Water diffusion through paint films

Water and oxygen diffuse through paint film at a relatively high rate. The protective ability of a paint-phosphate film is influenced by the properties of the phosphate film because the film is in contact with the diffused water and oxygen. Film properties of concern are solubility, uniformity, and porosity. 

The diffusion of water through paint films has been measured by various workers. The weight of water which could diffuse through three clear vehicles and eight paint films, each 0-1 mm thick, at 85-100% r.h. has been calculated on the assumption that the water would be consumed as soon as it reached the metal surface, i.e. that the rate-controlling step was the rate of diffusion of water through the film, and is shown in Table 14.3 ... 

Table 14.3 Diffusion of water through paint films of thickness 0-1 mm...

Kittelberger and Elm measured the rate of diffusion of sodium chloride through a number of paint films. Calculations based on their results showed clearly that the rate of diffusion of ions was very much smaller than the rate of diffusion of either water or oxygen. Furthermore, they found that there was a linear relationship between the rate of diffusion and the reciprocal of the resistance of the film. This relationship suggests that the sodium chloride diffused through the membrane as ions and not as ion pairs, since the diffusion through the film of un-ionised material would not affect the resistance, because if a current is to flow, either ions of similar charge... 

It is worthwhile drawing attention to health hazards associated with film infected water systems which also cause corrosion. Two of the most common are Legionnaires disease and so called humidifier fever . Because of strong adhesion of biofilms and diffusion rates through the film treatment based on cleaners and chemical sterilisers such as chlorine often fail similar considerations apply to other systems in industry, e.g. food, paint, oil and gas are examples where biofilm activities have given massive problems. 

Certain inorganic compounds, usually as the zinc salt, are used in paints as passivators to control rust formation. These inhibitors are slightly soluble in water, and are slowly dissolved from the paint film through moisture permeabihty. The ions formed are carried to the film-metal interface by moisture diffusion. There they cause the corrosion electrical potential to be elevated to the passive potential at which the corrosion rate is dramtically reduced by polarization at the anode. 

The most numerous researches have been carried out upon water-membrane systems, for which a summarising paper by Carson (1) has outlined the main methods of measurement. To prevent lateral diffusion of water through the edges of the specimen, or between it and its supports, various devices have been adopted. The edges have been moulded in wax, or compressed with a mixture of beeswax and resin. Shellac, petrolatum, and rubber have been used for the same purpose or the cell may be mercury sealed. Films of lacquers and paints may be made by painting the lacquer on suitable surfaces, such as amalgamated tin-plate (2), and then peeling off the dried membrane. 

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