Sulphuric anodising

Chromic and sulphuric anodising, as well as their variants, have been widely used. These surface treatments are specific to aluminium. They consist of growing oxide layers whose structure and properties differ from those of natural oxide layers on aluminium. Their thickness ranges from a few micrometers up to 100 p.m, i.e. 1000-10 000 times the thickness of the natural oxide layer, which is of the order of 5-10 nm [18]. 

Because of its good performance in mineral acids, there is little need or incentive to invoke anodic passivation techniques for zirconium. The metal can be anodised in sulphuric acid, but, again in contrast to the behaviour of titanium, it does not form a stable anodic film in chloride solutions, and even in neutral sodium chloride, zirconium rapidly corrodes if an anodic potential of 2 V is applied. 

The anodising procedures in general use are shown in Table 15.1, sulphuric acid being the most commonly used electrolyte. Treatment time is IS min to 1 h. 

Decorative self-coloured films can also be produced in sulphuric acid under conditions intermediate between normal and hard anodising. 

Fig. 15.2, taken from a paper by Lenz , shows the variation in density with thickness for steam-sealed anodic films produced in sulphuric acid on aluminium of 99-99 7o and 99-5% purity. A mean figure of 2-7 g/cm for sealed, and 2-5 g/cm for unsealed films is accepted by the British Standard for anodised aluminium... 

The protective action of sulphuric films is mainly controlled by the anodising conditions, compact films formed at temperatures below 20° C in 7<7o v/v sulphuric acid being more resistant than the films formed at higher temperatures in more concentrated acid. The wider pores of the latter result in less... 

Although there have been few changes in the basic anodising practices, and sulphuric acid is the electrolyte used in most plants, there have been many developments in the pretreatment, colouring and sealing processes associated with anodising. 

Architectural Anodising Sulphuric Acid Anodic Film Quality, British Anodising Association (1981)... 

Those experiments that did involve anodisation at more reasonable potentials, i.e. below oxygen evolution, suffered from an inability to characterise the initial PtOH species formed at coverages below a monolayer. Dickinson et ai (1975) systematically investigated the surface composition of a large number of Pt electrodes, polarised at various potentials in sulphuric acid, using XPS via the emersion approach. Figure 3.24 shows XPS spectra obtained from a Pt electrode after polarisation in sulphuric acid at 1.00 V and 1.5 V vs. SCE. 

We have found out the volume expansion coefficient could be reached 3.1 that is larger than previously reported maximum values of 1.6-1.7 [6]. The high volume expansion coefficient is caused by enhancing of the sulphuric content inside porous alumina formed at high field anodising. The high volume expansion also results in low porosity of alumina, which was about 3%. This is smaller than known minimal porosity of 5% for a sulphuric electrolyte [1]. 

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

The most-used commercial electrolytes for anodising are sulphuric or chromic acids, but the process can be carried out with phosphoric acid or with mixtures of these acids. A typical phosphoric acid solution for anodising aluminium may contain glycerine, acetic or hydroxyacetic acid ... 

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... 

The layer of aluminium oxide can be made thicker by electrolysis, to give even more protection. This process is called anodising. The aluminium is used as the anode of a cell in which dilute sulphuric acid is electrolysed. Oxygen forms at the anode and reacts with the aluminium, so the layer of oxide grows. 

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... 

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