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V,_ N Laminate Construction Composite products designed for corrosion resistance typically utilize a structural laminate and a corrosion barrier. This type of construction is necessary since the overall properties of a composite are derived from the widely differing properties of the constituent materials. Glass fibers contribute strength but have little or no corrosion resistance in many environments. Resins provide corrosion resistance and channel stress into the glass fibers and have little strength when unreinforced. Consequently, a resin -rich corrosion barrier is used to protect a glass -rich structural laminate. In accordance with general industry practice, corrosion barriers are typically 100-125 mils thick. They typically consist of a surfacing veil saturated to a 90% resin content, followed by the equivalent of a minimum of two plies of 1.5 -oz to 2 -oz/ ft chopped strand mat impregnated with about 70% resin. The structural portion of the laminate can be built with chopped strand mat, chopped roving, chopped strand mat alternating with woven roving, or by filament -winding. An additional ply of mat is sometimes used as a bonding layer between a filament -wound structural over -wrap and the corrosion barrier. Filament -wound structures have a glass content of approximately 70% and provide high strength combined with light weight. Because resin provides corrosion resistance, a resin - rich topcoat is often used as an exterior finish coat, particularly where occasional contact or spillage with aggressive chemicals might occur. UV stabilizers or pigments may be incorporated into top coats (to minim- ize weathering effects) or used in tanks designed to contain light sensitive products. A top coat is especial- ly useful forfilament-wound structures due to their high glass content. Surfacing Veil A well -constructed corrosion barrier utilizing surface veil is required for any polymer composite intended for corrosion service. Veils based on C -glass, synthetic polyester fiber and carbon are available. C -glass veils are widely used because they readily conform to complex shapes, are easy to wet out with resin and provide excellent overall corrosion resistance. Synthetic veils are harder to set in place and wet out, but can provide a thicker, more resin -rich corrosion barrier. The bulking effect of synthetic veil allows the outer corrosion barrier to have a very high resin content, which has both benefits and drawbacks. Higher resin concentration can extend resistance to chemical and abrasive attack, but also yields a corrosion barrier that is more prone to cracking in stressed areas. This can be an issue in corrosion barriers where multiple plies of veil are used, and in areas where veil layers overlap. Should the resin -rich veil portion of a corrosion barrier crack, the barrier is breached and all of the benefits of using multiple veils are lost. Furthermore, multiple plies of synthetic veil can be more difficult to apply and often lead to an increase in the number of air voids trapped in the corrosion barrier. Many composite specifications, including ASME RTP -1, impose a maximum allowable amount of air void entrapment in the corrosion barrier. Attempts to repair air voids are time-consuming and can reduce the corrosion resistance of the composite. Fabricators utilizing two plies of synthetic veil should carefully follow the veil manufacturer's instructions and also take special caution to ensure that no excessively resin -rich areas are formed. Where a two-ply corrosion barrier is desired, C -glass veil can be used for one or both plies. This provides a degree of reinforcement to the corrosion barrier, reduces resin drainage, and creates a corrosion barrier that is less prone to interlaminar shear cracking.