Intergranular Corrosion Susceptibility Research Articles

Sensitization characteristics for type 445 ultra-pure ferritic stainless steel with an electrochemical reactivation method

The sensitization characteristics of type 445 ultra-pure ferritic stainless steel (FSS) under different aging conditions were investigated by the optimized double loop electrochemical potentiodynamic reactivation (DL-EPR) test and microstructure observation. The solution 2 mol L−1 H2SO4 + 0.01 mol L−1 KSCN +0.5 mol L−1 NaCl and a scan rate of 100 mV min−1 are suitable for the DL-EPR test of 445 FSS. With the change of aging temperature from 500 to 900 °C, the degree of sensitization (DOS) slightly increases at first and then decreases, which reaches a maximum value at about 700 °C. Under the aging condition at 700 °C for 10 min to 2 h, the DOS value gradually increases, but it is less than 3% due to the low intergranular corrosion susceptibility. The aging treatment at 700 °C results in the precipitation of Laves phase and (Nb, Ti)(C, N), but no Cr-carbides. The sensitization may be related to the segregation of Cr atoms around (Nb, Ti)-precipitates at the grain boundaries.

Localized corrosion of thermally aged Cu-containing Ni-13Cr-10Fe alloys: The role of Cu enrichment induced grain boundary sensitization

This study reports a new mechanism of grain boundary (GB) sensitization without precipitation of Cr-rich phases at GBs in Cu-containing Ni-Cr-Fe alloys. Copper segregation at GBs during annealing can deplete the boundaries of Cr due to the repulsive interaction between Cr and Cu. The depletion of passivating Cr along GBs can cause intergranular corrosion susceptibility, as evidenced by electrochemical testing and post-mortem morphological characterization. The implications of this new sensitization mechanism on the localized corrosion resistance of Cu-containing commercial alloys are discussed.

Localized corrosion characteristics of Al-Mg-Si aluminum alloy with aging time and pre-delayed aging condition

The effects of single aging, delayed aging and pre-delayed aging treatment on the corrosion resistance of Al-Mg-Si alloy were studied by scanning Kelvin probe force microscopy (SKPFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), three-dimensional X-ray tomography (3D-XRT), confocal laser scanning microscopy (CLSM) and electrochemical measurements. The surface potentials of Al(MnFe)Si, Mg2Si, and Si-rich remnant phases are determined, identifying Al(MnFe)Si phase as the crucial cathodic site for corrosion initiation, and the increase in surface potential implies an accelerated corrosion rate after immersion. The intergranular corrosion susceptibility of the alloy is predominantly influenced by the microstructure caused by aging conditions. The factors affecting corrosion resistance are summarized as (i) adjustment of electrochemical reaction activity by controlling the area fraction of Al(MnFe)Si phase, (ii) the spacing of grain boundary precipitates (GBPs), and (iii) the width of the precipitation-free zone (PFZ). The delayed aging (DA) sample exhibits inferior corrosion resistance due to the continuous GBPs and extended sub-grain boundaries corrosion propagation paths. The pre-delayed aging treatment successfully mitigates the corrosion degree of DA by widening the GBPs spacing and narrowing the PFZ width. This suggests that pre-aging treatment is a potent strategy for enhancing corrosion resistance without compromising the alloy's strength.

Inhibition of the second phase precipitation and improvement of intergranular corrosion resistance by boron segregation at the grain boundary of S31254 superaustenitic stainless steel

The inhibition of Mo segregation and phase precipitation is vital for improving the hot workability and corrosion resistance of superaustentic stainless steels (SASS). The boron non-equilibrium segregation of S31254 SASS was implemented through solid solution, air cooling, and diffusion at low-temperature treatment (SADT). The precipitation process and intergranular corrosion (IGC) of S31254 SASS with various boron distributions were researched at a sensitive temperature. The second phases were observed and identified by SEM and TEM. IGC susceptibility was evaluated by double-loop potentiodynamic reactivation (DL-EPR) measurements. The SADT treatment promoted more segregation of B at the grain boundary, leading to lower amounts of grain boundary precipitation before aging for 6 h. The decrease of σ phases in B-regulated samples enhances the IGC resistance, compared with the samples without B addition specimens.

Effect of Cu content on intergranular corrosion and exfoliation corrosion susceptibility of Al−Zn−Mg−(Cu) alloys

The effect of Cu content on intergranular corrosion (IGC) and exfoliation corrosion (EXCO) susceptibility of Al−Zn−Mg−(Cu) alloys was investigated by electrochemical test, immersion test, electron backscattered diffraction, optical microscope, scanning electron microscope, scanning transmission electron microscope and scanning Kelvin probe force microscope. As Cu content increases from 0 to 2.6 wt.%, IGC susceptibility increases, while EXCO susceptibility first increases and then decreases, reaching the maximum at Cu content of 1 wt.%. With the increase of Cu content, the area fraction of recrystallized grains increases, and the aspect ratio of recrystallized grains first decreases and then increases; moreover, the Volta potential difference between grain boundary precipitates (GBPs) and the matrix increases, making GBPs more susceptible to corrosion. The initiation and propagation of IGC and EXCO are discussed mainly based on the features of GBPs, grain structure, and the Volta potential difference between GBPs and the matrix.

Effects of heat treatments on the microstructure and localized corrosion behaviors of AA7075 aluminum alloy

Five different heat treatments were applied to AA7075 aluminum alloy, resulting in distinct localized corrosion behaviors in 5 wt% NaCl solution. Some treatments exhibited higher intergranular corrosion (IGC) susceptibility due to the anodic nature of grain boundaries (GBs) caused by Cu depletion in the precipitate free zones (PFZs) along the GBs. Cu-enrichment and an increase in intervals between η-phase grain boundary precipitates (GBPs) did not improve IGC resistance. The open circuit (OCP) and galvanostatic (GL) tests could distinguish differences in corrosion behaviors, suggesting the potential use of GL testing as a faster method for predicting localized corrosion behaviors.

Influence of additional Mg on intergranular corrosion susceptibility of brazed Al-Mn-Si-Cu alloys and improvement effect by Ti

Influence of additional Mg on intergranular corrosion susceptibility of brazed Al-Mn-Si-Cu alloys and improvement effect by Ti

Corrosion behavior and mechanism of 4004/Al-Mn-Cu-Mg-Si/4004 aluminum brazing sheet in sea water acidified accelerated test

The microstructure and corrosion properties of a novel 4004/Al-Mn-Cu-Mg-Si/4004 aluminum brazing sheet were investigated in comparison with the traditional 4004/3003/4004. It revealed that, the microstructure after brazing of two aluminum brazing sheets from outer to center were both residual Al-Si eutectic, precipitate-free zone (PFZ), band of dense precipitates (BDP) and core material, respectively. However, the alloying element distribution and the types and amounts of second phases were quite different. The sea water acidified accelerated test showed that 4004/Al-Mn-Cu-Mg-Si/4004 exhibited typical exfoliation corrosion, while the corrosion quickly spread deep into the core material of 4004/3003/4004. The formation of Cu concentration gradient and the strip distribution of Al-Mg-Cu-Si precipitates in the BDP region contributed to the exfoliation corrosion of 4004/Al-Mn-Cu-Mg-Si/4004. The corrosion behaviors of Al-Mn-Cu-Mg-Si and AA3003 core materials are both in form of intergranular corrosion (IGC)，and the later was more severe than the former. For Al-Mn-Cu-Mg-Si core material, high-density fine dispersed cathodic α-Al(Mn, Cu)Si precipitates promoted the formation of a profusion of micro-galvanic cells in the Al matrix, which would retard any effects between the grain boundary precipitations and the adjacent relatively PFZs. Hence, the IGC susceptibility of Al-Mn-Cu-Mg-Si core material was decreased. Besides, less large-sized α-AlFeMn(Cu)Si phases in core material also reduced the localized corrosion attack. At last, the corrosion in depth direction of 4004/Al-Mn-Cu-Mg-Si/4004 was retarded, compared to its counterpart.

Investigations on weld joints of 7075 alloy with Al–5Mg–1Zn composite fillers by arc welding

In this work, the semisolid stirring casting assisted ultrasonic treatment and multi-pass rolling methods were applied to fabricate TiCp/Al–5Mg–1Zn filler materials for tungsten inert gas welding of 7075-T651 aluminum alloy. After welding, various performances and microstructures of the as-weld samples were studied via multifarious electron microscopy and extensive tests. As a result, the TiC nanoparticles which distributed uniformly minished the size of α-Al grains and modified secondary phase owing to the heterogeneous nucleation and retardation of growth. The hardness and tensile strength of joints were enhanced as TiC particles were added, compared to those of 5183 commercial fillers. Besides, both the stress corrosion cracking and intergranular corrosion susceptibility of the weld joints were reduced.

Effect of Sr microalloying on microstructure and corrosion resistance of Al–4.6Mg alloy

AbstractThe effect of Sr on the microstructure of Al–4.6Mg alloy is studied via optical microscopy, scanning electron microscopy‐energy‐dispersive analysis, electron backscattered diffraction, transmission electron microscopy, and the intergranular corrosion susceptibility is evaluated according to ASTM G67. The main precipitation phases of Al–4.6Mg alloy are β‐Al3Mg2 and Al6Mn, with a new square phase of Al3Mn also being formed. After the addition of Sr, the β‐Al3Mg2 phase of the alloy decreases, the morphology changes, and Al4Sr and τ‐Al38Mg58Sr4 phases are formed. Accordingly, the nanoscale precipitation phase τ‐Al38Mg58Sr4 is uniformly distributed inside the grains. Microstructure analysis shows that Sr promotes the dynamic recovery and dynamic recrystallization of the alloy, inhibits the growth of recrystallized grains, and reduces the alloy grain size from 14.294 to 10.273 μm. Additionally, the low‐angle grain boundaries increase, the high‐angle grain boundaries decrease, the texture of the rotated cube r‐{001}<110> improves, and the deformation texture decreases. The IGC susceptibility of the alloy is reduced with the addition of Sr, and the corrosion resistance in an electrolyte containing NaCl is enhanced.

Investigation on intergranular corrosion susceptibility of a nitrogen-containing austenitic stainless steel QN2109 by optimized DL-EPR tests

PurposeThe purpose of this study is to investigate the intergranular corrosion (IGC) susceptibility of a nitrogen-containing austenitic stainless steel QN2109. The intergranular corrosion (IGC) susceptibility of a nitrogen-containing austenitic stainless steel QN2109 was investigated.Design/methodology/approachThe double-loop electrochemical potentiodynamic reactivation (DL-EPR) tests were carried out. Scanning electron microscopy and atomic force microscopy were used to characterize the microstructure.FindingsThe optimized test condition for QN2109 was 1 M H2SO4 + 0.01 M NH4SCN at 40°C. The nose temperature of the temperature–time–sensitization (TTS) curve of QN2109 plot was approximately 750°C. Moreover, the IGC susceptibility started to appear at approximately 120 min. The Cr-depletion zone of QN2109 was generated by the formation of M23C6 rather than by the addition of nitrogen. The depth–width ratio of the grain boundaries after the DL-EPR tests decreased as the aging temperature increased. The degree of Cr depletion and size of the Cr-depletion zone at the grain boundary were reflected by the degree of sensitization and depth–width ratio, respectively.Originality/valueThe optimized test condition for DL-EPR tests of a nitrogen-containing austenitic stainless steel QN2109 was investigated. The TTS curve of QN2109 was first plotted to avoid IGC failure. The morphology of the Cr-depletion zone was reflected by the depth–width ratio.

On the Intergranular Corrosion Susceptibility of 304 Stainless Steel with Ultrafine Grains and Comparison with Micrometer Austenitic Grains Counterpart

The intergranular corrosion (IGC) susceptibility of ultrafine grains (∼430 nm) and micrometer grains (∼3.1 µm to 9.8 μm) 304 stainless steel obtained by cryogenic rolling and reversion annealing treatments were studied. Transmission electron microscopy results showed that after sensitized treatment at 650°C for 2 h, the micrometer grains were sensitized with many M23C6 precipitates at the grain boundaries, while no precipitates were in the ultrafine grains. The immersion corrosion tests in H2SO4-CuSO4 solution showed that ultrafine grains exhibited weaker IGC attacks than micrometer grains. The double-loop electrochemical potentiokinetic reactivation tests demonstrated the degree of sensitization decreased from 26.61% to 1.52% with the grain ultra-refinement from micrometer to ultrafine. Corrosion studies indicated that the ultrafine grains exhibited lower IGC susceptibility compared with micrometer grains. According to the findings, the large number of grain boundaries generated by grain ultra-refinement inhibited M23C6 precipitates at the grain boundaries during the sensitized process, thereby reducing the susceptibility of ultrafine grains to IGC.

The corrosion behavior and mechanical properties of 5083 Al-Mg alloy manufactured by additive friction stir deposition

Corrosion behavior and mechanical property of 5083 Al-Mg alloy manufactured by additive friction stir deposition (AFSD) were investigated compared with 5083-H112 alloy. AFSD is beneficial to reduce the pitting corrosion sensitivity of 5083 alloy due to refinement and re-dissolution of Mn rich secondary phase particles. But the intergranular corrosion (IGC) susceptibility of AFSD alloy is more significant due to the exfoliation of fine equiaxed grains after sensitization. This can be ascribed to that the grain boundaries of AFSD alloy after sensitization are dotted with β-phase which tends to be attacked by acid.

Sensitization of 316L Stainless Steel made by Laser Powder Bed Fusion Additive Manufacturing

Additively manufactured (AM) 316L stainless steel (SS) manufactured by laser powder bed fusion (L-PBF) and wrought 316L SS were subjected to sensitization heat treatments at 700°C up to 100 h. Using two evaluation methods, double-loop electrochemical potentiokinetic reactivation (DL-EPR) and ditching tests, degree of sensitization (DOS) and intergranular corrosion (IGC) susceptibility was evaluated. It was found that the wrought samples showed slightly lower IGC susceptibility compared to their AM counterpart. DOS and IGC attacks increased with sensitization time for all samples. Dislocation cellular structures were found to have little to no impact on DOS and IGC for the AM samples. Sensitized at 100 h, the AM sample showed significant Cr depletion along high-angle grain boundaries (12.35 wt% on average) and exhibited Cr carbide precipitation. Mo-rich particles along grain boundaries were also observed. The DL-EPR test attacks the surface oxide film and grain boundaries while the ditching test attacks the melt pool boundaries and grain boundaries (IGC and pitting). Changes to the DL-EPR and ditching standards for AM application have been proposed in this work.

Microstructure, mechanical properties and corrosion resistance of friction stir welded joint of Al–Mg–Mn–Zr–Er alloy

Abstract The microstructure, mechanical properties and corrosion resistance of friction stir welded joint of Al–Mg–Mn–Zr–Er alloy were investigated under three FSW parameter sets: 800 rpm × 100 mm min−1, 800 rpm × 80 mm min−1 and 600 rpm × 80 mm min−1. The microstructural transformation sequence of welded joint is discontinuous dynamic recrystallization, dislocation introduction, dynamic recovery and continuous dynamic recrystallization. The mechanical properties of welded joint are highly correlated with the welding process parameters. The welded joint can obtain good tensile strength and elongation under the parameters of 800 rpm × 100 mm min−1. Corrosion resistance of welded joints has little correlation with welding parameters, but is mainly related to different positions of welded joints. The main influencing factor is grain structure state. The intergranular corrosion susceptibility is related to β(Al3Mg2) phase.

Effect of microstructure on corrosion behaviour of Al-Cu-Li alloys

The corrosion behaviour and its relationship with microstructure, especially the distribution of precipitates at grain boundaries and within grains, of two Al-Cu-Li alloys with different Cu/Li ratio were investigated using accelerated immersion testing, advanced microscopy characterization and quasi-in-situ corrosion observation. The Al-2.23Cu-0.96Li alloy with low Cu/Li ratio exhibited excellent intergranular corrosion resistance and its corrosion modes unusually remained consistent intragranular corrosion regardless of the ageing stage. The corrosion behaviour of the counterpart Al-4.03Cu-0.91Li alloy with high Cu/Li ratio was found to follow the general evolution regulation, from intergranular corrosion to intragranular corrosion with the ageing time. Microstructure characterizations demonstrated that the coarsening and discontinuous distribution of grain-boundary precipitates played the key role in lowering the intergranular corrosion susceptibility of the two Al-Cu-Li alloys. Further quasi-in-situ corrosion observation of the Al-2.23Cu-0.96Li alloy showed that the dealloying process of the grain-boundary precipitates led to the dissolution of the precipitate-free zone, which meant that the broadening of the precipitate-free zone along the grain boundary could reduce the intergranular corrosion resistance of the alloy. Besides, the thin T 1 precipitates that were uniformly distributed within the grains did not suffer preferential dissolution, but they still affected the intragranular corrosion propagation at the nanoscale. Specifically, the corroded T 1 precipitates could become {111} Al -oriented sidewalls of the intragranular corrosion channel. • The Al-2.23Cu-0.96Li alloy kept low IGC susceptibility regardless of the ageing stage. • Intragranular T 1 precipitates do not preferentially dissolve due to dealloying. • Nanoscale corrosion behaviour was investigated through quasi-in-situ TEM experiment.

Enhanced mechanical property and corrosion resistance of alloy 5182 FSW joints by Sc and Zr alloying

For conventional work-hardened Al Mg alloys, Friction-Stir Welding (FSW) can soften the as-welded joint, and increase the susceptibility to intergranular corrosion (IGC). This study found that small Sc and Zr additions (0.1wt.%Sc, 0.1wt.%Zr) increased the yield strength of the Al Mg alloy FSW joint by ~50 MPa, and decreased the IGC susceptibility dramatically. The microstructure characterization revealed that strengthening effects derived from fine coherent Al 3 (Sc,Zr) dispersoids and extensive grain refinement contributed to the higher strength of the joint. The improved corrosion resistance in both stir zone (SZ) and heat-affected zone (HAZ) was attributed to the pronounced grain refinement (in SZ) and subgrain structure (in HAZ), respectively. The strong pinning effect of Al 3 (Sc,Zr) dispersoids inhibits recovery and recrystallization in the highly deformed weld microstructure. • Sc and Zr alloying improved strength/corrosion resistance of Al Mg alloy FSW joint. • Coherent Al 3 (Sc,Zr) dispersoids can be preserved in Sc/Zr-added alloy 5182 FSW joint. • Al 3 (Sc,Zr) precipitation and grain boundary strengthening contributed to strength increment. • Both subgrain and finer grain structure can suppress intergranular corrosion.

Evaluate Effect of Pulsed Current Gas Tungsten Arc Welding Process Parameter on Intergranular Corrosion of SS304L Weld

Austenitic stainless steel (ASS) is the most common type of stainless steel which offers excellent weldability and mechanical properties. ASS is being used for various applications i.e. automotive, oil and gas and chemical industries in which the welding process plays a prominent role. Welding process selection is the main factor that emphasizes mechanical and corrosion resistance properties in various aggressive environments. There are various corrosion occurs in ASS but intergranular corrosion (IGC) forms during welding at elevated temperatures. IGC mainly occurs at grain boundaries of structure and resulting chromium depletion due to precipitation of chromium carbide at the grain boundary. In present work pulsed current gas tungsten arc welding (PCGTAW) process was used to investigate intergranular corrosion by oxalic acid test as per ASTM A262 Practice A. Experiments performed based on Taguchi L9 using design of experiments and corrosion rates are evaluated at base metal, heat affected zone and weld zone. This work is aimed to optimize process parameters followed by regression analysis to IGC susceptibility in the weldment. In this investigation, it has been found from ANOVA and main effects plots that peak current and base current are the most significant parameters in the PCGTAW process. The results of the corrosion test revealed that heat affected zone is more susceptible to IGC. At the end, it has been observed that the optimum value of peak current, base current and frequency based on regression analysis are 100 A, 50 A and 6 Hz respectively.

Grain growth and precipitation behaviour of AISI 430 ferritic stainless steel subjected to pulsed laser beam welding using free-form pulse shaping

Ferritic stainless steels are prone to grain coarsening and precipitation of chromium-rich grain boundary phases during fusion welding, which increase intergranular corrosion susceptibility. State-of-the-art techniques to overcome these challenges mainly feature heterogeneous nucleating agents with regard to grain coarsening or alternating alloy concepts as well as post-weld heat treatments as for restoration of intergranular corrosion resistance. The present investigation seeks to depart from these traditional approaches through the use of a tailored heat input during pulsed laser beam welding by means of free-form pulse shaping. Grain size analysis using electron backscatter diffraction shows a substantial reduction of grain size as compared to continuous-wave lasers due to a distinctive columnar to equiaxed transition. Moreover, phase analyses reveal the overcoming of chromium carbide precipitation within the heat-affected zone. As corrosion tests demonstrate, intergranular attack is therefore concentrated on the weld metal. In comparison to continuous-wave laser beam welding, intergranular corrosion susceptibility is substantially reduced for very short pulse durations. From these results, it can be derived that pulsed laser beam welding using free-form pulse shaping enables direct control of heat input and, thus, tailored grain growth and precipitation formation properties.

Effects of artificial aging on precipitation behavior, mechanical properties and corrosion resistance of Si3N4/6061Al composite fabricated by sintering and hot extrusion processes

Artificial aging plays important roles on the final performances of the manufactured components of Al matrix composites, and it is of great importance to find the appropriate aging parameters to obtain a good combination of mechanical properties and corrosion resistance. In this study, the Si3N4 particle reinforced 6061Al (Si3N4/6061Al) composite was produced by sintering and hot extrusion, and both single and two-step aging were carried out to examine the precipitation behavior. It was found that the grain structure of Si3N4/6061Al and 6061Al did not show a notable change after solution treatment, while Mg element inclined to concentrate at the Si3N4/6061Al interface. The compounds containing Mg and Cu formed at Si3N4/6061Al interface during aging process, and β″ and β′ phases precipitated after peak and two-step aging, respectively. β″ phase contributed to improving the hardness, strength and wear resistance, while β′ phase caused an opposite trend. The Si3N4 particles could also enhance the mechanical properties due to the load transfer and dislocation strengthening. The samples treated by solution exhibited the best corrosion resistance, while the peak-aged ones were sensitive to intergranular corrosion owing to the continuously distributed precipitates and the precipitate free zones at grain boundaries, and two-step aging could alleviate the intergranular corrosion susceptibility. Due to the preferential dissolutions of Mg element in Mg-containing layers and Mg/Cu-containing phases, the Al matrix around Si3N4 particles was seriously corroded.