The Corrosion Resistance of Thermoset Composites in Alkaline Environments

ABSTRACT Corrosion engineers need guidelines for selecting thermoset resins for aggressive applications such as hot alkali and alkaline peroxide. The suitability of fiberglass-reinforced plastic (FRP) for alkaline service depends on factors such as the ester content of the resin, the unsaturated monomer composition, and the cure system. The purpose of the present paper is to show the effect of these factors on the alkaline corrosion resistance of FRP and provide corrosion engineers with the guidance needed for selecting the best epoxy vinyl ester resins for alkaline environments. INTRODUCTION Sodium hydroxide is the seventh largest volume chemical produced in the United States. It is used in numerous industrial applications due to its chemical reactivity with inorganic and organic materials. Fiberglass-reinforced plastic (FRP) equipment can be a cost effective way to handle hot sodium hydroxide. However, care must be taken when choosing FRP for sodium hydroxide service, because sodium hydroxide can hydrolyze ester linkages in ester containing resins. Epoxy vinyl ester resins have good chemical resistance to caustic because there are no more than two ester groups per molecule. However, the percentage of ester groups for a given molecule can vary depending on the molecular weight. With the introduction of a variety of new epoxy vinyl ester resins, chemical resistance testing was needed to determine their suitability in hot sodium hydroxide. Brown Stock process streams from Kraft pulping have a high pH of around 12,and are usually hot. In the past Brown Stock system equipment, vat, tanks, and transfer lines, was made of carbon and/or stainless steel. However, since pulp mills are continuing to recycle more process streams of various contents as make-up and wash water, many times carbon steel does not provide the corrosion resistance required. Even stainless steel is precluded when recycled process streams contain high enough levels of chloride salts. In anticipation of an environmentally driven future demand for totally chlorine free (TCF) pulp, the pulp and paper industry is investigating alkaline hydrogen peroxide pulp bleaching. Because it would be economically desirable to use existing equipment designed for chlorine dioxide bleaching, it is important to determine the corrosive effects on the materials of construction normally used in chlorine dioxide bleaching processes. One study showed that titanium is inadequate for extended use in an alkaline hydrogen peroxide bleaching environment 1. ] Fiber-reinforced plastic composites made from epoxy vinyl ester resins, and in particular brominated epoxy novolac vinyl ester resins have shown excellent resistance to chlorine dioxide bleaching environments 3?4.The present study was initiated to determine the effect of alkaline hydrogen peroxide on these same fiber-reinforced plastic composites. If the results were positive, this would provide an economical material of construction that can be used in both chlorine dioxide and alkaline hydrogen peroxide bleaching environments.