Development of Novel Chromate-Free Anodising Processes for the Structural Bonding of Aluminium Alloys

Researchers: Dr Keith Yendall, Dr Tim Cartwright, Dr Ralf Dahm
Supervisors: Dr Gary Critchlow, Prof Ian Ashcroft

The attainment of durable metal bonds is of particular interest to the defence, aerospace, and automotive sectors. The usage of surface treatments is well established to help offer optimised reliability and the best available prebond processes presently available for aluminium bonding. In particular hexavalent chromium containing processes are employed for aluminum, for example the Bengough-Stuart chromic acid anodise (CAA).

Scope of the Project

Due to safety, cost, health and environmental factors, there is a lot of pressure to use alternatives to CAA and hexavalent chromium. This project has strived to develop innovative chromate-free processes that may be considered as drop-in replacement for the CAA process. The study focus is the processing of industrially-significant aluminum alloys. The objective is to produce microstructures in the alloy surfaces similar to the CAA or through the use of duplex structures to enhance the physicochemical characteristics imparted by this process.

The HTBSAA Process

Initial studies were aimed at varying the comparatively benign boric sulphuric acid anodisation process (BSAA). Several approaches were successful including the development of high temperature BSAA (HTBSAA). Studies done both in Loughborough University and at Bombardier Aerospace proved that this is a suitable drop-in replacement for CAA. The HTBSAA process, however, is time-consuming and expensive to perform comparable to CAA.

The ACDCPSAA Process

The need for more optimised processes led to the patented alternating current-direct current (ACDC) anodisation process using a mixed low concentration phosphoric-sulphuric acid (PSAA) electrolyte.

The ACDCPSAA process offers a route to producing duplex oxides. The outer porous AC layer can successfully form an interphase with adhesives or primers applied subsequently while the inner more compact DC layer provides optimised corrosion protection as shown in Figure 1. Adhesion tests, including those carried out under dry fatigue conditions, have shown that excellent joint strengths and durability are possible using this anodising treatment with values equivalent to CAA. In terms of corrosion resistance, linear polarisation data again showed ACDCPSAA to be at least comparable to CAA. The ACDCPSAA process also shows improved bare metal fatigue performance over CAA. Furthermore, the optimised ACDCPSAA process is comparatively quick, enabling a four-fold increase in productivity when compared with CAA.

(a)

(b)

(c)

Figure 1. (a) cross-section through the oxide on 2024-T3 aluminium post ACDCPSAA treatment; (b) to illustrate the porous outer AC layer, and; (c) to illustrate the compact DC layer.

Research Impact

The processes developed in these studies provide optimised surfaces for structural metal bonding. These processes offer chemically benign replacements for the presently-used, highly toxic and carcinogenic chromic acid anodise which is favored by the European aerospace industry. Particularly, the patented ACDCPSAA process, which offers a number of advantages to the industry is currently being up-scaled for commercial use by Bombardier Aerospace.

Loughborough University

The Department of Materials has roots going back virtually 40 years and throughout this time has been contributing to the advancement and application of knowledge in Materials Science and Engineering by means of teaching, scholarship and research. Our philosophy is based on the engineering application and use of materials which, when processed, are altered in structure and properties. This encompasses design considerations and business implications.

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