Papers by Keyword: Exfoliation Corrosion

Abstract: The effect of grain size on the initiation behavior of exfoliation corrosion in cold-rolled Mg-14mass%Li-3mass%Al alloy was examined. Exfoliation corrosion initiated after 30 minutes in the coarse-grained structure (279μm), whereas it was delayed to 60 minutes in the fine-grained structure (75μm) and further to 75 minutes in the ultrafine-grained structure (39μm). This delay is attributed to the suppression of shear band formation and localization during cold rolling with decreasing grain size, which enhances the uniformity of surface reactions in the corrosive environment and promotes faster and denser formation of the protective film at the early stage. Accordingly, the improved condition of the initial corrosion film is considered the primary factor responsible for the delayed onset of exfoliation corrosion.

Abstract: Mg-Li alloys have much better formability than traditional Mg alloys. However, the corrosion resistance of Mg-Li alloys are low due to Li addition and cold-worked Mg-Li alloys sustain exfoliation corrosion, which is serious problem for practical use of Mg-Li alloys [1]. According to our previous report, the elongated grains may be the cause of the exfoliation corrosion. In this study, the exfoliation corrosion behavior of Mg-14mass%Li-1mass%Al (LA141) alloy processed by equal channel angular extrusion (ECAE) is characterized to see the corrosion morphology of the samples introduced more grains than rolling. The specimens processed by rolling showed only elongated grains and the ones processed by ECAE (Route A) at 2 and 4 times showed elongated grains and partially equiaxial grains and in the samples fabricated by ECAE (Route B_c) at 2 and 4 times, equiaxial grains were found by optical microscopy. In corrosion examination, the specimens of rolling and ECAE (Route A) developed exfoliation corrosions and the samples of ECAE (Route B_c) didn’t sustain exfoliation corrosion. This results were consistent with our previous report. Then the corrosion behavior of rolled and ECAEed samples were focused. The evaluation of the time until crack initiation and the number of cracks were employed to see the corrosion morphology of the samples

Abstract: Relationship between microstructure and exfoliation corrosion in Mg-14mass%Li-3mass%Al cold rolled material were investigated by controlling microstructure using heat treatment. The exfoliation corrosion was exhibited on the specimens with elongated microstructure by cold-rolling. Heat treatment at 200 C only recovery occurred. Recovery and recrystallization were caused by heat treatment at 300 C However, heat treated at 300 C decreased precipitate fraction. Corrosion rate of heat-treated at 200 C decreased by release the residual strain. However, heat treatment at 300 C has lower corrosion resistance than heat-treated at 200 C because it has less precipitate density than as-rolled. Exfoliation corrosion was shown in as-rolled and heat-treated at 200 C. In as-rolled exfoliation corrosion was shown more significant when reduction rate increase. In as-rolled, exfoliation corrosion was shown more significant than heat-treated at 200 C. After heat treatment to change the grain shape from elongated to equiatial, exfoliation was suppressed by the distribution of AlLi phase.

Abstract: In this paper, the effect of Sc addition (0.06 wt%) on the corrosion behavior of medium strength Al-Zn-Mg alloy is investigated by mass loss measurements, electrochemical experiment, intergranular corrosion and exfoliation corrosion tests. The results indicate the addition of Sc reduces the relative weight loss and enhances pitting performance as a result of grain refinement. The improved intergranular corrosion and exfoliation corrosion resistance caused by minor Sc addition are mainly attributed to the delay in both the initiation and advance stages of local corrosion.

Abstract: A final thermomechanical treatment (FTMT) including peak aging and subsequent dynamic aging was proposed to prepare 7055 Al alloy sheets. The optimization was based on nine well-planned orthogonal experiments. Three main processing conditions in the thermomechanical treatment for obtaining the optimum synthetic properties of 7055 (i.e. preheating temperature, final rolling temperature and deformation degree) were investigated. It was shown that the final rolling temperature is the most important factor among the three parameters, and the optimum properties (yield strength: 651 MPa, ultimate tensile strength: 660 MPa) of 7055 Al alloy sheet can be gained with preheating at 140oC and 40% deformation at 170oC. With dynamic aging, grain boundary precipitates became discontinuous without much coarsening of matrix precipitates, while they were continuously distributed after T6 aging. The present optimal FTMT process can improve the intergranular / exfoliation corrosion resistance without sacrificing the strength compared to T6 tempering. The present FTMT process as a good alternative can produce high-strength Al alloy sheets with high strength and good corrosion resistance efficiently and economically.

Abstract: The thermal stability of constituent particles in both as-rolled and solution treated in AA7A55 has been investigated by means of scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). Three types of solution treatments were used to dissolve particles constituents. Whereas the degree of recrystallization aggravates which deteriorates the properties with increasing the solution temperature, a significant amount of recrystallization exists after utilizing 480°C solution treatment whereas a much smaller extent from 480°C to 490°C. And electrochemical and exfoliation corrosion matched with the degree of recrystallization. With increasing the recrystallization the corrosion become worse.

Abstract: Mechanical properties, microstructure and exfoliation corrosion (EXCO) behavior of spraying forming 7075 alloy underwent retrogression and re-aging (RRA) were studied by tensile test, transmission electron microscopy and EXCO test, and compared with those of T6 peak aging and T73 overaging treatments. The results show that after T6 treatment, abundance transgranular dispersive η phases make tensile strength of the alloy reach 760MPa, elongation and EXCO rating are only 4.8% and ED respectively by dint of continuous η phases at grain boundaries and narrow precipitate free zones (PFZ). After T73 treatment, interrupted η phases at grain boundaries and wide PFZ can improve elongation and EXCO resistance, but depressed volume fraction of coarsening transgranular η phases reduce tensile strength to 676MPa. After RRA treatment (120°C/24h + 200°C/10min + 120°C/24h), abundance transgranular dispersive η phases separate out again, η phases at grain boundaries interrupt, and PFZ widen slightly. Tensile strength, elongation and EXCO rating of alloy are 758MPa, 8.4% and EA respectively.

Abstract: Based on initial discontinuity state (IDS) of material, this paper presents a preliminary analytical model which was developed to evaluate the effect of exfoliation corrosion on the residual fatigue life of upper wing skins. A life prediction was then carried out using constant amplitude compression dominated loading for various exfoliation corrosion levels by AFGROW, and the prediction agreed reasonably with the available test data. The result suggest that exfoliation can be treated as a process zone located initially at a surface, and then at the base of an evolving general stress concentration representing the loss of material in the exfoliation region. The major effect of the exfoliation corrosion on fatigue life is to cause a dramatic reduction in life with small corrosion depths, and most of the fatigue effect is associated with the introduction of small pits, rather than more general stress concentration associated with the bulk of the exfoliation attack. But the effect of pit on fatigue life is gradually reduced with increasing pit size. A pit with a constant depth can be applied to the model described in this paper for long exposure structure. A preliminary recommendation for the pit depth is about 0.3mm for LY12CZ.

Abstract: Results are summarized of feasibility of using model for description of exfoliation corrosion (EC) in modeling of structural degradation of AlCu4Mg1 (Al2024) on extruded stringer type specimens in EXCO solution for different times of exposure. The effect of time for time development of this localized form of corrosion was assessed by using the approach of Robinson. Briefly, this approach makes possible description of intergrannular penetration observed under blisters formed by voluminous corrosion products. However, intergrannular stress supposed to be resulting from the expansion of corrosion products along dissolved grain boundaries is not taken into account in this model. Possible application of concept of intergrannular stress incorporation into EC model is discussed for modeling of structural degradation of Al2024 in EXCO tests.

Abstract: The effects of two-step aging temperature on exfoliation corrosion of an Al-Zn-Mg-Cu alloy were characterized by EXCO test, electrochemical test and TEM observations. Results suggested that the partial recrystallized structure is susceptible to exfoliation corrosion which is greatly influenced by intergranular corrosion. The transfer current of the sample aged at 170°C (second-step aging temperature) is higher than that of the sample aged at 155°C. The second-step aging temperature exhibits more contributation to the exfoliation corrosion in this investigation, compared to the first-step aging temperature.