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.     

Artificial neural networks for pitting potential prediction of resistance spot welding joints of AISI 304 austenitic stainless steel

The aim of this work is to predict the influence of welding process on pitting corrosion behaviour (PCB) of resistance spot welding (RSW) joints of AISI 304 austenitic stainless steel (ASS). PCB is estimated from the pitting potential (E_pitt), which is predicted from three RSW parameters. The prediction is carried out by artificial neural networks (ANNs) because the phenomena that relate the heat generated in RSW process to the PCB are highly complex and nonlinear. The study of PCB of RSW joints of ASS by ANNs is a novel research field in which this work makes a significant contribution.     

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.     

Comparative study between large-scale and small-scale electrochemical potentiokinetic reactivation performed on AISI 316L austenitic stainless steel

This work optimizes the application of electrochemical potentiokinetic reactivation (EPR) to assess the degree of sensitization (DOS) of AISI 316L and compares the large-scale and small-scale EPR with the aim of improving the study of the different zones of AISI 316L welded joints by using an electrochemical minicell. The optimized EPR allows to discriminate better than the standardized EPR among different DOS. Small-scale EPR shows greater sensitivity to assess the DOS than large-scale EPR: (i) at lower deformation levels; (ii) for shorter sensitization times; (iii) when localized microstructural regeneration is caused by the combined effect of deformation and subsequent sensitization.     

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.     

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.     

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.     

Microelectrochemical corrosion study of super martensitic welds in chloride-containing media

Pitting corrosion was studied in welded joints of supermartensitic stainless steels. The electrochemical behavior of three different weldment zones (weld metal (WM), parent metal (PM) and heat affected zone (HAZ)) was characterized independently using an electrochemical minicell through potentiodynamic and potentiostatic techniques. The minicell design selected for this work has the additional advantage of showing promise for service application. Macro-electrochemical characterization was also used to provide a basis for comparison and check the effectiveness of minicell techniques in characterizing the corrosion resistance of welded super martensitic stainless steels. Minicell experiments exhibit a great capacity to discriminate between the different welding zones. The electrochemical response can be correlated with the microstructure of the different zones. Thermal cycles resulting from the welding procedure do not have an important effect on the amount of retained austenite or on the values of the pitting potential E_P in the HAZ.     

Effects of microplasma arc AISI 316L welds on the corrosion behaviour of pipelines in LiBr cooling systems

The effect of microplasma arc welding (MPAW) on the electrochemical and corrosion behaviour of AISI 316L stainless steel tubes has been studied. Scanning electrochemical measurements were performed in sodium chloride to evaluate the difference in the electrochemical activity of base (non-welded) and welded samples. Oxygen reduction rates increase in AISI 316L due to the heat treatment effect induced by welding, indicating a higher electrochemical activity in the welded samples. Additionally, the use of MPA weldments in lithium bromide (LiBr) absorption machines was also analysed at typical operating temperatures and Reynolds numbers. The welding process increases corrosion rates, hinders passivation and increases the susceptibility to pitting attack in LiBr. However, zero-resistance ammeter and localization index measurements show that the galvanic pair generated between the base and welded alloys is weak, both electrodes being in their passive state. Temperature greatly affects the corrosion process.     

Correlation between the degree of sensitization and stress corrosion cracking susceptibility of type 304H stainless steel

Austenitic stainless steel 304H is extensively used in the super heater tubes of power boiler due to its superior mechanical properties at elevated temperatures. However, its relatively high carbon content increases the susceptibility to sensitization and subsequent stress corrosion cracking.This work is concerned with investigation of the sensitization and stress corrosion cracking (SCC) of austenitic stainless steel grade 304H. Electrochemical potentiokinetic reactivation (EPR) test was used to evaluate the degrees of sensitization (DOS) of the studied alloy at various temperatures and periods of time. DOS increased with increasing sensitization time and temperature. This was confirmed by microstructure examination after EPR test. Boiling magnesium chloride (MgCl₂) test was used to evaluate the susceptibility of 404H stainless steel to stress corrosion cracking. DOS and test stress level had negative effects on time to failure in boiling MgCl₂. The correlation between DOS and SCC was also discussed.     

Correlation between critical pitting temperature and degree of sensitisation on alloy 2205 duplex stainless steel

In this study effect of different ageing conditions on intergranular corrosion, pitting corrosion and relation between critical pitting temperature (CPT) and degree of sensitisation (DOS) was investigated by potentiostatic polarisation and double loop-EPR methods. The results showed by increasing sensitisation time, DOS increased and measured CPT value decreased. In addition the values of DOS and CPT of specimens aged at 650 °C showed almost liner relation while this correlation was diminished for the specimens aged at 800 °C. The results may be attributed to the further formation of precipitates on specimens aged at temperature of 800 °C.     

Evolution of intergranular corrosion resistance for HR3C heat-resistant austenitic stainless steel at elevated temperature

An attempt has been made to investigate the evolution of intergranular corrosion (IGC) resistance of HR3C heat-resistant austenitic stainless steel. The double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests were conducted to evaluate the IGC resistance of HR3C steel. The results show that the side peaks can be observed in active–passive region of DL-EPR curves due to inhomogeneous distribution of the chromium dissolved in matrix. The variation of the degree of sensitisation (DOS) during sensitising at 800°C can be divided into three regions (sensitisation dominant region, desensitisation dominant region and equilibrium region). When HR3C steel is sensitised at 800°C for over 24?h, the equilibrium state in DOS can be reached. Simultaneously, the IGC morphologies after DL-EPR tests can correlate well with the DOS. On the basis of the equilibrium state in DOS, proper heat treatments and corrosion protection measures can be taken to avoid IGC of HR3C steel.     

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.     

2219铝合金FSW和TIG焊接头力学与腐蚀行为

评价了2219-CS铝合金FSW和TIG焊接头的力学性能,并采用晶间腐蚀、剥落腐蚀及电化学腐蚀方法对2219-CS铝合金FSW和TIG焊接头进行了整体及分区域腐蚀行为表征,借助光学显微镜和扫描电子显微镜分析了表面腐蚀形貌. 结果表明,搅拌摩擦焊接头焊核区的细晶结构和弥散再分布的沉淀相大幅度提高了其耐腐蚀性能,热影响区中沉淀相回溶及粗化较TIG焊接头热影响区较弱,耐腐蚀性能有一定提高,搅拌摩擦焊接头的高电化学均质性宏观表现为其整体腐蚀速率的显著降低.     

Heat treatment effect on the corrosion behaviour of black passivated ZnNi alloys

Electrochemical corrosion behaviour of ZnNi and black passivated ZnNi alloys in aerated and partially deaerated 5% NaCl solution was studied using dc and ac techniques. The heat treatment (24 h, 120 °C) effect on black CCC properties was analyzed. No changes in the R_p values were observed, but differences in the potentiodynamic curves and in EIS diagrams suggested a change in the corrosion mechanism. Comparative studies by AFM and electrochemical techniques showed that these changes in the corrosion behaviour could be explained by taking into account the different effect of heat treatment on the inner and outer layers of the CCC.     

Enhanced corrosion protection of MgO coatings on magnesium alloy deposited by an anodic electrodeposition process

MgO coatings were prepared on magnesium alloy surface by an anodic electrodeposition process in concentrated KOH solution followed by heat treatment in air. The phase composition and microstructure of the as-formed MgO coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behavior of the MgO-coated samples was evaluated by electrochemical measurements and immersion tests in Hanks’ solution. The results showed that the MgO-coated Mg alloy exhibited a much superior stability and lower corrosion rate, and thus enabled to improve the corrosion resistance, whereas the bare Mg alloy suffered from severely localized corrosion attack.     

Annealing temperature effect on the pitting corrosion resistance of plasma arc welded joints of duplex stainless steel UNS S32304 in 1.0 M NaCl

Pitting corrosion resistance of 2304 duplex stainless steels after autogenous plasma-arc welding and subsequent short-time post-weld heat treatment at different temperatures, determined by critical pitting temperature in 1.0 M NaCl solution, has been investigated. The results showed that the as-welded joint displayed impaired pitting corrosion resistance and that pitting preferentially occurred at ferrite grain in heat-affected zone near the fusion line. Short-time annealing treatment at 1020–1120 °C has a beneficial effect on the pitting corrosion resistance of welded joint. The most favorable annealing temperature for the analyzed welded joints was found to be 1080 °C, at which the joint restored the pitting corrosion resistance lost during welding entirely.     

Effect of ageing heat treatments on the microstructure and intergranular corrosion of powder metallurgy duplex stainless steels

The influence of ageing heat treatments (675 and 875 °C for 1.5 to 48 h) on the microstructure and intergranular corrosion resistance of sintered in nitrogen duplex stainless steels was investigated. The materials were obtained by sintering mixtures of austenitic AISI 316L and ferritic AISI 430L powders. Corrosion behaviour was evaluated by using electrochemical techniques. The beneficial effect of nitrogen on corrosion behaviour of solution annealed samples was established. During ageing, secondary phases were precipitated and the intergranular and transgranular corrosion resistance significantly decreased though repassivation was observed in specimens aged at 875 °C for times up to 8 h.     

Corrosion behaviour of duplex stainless steels sintered in nitrogen

Duplex stainless steels obtained through powder metallurgy (PM) technology from austenitic AISI 316L and ferritic AISI 430L powders were mixed on different amounts to obtain biphasic structures with austenite/ferrite ratio of 50/50, 65/35 and 85/15. Prepared mixes of powders have been compacted at 750 MPa and sintered in N₂-H₂ (95% and 5%) at 1250 °C for 1 h. Corrosion behaviour, using electrochemical techniques such as anodic polarization measurement, cyclic anodic polarization scan and electrochemical potentio-kinetic reactivation test and double loop electrochemical potentio-kinetic reactivation double loop test were evaluated. For duplex stainless steels, when austenite/ferrite ratio increases the corrosion potential shifts to more noble potential and passive current density decreases. The beneficial effect of annealing solution heat treatment on corrosion behaviour was established and was compared with corrosion behaviour of vacuum sintered duplex stainless steels. The results were correlated with the microstructural features.