Novel methods, incorporating pre- and post-anodising steps, for the replacement of the Bengough–Stuart chromic acid anodising process in structural bonding applications

The present investigation focuses on novel anodising processes which are being developed for the replacement of the hexavalent chromium containing 40/50 V Bengough–Stuart process, with particular emphasis on their resultant performance in structurally bonded systems used in the demanding and harsh environments encountered on operational aircraft. An electrolytic phosphoric acid-based deoxidiser (EPAD) has been studied in combination with a standard sulphuric acid anodise. It has been shown that the EPAD provides an open porous structure in order to enhance adhesion to the modified sulphuric acid anodised (SAA) surface. Additionally, a post-anodising (PAD) treatment has been used to aid structural adhesion in combination with the SAA processes.As a control, the standard 40/50 V Bengough–Stuart chromic acid anodising (CAA) has been used as a baseline performance indicator in adhesion tests. Single-lap shear (SLS) and modified Boeing wedge tests were used to determine adhesion performance. SLS tests were used to determine initial, dry joint strengths whilst wedge test joints immersed in deionised water for up to 100 h gave a measure of joint durability. Overall, excellent initial joint strengths and durability have been found with both EPAD plus SAA and PAD plus SAA processes suggesting that these environmentally benign treatments may be used as possible drop-in replacements for the currently used hexavalent chromium process. Electron microscopy has been used to investigate the topographical changes introduced to the surface by the various surface pretreatments under investigation to provide an explanation for the observed adhesion test results.