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.     

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.     

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.     

Effect of magnetic field applied during gas metal arc welding on the resistance to localised corrosion of the heat affected zone in AISI 304 stainless steel

Resistance to localised corrosion at the heat affected zone (HAZ) of AISI 304 stainless steel gas metal arc welded (GMAW) in presence of magnetic fields of different intensity was studied. Samples of HAZ, 8 mm away from the weld centre, were subjected to electrochemical potentiokinetic reactivation to assess the degree of sensitisation (DOS). The application of magnetic field during welding induced lower DOS and better resistance to pitting corrosion in 3.5% NaCl solution than samples welded without it. Experimental findings suggest that magnetic fields enable Cr redistribution in the austenitic base metal during the welding thermal cycle reducing Cr depletion.     

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.     

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.     

Quantitative analysis of Cr-depleted zone morphology in low carbon martensitic stainless steel using FE-(S)TEM

A new type of intergranular stress corrosion cracking (IGSCC) has been observed in low carbon martensitic stainless steel at heat affected zone (HAZ). Nano level microstructural analysis was carried out to investigate IGSCC factors. It was found that cracks propagate along prior austenite grain boundaries where a row of carbides had been formed. Cr-depleted zones at the grain boundaries were characterized by a STEM-EDX analysis and that morphology was obtained by deconvoluting this results. It is concluded that Cr-depleted zones only a few nanometers in width are enough to cause IGSCC at HAZ in this steel under certain circumstances.     

Application of the modified electrochemical potentiodynamic reactivation method to detect susceptibility to intergranular corrosion of a newly developed lean duplex stainless steel LDX2101

The present work aimed at defining optimal conditions using double loop electrochemical potentiokinetic reactivation (DL-EPR) method for evaluating intergranular corrosion (IGC) susceptibility of lean duplex stainless steel (LDX2101) aged at 700 °C between 3 min and 300 h. The results demonstrated that the modified DL-EPR measurement (solution of 33% H₂SO₄ + 0.1% HCl at 20 °C and scan rate of 2.5 mV/s) could successfully characterize the interactions between precipitation, chromium depletion and IGC of LDX2101 with high sensitivity and reproducibility. In addition, there was no indication of healing because the effect of formation of chromium-enriched precipitates was more dominative than that of redistribution of chromium in depleted zones.     

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.     

Corrosion behavior of TiC particle-reinforced 304 stainless steel

TiC particle-reinforced 304 stainless steels were prepared using a new developed in situ technology and their corrosion behavior was compared with that of 304SS in 5 wt.% HCl solution. As compared to 304SS, E_corr, E_pit and E_rp values had shifted to a more negative region in 304SS containing TiC, indicating faster corrosion rate by TiC addition. The addition of TiC particles to 304SS resulted in no rapid pit propagation but maintained a high corrosion rate in the whole immersion time investigated.     

Sensitization control in AISI 316L(N) austenitic stainless steel: Defining the role of the nature of grain boundary

Two grades of AISI 316L(N) austenitic stainless steels differing only in copper content (0.083 and 0.521 wt.%), showed remarkable difference in resistance to sensitization and susceptibility to intergranular corrosion. Different thermal treatments were carried out with an overall objective of altering the nature of the grain boundary. An attempt was made to correlate the degree of sensitization (DOS) with various microstructural parameters such as grain size and grain boundary nature. No clear trend could be established between the individual parameters and DOS. Effective grain boundary energy (EGBE), which is a combined parameter showed clear trend with DOS. The presence of 0.521 wt.% of copper brings down EGBE remarkably leading to improved resistance to sensitization.     

Investigation into the influence of laser melting on the sulphide inclusions in AISI 416 stainless steel

AISI 416 is a high sulphur martensitic stainless steel with elliptical shaped sulphide inclusions over 2 μm in length. After laser melting the sulphide inclusions became spherical and reduced in size to below 0.5 μm in diameter. SEM investigations showed that the total number of inclusions increased dramatically and it was calculated that the total volume of sulphur in the stainless steel had not changed during the laser treatment. The pitting potential was increased by over 200 mV with an inverse linear dependency on the average inclusion size. The frequency of metastable pitting events decreased by a factor of 100.     

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.     

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.     

Inhibition effect of chromate on the passivation and pitting corrosion of a duplex stainless steel in LiBr solutions using electrochemical techniques

The electrochemical behavior of duplex stainless steel (DSS) in LiBr media was investigated by anodic cyclic polarization curves and AC impedance measurements. The effect of bromide concentration and the presence of chromate in the solutions on the corrosion behavior of AISI 2205 was studied. Cyclic polarization curve analyses showed that there was different pitting susceptibility of passive films depending on the LiBr concentration. Pitting potential decreases with LiBr concentration in a semilogarithmic scale following two different slopes. Chromate presence displaces pitting potentials towards more positive values at low LiBr concentrations but it has no effect when LiBr concentration increases.The comparative analysis carried out in LiBr and LiBr chromate-containing solutions at two different concentrations, 0.016 M and 0.032 M, verifies the assumption that halogen ions facilitate inhibitor adsorption. The addition of halides strongly increased the inhibition efficiency of chromate. The passive film becomes more resistant when bromide concentration increases, although film thickness decreases.     

Intergranular corrosion susceptibility in supermartensitic stainless steel weldments

The intergranular corrosion susceptibility in supermartensitic stainless steel (SMSS) weldments was investigated by the double loop – electrochemical potentiokinetic reactivation (DL-EPR) technique through the degree of sensitization (DOS). The results showed that the DOS decreased from the base metal (BM) to the weld metal (WM). The heat affected zone (HAZ) presented lower levels of DOS, despite of its complex precipitation mechanism along the HAZ length. Chromium carbide precipitate redissolution is likely to occur due to the attained temperature at certain regions of the HAZ during the electron beam welding (EBW). Scanning electron microscopy (SEM) images showed preferential oxidation sites in the BM microstructure.     

Oxidation of stainless steel 304L in carbon dioxide

Oxidation of 304L stainless steel in a carbon dioxide atmosphere at 10⁵ Pa has been studied. Between 1193 and 1293 K the oxidation kinetics exhibit first a rapid increase, then a parabolic behaviour with apparent activation energy of (209 ± 8) kJ mol⁻¹ and obeys a Langmuir pressure law. After 1.15 mg cm⁻², the kinetics become almost linear.The reaction products are chromia at the grain boundaries, wüstite (Fe_1−xO) on the surface for weight gains greater than 0.30 mg cm⁻² and chromite. The very complex reaction mechanism takes into account random buckling for weight gains >1.15 mg cm⁻².     

Detection and characterization of stress-corrosion cracking on 304 stainless steel by electrochemical noise and acoustic emission techniques

This paper focuses on the corrosion process of 304 stainless steel in acidic NaCl solution during slow strain rate testing experiment by using electrochemical noise (EN) and acoustic emission (AE) techniques. Meanwhile, the EN and AE characteristics of corrosion process were studied. The results show that stress corrosion occurs easily in the experimental system, and corrosion forms develops gradually from localized corrosion including stress corrosion and pitting corrosion to general corrosion. The AE signal characteristics of pitting corrosion, crack and bubble break-up are significantly different during the corrosion process.     

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.