Acid Anodising

This process is widely used on aeronautical alloys of the 2000 and 7000 series, in order to improve their corrosion resistance in service. The thin anodic layer (5 pm), which is normally grey on 1050A, can assume more or less dark shades depending on the alloys. It provides a good hold for paint and adhesives. 

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Boeing Process Specification, Phosphoric Acid Anodising of Aluminum for Structural Bonding, BAC 5555, revision A, 1975. 

Chromic acid anodise O Chromic-sulphuric acid etch A Sulphuric acid anodise A Vapour degrease... 

Surface-sensitive techniques for use in the study of adhesive bonding are discussed, including X-ray photoelectron spectroscopy and auger electron spectroscopy/scanning auger microscopy. Data analysis is considered, with reference to quantification, chemical-state information, depth-distribution information and surface-behaviour diagrams. Applications to adhesive bonding are described, particularly failure analysis, hydration of phosphoric acid-anodised aluminium and adsorption of hydration inhibitors. 100 refs. 

This mechanism has been clearly identified in the case of the durability of aluminium-alloy joints. One example [10] is that of aluminium-alloy joints bonded using an aerospace epoxy-film adhesive where the aluminium-alloy substrate was subjected to a phosphoric-acid anodising (PAA) surface treatment, but where the primer (which is normally used in such an adhesive system) was omitted. Under... 

Fig, 6. Logarithmic crack growth rate per cycle, da/dN, versus logarithmic Gmax for aluminium-alloy substrates (phosphoric-acid anodised (PAA) pretreatment) bonded using an epoxy-film adhesive. Cicles are for tests conducted in a dry environment of 23°C and 55% relative humidity and dots for tests conducted in a wet environment of distilled water at 28 C. Diflerent styles of the symbols represent replicate tests [10]. 

Fig. 11. Micrograph from energy-filtered transmission electron microscopy of a cross-section of the interphase region of a phosphoric-acid anodised and primed (PAAP) pretreated joint [10].

ASTM D 3933-80 Practice for Preparation of Aluminium Surfaces for Structural Adhesives Bonding (Phosphoric Acid Anodising). 

BAC 5555 Phosphoric Acid Anodising of Aluminium Alloys for Structural Bonding. 

The Phosphoric Acid Anodisation process utilising phosphoric acid solutions. This was originally developed by the Boeing Company and is the treatment of choice for critical applications in the USA. 

Chromic Acid Anodisation is widely used to improve the corrosion protection of bare aluminium surfaces such as in window frames and other architectural applications. It is also the commonest pre-treatment process used for aerospace bonding in Europe. 

For metallic adherends most of these prelreatments either involve acid etching or an acid etch followed by an acidic anodising process. However, individual alloys and the particular surface structures caused by different heat treatments may respond differently to a given pretreatment for example, aluminium clad aluminium alloys (Alclad ) as opposed to the bare , unclad alloys. 

Electrochemical chromic acid anodising Electrochemical phosphoric acid anodising Electrochemical sulphuric acid anodising Electrochemical hard anodising Electrochemical boric acid/sulphuric acid anodising Chemical sol-gel procedures Activated plasma... 

Anodising in chromic acid is the electrochemical pretreatment procedure favoured in Europe. European aerospace companies view this as giving a less friable oxide film than the phosphoric acid anodising process favoured in the United States and believe that it imparts much better durability to the bonded joint. This latter statement is somewhat debatable in the light of both experience in the laboratory and the actual chemical stmcture of the oxide films themselves. 

Sulphuric acid anodising techniques are more generally used for the production of decorative aluminium sheets to improve the durability of the aluminium (cf. chromic acid anodising when 20 ixm, or greater, oxide films are grown and then sealed in hot water/steam) and for the preparation of surfaces prior to priming... 

This is a variant of the standard sulphuric acid anodising procedure where the bath temperature, anodising time and current density parameters are somewhat modified to give the densest oxide film growth of all. The hardness of the oxide film increases and the pore size is significantly reduced. 

The cleaning, alkaline etch and desmutting stages are carried out as for conventional sulphuric acid anodising. 

As already indicated, it is generally accepted that the oxide film produced either by conventional sulphuric acid anodising or by hard anodising in sulphuric acid, does not give the optimum surface structure for structural adhesive bonding, which is why these techniques seem to be rarely used in the aerospace industry. 

Boric acid-sulphuric acid anodising (BSAA)... 

Research by Critchlow and co-workers [35] has shown that these pretreated substrates can be made suitable for structural adhesive bonding if either the anodising temperature is raised to 35 C or the anodised adherends are dipped in dilute phosphoric acid prior to bonding - i.e. similar to the techniques employed by Arrowsmith et al. [34] when using sulphuric acid anodising. 

It should be noted, however, that the resultant bonds durability performance is not as good as for those produced by chromic acid anodising or the novel sol-gel pretreatment. This is perhaps not surprising when the resultant surface topography is examined (Fig. 20). 

Unlike aluminium, DC sulphuric acid anodising is an acceptable pretreatment prior to bonding titanium components. 

British Standards Institute, prEN 2101, Aerospace Series (1998). Chromic Acid Anodising of Aluminium and Wrought Aluminium Alloys, Draft Issue 2 (Type A). 

In all cases, the aluminium substrates are pretreated using conventional chromic acid anodising techniques, often this is augmented with the application of a pretreatment protection primer. The carbon composites use peel ply techniques to prepare the surfaces for bonding. 

Jaguar. In-board and out-board flaps, trailing edges, tips and slats are constructed from BR-227 primed, chromic acid anodised aluminium bonded with FM-61, a 175°C curing modified epoxy film adhesive. 

Further examples can be taken from the Jaguar, Tornado and the BAE Hawk where unprimed, chromic acid anodised aluminium is bonded with another 175°C curing film adhesive - Redux 308A NA - to produce the sandwich structures needed for the control surfaces. All three aircraft also use EA 934 as a shimming adhesive. 

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