Anodic films

Highly protective layers can also fonn in gaseous environments at ambient temperatures by a redox reaction similar to that in an aqueous electrolyte, i.e. by oxygen reduction combined with metal oxidation. The thickness of spontaneously fonned oxide films is typically in the range of 1-3 nm, i.e., of similar thickness to electrochemical passive films. Substantially thicker anodic films can be fonned on so-called valve metals (Ti, Ta, Zr,. ..), which allow the application of anodizing potentials (high electric fields) without dielectric breakdown. 

Operation and Control. The amount of current appHed to the part determines the speed of which the anodic film is formed. Generally, a current density of 12.9—25.8 m A/cm (12—24 A/ft ) is appHed to produce a coating thickness of 10—20 p.m (0.4—0.8 mils) per 20 minutes. Most coatings range in thickness from 5—50 p.m (0.2—2.0 mils). 

The temperature of the anodizing solution also has an effect on the anodic film stmcture. Thus chillers are used to maintain a consistent temperature in the bath while the part is being anodized, and air agitation is continuously appHed to ensure a uniform temperature. The result is a consistent anodic film density of uniform pore size. 

In the anodizing stage electrolytic reactions produce a uniform aluminum oxide layer across the aluminum surface. This anodic film is transparent and porous. The underlying matte or bright surface can be seen. After anodizing, the aluminum part can be colored or sealed. 

The use of anodic films on aluminum alloys is only applicable to some mildly corrosive environments, including architectural purposes and where abrasion resistance is required. 

Zahavi, J. and Metzger, M., Electron Microscope Study of Breakdown and Repair of Anodic Films on Aluminium , J. Electrochem. Soc., 119, 1479 (1972)... 

The dissolution of passive films is, in the main, controlled by a chemical activation step in contrast to film-free conditions at. Many protective anodic films are oxides and hydroxides whose dissolution depends upon the hydrogen ion concentration, and the rate follows a Freundlich adsorption equation ... 

King, J. F., Adamson, K. G. and Unsworth, W., Impregnation of Anodic Films for the Protection of Magnesium Alloys, Ministry of Defence D.Mat. Report No. 193, February (1973) Adamson, K. G., King, J. F. and Unsworth, W., Evaluation of High Temperature Resistant Coatings for the Protection of Magnesium Alloys, Ministry of Detence D.Mat. Report No. 196, July (1973)... 

Anodic passivation also allows titanium to be employed as a Jig for aluminium anodising baths ", because the protective anodic film formed on titanium allows passage of electronic current to the metal contact while virtually suppressing flow of ionic current through the anodically-formed surface film. This aspect is discussed in more detail in relation to special applications. 

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. 

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

In general, there are two types of surface contamination (1) organic contamination—such as oils, greases, paint coatings etc. and (2) inorganic contamination —such as rust, oxide films, corrosion products, scale, anodic films etc. Although these two types of contaminant can be removed simultaneously, it is simpler to consider the cases separately. 

Hard anodic films, 50-100/rm thick, for resistance to abrasion and wear under conditions of slow-speed sliding, can be produced in sulphuric acid electrolytes at high current density and low temperature. Current densities range from 250 to 1 000 Am , with or without superposed alternating current in 20-100g/1 sulphuric acid at —4—I- 10°C. Under these conditions, special attention must be paid to the contact points to the article under treatment, in order to avoid local overheating. 

The structure of the anodic film, according to present views, is shown diagrammatically in Fig. 15.1. 

Density Owing to the variable degree of porosity of the anodic film, it is only possible to determine the apparent density, which varies with the anodising conditions and also with the film thickness. 

Fig. IS.2 Apparent density of anodic film as a function of film thickness (courtesy Aluminium, Berl., 32, 126 (1938))...

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... 

Hardness It is not possible to obtain a reliable figure for the hardness of anodic coatings with either the indentation or scratch methods, because of the influence of the relatively soft metal beneath the anodic film, and the presence of a soft outer layer on thick films. On Moh s Scale, the hardness of normal anodic films lies between 7 and 8, i.e. between quartz and topaz. 

Flexibility The normal anodic film begins to crack if subjected to an extension exceeding about 0 - 5%. Thinner films up to 5 nm in thickness appear to withstand a greater degree of deformation without obvious failure, and are often used for dyed coatings on continuously anodised strip from which... 

Refractive index The refractive index of the clear anodic film produced on aluminium of the highest purity in sulphuric acid is 1-59 in the as-formed condition, rising to 1-62 after sealing . 

Table 15.6 Effect of metal purity and anodic film thickness on reflectivity...

Friction The coefficient of friction of the sealed anodic film is 0 76, falling to 0-19 after impregnation with silicone oil . These results were obtained with anodised wire. 

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