This thesis is concerned with the corrosion protection of aluminium. Specifically, it investigates alternatives to chromate containing conversion coatings for aluminium extrusions of alloy 6063, used in architectural applications, and also examines these coatings on sheet alloy 3105.
A huge financial cost is incurred due to corrosion damage each year. Chromate
conversion coatings have been applied successfully to aluminium extrusions before subsequent powder coatings for decades, achieving a high standard of corrosion protection and paint adhesion. However the toxic and potentially carcinogenic nature of hexavalent chromium compounds has led to a drive towards environmentally friendly conversion or coating systems.
This thesis discusses the corrosion protection of aluminium, describing the processes undertaken to produce a novel corrosion protection pretreatment. The introduction describes the importance of aluminium, the different alloys types and the classification system, the detrimental effects of corrosion upon aluminium, current protection systems in place, and some of the systems that are being investigated as alternatives.
Chapter 2 looks in detail at two the aluminium alloy types that have been used
throughout this thesis, aluminium alloys 3105 and 6063. These are important
commercial alloys but their behaviour under etching conditions has been less well studied to date. On this basis, investigations into alloy morphology in the as received condition and following etching prior to the pretreatment step have been carried out.
These have direct consequence to the subsequent application of the chrome-free pretreatment coatings described in Chapter 3 and in later chapters.
Chapter 3 analyses the novel chrome free pretreatments based around a pseudo-sol gel process involving the use or zirconium and/or titanium precursors. These initial coatings have been analysed in plan view using SEM/EDX. Selected coatings have also been powder coated, exposed to Lockheed testing and then ultramicrotomed so that cross-sections can be analysed using TEM and EDX. The microscopy data have been compared to ascertain surface coverage and to gain a qualitative understanding of adhesion between the different layers. The addition of polymers to the pretreatments showed an improvement in some of the pretreatments, with the concentration playing an
important part. Good surface coverage was obtained with two pretreatments in
particular.
Chapter 4 discusses the results of industry-standard, accelerated corrosion testing and adhesion testing for each of the pretreatments described in Chapter 3. The data from Chapters 3 and 4 have then been considered together to try to establish understanding which can be used to deve lop suitable coatings that may be optimized for commercial use. In agreement with Chapter 3, the addition of polymer showed an improvement in the pretreatment performance. Three of the pretreatments showed excellent corrosion resistance, with two of these also exhibiting excellent adhesion.
In Chapter 5, electrochemistry, academically favoured over the accelerated corrosion testing described in Chapter 4, is utilized to assess the resistance to potential of some of the pretreatments. These data have been considered in the light of the conclusions drawn in Chapters 3 and 4, and suggests a different mechanism of corrosion protection in the new pretreatments compared to that observed on chrome based systems.
Chapter 6 looks at the development of a novel technique for qualitative screening of potential alloy pretreatments in a short period of time. The aim here is to develop a rapid test which can identify suitable candidates for more detailed corrosion testing and analysis.
Chapter 7 presents the details of all the experimental detail of the research carried out, while Chapter 8 brings together conclusions from the thesis and also discusses future work that stems from this research.