The process of combining iron and oxygen called

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The process of combining iron and oxygen, called oxidation, is accompanied by the production of a measura- ble quantity of electrical current, which is why this is called an electrochemical reaction. For the reaction to pro- ceed, an anode, a cathode and an electrolyte must be present. This is termed a corrosion cell. In a corrosion cell, the anode is the negative electrode where corrosion occurs (oxidation), the cathode is the positive electrode end, and the electrolyte is the medium through which an electrical current flows. Coatings in Corrosion Control A coating may be defined as a material which is applied to a surface as a fluid and which forms, by chemical and/or physical processes, a solid continuous film bonded to the surface. Eliminating any of the reactants in the process can interrupt corrosion. If a barrier is put on to the iron that pre- vents oxygen and/or water from coming in contact with steel, the corrosion process can be prevented. Steel is not the only surface protected by such barriers. Other alloys and metals such as stainless steel, brass, aluminum and other materials such as concrete, wood, paper, and plastic are also protected from the environment with coatings. Protective coatings that serve as barriers are the principal means of protecting structures. COMPOSITION OF COATINGS Most coatings are made up of four principal parts: pigments; non-volatile vehicles (resins or binders); volatile vehicles (organic solvents, water or the combination of both); and additives (specialty chemicals which make the coating function). All of the components of a coating interact to accomplish the purpose for which the coating was designed. Pigments Pigments are included in coatings to perform any of the following functions: ! Add color ! Adjust the flow properties of wet coatings ! Resist light, heat, moisture, chemicals ! Inhibit corrosion ! Reflect light for opacity or hiding ! Contribute mechanical strength Pigments whose prime function is to contribute opacity to coatings are called hiding or prime pigments. The prin- ciple white-hiding pigment is titanium dioxide. There are hundreds of colored-hiding pigments which, when used alone or combination with other pigments, give coatings their variety of colors. Hiding pigments can be very
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PAGE 27 SYSTEMS expensive. In order to make the paint less costly, non-hiding or extender pigments are used. Certain colors, such as light-stable reds, are more expensive. Determine costs from your coating supplier prior to writing the project specification. Pigments are used to adjust the viscosity and flow properties of the paint in order to obtain paint that won't sag at high film builds. Using pigments with low oil absorption can decrease the amount of solvents in the paints. Pigments used to reduce or prevent corrosion of a coated surface are called inhibitive pigments.
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  • Structural steel

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