Metallic Barrier Pigments

Besides reducing the permeability of water vapor, oxygen, and other corrosive media, alnminnm pigment also reflects UV radiation and can withstand elevated temperatures. 

Nickel flake-filled coatings can be useful for strongly alkaline environments. Cupronickel flakes (Cu - 10% Ni - 2% Sn) are used in ship protection because of their outstanding antifouling properties. The alloy pigment is of interest in this application because its resistance to leaching is better than that of copper itself [109]. 

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Some of the metallic inorganic pigments are used for their non-optical effects. They can provide a complete optical barrier against visible, infrared (IR) and UV radiation. 

As a result, barrier pigments can be incorporated into primer, intermediate coat, or top-coat since they are chemically inert and do not react with the metal, unlike inhibitive or sacrificial... 

Inhibitive Pigment a pigment which retards or prevents the corrosion of metals by chemical and/or electrochemical means, as opposed to a purely barrier action. Red lead and zinc chromate are examples of inhibitive pigments as opposed to red iron oxide which has little or no inhibitive action. 

From a practical point of view, the attainment of the specified dry film thickness is important. In many cases, a minimum thickness must be achieved or protection will fail in a short period. This aspect demands skill in application and close inspection and control. For metallic zinc paints, the thickness would normally be of the order of 60-100/im, for anticorrosive pigmented paints, together with top coats, 100-200 //m, and for inert barrier types 150-300/im. For special environments or projects, these thicknesses may be varied but it should not be presumed that protection will always be improved or be more effective over a longer period of time by increasing the film thickness. 

Leafing pigments float on the surface of the wet film, orienting themselves parallel to the surface and thereby forming a dense metallic layer on top of the coating. This layer has a high reflectivity and provides excellent barrier protection of the film (resin). 

While barrier-effect pigments solely increase the diffusion path of corrosive species and the coating practical adhesion of the coating, active pigments such as chromates additionally inhibit electrochemical reactions at the metal-polymer interface as well in defects of organic coatings. 

Heavy-duty coating systems generally comprise two primer coats and two topcoats, modern system sometimes consist of one primer coat and two topcoats. The total dry film thickness (DFT) of such anticorrosive paint systems is 150 - 200 pm, each layer has a minimum DFT of 40-50 pm. Due to their excellent adhesion, the first and second primers prevent corrosion of the metal surface. The pigments and extenders allow the primers to react with ions (CD and SO4") that diffuse into the film from the atmosphere. The pigmented organic film also forms a barrier against humidity that may otherwise initiate a corrosive process. 

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