Steam Initiated Surface Modification of Aluminium Alloys

The extensive demand of aluminium alloys in various industries such as in transportationis mainly due to the high strength to weight ratio, which could be translated into fuel economy and efficiency. Corrosion protection of aluminium alloys is an important aspect for all applications which includes the use of aluminium alloys in the painted form requiring a conversion coating to improve the adhesion. Chromate based conversion coating processes are extremely good for these purposes, however the carcinogenic and toxic nature of hexavalent chromium led to the search for more benign and eco-friendly alternative processes.
In the present work high temperature steam-based process has been investigated as a possible chromate free conversion coating. Investigations in the thesis includes the effect of alloy type, substrate microstructure, surface finish, and various chemistries on the coating formation, and detailed analysis of the structure and morphology of the coating, and interface structure with and without organic top coat. Corrosion performance of the coatings was investigated using electrochemical methods, AASS, and FFC test. The morphology, microstructure, chemical composition, adhesion, and interface structure of the coatings were analysed using SEM, FIB-SEM,TEM, GI-XRD, FTIR, XPS, AFM, contact angle, and boiling test.
Chapter 1 of this thesis provides a background to the work and available literature information. Materials and experimental methods are outlined in chapter 2. The chapters 3 to 12 present various experimental results in the form of appended papers. The chapters consist of the experimental results obtained by the use of steam-based process and its effect on microstructureand corrosion resistance of the alloy as a function of steam pressure, use of various chemicals, crystalline nano-particles, role of steam-based treatment on adhesion of industrially applied powder coating, and investigations of a failed painted aluminium window profile due to defects in the extruded profile. Chapters 13 and 14 describe the overall discussion, conclusions and future work based on the results presented in the appended papers.
Overall the results presented in the thesis show the feasibility of using steam to generate thicker oxide coating on aluminium, which can be useful as a standalone coating or for further top coat. The steam process can be carried out either as a batch process using autoclave or using spray system, and with or without various chemistries as accelerators. In general, results show the formation of 650 nm – 3000 nm thick conversion coating, where the thickness depend on the treatment parameters and steam chemistry. Further, the formed coating provide good coverage on the aluminium matrix and second phase particles, with a performance matching in several cases quality standards of chrome based coating and alternatives used today.