In this work, acoustic emission (AE) monitoring was correlated with in-situ optical microscopy observation and electron backscatter diffraction (EBSD) measurements to investigate the evolution of a single stress corrosion crack in SUS420J2 stainless steel subjected to chloride droplet corrosion. A single dominant crack evolution was observed to transition from a slow initiation of active path corrosion-dominant cracking to a rapid propagation of hydrogen-assisted cracking. The initiation-to-propagation was concomitant with a significant increase in the number of AE events. In addition, a cluster analysis of the AE features including traditional waveform parameters and fast Fourier transform (FFT)-derived frequency components was performed using k-means algorithms. Two AE clusters with different frequency levels were extracted. Correlated with the EBSD-derived kernel average misorientation (KAM) map of crack path, low-frequency AE cluster was found to correspond with the location of plastic deformation in the propagation region. High-frequency AE cluster is supposed to be from the cracking process. The correlation between AE feature and SCC progression is expected to provide an AE signals-based in-situ insight into the SCC monitoring.

In this study, the effects of Fe 3+ on corrosion behavior of high-purity aluminum (Al) were investigated in aqueous NaCl solutions containing Fe 3+ using immersion tests, electrochemical analysis, and surface observations. For high-purity Al immersed in an Fe 3+ solution, the open circuit potential was much lower and the corrosion rate was higher than that of high-purity Al in a solution without Fe 3+ . The corrosion morphology of high-purity Al after immersion in the test solutions containing Fe 3+ was mainly general corrosion; this differed significantly from the morphology of the Al immersed in a NaCl solution without Fe 3+ . The effects of Fe 3+ on the corrosion behavior of high-purity Al were due to a less-protective passive film composed of Al(OH) 3 and Fe(OH) 3 that formed on high-purity Al during immersion.

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An accurate evaluation of stress corrosion cracking (SCC) in 13Cr martensitic stainless steel (MSS) is still missing due to the lack of an in-situ insight into the process evolution and full characterization of the corrosion morphology. In this work, two main regimes involved in the SCC progression, including localized corrosion and cracking, were comparatively studied using in-situ acoustic emission (AE) monitoring and three-dimensional (3D) X-ray computed tomography (XCT) scanning. The stress corrosion tests were conducted with u-bent smooth specimens subjected to a single droplet of 1 μL 1% neutral NaCl solution. Localized corrosion and cracking evolution were controlled in tempered and quenched steel specimens, respectively. From XCT scanning, localized corrosion was featured by an irregular corrosion pit with deposited corrosion products containing cracks. The single dominant SCC crack was observed to initiate from corrosion pit and propagate with a 3D tortuous and discontinuous morphology. AE signals were detected in both cases. Correlated with in-situ observations and clustering analysis, source identification of AE signals was proposed. AE signals during localized corrosion were assessed to be mainly from cracking within the deposited corrosion products. Comparatively, hydrogen-bubble evolution, plastic deformation, and crack-branches coalescence were proposed as the AE sources of cracking evolution.

Thermal sprayed aluminum coated steels were exposed to subtropical climate at the University of the Ryukyus, Okinawa. They were found to exhibit good corrosion resistance even after 25 years. The results of the present study indicate that the thickness of the thermal sprayed aluminum film need not necessarily decrease, rather it may increase with the duration of exposure, but the porosity clearly increased. Furthermore, a film of alumina was formed on the rust where damaged steel was exposed to the atmospheric environment. Therefore, the thermal sprayed aluminum film is believed to provide 1) environment interception and 2) electrolytic protection.