Paint resistance values

To avoid galvanic problems, different materials of con-stmction may have to be electrically isolated or at least the electrical resistance between them increased to a level sufficient to reduce the corrosion current to an acceptable value. In some instances it is more practical to paint or otherwise coat the more cathodic of the two parts of the couple. The anodic material should not be coated, since even more rapid penetration would occur at any breaks in the coating. 

The corrosion resistance of zinc is discussed in Section 4.7, and it is only necessary here to say that zinc is protected against further attack by a film of corrosion products. It is remarkably resistant to atmospheric corrosion except perhaps in the most heavily contaminated industrial areas, and even there its use as a protective coating is still a sound practical and economic proposition. The value of zinc coatings as a basis for painting under very aggressive conditions has been clearly demonstrated. 

It has been shown that paint films are so permeable to water and oxygen that they cannot affect the cathodic reaction, and that the anodic reaction may be modified by certain pigments. There are, however, many types of protective paint which do not contain inhibitive pigments. It is concluded that this class of paint prevents corrosion by virtue of its high ionic resistance, which impedes the movement of ions and thereby reduces the corrosion current to a very small value. 

When samples of about 1 cm were taken from a single cast film of 100 X 200 mm of a number of paint and varnish films, their resistances varied with the concentration of potassium chloride solution in one of two ways (Fig. 14.2). Either the resistance increased with increasing concentration of the electrolyte (inverse or / conduction) or the resistance of the film followed that of the solution in which it was immersed (direct or D conduction). The percentage of / and D samples taken from different castings varied, but average values for a number of castings were 50% D for the pentaerythritol alkyd and the tung oil phenol formaldehyde varnishes, 57% for urethane alkyd, 76% for epoxypolyamide and 78% for polyurethane varnishes... 

The situation has now been clarified , and it has been shown that, with a.c., the values obtained are controlled by the capacitance until the frequency has fallen to about 1 Hz. It was shown that under these circumstances, in the absence of corrosion, the resistances of paint films measured by d.c. or a.c. were the same furthermore, no polarisation resistance was detected. 

Impact tests Such tests reveal the resistance of coatings to deformation and destruction by concentrated sudden stresses. They thus throw considerable light on the integrity of the metal-coating bond. Changes in adhesion through chemical reaction at the paint/metal interface will be reflected in the impact-test values. 

Although hardness is a somewhat nebulous term, it can be defined in terms of the tensile modulus of elasticity. From a more practical side, it is usually characterized by a combination of three measurable parameters (1) scratch resistance (2) abrasion or mar resistance and (3) indentation under load. To measure scratch resistance or hardness, an approach is where a specimen is moved laterally under a loaded diamond point. The hardness value is expressed as the load divided by the width of the scratch. In other tests, especially in the paint industry, the surface is scratched with lead pencils of different hardnesses. The hardness of the surface is defined by the pencil hardness that first causes a visible scratch. Other tests include a sand-blast spray evaluation. 

Tests conducted on specimens i osphated for 5 minutes but provided with additional paint thickness using Tremclad or Marlnox SR-2 are summarized in Table II. Specimens ( 18) rinsed with phospho-chromic solution show higher values of resistance and lower values of capacitance with respect to non-rlnsed specimens ( 20) both having 80 ym of paint thickness. 

Determination of R does not require that the current and potential signal be correlated. However, if measurements are made at different times, they should be made with similar working electrodes and similar exposure conditions. R values have been used to assess the degradation of organically coated metals. In these studies, R data appeared to correlate with the film resistance (impedance magnitude at low frequency) values determined from EIS (145), or with visual inspection of painted samples subject to various cabinet exposures (146). 

Since the alcohol is regenerated and the net result is the same as though the original silicon halide had been hydrolyzed directly, it is obvious that the esters may be regarded as hydrolytic intermediates which allow the over-all process of hydrolysis to be controlled and directed. The commercial value of the esters therefore lies in their ability to deposit silica in a desired form and at a well-defined rate. Besides their use as stone preservatives, esters like ethyl silicate are used as sources of pure ion-free silica and as additives to organic paints and lacquers to provide a harder, more resistant film. A disperse sol of silica in n-propanol (possibly derived from propyl silicate) is available commercially for related applications. 

In the case of the tray of water for example, this last factor might be visualised as affected by impurities at the water surface. This surface might even be entirely covered up by paraffin, analogously to the behaviour of a painted particle board. The surface has, so to say, a certain resistance for mass transfer. The reciprocal value of this resistance is called "mass transfer coefficient". 

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