Commercial aluminum (Al) alloys are often precipitation strengthened to improve strength and wear resistance. However, localized corrosion due to the galvanic coupling between the precipitates and Al matrix often leads to degraded performance when these alloys are exposed to corrosive environment. In this work, Al-based solid solution was synthesized to simultaneously improve the strength and corrosion resistance of Al alloys, which ultimately led to high tribocorrosion resistance. Specifically, the effects of testing condition (e.g. sliding frequency) and alloying effects (e.g. Mn and Mo) on the corrosion and tribocorrosion behavior of Al-based binary and ternary solid solutions were studied.
To understand the effects of wear condition on the depassivation-repassivation kinetics during tribocorrosion, in the first study, the tribocorrosion behaviors of Al-20 at.%Mn alloys were investigated in simulated seawater by changing the sliding frequency from 0.05 to 1 Hz in reciprocal motion. The results show that the depassivation rate of passive film increased with increasing sliding frequency. Mechanical wear also increased with increasing sliding frequency, which was mainly related to the increase of coefficient of friction and real contact area. Chemical wear tended to increase with scratching frequency, most likely due to faster repassivation kinetics at lower frequency. The surface layer was analyzed by cross-sectional transmission electron microscopy, indicating the passive film was primarily consisted of aluminum oxide where manganese was selectively dissolved.
Despite extensive past research, the fundamental understanding of the alloying effects on the atomistic structure, composition, and chemical state of the passive layer of Al alloys and their formation mechanism is still not well understood. In the second study, the effects of Mn on the aqueous corrosion of Al-Mn alloys were investigated. It was confirmed that Mn alloying could enhance the corrosion resistance of Al without participating in the surface oxidation. Atom probe tomography analysis confirmed the absence of Mn in the anodized and corroded surface of Al-Mn alloys. The selective dissolution of Mn in these alloys was believed to increase the free volume at the metal/oxide interface to facilitate the formation of a denser, thinner oxide layer with closer to stoichiometry composition, leading to its enhanced corrosion resistance than pure Al.
Lastly, to better understand the corrosion and tribocorrosion resistance of Al-based lightweight concentrated alloys and the effects of alloying concentrations on the structure and property of the passive layer, the third study investigated the effects of a passive element (Mo) and non-passive element (Mn) on the corrosion and tribocorrosion behavior of Al-Mn-Mo alloys. Specifically, Al80Mn8Mo12 exhibited higher corrosion resistance than Al80Mn20 due to the formation of a more compact and less defective passive film, as explained by the roles Mo played in both the substrate and the passive film. It was found that the pitting potential and corrosion current density of Al-Mn-Mo increased with Mo%. The effect of Mo alloying concentration on the tribocorrosion behavior of Al-Mn-Mo alloys was investigated as well. Adding Mo to Al-Mn alloys led to a lower wear and tribocorroison resistance of Al-Mn-Mo alloys. In addition, decreasing Mn and Mo concentrations resulted in a reduction of the tribocorrosion resistance in the ternary alloy, which was mainly dominated by the mechanical response under the selected testing conditions. / Doctor of Philosophy / Various critical current and future applications in the fields of aerospace, transportation, energy, and biomedical industries require not only a strong and tough metal, but one that is robust and reliable when interacting with some very corrosive environment. Such corrosive environment is testing the limits of most engineering metals and challenging the current understanding of the underlying degradation mechanism. For example, strength and wear resistance in most precipitation-hardened Al (aluminum) alloys is often achieved at the expense of sacrificed corrosion resistance, mainly due to micro-galvanic coupling between the soft matrix and hard precipitates. In addition, the performance of Al alloys deteriorates dramatically when there is combined wear and corrosion, i.e. tribocorrosion attack at the surface, due to the depassivation on the wear track as a result of mechanical removal of the passive film.
Recent study shows that alloying Al with appropriate transition metals in supersaturated solid solution simultaneously improves the corrosion and wear resistance of Al. In this thesis, Al-Mn and Al-Mn-Mo solid solutions was synthesized and studied to understand the effects of testing condition (e.g. sliding frequency) and alloy composition (e.g. Mn and Mo concentration) on the corrosion and tribocorrosion behavior. First, the depassivation mechanism during tribocorrosion of Al-Mn alloys was investigated by performing tribocorrosion test using different sliding frequency from 0.05 to 1 Hz in 0.6 M NaCl aqueous solution. Results showed that both chemical and mechanical wear increased with increasing frequency. The mechanical wear increased with scratching frequency due to faster depassivation rate and increased real contact area, while chemical wear increased with frequency due to higher repassivation kinetics. Secondly, the effects of Mn on the aqueous corrosion and passivation of Al-Mn solid solution alloys were investigated by electrochemical experiments and advanced surface characterization. It was found that Mn addition enhanced the corrosion resistance of Al without participating in the surface oxidation. A denser, thinner oxide layer was formed on Al-Mn due to the increased free volume at the metal/oxide interface as a result of Mn dissolution. Lastly, the effects of alloying concentration on the aqueous corrosion and tribocorrosion of Al-Mn-Mo alloys were studied experimentally. The pitting potential and corrosion current density of Al-Mn-Mo were found to increase with Mo%. The passive film thickness depended on the total alloy concentration, while its compactness and defect density on the individual ones. The tribocorrosion resistance of Al-Mn-Mo alloys decreased with increasing Mn and Mo concentrations. In summary, the results from this thesis develop mechanistic understanding of the corrosion and tribocorrosion mechanisms of Al-based solid solution alloys, which sheds light on a new alloy design strategy for making lightweight, strong, and corrosion-resistant metals.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/106788 |
| Date | 30 November 2021 |
| Creators | Chen, Jia |
| Contributors | Materials Science and Engineering, Cai, Wenjun, Jia, Xiaoting, Corcoran, Sean G., Bai, Xianming |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | Creative Commons Attribution-NonCommercial 4.0 International, http://creativecommons.org/licenses/by-nc/4.0/ |
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