Development of cold sealing processes for anodized aluminium
This thesis is concerned with the development of cold sealing treatments for porous anodized aluminium. The need for such treatments arises because of the need to conserve energy resources and the rapid rise in energy costs over recent years. The extensive literature has been reviewed with special emphasis on recent studies of chemical/cold sealing processes. The review has considered both theoretical aspects, practical considerations of pore sealing and the substantial patent literature. It is shown that: (a) virtually all cold sealing solutions are based on nickel fluoride and operate in a temperature range of I5-60oe for 5-35 minutes and at a pH of 5.0- 6.5; (b) anodized aluminium sealed in the above solution(s) can not be tested for seal quality immediately. Original research data has been obtained by studying the nickel fluoride solution. A study of the ageing phenomenon was carried out An additional process after cold sealing namely 'post-treatment' was considered. Once these criteria for good sealing were established the next stage was to study the effect of additives in the nickel fluoride cold seal bath. As the importance of a surfactant in cold seal baths was suggested by one of the patents and claimed to be crucial for successful sealing, various types of surfactant such as ionic and nonionic were studied. Various analytical techniques and seal quality tests were used in studying the mechanism of nickel fluoride cold sealing and a new mechanism of cold sealing was proposed. \ Studies were carried out using other metal ions than nickel including copper, cobalt, zinc and zirconium with fluoride. Most were used at room temperature bearing in mind the need to save energy. It was found that the nickel fluoride cold sealed finishes performed well with ageing. However, depending on the test method used, sufficient ageing (from 5 hours to 28 days at room-temperature) must be allowed prior to carrying out seal quality tests which are designed for conventional hydrothermal sealing. Accelerated ageing by post-treatment was found to improve substantially the seal quality of anodized aluminium. It was found that the degree of sealing depends on the method of post-treatment carried out. The study of surfactant in cold seal baths showed that seal quality is marginally improved with the addition of surfactant and depends on the type and concentration of surfactant used. Amongst the alternative metal fluorides examined, copper, zinc and cobalt resulted in partial sealing whereas zirconium produced a good seal quality superior to nickel fluoride and hydrothermal sealing in terms of acid dissolution and corrosion tests. However, it produced a silvery white appearance and high admittance value. Such problems were reduced by using potassium fluorozirconate.