Intergranular Corrosion Propagation Research Articles

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.

Understanding the mechanisms of intergranular corrosion in 2024 Al alloy at the polycrystal scale

This paper provides a new insight into the understanding of the susceptibility to intergranular corrosion (IGC) of 2024 Al alloy. Whatever the exposure conditions, coincidence site lattice high-angle grain boundaries (CSL HAGBs) slightly contributed to IGC damage because of their low contribution to the cumulative length of GBs. Low-angle grain boundaries (LAGBs) showed intrinsic susceptibility to corrosion initiation almost similar to random HAGBs and lower susceptibility to IGC propagation. However, cyclic corrosion tests promoted both corrosion initiation and propagation at LAGBs. This was mostly due to the trapping of corrosion-induced hydrogen in the core of the dislocations defining the LAGBs.

The corrosion behavior of the 6061 Al alloy in simulated Nansha marine atmosphere

The corrosion behavior of the extruded 6061 Al alloy within 40 days of exposure in simulated Nansha marine atmosphere is investigated by electrochemical measurements, microscopic observations and X-ray photoelectron spectroscopy (XPS) tests. It is found that severe intergranular corrosion (IGC) and intragranular corrosion occur on the sample surface. Mg2Si phase and AlFeSi particles are the main intermetallic compounds in the 6061 Al alloy, which are closely related to the initiation and propagation of IGC. Moreover, the preferential dissolution of Mg from Mg2Si phase is observed and the reasons are explained from the perspective of thermodynamics. This study provides some valuable information for comprehensive understanding the corrosion behavior and mechanism of the 6061 Al alloy in simulated Nansha marine atmosphere. It is anticipated that we can use this valuable information to develop and design new Al alloys to make them more resistant to corrosion in this environment in the future.

Role of constituent intermetallic phases and precipitates in initiation and propagation of intergranular corrosion of an Al-Li-Cu-Mg alloy

The corrosion behaviour of a recently developed Al-Li-Cu-Mg alloy thick plate was investigated using correlative immersion testing in 3.5% NaCl solution with analytical electron and ion microscopy. Intergranular corrosion was observed along grain boundaries and the particle-matrix interface of Al7Cu2(Fe, Mn) and Al20Cu2Mn3 phases from 7 min of immersion. The Li-containing Al7Cu2(Fe, Mn) phases are electrochemically more active and susceptible to de-alloying than the Al20Cu2Mn3 phases. Intergranular T1 (Al2CuLi) precipitates are attacked via selective dissolution of Li and Al. In contrast to previous observation, the dissolution rates of T1 precipitates and the adjacent alloy matrix are almost the same.

Effect of Mg on Intergranular Corrosion of Al-Cu Alloy

Due to high strength and light weight, 2000 series Al alloys (Al-Cu-Mg) are widely utilized in several applications such as aircrafts and automobiles. However, 2000 series alloys have limitations to use for some applications because corrosion resistance of these alloys is relatively low in commercial Al alloys. The localized corrosion is sometimes initiated at intermetallic particles. Kakinuma et.al. studied the pitting corrosion initiation at Al-Fe-Si intermetallic particles using a micro-scale electrochemical method. They clarified the effect of pH on pitting initiation 1. Also, they found that open circuit potential was changed by pit initiation and propagation 2. In addition to pitting, intergranular corrosion (IGC) should be concerned in the case of 2000 series alloys. It has been known that compositional heterogeneity related to precipitations at grain boundaries affects the occurrence of IGC.It has been known that Mg in 2000 series alloys improves IGC resistance. However, it is not clear whether Mg inhibits the initiation of IGC or the propagation of IGC. In this study, the comparison of IGC on Al-Cu and Al-Cu-Mg alloys was performed.To clarify the effect of Mg on corrosion behavior Al-Cu alloy, open circuit potential of Al-Cu and Al-Cu-Mg alloys in 0.1 M NaCl (pH 6.0) were measured for 1 h. After that, the total number of IGC and length of IGC at each site in electrode area (ca. 1 cm × 1 cm) was measured using an optical microscope. Total number of IGC on Al-Cu-Mg alloy was almost same as that on Al-Cu alloy. On the other hands, total length of IGC on Al-Cu-Mg alloy was two times shorter than that on Al-Cu alloy. It was suggested that Mg effectively suppresses propagation of IGC.Reference Kakinuma, I. Muto, Y. Oya, Y. Kyo, Y. Sugawara, N. Hara: J. Electrochem. Soc., 166(2) (2019), C19-C32Kakinuma, I. Muto, Y. Oya, T. Momii, Y. Sugawara, N. Hara: J. Electrochem. Soc., 168 021504 (2021)

Three-dimensional Computational Modeling and Simulation of Intergranular Corrosion Propagation of Stainless Steel

In oxidizing nitric acid solutions, stainless steel undergoes intergranular corrosion accompanied by grain dropping and changes in the corrosion rate. For the safe operation of reprocessing plants, this mechanism should be understood. In this study, we constructed a three-dimensional computational model using a cellular automata method to simulate the intergranular corrosion propagation of stainless steel. The computational model was constructed of three types of cells: grain (bulk), grain boundary (GB), and solution cells. Model simulations verified the relationship between surface roughness during corrosion and dispersion of the dissolution rate of the GB. The relationship was investigated by simulation applying a constant dissolution rate and a distributed dissolution rate of the GB cells. The distribution of the dissolution rate of the GB cells was derived from the intergranular corrosion depth obtained by corrosion tests. The constant dissolution rate of the GB was derived from the average dissolution rate. Surface roughness calculated by the distributed dissolution rates of the GBs of the model was greater than the constant dissolution rates of the GBs. The cross-sectional images obtained were comparable to the corrosion test results. These results indicate that the surface roughness during corrosion is associated with the distribution of the corrosion rate.

Evolution of Connectivity Among CSL Boundaries During Hot Plate Rolling of 316H Stainless Steel and the Effect on Intergranular Corrosion Resistance

Effects of the topological grain boundary network in hot rolled plates of 316H on intergranular corrosion (IGC) propagation were studied, which demonstrated that the connectivity among low-Σ coincidence site lattice (CSL) boundaries, instead of the CSL fraction, played the dominant role in arresting the IGC propagation. It was worked out that original dynamic recrystallized structures with homogeneous CSL boundaries and larger grain size were the efficient way to improving the CSL connectivity, which can be realized by optimizing the processing parameters during hot plate rolling.

Intergranular corrosion behavior of extruded 6005A alloy profile with different microstructures

The intergranular corrosion (IGC) behavior of an extruded 6005A alloy profile with the coexisting of peripheral coarse grain (PCG) structure and partial recrystallized grain (PRG) structure was investigated by using an accelerated corrosion test, electrochemical impedance spectroscopy and a quasi in situ examination of IGC process. PCG structure was found to have a unique IGC behavior that pitting corrosion and subsequent IGC are less severe and will not be transformed into intragranular corrosion as they were found in PRG structure. Microstructure characterization reveals that the microstructural differences in grain boundary precipitates, primary α-AlFeMnSi intermetallic particles and grain characteristic between PCG structure and PRG structure are the reason for these phenomena. Further analysis indicates that the grain boundaries decorated with more AlFeMnSi particles and Q phase precipitates are more sensitive to corrosion, where Q phase precipitates are the primary cathodes and the most important factor affecting IGC; AlFeMnSi particles are supposed to initiate pitting corrosion since they are dissolved as anodes in the early stage of corrosion. With the development of corrosion, they are transformed into cathodes and become the bridges of IGC propagation by connecting the Q phase precipitates at grain boundary. In addition, grain characteristic was also found to have great effects on IGC. With the decrease in grain size and the increase in the frequency of high-angle grain boundaries and the dislocation density, corrosion becomes more severe and more likely to be transformed into intragranular corrosion.

The corrosion behavior of chromated AA2024 aluminum alloy in 3.5% NaCl solution

PurposeA chromate conversion coating was prepared on the surface of bare AA2024 aluminum alloy by direct immersion in the chromating treatment bath, and the corrosion behavior of chromated AA2024 aluminum alloy in 3.5 per cent NaCl solution was studied by electrochemical measurement and microstructural observation.Design/methodology/approachAccording to the polarization curve test and the scanning electron microscope observation, the corrosion evolution of chromated AA2024 in 3.5 per cent NaCl solution was divided into the following three stages: coating failure, pitting corrosion and intergranular corrosion (IGC).FindingsIn the first stage, the chromate coating degraded gradually due to the combined action of chloride anions and water molecules, resulting in the complete exposure of AA2024 substrate to 3.5 per cent NaCl solution. Subsequently, in the second stage, chloride anions adsorbed at the sites of θ phase (Al2Cu) and S phase (Al2CuMg) on the AA2024 surface preferentially, and some corrosion pits initiated at the above two sites and propagated towards the deep of crystal grains. However, the propagation of a pit terminated when the pit front arrived at the adjacent grain boundary, where the initiation of IGC occurred.Originality/valueFinally, in the third stage, the corrosion proceeded along the continuous grain boundary net and penetrated the internal of AA2024 substrate, resulting in the propagation of IGC. The related corrosion mechanisms for the bare and the chromated AA2024 were also discussed.

Mechanism for Propagation of Intergranular Corrosion in Pipeline Steel

"High-pH" (pH 8 to 10.5) stress corrosion cracking (SCC) and intergranular corrosion (IGC) of low-carbon pipeline steels both occur in the active-passive transition potential region, suggesting that both processes are driven by metal dissolution (1). In non-sensitized pipeline steels, there are no grain boundary (GB) precipitates and no consensus on the mechanism of IGC. IGC in non-sensitized steels forms triangular wedges of corrosion product around GBs (2,3). The wedges display characteristic angles determined by enhanced rates of dissolution at the GB compared to that of the grain surface (2,3). Recently we reported the formation of such corrosion product wedges around GB triple junctions during high-pH IGC of X70 pipeline steel in 1 M sodium bicarbonate solution (4). Nanoindentation measurements detected reduced hardness in the metal around corroded GBs, which was suggested to arise from softening by vacancies produced by anodic oxidation of reactive Si solute atoms (5). In the present work, a mathematical model was developed for evolution of GB morphology during IGC of pipeline steel. The model incorporates generation of vacancies at the dissolving steel interface and vacancy diffusion to GBs. The vacancy concentration at the corroding surface is taken to be the same as that of Si solute, 0.77 at. %. Vacancies reaching GBs are assumed to diffuse rapidly to the steel surface and contribute to GB recession, thus accounting for enhanced penetration of corrosion along the boundary. Finite-element simulations were performed to predict GB shape evolution. The predicted shapes were compared with measurements from cross sections of X70 steel samples corroded in 1 M NaHCO3at -0.521 V vs Ag/AgCl. Short corrosion exposures of 2 hr produced GB wedges with flat triangular sides having angles of 120o- 140o(5). Calculations with the vacancy diffusion model showed that these angles are consistent with a vacancy diffusivity of 3 x 10-18m2/s, one order of magnitude smaller than the diffusivity of 6 x 10-17m2/s obtained by extrapolation from high-temperature measurements in bcc Fe (6). The lower diffusivity in ferritic steel is considered reasonable in view of likely vacancy trapping in the alloy. For longer corrosion exposures of 5 hr, the current density decreased by two orders of magnitude due to the formation of a precipitated corrosion product on the outer steel surface. Pronounced sharpening of the GB wedges was observed to angles of 20o- 30o. For comparison, calculations with the vacancy diffusion model incorporated the diffusivity from short-time experiments and included the transient decay of the corrosion rate. The calculations demonstrated dramatic sharpening of the GB wedges in quantitative agreement with experiment (Fig. 1). Sharpening is caused by an enhanced contribution to GB morphology evolution from vacancy diffusion at low current densities, relative to that from grain surface corrosion. Further, sample cross sections after long corrosion exposures showed evidence of GB crack initiation. It is suggested that wedge angle sharpening leads to intensification of tensile wedging stress due to GB corrosion product (5), which eventually initiates cracks. Therefore, the vacancy diffusion model explains IGC morphology evolution and also provides an explanation for the transition of IGC to early-stage SCC. REFERENCES R. N. Parkins, S. Zhou, Corros. Sci., 52, 347 (1996). B. Gwinner et al., Corros. Sci., 107, 60 (2016). L. Beaunier, M. Froment, C. Vigneaud, Electrochim. Acta, 25, 1239 (1980). D. Yavas et al., Electrochem. Commun., 88, 88 (2018). D. Yavas et al., Electrochim. Acta, 285, 336 (2018). M. I. Mendelev, Y. Mishin. Phys. Rev. B, 80, 144111 (2009). ACKNOWLEDGMENTS Support was provided by the USDoT PHMSA under Competitive Academic Agreement Program No. DTPH5614HCAP03 and DTPH5614HCAP01. Figure 1. Grain boundary wedge apex angle variations over time. Curves (a), (b) and (c) are calculated from the FE simulation using time-dependent corrosion rates from experimental current transients. Data points (a), (b) and (c) are experimental wedge angles from the same corrosion experiments. Error bars represent standard deviations of measured wedge angles in each experiment. Figure 1

Beneficial effect of reversed austenite on the intergranular corrosion resistance of martensitic stainless steel

In this work, the influence of reversed austenite on the intergranular corrosion of AM355MSSs (martensitic stainless steels) was investigated. The combined results revealed that the reversed austenite exhibited an inhibiting effect on the intergranular corrosion of AM355MSSs. The inhibiting effect of reversed austenite on the intergranular corrosion was summarized as follows. Firstly, the formation of reversed austenite at the Cr-depleted regions reduced the number of nucleation sites for intergranular corrosion. Secondly, the reversed austenite always distributed along the martensite lath boundaries, preventing the propagation of intergranular corrosion.

Corrosion behaviour of AA6082 Al-Mg-Si alloy extrusion: The influence of quench cooling rate

The influence of quench cooling rate on corrosion behaviour of AA6082-T6 Al-Mg-Si alloy extrusion is investigated. It is revealed that different quench cooling rates result in significant difference in grain boundary precipitation and intergranular corrosion (IGC) susceptibility. Transmission electron microscopy is employed to probe corrosion front with the aim of precisely determining the role of each individual grain boundary precipitate in IGC propagation. However, it is observed that Mg2Si precipitate doesn’t significantly affect the process of corrosion propagation along the grain boundary and the presence of Cu-containing precipitates is not a necessary condition for IGC to occur in the alloy.

Thin Foil Penetration Technique at a Local Scale: Towards an Analysis of the Current Transients during Intergranular Propagation on AA2024 Alloy

A local electrochemical probe was combined with the thin foil penetration technique to study the corrosion propagation of a reduced number of grain boundary traces (in-plane) propagating inside a 60μm thick foil of AA2024. This local and “in-volume” approach allowed analysis of the current transient resulting from the full penetration of the selected intergranular paths. By coupling a 2D spatially resolved electrochemistry to digital optical observation on the back side of the foil, it was possible to relate the current transient to the successive steps of the “in-volume” propagation of intergranular corrosion.

Progress in Understanding Initiation of Intergranular Corrosion on AA6005 Aluminum Alloy with Low Copper Content

As part of an extensive research program to study recent, unexpected intergranular corrosion (IGC) on 6xxx series aluminum alloys (AlMgSi), this paper investigates the mechanism of initiation and early propagation of IGC on the extruded AA6005-T5 alloy with small Cu content (0.1 wt%) by use of advanced electron microscopy techniques applied for near surface characterization. Corrosion testing was restricted to the accelerated IGC test according to the standard BS ISO 11846, involving exposure to acidified chloride solution. The effect of modifying the as-received extruded surface by metallographic polishing, argon sputtering, and alkaline etching was investigated. Initiation of IGC was delayed on the as-received surface compared to the modified surface, caused by the presence of an approximately 8 nm thick crystalline oxide layer formed during extrusion. IGC initiated at the primary α-Al(Fe,Cu,Mn)Si particles for all types of surfaces. However, these particles corroded rapidly in the test solution forming a residue of Cu and Si on the exposed particle surface. This phenomenon, as well as enrichment of Cu on the Al matrix surface by dealloying, contributed increasingly to the formation of new effective cathodic sites and continuing propagation of IGC. The AlMgSiCu (Q) phase, present as primary and secondary particles, was relatively inert against both oxidation and reduction.

Influence of hydrogen on the propagation of intergranular corrosion defects in 2024 aluminium alloy

To study the influence of hydrogen on the intergranular corrosion mechanism of a 2024 aluminium alloy, samples were hydrogen precharged by cathodic polarisation and then exposed to a NaCl solution. EBSD analyses and SEM observations showed that hydrogen increased the number of corroded interfaces and led to the embrittlement of low-angle grain boundaries which were not susceptible to corrosion without hydrogen precharging. The increase of the reactivity of the 2024 aluminium alloy in the presence of hydrogen gave a new insight into the intergranular corrosion mechanism: corrosion-induced hydrogen promoted the intergranular corrosion propagation and partially controlled the corrosion defect morphology.

Improvement of corrosion resistance of AA2024-T3 using femtosecond laser peening without protective and confining medium

This work reports for the first time the effect of femtosecond laser shock peening (fLSP) without a sacrificial overlay under atmospheric conditions (no confinement) on the corrosion behaviour of AA2024-T3. After fLSP and additional exposure to air a superhydrophobic state (θ=160 ± 4°) is achieved. Moreover, fLSPed corrosion-resistant surface contributes to increased polarization resistance, reduced corrosion current and lower anodic dissolution, with long-term stability in aggressive chloride solution. Furthermore, SEM/EDS characterisation reveals reduction in pitting and complete eradication of intergranular corrosion (IGC) attack due to reduced metal/electrolyte contact area and homogeneous, refined microstructure which prevents chain-link IGC propagation.

Local Electrochemistry on Thin Foils to Investigate Intergranular Corrosion in AA2024

Intergranular corrosion (IGC) on Al-Cu alloys, is controlled by the preferential anodic dissolution of the Cu depleted zone induced by the structural hardening treatments [1].The main dissolution process affects the active anodic head at the tip of the grain boundary and for a minor part the faces of the surface of the grain « crevice » behind the active anodic head [2]. Therefore it could be assumed that the measure of the anodic part of the total current during potentiostatic experiments would be useful to establish charge and mass balance to evaluate the IGC propagation rate. Actually electrochemical analysis is not largely spread in IGC studies of aluminum alloys and physical based methods like the foil penetration technique were preferred to define IGC propagation rate [3,4]. In a recent work [5], we combined local electrochemical probe and thin foil penetration techniques to select only one grain boundary propagating on the longitudinal direction (rolling induces long grains, up to 700µm length, leading to a columnar microstructure). In this presentation, the combination of the specially designed microelectrochemical cell controlled by a SECM stage with a direct optical interrogation of the backside of a 50µm thick foil AA2024 will be described. This local and “in volume” approach allowed us to analyze the current transient resulting from the full penetration of a well-selected intergranular path. By linking the 2D spatially resolved electrochemistry to optical observation on the backside of the foil (1D), it was possible to relate the total current to the successive steps involved during the “in volume” attack (3D) of a well-defined grain boundary trace. It will be discussed how this approach could be applied to deduce the IGC propagation rate from mass balance using the Faraday law and could support more robust assumptions on the effect of microstructure on IGC propagation rate because it would be easier to manage a spatial control of the location of the IG attack. Acknowledgements This work is supported by French ANR-14-CE07-0027-01 – M-SCOT: Multi Scale COrrosion Testing.

Impact of Annealing Prior to Solution Treatment on Aging Precipitates and Intergranular Corrosion Behavior of Al-Cu-Li Alloy 2050

The influences of annealing prior to solution treatment on the grain structure, subsequent aging precipitates, and intergranular corrosion (IGC) of Al-Cu-Li alloy (AA2050) sheet with T6 aging at 448 K (175 °C) were investigated. Annealing impedes the full recrystallization during solution treatment, increasing the population density of T1 (Al2CuLi) precipitates, but decreasing that of θ′ (Al2Cu) precipitates, of the aged alloy. Meanwhile, annealing leads to the heterogeneous distribution of T1 precipitates, increasing the alloy hardness, and decreasing the open-circuit potential of the aged alloy. With prolonged aging time, the corrosion mode of the aged AA2050 samples with and without annealing evolved in a similar manner. The corrosion mode as a function of aging may be summarized as local IGC with pitting and general IGC with pitting (following initial aging and under the underaged condition), pitting corrosion (later in the under-aging stage), pitting with slight IGC (near the peak-aged condition), and pitting with local IGC (under the overaging condition). The annealing treatment hinders IGC propagation on the rolling surface while accelerating the IGC on transverse surfaces.

Tracking the evolution of intergranular corrosion through twin-related domains in grain boundary networks

Tailoring the grain boundary network is desired to improve grain boundary-dependent phenomena such as intergranular corrosion. An important grain boundary network descriptor in heavily twinned microstructures is the twin-related domain, a cluster of twin-related grains. We indicate the advantages of using twin-related domains and subsequent statistics to provide new insight into how a grain boundary networks respond to intergranular corrosion in a heavily twinned grain boundary engineered 316L stainless steel. The results highlight that intergranular corrosion is typically arrested inside twin-related domains at coherent twins or low-angle grain boundaries. Isolated scenarios exist, however, where intergranular corrosion propagation persists in the grain boundary network through higher-order twin-related boundaries.

A closer inspection of a grain boundary immune to intergranular corrosion in a sensitised Al-Mg alloy

The intergranular corrosion of Al-Mg alloys arises from sensitisation due to thermal exposure. Here, a protected low angle (8.7°) grain boundary which hinders the intergranular corrosion propagation in a sensitised (150°/7 d) Al-Mg alloy was studied in detail. Discrete β-phase was observed on this protected boundary, but with a lower continuity compared to the other fully (or partially) dissolved grain boundaries. The results herein seek to unify the relationship between β-phase continuity and grain boundary misorientation angle as critical for intergranular corrosion propagation in the studied alloy.