The objective of this study was to quantitatively study localized corrosion, especially exfoliation corrosion (EFC) of high strength aluminum alloys and to investigate the mechanism of exfoliation corrosion with a focus on the effects of alloy temper, microstructure, relative humidity (RH) and mechanical stress. A new technique, Exfoliation of Slices in Humidity (ESH), was developed for the determination of exfoliation corrosion (EFC) susceptibility and quantification of EFC kinetics. This technique involves in exposing properly oriented and unconstrained samples to high humidity following an electrochemical pretreatment. The EFC kinetics was determined by measuring the width of the central unattacked region of the samples. The ESH results show the capability of the ESH test to discriminate between plates of varying susceptibility and to determine EFC rates quantitatively. Optical microscopy and analytical TEM were used to investigate the effects of microstructure and local chemistry at grain boundary on EFC susceptibility. Alloys with more elongated grain shape are more susceptible to EFC and a high Zn content in grain boundary precipitate free zone relates to a high susceptibility.
The effects of RH, temper and applied stress on EFC kinetics of AA7178 were investigated by ESH tests. The critical RH for EFC propagation in AA7178 was found to be about 56% and the EFC kinetics increased with RH. ESH tests provide a quantitative description of the temper effect on EFC kinetics. The effects of applied compressive and tensile stresses on EFC kinetics were studied using a four-point bending jig. Compression accelerated EFC significantly and tension reduced kinetics. An equation describing the effects of RH, stress and time on EFC kinetics was developed based on the ESH results using Eyring model.
In situ X-ray radiography was used to characterize intergranular and exfoliation corrosion in high strength Al alloys. The samples were either exposed to sodium chloride solution (NaCl) at a controlled potential or to high humidity after an electrochemical pretreatment in NaCl solution. In situ X-ray radiography of intergranular corrosion attack provides a wide range of IGC kinetics including the fastest growing sites. This method is a good approach for visualizing the EFC process.
|School:||The Ohio State University|
|Department:||Materials Science and Engineering|
|School Location:||United States -- Ohio|
|Source:||DAI-B 79/09(E), Dissertation Abstracts International|
|Keywords:||Aluminum alloys, Exfoliation corrosion, Exfoliation of Slices in Humidity (ESH), Kinetics, Relative humidity, Temper|
Copyright in each Dissertation and Thesis is retained by the author. All Rights Reserved
The supplemental file or files you are about to download were provided to ProQuest by the author as part of a
dissertation or thesis. The supplemental files are provided "AS IS" without warranty. ProQuest is not responsible for the
content, format or impact on the supplemental file(s) on our system. in some cases, the file type may be unknown or
may be a .exe file. We recommend caution as you open such files.
Copyright of the original materials contained in the supplemental file is retained by the author and your access to the
supplemental files is subject to the ProQuest Terms and Conditions of use.
Depending on the size of the file(s) you are downloading, the system may take some time to download them. Please be