Coatings oxide films

Sheet aluminium can be given a colour by a similar process. The aluminium is first made the anode in a bath of chromic acid (p. 377) when, instead of oxygen being evolved, the aluminium becomes coated with a very adherent film of aluminium oxide which is very adsorbent. If a dye is added to the bath the oxide film is coloured, this colour being incorporated in a film which also makes the remaining aluminium resistant to corrosion. This process is called anodising aluminium. 

Triphenyl phosphate [115-86-6] C gH O P, is a colorless soHd, mp 48—49°C, usually produced in the form of flakes or shipped in heated vessels as a hquid. An early appHcation was as a flame retardant for cellulose acetate safety film. It is also used in cellulose nitrate, various coatings, triacetate film and sheet, and rigid urethane foam. It has been used as a flame-retardant additive for engineering thermoplastics such as polyphenylene oxide—high impact polystyrene and ABS—polycarbonate blends. 

Transparent electroconductive coatings of stannic oxide are deposited on nonconductive substrates for electrical and strengthening appHcations. However, the agents used to deposit the oxide film are actually stannic chloride. More recently, some organotin compounds have been employed. 

The lifetime of a conventional exhaust system on an average family car is only 2 years or so. This is hardly surprising - mild steel is the usual material and, as we have shown, it is not noted for its corrosion resistance. The interior of the system is not painted and begins to corrode immediately in the damp exhaust gases from the engine. The single coat of cheap cosmetic paint soon falls off the outside and rusting starts there, too, aided by the chloride ions from road salt, which help break down the iron oxide film. 

Biocorrosion of stainless steel is caused by exopolymer-producing bacteria. It can be shown that Fe is accumulated in the biofilm [2.62]. The effect of bacteria on the corrosion behavior of the Mo metal surface has also been investigated by XPS [2.63]. These last two investigations indicate a new field of research in which XPS can be employed successfully. XPS has also been used to study the corrosion of glasses [2.64], of polymer coatings on steel [2.65], of tooth-filling materials [2.66], and to investigate the role of surface hydroxyls of oxide films on metal [2.67] or other passive films. 

Discontinuities in conducting oxide film or scale or discontinuities in applied metallic or non-metallic coatings. Exposed substrate (provided that this is more electrochemically active than the coating). 

The values in Table 2.16 show how the potentials obtained under service conditions differ from the standard electrode potentials which are frequently calculated from thermodynamic data. Thus aluminium, which is normally coated with an oxide film, has a more noble value than the equilibrium potential 3 + / = — 1-66V vs. S.H.E. and similar considerations apply to passive stainless steel (see Chapter 21). 

Platinised Ti, Ta or Nb < 1000 Am/m (consumption) Discontinuities in Pt coat protected by oxide film on subtrate sensitive (< lOOHz) a.c. ripple in d.c. or negative current spikes causing electrode consumption maximum operating potential with Ti substrate 9 V... 

It is a valve metal and when made anodic in a chloride-containing solution it forms an anodic oxide film of TiOj (rutile form), that thickens with an increase in voltage up to 8-12 V, when localised film breakdown occurs with subsequent pitting. The TiOj film has a high electrical resistivity, and this coupled with the fact that breakdown can occur at the e.m.f. s produced by the transformer rectifiers used in cathodic protection makes it unsuitable for use as an anode material. Nevertheless, it forms a most valuable substrate for platinum, which may be applied to titanium in the form of a thin coating. The composite anode is characterised by the fact that the titanium exposed at discontinuities is protected by the anodically formed dielectric Ti02 film. Platinised titanium therefore provides an economical method of utilising the inertness and electronic conductivity of platinum on a relatively inexpensive, yet inert substrate. 

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. 

Electroplating passive alloys Another application of strike baths reverses the case illustrated in the previous example. The strike is used to promote a small amount of cathode corrosion. When the passivation potential of a substrate lies below the cathode potential of a plating bath, deposition occurs onto the passive oxide film, and the coating is non-adherent. Stainless steel plated with nickel in normal baths retains its passive film and the coating is easily peeled off. A special strike bath is used with a low concentration of nickel and a high current density, so that diffusion polarisation (transport overpotential) depresses the potential into the active region. The bath has a much lower pH than normal. The low pH raises the substrate passivation potential E pa, which theoretically follows a relation... 

The deposit resists atmospheric tarnish even in the presence of high pollution by sulphur dioxide (in contrast to nickel) and hydrogen sulphide, and coatings exposed to the outdoor atmosphere remain bright indefinitely, sometimes taking on a slightly more pink colour as the oxide film thickens. 

Metals in practice are usually coated with an oxide film that affects the potential, and metals such as Sb, Bi, As, W and Te behave as reversible A//A/,Oy/OH electrodes whose potentials are pH dependent electrodes of this type may be used to determine the solution s pH in the same way as the reversible hydrogen electrode. According to Ives and Janz these electrodes may be regarded as a particular case of electrodes of the second kind, since the oxygen in the metal oxide participates in the self-ionisation of water. 

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