Friendlier surface treatments - for metals

2012-04-04T12:55:55Z (GMT) by Gary Critchlow Keith Yendall J. Lu
With only a few exceptions, some degree of surface treatment is applied to all metal surfaces prior to adhesive bonding. The surface treatment applied will depend upon the requirements of the bond and service conditions that it will see and will generally be chosen on a “fit-for-purpose” basis. The minimum preparation which is usually carried out might include a simple degrease to remove processing aids, such as oils and waxes, and contaminants. However, it is recognised that the current state-of-the-art processes for structural or semi-structural metal bonding are highly complex, multi-stage treatments including conversion coating and anodic oxidation. Alternatives to the commonly-used degreasing processes are sought for many reasons, for example: established processes may not be adequate for difficult-to-remove materials; the processes may use VOCs; they can be carcinogenic or ozone depleting. Regarding the higher treatments, the anodising processes, in particular, are difficult, time consuming and costly to carry out. There are also legislative drivers which make utilisation of the more complex processes, especially those which utilise hexavalent chromium, highly undesirable. Other factors such as energy and chemical disposal costs also deserve consideration when considering the need for environmental or operator “friendly” processes. This paper will consider a number of alternative friendly surface treatments which might be considered as drop-in replacements for the current industrial standards. The friendlier surface treatments include two simple cleaning methods, namely: seaweed-based cleaners and CO2 laser ablation. In addition, to cover the spectrum of processes, two novel anodising methods will also be discussed. These are based upon electrolytic phosphoric acid deoxidising plus sulphuric acid anodising (EPAD+SAA) and alternating currentdirect current (ACDC) anodising.