Anodised Surfaces

Cleaning should not attack the anodic layer that has dissolved in acidic and alkaline media, for example, the natural oxide layer. Normalised procedures can assess products for cleaning anodised surfaces [4, 5]. 

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Dos Santos et al. (2014) deposited titanium oxide coatings by PEO in a Ca-P-based electrolyte. The crystalline part of the sample consisted predominately of anatase with minor amounts of rutile, whereby Ca2+ and P043- ions were incorporated into the oxide phases. Cross-sectional images of the coating-substrate interface revealed the presence of voids elongated along the interface. Osteoblast cell cultured at the PEO coating verified the cytocompatibility of the anodised surface. The results of the cytotoxicity tests show satisfactory cell viability of the titanium dioxide films produced in this study. 

Anodising of Zn, Al, Sn and Mg may be carried out with various solutions, a common component of which is trisodium phosphate, Na3P04 [24]. Among the methods available for the sealing of anodised surfaces is one employing chromium acid phosphate, Cr(H2P04)3 [25]. 

Like natural oxides, anodic coatings have a poor resistance to acidic or aUcaline media (Figure B.1.18). Consequently, specific cleaning agents must be used for the maintenance of anodised surfaces. It should also be remembered that anodic coatings do not eliminate the risk of galvanic corrosion. 

Since the late 1960s, it has generally been admitted that anodised surfaces exposed to weathering must be cleaned in order to preserve their appearance and to avoid pitting corrosion of surfaces that are insufficiently cleaned by rain. 

Like uncoated metal, an anodised surface will always corrode more on the reverse side and on faces that are poorly cleaned by rain. Dust and dirt enhance corrosion. For aU these reasons, it is not possible to rely on the effect of rain or cleaning by plain water, although this was considered sufficient in the past. 

Regular cleaning with appropriate neutral detergents that do not alter the anodic coating is necessary for the maintenance of anodised surfaces exposed to weathering and dust. 

The absence of cleaning over several years can lead to severe damage of anodised surfaces that is difficult to recover. 

An innovative technology has been developed by UK anodisers (HMF Ltd. and ASE Ltd.) that allow for the production of anodised surfaces that prevent micro-crack formation and can make possible the production of aluminium tools that have improved wear resistance with cyclic heat resistance up to 500°C and, in addition, very high thermal emissivity. This anodising treatment is referred to as the ALAMO anodising process. 

The second category was concerned with adhesion to porous or microfibrous surfaces on metals. Aluminium may be anodised to form an oxide surface comprising pores of diameter of tens of nanometers. Electroforming and chemical oxidation can be used to produce microfibrous or needle-like coatings on metals, including copper, steel and titanium. The substrate topography was demonstrated to play an vital part in adhesion to these surfaces [45-48]. 

Similar considerations also apply to the dielectric films formed on the metal surface during anodising, and, for example, in the case of the valve metals (Al, Ti, Ta, Nb, etc.) IR drops of hundreds of volts may be produced by the anodic oxide film formed on the metal surfaces. Paint films applied to a metal surface also exert resistance control see Section 14.3). 

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. 

Drawing Tantalum has a tendency to gall and is normally anodised to provide a surface which will carry a drawing lubricant. Seamless tube is produced by cupping followed by drawing or by hollow shells. 

The attainment of a clean surface prior to the application of any subsequent treatment or coating is essential, whether this subsequent operation is electroplating, anodising, chemical treatment or organic coating. 

As the surface smoothing and levelling effects are somewhat limited, the use of acid cleaners prior to anodising or electropainting, where surface defects can be enhanced, is not common. 

The more or less regular pattern of pores imposes a cellular structure on the film, with the cells approximating in plan to hexagons, each with a central pore, while the bases which form the barrier-layer, are rounded. The metal surface underlying the film, therefore, consists of a close-packed regular array of nearly hemispherical depressions which increase in size with the anodising voltage. The thickness of the individual cell walls is approximately equal to that of the barrier-layer... 

Emissivity Table 15.5 shows the total heat emissivity of various aluminium surfaces, as a percentage of that of a black body. The figures have been recalculated from the data of Hase. The emissivity of anodised aluminium rises rapidly with film thickness up to 3 fim after which the rate of increase diminishes. 

Reflectivity The total and specular reflectivities of an anodised aluminium surface are controlled by both the condition of the metal surface, polished... 

The hardness and abrasion resistance of anodic coatings have never been easy properties to measure, but the development of a British Standard on hard anodising has made this essential. Film hardness is best measured by making microhardness indents on a cross-section of a film , but a minimum film thickness of 25 tm is required. For abrasion resistance measurements, a test based on a loaded abrasive wheel , which moves backwards and forwards over the film surface, has improved the sensitivity of such measurements. 

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. 

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