Application of Sol-Gels as anticorrosion coatings
Document Type Theses, Ph.D
Sucessfully submitted for the award of Doctor of Philosophy to the Dublin Institute of Technology, 2009.
The main aim of the thesis work is to develop a novel chrome-free coating system through sol-gel process for protecting the aluminium alloys used for various applications from corrosion. Traditional chromate-based conversion (Cr (VI)) coatings are very efficient and offer many valuable coating properties. However the “End of Life Vehicle” directive issued in 2000 restricts the use of chromates because of toxicity and suspected carcinogenic danger of Cr (VI). The environmental concerns dictate the elimination of current chromate-based surface treatment technologies and development of fundamentally new chromate-free coating systems that can provide long-term and environmentally benign corrosion protection. One of the most promising alternatives being investigated are organosilane based sol-gel. Chapter 1 is a general introduction about corrosion and current state of art, techniques/methods to prevent it occurring on the aluminium alloy AA 2024-T3. This chapter gives a brief introduction to the types of engineering alloys and the associated corrosion issues and the related economic costs. The current states of art anti corrosion technologies are then discussed. Chapter 2 discusses sol-gel chemistry in detail and review literature of previous works by various research groups of sol-gel coatings on AA 2024-T3 alloy. Chapter 3 introduces and discusses the characterization techniques used to evaluate the structural, microscopical and barrier performance of sol-gel coating on AA2024-T3 grade aluminium panels. Chapter 4 presents the first experimental results and studies the anti corrosion properties of methyltriethoxysilane (MTEOS) sol-gel films doped with magnesium (II) nitrate (Mg (NO3)2) as a inhibitor in different concentrations (0.1% - 1.0 wt %) on AA2024-T3 alloy iii and are compared with AlodineTM 1200 (the established Cr(VI) pre-treatment). The electrochemical studies showed that the optimum performance was achieved with Mg (NO3)2 level of 0.7% w/w. It is proposed that the superior anticorrosion properties of the Mg2+ rich sol-gel are due to the pore blocking mechanism of insoluble magnesium precipitates formed during the hydrolysis process. Chapter 5 investigates the corrosion protection properties of functional silane/zirconium hybrid coatings. The corrosion protection properties of the hybrid coating are then compared with that of epoxy functionalized silane and non-functional sol-gels coatings. The studies indicated that hybrid coating with 20 mol% zirconium showed good corrosion protection properties when compared to non-functionalised sol-gels. Chapter 6 is an extension of work from Chapter 5, which determines the effect of various zirconium ligands on the structural and barrier properties of hybrid coatings. Thermal stability data and electrochemical studies indicate that the acid ligand modified coatings provided the best performance followed by acetyl acetone (acac) modified coating, while 2,2-bipyridine (Bipy) was the poorest. In all cases the zirconium nanoparticle improved the protective properties of the sol-gel coating. Chapter 7 focuses on the addition of aluminium/zirconium alkoxide precursors to the systems in Chapter 6 with the aim of improving the alkaline stability and corrosion protection properties of final coating. The best anticorrosive protection afforded to AA2024 in alkaline pH 10 (in 3.5% NaCl) electrolyte was provided by 15 mol % aluminium containing sol-gel coating. Chapter 8 concentrates on developing tetrazines as anti corrosion additives in sol-gel coatings, through doping and also engineering as zirconium precursor ligands. Some of the coatings developed were over-coated with full aerospace paint systems (with the aid of an industrial partner) and compared with the current state of the art using traditional iv accelerated techniques. Accelerated corrosion test results showed that hybrid coating doped with 0.3wt% of DPTZ (with Chrome primer and a top coat) on AA 2024-T3 passed 2000 hr along with other chrome based industrial coatings. Chapter 9 gives an overall conclusion of work presented in this thesis. Recommendations for future work are also suggested.