Anodising in Phosphoric acid

To obtain a more open oxide film, i.e. one which is more readily penetrated by the adhesive being used, aluminium adherends can be anodised in phosphoric acid. Certainly, the resultant oxide films are more friable than their CAA... 

Figure 14 AC anodising in phosphoric acid ultramicrotomed/TEM micrograph through anodised 6060-T6 alloy substrates [39] (Courtesy Elsevier Limited).

Johnsen et al. found that the durability of the bonded joints, assessed by the Wedge Test [40] (Fig. 16), almost irrespective of electrolyte used, to be nearly as good as obtained using conventionally pretreated adherends, which had been CSA pickled and then DC anodised in phosphoric acid [38]. Bjprgum et al., however, found that an AC PAA pretreatment yielded joints which failed cohesively whereas the AC SAA joints tended towards adhesion failure [39]. 

The formation of defects in porous anodic alumina films fabricated in phosphoric acid solutions has been studied. TEM and SEM examinations revealed that defects appear at the triple point junctions of the cell boundaries, where neighbouring celis meet. The defects represent voids in the anodic alumina and develop into spatial structures that comprise a central void at the triple-point junction extending into tubular branches that pass toward the pore wall. The defects are generated continuously during the anodic film growth under appropriate anodising conditions. Additional post-anodising treatment also results in periodic arrays of circular nanoholes in the pore walls. 

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. 

It is possible to obtain conditions in which the anodic him continues to grow to form a blue or black layer, and this, although not exceptionally protective, has uses in the treatment of baking pans. A typical anodising solution contains 100g/1 Na2HP04 IZHjO and 20 ml/1 phosphoric acid, and is used at 350 A/m at 60-90°C for about 10 min. 

Among the newer uses of phosphoric acid are as a metal cleaner in printed circuit board manufacture, and as a metal etchant in the production of semiconductor materials. A 1% solution of hexafluorophosphoric acid, HPFg, can be used to clean aluminium, prior to anodising [23]. 

Brewis. Abrasion and solvent wiping have already been mentioned, but other techniques involve treating plastics in a gas flame, corona discharge for the treatment of both plastics and metals, and the use of acid-etch baths (chromic or phosphoric acid) for metals and plastics or anodising in acid baths for metals. 

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]. 

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. 

In this procedure the first step, once again, is to pickle the aluminium substrates in accordance with the specifications quoted above. Under certain circumstances it may be sufficient to use only an alkaline degrease before anodising. The etched panels are then clamped to the anode of a standard anodising bath and are immersed in a solution of phosphoric acid at 25°C. Anodising then takes place in accordance with one or other of several European and American specifications [29,30]. A typical bath composition is ... 

However, Arrowsmith et al. [34] formd that the situation could be improved by dipping the clean, anodised substrate in a solution of phosphoric acid to dissolve away some of the anodic oxide layer in order to reveal a more open structure more amenable to adhesive bonding. This can be seen in Fig. 12. 

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. 

The panels were washed in solvent to remove surface dirt and then immersed in 10% phosphoric acid at 50°C and AC anodised for 30 s using a current density of 4 A/dm. Another approach was to wash the panels in solvent to remove surface... 

In both cases the anodised substrates were washed in water and dried at 60°C. AC anodising of aluminium produces significantly thinner oxide films than by the more conventional DC processes. Johnsen et al. estimated the thicknesses as 0.2 p.m for sulphuric acid and 0.1 p.m for phosphoric acid. 

Velterop L. (2003), Phosphoric sulphuric acid anodising An alternative for chromic acid anodising in aerospace applications , ri7BA7era//Mrgie, 43, 284-9. 

Aluminium is usually anodised electrolytically, in the presence of an acid, either sulphuric, chromic or phosphoric, to give a tough resistant oxide film, which generally forms good bonds with the usual bonding systems. The anodising must be carried out with care and the type of crystalline structure being formed on the aluminium surface must be considered. A uniform reticulated structure is desired, not a... 

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