISI 304 stainless steel was determined by two non-destructive electrochemical tests: double loop electrochemical potentiokinetic reactivation technique (DLEPR) and local electrochemical impedance spectroscopy (LEIS). Welding was carried out using the shielded metal arc with two selected welding energies: the first one (0.7 kJ mm⁻¹) does not promote the sensitization of the 304 steel and it constitutes the reference sample and the second one (2.2 kJ mm⁻¹) which leads to the precipitation of chromium carbides in the grain boundaries after the welding process. The non-destructive DLEPR and LEIS tests allowed the length of the SZ to be determined and a good agreement between the two techniques and the microstructure of the two welded samples was shown. The presence of an inductive loop on the local impedance diagrams seems to reflect a galvanic coupling between the weld string (anode) and the welded stainless steel plates (cathode) which will be very prejudicial to a good corrosion resistance of the welded system. The results showed that the two electrochemical tests could be applied in practical cases in industrial field.     

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.     

Corrosion and passivation of low-temperature nitrided AISI 304L and 316L stainless steels in acidified sodium sulphate solution

304L and 316L steels were nitrided at 425 °C for 30 h and examined at various depths in 0.1 M Na₂SO₄ acidified to pH 3.0. In the near-surface region with about 7-14 wt% N, at potentials of active state anodic currents were much higher than those for untreated steels, whereas in deeper regions with <7 wt% N the currents were only slightly increased in comparison with untreated steels or they were even lower in passive and transpassive states. Surface films were composed of oxygen-containing species on top and of Cr-N species in deeper layers. It is suggested that strong corrosion of near-surface regions is associated with nitride precipitates. Beneficial effect of low nitrogen concentrations can be due to initially accelerated corrosion which leads to larger amounts of passivating species and to the accumulation of corrosion resistant chromium nitrides.