October_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 7 3 1 reaction at the interface. The extent of corrosion damage depends on the type of outdoor exposure with various types of corrosive environments (Fig. 1) lead- ing to different corrosion mechanisms. The rate, extent, damage, and leading mechanism of corrosion also depend on the substrate. Thus, the dominating factors influencing the development of anticorrosion systems include the sub- strate material and environment. Coatings are applied to steel struc- tures, such as buildings and bridges in coastal areas, to mitigate corrosion. These corrosion protection coatings usually consists of multilayered coat- ings with each layer having different properties and purposes. Individual coats can be metallic or nonmetallic. In a highly corrosive marine atmosphere, the coating consists of a primer, one or more intermediate coats, and a top- coat. Factors to consider in the design and selection of protective coatings for steel structures include the chemical, mechanical, and physical properties of the substrate as well as the coating system. Figure 2 shows the complexity of the variables involved in the design, material selection, and performance of coatings. Several techniques are used to protect steel bridges and structures from corrosion damage. Three pro- tective mechanisms of modern an- ticorrosion coatings include barrier protection, inhibitive protection, and galvanic protection, which are the ba- sis for development of metallic, organ- ic, and inorganic coatings. In barrier protection coatings, the extremely low permeability of the coating impedes diffusion of corrosive media into the substrate surface. Barrier coatings usu- ally contain titanium dioxide, glass flakes, and lamellar aluminum, and are mainly used in immersive marine envi- ronments [2] . The protection potential of barrier coatings is highly dependent on coating thickness; typical thickness is between 150 and 250 µm [3] . Chemical passivation of the substrate is imparted by adding inhibitive pigments that re- act with the substrate to protect against corrosive media. Inorganic salts such as phosphates, chromates, nitrates, and silicates are most commonly used in inhibitive coatings [2] . The galvanic ap- proach is one of the most widely used techniques for corrosion protection. In this case, protection is obtained by sac- rificing an electrochemically more ac- tive metal by installing it in electrical contact with the substrate to complete the electrochemical process. Use of me- tallic zinc powder in zinc-rich primers is an example of sacrificial protection [2] . Several coating systems are cur- rently used to provide corrosion protec- Fig. 2 — Factors influencing design, development, and application of a coating system. TABLE 1 — RELATIVE EFFICACY OF SELECT COATING-SYSTEM PROPERTIES AND CHARACTERISTICS Coating system Drying time Dry-film thickness Hardness Abrasion resistance Corrosion resistance Three-coat High Moderate High Moderate High EM (epoxy mastic) Moderate Low High High High WBC (waterborne coatings) High Moderate Low Low Low CSA (calcium sulfonate alkyd) High Moderate Low Low Excellent GFP (glass flake-reinforced polyester) Moderate High High High Moderate SLX (siloxane) Moderate Low High High Low Polyurea Low High High Excellent Excellent