- Title
- Characterisation of pitting corrosion for aluminium alloys in natural seawater and freshwater immersion environments
- Creator
- Liang, Mengxia
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Aluminium alloys are increasingly being used in structural and commercial infrastructure, such as aerospace, transportation, architecture, defence and marine industries, due to their low density, desired strength, excellent corrosion resistance and other combined properties. For this reason, structural safety is paramount as it can be influenced by the long-term exposure of aluminium alloys to various environments. In wet environments or when in contact with water, aluminium alloys are prone to pitting corrosion, which can be very detrimental. Pitting leads to rapid attacks of discrete areas on the metal surface, while the adjacent surface can remain virtually unaffected. It is vicious and insidious and may result in perforation or contribute to stress-induced failure. For safety and economic reasons, it is important to be able to predict future corrosion and estimate the lifetime of aluminium components. This requires a thorough understanding of the progression of pitting and reliable corrosion data, measured in years, which is essential to building an accurate corrosion prediction model. Many decades of research and investigation have provided for a considerable understanding of the onset and early development of pitting. Unfortunately, relatively few investigations have been dedicated to longer-term (especially that expressed in years) pitting behaviour. In addition, recent research on AA6060-T5 aluminium alloys that are continuously immersed in natural seawater for 2(+), 3, 3.5 and 4 years showed that the long-term pitting corrosion of aluminium alloys is a very complex process. Several different types of pitting morphologies were observed on the metal surface. The development of mass loss at the maximum pit depths as a function of time showed a trend similar to the early part of a bi-modal model, as previously proposed based on long-term pitting corrosion data of steels. In order to have a better understanding of how these complex corrosion morphologies develop and whether this bi-modal model trend can also be applied to other series of aluminium alloys, a group of long-term test programs were performed. This thesis reports observations on the complex, long-term pitting corrosion of AA5005-H34 aluminium alloys in six different immersion environments, all in natural seawater or freshwater. Test coupons were recovered periodically after exposure periods of 1 month, 3 months, 6 months, 12 months, 18 months and 2 years. The corrosion properties including pitting morphologies, corrosion products, mass loss and maximum pit depths for each exposure period were carefully investigated. The changing pitting morphologies and their relationship with the development of mass loss and pit depth as a function of time were analysed and discussed. Extreme value theory was used to analyse the uncertainty of the pit depth data. Also, the likely factors influencing pitting corrosion were discussed. The results showed that pitting morphologies varied significantly in different immersion environments. Within the same immersion environment, the dominant pitting morphology also changed after different amounts of exposure. It was found that the mass loss, the average of the 15 deepest pit depths and the maximum pit depths tended to develop in stages as exposure time increased. Overall, the trends of the datasets for the mass loss and pit depths were similar to that of the early part of a bi-modal trend as a function of time. Corrosion products played an important role in the bi-modal development of pitting. Also, for the deeper pits, specifically those ‘dimple’-like pits that developed in a long-term steady phase, Frechet extreme value distribution showed a slightly better distribution than Gumbel distribution.
- Subject
- aluminium alloys; pitting corrosion; SEM; extreme value theory
- Identifier
- http://hdl.handle.net/1959.13/1407935
- Identifier
- uon:35795
- Rights
- Copyright 2019 Mengxia Liang
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 24 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 114 KB | Adobe Acrobat PDF | View Details Download |