Statistical model for intergranular corrosion growth kinetics

A statistical model has been developed to predict the anisotropy of intergranular corrosion (IGC) kinetics and effect of microstructure on IGC kinetics in high strength AA2024-T3 alloys. The methodology is to estimate the statistical distribution of minimum IGC path length that will be found through the thickness of a given thickness ST sample using the principle of order statistics. A brick-wall model is adopted for wrought Al alloy microstructure and different distributions of grain dimension are assumed for the purpose of the estimation of total IGC path length. Numerical stimulation using a gamma distribution model stimulated the anisotropy of localized corrosion in AA2024-T3 alloys, which quite agrees with the growth rate anisotropy experimentally determined by the foil penetration technique. Some limitations regarding the statistical model are discussed. The statistical model provides a new approach to predicting and quantifying localized corrosion kinetics on basis of the alloy microstructure.     

Effects of compressive stress on localized corrosion in AA2024-T3

The effect of compressive stress on intergranular corrosion (IGC) of AA2024-T3 was studied using a constant load and simultaneous electrochemical measurement. A specially designed electrochemical cell was used to compress a pillar-shaped sample and control the potential at a value that promoted IGC. The extent of IGC was assessed by metallurgical cross-sectional images. The effect of the compressive stress depended on the orientation of the stress relative to the elongated microstructure. Application of a compressive stress halfway to yield in the S or through-thickness direction significantly reduced the growth kinetics of IGC in the longitudinal direction, but did not eliminate it totally. The strain change during exposure also was used to quantify the change in radius of the cylindrical sample as a function of time during IGC growth. The effect of compression on reducing IGC was also assessed by the current density measured during potentiodynamic and potentiostatic polarization. The effects of residual compressive stress on IGC were studied using samples treated by low plastic burnishing (LPB), which produces a surface layer with high residual compressive stress. The results depended on the plane of the LPB treatment. A micro-capillary cell was used to measure corrosion behavior at different zones of the section of an LPB-treated sample. The breakdown potential was significantly higher in the zone with residual compressive stress than in the interior of the sample.     

Comparison of the corrosion resistance of an Al–Cu alloy and an Al–Cu–Li alloy

ABSTRACT

In this study, the corrosion mechanisms of the AA2024-T3 and the AA2098-T351 were investigated and compared using various electrochemical techniques in 0.005?mol?L^?1 NaCl solution. The severe type of corrosion in the AA2098-T351 was intragranular attack (IGA) although trenching and pitting related to the constituent particles were seen. On the other hand, the AA2024-T3 exhibited severe localised corrosion associated with micrometric constituent particles, and its propagation was via grain boundaries leading to intergranular corrosion (IGC). Electrochemical techniques showed that the corrosion reaction in both alloys was controlled by diffusion. The non-uniform current distribution in both alloys showed that EIS was not a proper technique for comparing the corrosion resistance of the alloys. However, local electrochemical techniques were useful for the evaluation of the corrosion resistance of the alloys.     

Intergranular corrosion and stress corrosion cracking of an aged AlCu alloy in 1 N NaCl solution

The susceptibility of an aged pure AlCu alloy with 4 wt% Cu to intergranular corrosion (IGC) and to stress corrosion cracking (SCC) has been studied in 1 N NaCl solution. For this purpose the time to failure of thin flat tensile samples under constant load has been determined in dependence of the electrode potential and compared with the electrochemical behaviour of unstressed samples. Below the pitting potential of pure aluminum in 1 N NaCl solution the tensile specimens never failed. Above this potential, failure did occur, and time to failure decreased with increasing electrode potential. Unstressed samples showed intergranular attack at potentials at least 40 mV higher than the pitting potential of pure aluminum. Metallographic examinations showed that mechanical stress accelerates the intergranular attack by a factor 10 to 100. Current densities calculated from the velocity of penetration under stress are in good agreement with values known from the pitting of pure aluminum. From the results obtained in this work the conclusion is that SCC of the aged AlCu alloy can be regarded as stress accelerated IGC.     

The study of intergranular corrosion in aircraft aluminium alloys using X-ray tomography

Atmospheric corrosion is one of the leading causes of structural damage to aircraft. Of particular importance is pitting and intergranular corrosion, which can develop into fatigue cracks, stress corrosion cracks, or exfoliation. Therefore it is of interest to the Australian Defence Force (ADF) to understand how corrosion ensues in susceptible aircraft aluminium alloys, such as AA2024-T351 and 7050-T7451. However, there are many difficulties in measuring the extent of intergranular corrosion, since it is predominantly hidden below the surface. Traditionally, cross-sectioning has been used to view and measure the depth of attack. In the present work, 2 mm diameter pin specimens were contaminated with a droplet of 3.5% NaCl and exposed to constant humidity that resulted in intergranular corrosion. X-ray computed tomography was then used to non-destructively assess the depth and volume of corrosion both as a function of time in 97% relative humidity, and as a function of relative humidity after 168 h exposure. Both corrosion depth and volume increased with time, but there was evidence for a limiting depth in AA2024. Depth and volume also increased with relative humidity of the environment, for which the time-of-wetness and oxygen concentration of the droplets were considered the important factors in driving the corrosion process.     

In-situ observation of intergranular stress corrosion cracking in AA2024-T3 under constant load conditions

A specially designed setup was used to apply a constant load to a thin sheet sample of AA2024-T3 and, using microfocal X-ray radiography, to observe in situ the resulting intergranular stress corrosion cracking (IGSCC) from the exposed edge of the sample. The growth of and competition between multiple IGSCC sites was monitored. In many experiments twin cracks initiated close to each other. Furthermore, the deepest crack at the beginning of every experiment was found to slow or stop growing, and was then surpassed by another crack that eventually penetrated through the sample. These observations cannot be explained by the theory of fracture mechanics in inert environments. The possible mechanisms underlying the competition between cracks are discussed.     

Influence of heat treatments and chemical composition on SCC susceptibility during repairing procedure of overlaying of Inconel 182 by laser surface melting

Abstract

A practical repairing technique using laser surface melting (LSM) was developed to remove the stress corrosion cracking (SCC) in overlaying of Inconel 182. Influence of microstructure of different heat treatments performed during repairing process on intergranular cracking/intergranular stress corrosion cracking (IGC/IGSCC) susceptibility was discussed. The intergranular precipitate was identified as M₂₃ C₆ by TEM. The microstructure with no intergranular precipitate and refiner sub-grain after LSM process shows excellent IGC/IGSCC resistance. The stress relief heat treatment induced severe microstructure of high IGC/IGSCC susceptibility, owing to the semicontinuous intergranular precipitation. The influence of Nb/C ratio on IGC/IGSCC susceptibility of three nickel based superalloys after LSM process was also investigated. For both of the Inconel 182 alloys with different Nb content, the microstructure after LSM process and following sensitisation treatment showed precipitation free grain boundary. The results of corrosion tests also indicated that the material with higher Nb/C ratio showed higher IGC/IGSCC resistance after LSM process and following sensitisation treatment.     

Exfoliation and intergranular corrosion resistance of the 2198 Al–Cu–Li alloy with different thermomechanical treatments

In this study, the resistance to exfoliation and intergranular corrosion (IGC) of the 2198 Al–Cu–Li alloy submitted to different thermomechanical treatments (T3, T8, and T851) was investigated. The tests were carried out following the standard practices, ASTM G34-18 and ASTM G110-15, respectively. All the tested alloys showed susceptibility to exfoliation and some alloys showed susceptibility to IGC, but the artificially aged alloys presented a higher tendency to exfoliation. The extensive hydrogen evolution reaction (HER) was observed on the surfaces of artificially aged alloys when immersed in the EXCO solution. The HER resulted in an increase in solution pH with the time of immersion. Also, the weight losses related to the artificially aged alloys were higher than that of the naturally aged ones. The T8 treatment was the only condition that resulted in susceptibility to both, intergranular and transgranular corrosion, whereas the T851 treatment did not show IGC susceptibility, only transgranular corrosion. Finally, the 2198-T3 condition showed the highest corrosion resistance among the thermomechanical treatments tested. The results of the 2198 alloy subjected to various treatments were compared with that of the 2024-T3 alloy. This last alloy showed higher resistance to exfoliation and IGC as compared with the 2198 alloy.     

Corrosion behavior of 2195 and 1420 Al-Li alloys in neutral 3.5% NaCl solution under tensile stress

1 Introduction Al-Li alloys, compared with traditional Al alloys, possess many excellent properties, such as lower density, greater elastic modulus and higher specific strength. 2195 Al-Li alloy with high strength and good ability to weld, was applied to …     

Relationship between microstructure, microhardness and corrosion sensitivity of an AA 2024-T3 friction stir welded joint

Corrosion sensitivity of a friction stir welded (FSW) AA2024-T3 aluminum alloy has been investigated using both normalized intergranular corrosion test (ASTM-G110) and local electrochemical open circuit potential measurements. In addition, Vickers microhardness and microstructural analysis have been performed.The HAZ close to the TMAZ is the most sensitive to intergranular corrosion because of the presence of continuous lines of S′(S) intergranular precipitates at grain boundaries. Pitting corrosion is due to the intermetallic particles. Their fragmentation produced by stirring effect modifies the pitting corrosion behavior. Microhardness variations depend on the relative volume fraction of GPB zones and S′(S) intragranular precipitates.     

时效析出相微结构改变对AA2024铝合金应力腐蚀行为的影响

通过恒载荷应力腐蚀实验、扫描电镜（SEM）和透射电镜（TEM）等测试方法,研究了时效处理中析出行为的变化对AA2024铝合金应力腐蚀行为的影响。结果表明,合金在T3态时对应力腐蚀较为敏感,进行T8时效处理后,合金的应力腐蚀敏感性显著降低。利用高角环形暗场成像扫描透射电镜技术（HAADF-STEM）对合金进行准原位腐蚀实验观察,研究了合金T3和T8时效状态下的腐蚀过程和析出行为的变化情况,直观地展示了不同时效状态合金的腐蚀形态：T3态的合金为晶间腐蚀形貌,T8态的合金为晶间腐蚀和晶粒腐蚀相结合。由于析出行为和腐蚀机制的改变,不同时效状态的AA2024铝合金的应力腐蚀敏感性不同。     

Comparison of susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys in neutral chloride solutions using electrochemical noise analysis

The susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys was investigated in aqueous neutral chloride solution for the purpose of comparison using electrochemical noise measurement. The experimentally measured electrochemical noises were analysed based upon the combined stochastic theory and shot-noise theory using the Weibull distribution function. From the occurrence of two linear regions on one Weibull probability plot, it was suggested that there existed two stochastic processes of uniform corrosion and pitting corrosion; pitting corrosion was distinguished from uniform corrosion in terms of the frequency of events in the stochastic analysis. Accordingly, the present analysis method allowed us to investigate pitting corrosion independently. The susceptibility to pitting corrosion was appropriately evaluated by determining pit embryo formation rate in the stochastic analysis. The susceptibility was decreased in the following order: AA2024-T4 (the naturally aged condition), AA7475-T761 (the overaged condition) and AA7075-T651 (the near-peak-aged condition).     

The effect of welding parameters on the corrosion behaviour of friction stir welded AA2024-T351

The effect of welding parameters (rotation speed and travel speed) on the corrosion behaviour of friction stir welds in the high strength aluminium alloy AA2024-T351 was investigated. It was found that rotation speed plays a major role in controlling the location of corrosion attack. Localised intergranular attack was observed in the nugget region for low rotation speed welds, whereas for higher rotation speed welds, attack occurred predominantly in the heat-affected zone. The increase in anodic reactivity in the weld zone was due to the sensitisation of the grain boundaries leading to intergranular attack. Enhancement of cathodic reactivity was also found in the nugget as a result of the precipitation of S-phase. The results were compared with samples of AA2024-T351 that had been heat treated to simulate the thermal cycle associated with welding, and with samples that had been exposed to high temperatures for extended periods to cause significant over-ageing.     

基于晶格储存能的航空铝合金腐蚀行为研究

采用多种电子显微技术对AA2024-T3航空铝合金的局部腐蚀行为进行研究。结果表明:AA2024-T3铝合金的局部腐蚀主要以晶间腐蚀的形式出现,相对于晶界沉淀相,晶格储存能更为显著地影响晶间腐蚀的发展,促使其优先发生在晶格储存能更高的晶粒周围。研究结果有助于提高对航空铝合金晶间腐蚀机理的认识,并为航空铝合金的设计提供理论指导,对保障航空安全具有重要意义。     

A morphological study of filiform corrosive attack on chromated and alkaline-cleaned AA2024-T351 aluminium alloy

In order to characterise filiform corrosion on a commercial AA2024-T351 aluminium alloy, a detailed microscopical study using SEM and EDS was performed. One set of AA2024-T351 aluminium alloy samples was alkaline-cleaned and deoxidised and chromate conversion coated. Another set was alkaline-cleaned only. Both samples were similarly spray coated with a 42 μm clear polyurethane topcoat. Filaments were subjected to a range of specimen preparation techniques. Sections and top views examined by SEM revealed varying degrees of attack ranging from generalised etching without local attack to severe local attack in the form of pitting, resulting in grain etchout, grain boundary attack and subsurface etchout. EDS revealed the presence of chloride deep into the pits and the subsurface etchout.     

Corrosion behaviour of AA2024 aluminium alloy in different tempers in NaCl solution and with the CeCl3 corrosion inhibitor

The paper analyses the corrosion behaviour of naturally and artificially aged AA2024 alloy in NaCl solution and in the presence of an environment-friendly corrosion inhibitor, CeCl₃. On the basis of the values of polarisation resistance and corrosion current density, the corrosion resistance of the protective inhibitor film is established as well as the general corrosion resistance of this aluminium alloy. Resistance to pit formation is determined based on the difference in pitting and corrosion potentials while resistance to pit growth is determined based on the amount of charge consumed during pit growth. A scanning electron microscope is used to examine the morphology of the pits formed during the pitting corrosion testing, as well as to determine the cerium content on intermetallic particles and the matrix AA2024 alloy. The corrosion behaviour of AA2024 alloy is investigated after different test periods in NaCl solution and in the same solution with the CeCl₃ inhibitor. The corrosion resistance of both tempers of AA2024 alloy is more than one order of magnitude higher in the presence of CeCl₃. An explanation of the observed differences in the corrosion behaviour of the naturally and artificially aged AA2024 alloy is proposed. Different corrosion behaviour of the alloy after different test periods is also explained.     

Mechanisms of corrosion inhibition of AA2024-T3 by vanadates

The mechanisms of corrosion inhibition of AA2024-T3 by vanadates were studied using chronoamperometry, polarization curves and adsorption isotherms. The electrochemical behaviour of clear solutions containing metavanadates and orange solutions containing decavanadates was clearly distinctive. Metavanadates reduced the kinetics of oxygen reduction to an extent similar to chromates. Corrosion inhibition of AA2024-T3 by metavanadates was very rapid and it might occur by the formation of an adsorbed layer. Reduction of clear metavanadate solution was very slow. Approximately 35 min were required to develop a monolayer of a reduced vanadate species. The adsorption of the inhibitor likely blocked reactive sites on intermetallic particles, discouraging the oxygen reduction reaction (ORR). Adsorption of the inhibitor on the Al matrix could also displace Cl⁻ ions, increasing the stability of the passive film and reducing the breakdown of S-phase particles. In contrast, decavanadates were shown to be poor inhibitors of the ORR. A sharp current spike was observed after injection of decavanadates for both Cu and AA2024-T3 at various applied cathodic potentials. Integration of the current peaks suggested the formation of several monolayers of a reduced vanadate species. The formation of several monolayers was in line with the poor performance of decavanadates as inhibitors of AA2024-T3 corrosion.     

Precipitation and Cr depletion profiles of Inconel 182 during heat treatments and laser surface melting

Thermodynamic and kinetic modeling were conducted to simulate Cr depletion profiles near grain boundaries in Inconel 182 during heat treatments and laser surface melting (LSM) using Thermo-Calc and DICTRA code. The effect of Nb addition was also considered in the modeling. Based on the good agreement with Cr concentration distributions during the heat treatments measured experimentally, Cr depletion profiles adjacent to grain boundaries during the heat treatments and the LSM process were modeled. The Cr depletion profiles were evaluated using the Cr depletion area below the critical Cr concentration for intergranular cracking/intergranular stress corrosion cracking (IGC/IGSCC) susceptibility (12 mass%). Compared with the result of the Streicher test, the calculated Cr depletion areas showed good agreement with IGC/IGSCC susceptibilities. The sample after stress relief (SR) treatment had the largest Cr depletion area and showed the poorest IGC/IGSCC resistance. Cr depletion showed some recovery during subsequent low temperature sensitization (LTS). The sample after the LSM process had the smallest Cr depletion area and showed the best IGC/IGSCC resistance.     

Elimination of intergranular corrosion susceptibility of cold-worked and sensitized AlSl 316 SS by laser surface melting

Susceptibility to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) due to sensitization is one of the major problems associated with austenitic stainless steels. Thermal exposures encountered during fabrication (welding, hot working, etc.) and elevated temperature service may lead to sensitization of components of austenitic stainless steels. Laser surface melting (LSM) is an in-situ method to increase the life of a sensitized component by modifying the surface microstructure without affecting the bulk properties. In this paper, the results obtained in the attempt to improve IGC resistance of coldworked and sensitized 316 SS by LSM are presented. Type 316 SS specimens cold worked to various degrees ranging from 5 to 25% reduction in thickness and sensitized to different degrees by exposing at 898 K for different durations were laser surface melted using continuous wave (cw) CO₂ laser. ASTM standard A262 practice A, optical metallography, and ASTM standard G108 were used to characterize the specimens before and after LSM. Influence of prior deformation on the desensitization behavior was evaluated for the laser melting conditions adopted during the investigation. Complete dissolution of M₂₃C₆ due to laser melting and suppression of re-precipitation due to rapid quenching result in a desensitized homogenous microstructure, which is immune to IGC. Under identical laser melting conditions, the extent of desensitization decreases with an increase in the degree of cold work, and hence, higher power levels and an extended interaction time must be adopted to homogenize the sensitized microstructure with prior cold work.     

Effect of temperature on transgranular and intergranular stress corrosion crack velocity of Ag–Au alloys

The effect of temperature on both the transgranular and the intergranular stress corrosion crack velocity of silver–gold alloys in a 1 M KCl solution was studied for temperatures ranging from 25 to 80 °C by means of slow strain rate experiments. At a constant potential and a constant elongation rate, the crack propagation rate was higher the higher the temperature. Transgranular stress corrosion cracking (TGSCC) velocity was found to change with the temperature, as well as with the potential, in the same way as intergranular stress corrosion cracking (IGSCC) velocity. In the region of potentials where the crack velocity was not controlled by ion diffusion in the crack, it was concluded that TGSCC and IGSCC for Ag–Au alloys in KCl solutions were controlled by the same stress corrosion cracking mechanism.