Kinetics of sharp intergranular corrosion fissures in AA7178

Abstract

During service in structural aircraft applications, AA7178 has been found to develop sharp intergranular corrosion (IGC) fissures, a special form of attack. A new laboratory approach to quantify the kinetics of sharp IGC fissure growth was developed in this work. Sharp IGC fissures, similar to those formed in service, grew in samples that were given an electrochemical pre-treatment and then exposed in a humid environment. The time for the first sharp IGC fissure to penetrate the thin sample was determined by visual observation of the back side of the sample. The depth of the sharp IGC fissure was determined by serial sectioning. The kinetics measured by this approach did not represent the fastest rates, but rather the rates of long sharp IGC fissures. Relative humidity had no measurable effect on fissure kinetics, indicating that there was little connection of the local sharp IGC fissure region with the outside environment. AA7178 in the as-received and T7 conditions exhibited slower sharp IGC fissure rates than in the T6 condition. The sharp IGC fissures were found to be filled with corrosion products, which possibly exert a stress that could play an important role in driving fissure growth. It is suggested that the sharp IGC fissure could be a form of SCC.

Corrosion Damages Induced by Cyclic Exposure of 2024 Aluminum Alloy in Chloride-Containing Environments

“…Evaluation of the mechanical stresses inside the corrosion defects.-In the literature, some authors assumed that the corrosion products trapped inside the corrosion defects could generate mechanical stresses leading to enhanced corrosion damage. [49][50][51][52] Mechanisms proposed in the literature for exfoliation, which is promoted in humid or cyclic exposure conditions, showed that crack opening stresses are possible during cyclic tests because of partial drying of the corrosion products. Moreover, results from the literature showed that exfoliation corrosion also occurred during full immersion in EXCO solutions, which indicated that even the wet corrosion products can generate enough opening stress to accelerated intergranular attack.…”

“…51 Therefore, during the air exposure periods and, for long corrosion defects during the immersion period, the main reaction was H þ reduction inside the corrosion defects and the propagation rate of the corrosion defects mainly depended on local chemical conditions. 48,51 There was in this case no limitation by H þ transport processes. Moreover, due to the formation of a Cu-rich film inside the corrosion defects, H þ reduction was enhanced since its overpotential (and even that of oxygen reduction) was lower on Cu than on Al surface.…”

The effect of droplet size on the localized corrosion of high‐strength aluminum alloys

“…, corrosion follows sheet‐like growth predominantly in the longitudinal (rolling) direction and the long‐transverse directions. This selective grain attack as it is sometimes called means corrosion will follow the elongated flat grain structure and it will not likely result in classical hemispherical pitting. The tendency for intergranular to follow the rolling direction makes it easily identifiable on the long‐transverse and short‐transverse faces.…”

Erosion Corrosion Behavior of Aluminum in Flowing Deionized Water at Various Temperatures

“…The electrochemical corrosion rate of aluminum in deionized water was slow. To more quickly obtain the corrosion surface and corrosion products of aluminum, the samples for scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared (FTIR) and powder X-ray diffraction (XRD) were subjected to accelerated corrosion by potentiostatic anodic oxidation at 0.5 V (vs. Pt) for 0.5 h [17,18]. Large area platinum electrodes were used as the cathodes, 1 cm × 1 cm aluminum foils after polishing and washing were used as the anode, and the flowing deionized water with 3 m s -1 was used as the electrolyte at 10, 20, 30, 40, and 50 • C.…”