X. Zhong

Low-Temperature Environmentally Assisted Cracking of Grade 2205 Duplex Stainless Steel Beneath a MgCl2:FeCl3 Salt Droplet

The corrosion and environmentally assisted cracking susceptibility of grade 2205 duplex stainless steel beneath a FeCl3:MgCl2-containing salt deposit has been investigated. Long-term exposure to atmospheric environment at 50°C and 30% relative humidity resulted in different forms of corrosion and the formation of cracks depending on the location under the salt-laden droplet. Selective dissolution with closely-spaced microcracks of the ferrite suggested hydrogen embrittlement toward the center of the droplet, with chloride-induced stress corrosion cracking and selective dissolution of the austenite observed toward the rim of the deposit. Cracks in the ferrite had cleavage-like appearances, typically forming within existing cracks, whereas the austenite had branched cracks, initiating from crevices and pits. These observations are discussed in light of expected electrochemical potential variations beneath the droplet.

Liquid In Situ Analytical Electron Microscopy: Examining SCC Precursor Events for Type 304 Stainless Steel in H 2 O

In situ transmission electron microscopy (TEM) has become an increasingly important research area in materials science with the advent of unique microscope platforms and a range of specialized in situ specimen holders. The ability to image and perform x-ray energy dispersive spectroscopy (XEDS) analyses of metals in liquids are particularly important for detailed study of the metal-environment interactions with specific microstructural features. In particular, these capabilities now make it feasible to explore what has been termed “SCC precursor phenomena” – that is, those sub-micron scale reactions between an alloy of interest and the environment. This topic is especially timely for this Swann Memorial Symposium, as Peter Swann’s activities in this area, particularly in the 1970’s concerning the initiation and early stages of transgranular SCC in austenitic stainless steels, were clearly prescient. In this study, we have used liquid cell TEM with XEDS to explore the “precursor phenomena” that can promote the development of defect initiation in Type 304 austenitic stainless steel. For this work, the FIB lift-out technique was used to extract specimens to be studied in the liquid cell TEM specimen holder. This technique has been applied to examine the localised dissolution of MnS inclusions, which can lead to pit initiation.

Effect of Reinforcements on the Corrosion Behavior of SiCp/AA2124 Metal Matrix Composites

In the present study, two composites, reinforced with 25 vol% SiC particles with average sizes of 3 μm (micrometer-sized, composite A) and 0.7 μm (submicrometer-sized, composite B), respectively, were evaluated to investigate the effect of reinforcements on the corrosion behavior of SiCp/Al Alloy 2124 (UNS A92124) metal matrix composites. Potentiodynamic polarization and electrochemical impedance spectroscopy measurements performed in a 0.6 M NaCl solution revealed that composite B, whose corrosion resistance was close to that of AA2124, exhibited higher corrosion resistance than composite A. It was found that the composition and distribution of the intermetallic particles in the aluminum matrix were altered by the addition of SiC particles. Preferential precipitation of Al2Cu (θ-phase) and Mg segregation in the SiC/Al interfacial regions were observed and are believed to be responsible for the inferior corrosion performance of composite A. The improved corrosion resistance of composite B was attributed t...