A Corrosive Containment Revolution
Fiberglass composites have been offering corrosion resistance in water and wastewater treatment for more than 40 years
Fiberglass composites have become the material of choice in many water or wastewater treatment operations. For mild to severe corrosion service, they can often be the materials of choice for economy and durability.
True, there have been problems, but they are usually avoidable by using proper installation, with good design and quality assurance practices. The experience of more than 40 years has demonstrated that many fiberglass tanks, piping, ducts or other items can survive longer than higher cost metal or coated metal equivalents.
Types of Composites and History
Primarily, composite materials consist of fibrous reinforcements with an organic resin matrix. In the corrosion-resistant materials field, the fibers are glass and the resins are any of several thermosetting polymers. Other fibers (such as carbon or aramid) usually are not the most economical choice for corrosion service.
Glass fibers began commercial production in the mid-1930s with a joint venture of Owens-Illinois and Corning Glass creating Owens-Corning Fiberglass. The marriage of fiberglass reinforcements and thermoset resins began during WWII, primarily with the need to shelter radar equipment from the environment, since both fibers and resins are transparent to the electromagnetic signals. The small boat industry began soon thereafter and blossomed rapidly during the 1950s. As a result of successful boat applications, some enterprising fabricators began building small tanks and covers for acid containing equipment, mainly in competition with rubber-lined steel. Bill Gerstacker, one of the fiberglass sales pioneers in western Pennsylvania, stated that one of the earliest equipment applications was the use of hand layup fiberglass covers for metal plating baths in the early 1950s1. He also commented that the success of fiberglass covers was that the competitive rubber-lined steel covers were not even offered until about 15 years later.
The earliest applications in water and wastewater treatment are not known to this writer; but certainly by the late 1950s, there were fiberglass tanks for storage of corrosive chemicals such as sulfuric, nitric or hydrochloric acids. Pipe and gas ducts were also beginning to be reported in the same period. The Reinforced Plastics Division of the Society of the Plastics Industry (SPI) began holding meetings in 1945, and the first printed papers began in 1950. The first printed paper on fiberglass pipe was by aU.S. Navy researcher in 1952.2A description of a fiberglass tank for potable water storage at an U.S. Army installation was published in 1953.3
The earliest fabrication method for WWII radar domes was by hand layup of fiberglass fabric with polyester resin; the B-29 Superfortress had an airborne radar unit with a fiberglass radome (the author flew in one of those). Epoxy resins came into play in the early 1950s with superior properties for electrical laminates. Resins with greater corrosion resistance were introduced in the mid- to late-1950s and vinyl ester resins (the dominant type used now for corrosive environments) first appeared about 1964.
Glass Reinforcement Types
The forms of fiberglass reinforcements used for corrosion are:
- Roving (bundles of 800 to 4,000 or more continuous parallel filaments).
- Woven roving (a heavy fabric with a normal ratio of 5 warp strands per inch and 4 fill strands per inch). The most favored type weighs 24 ounces per square yard.
- Chopped strand mat (made from random scattered small bundles of about 200 filaments cut to about 2 inch lengths, and bonded together with a binder soluble in polyester resin). The most popular weight is 1.5 ounces per square foot.
- Sprayup roving (~200 filament strands gathered as a roving for passage through a chopper gun to produce a laminate similar to chopped strand mat).
- Continuous strand mat (made by swirling continuous fibers in small strands during manufacture).
- Fabric of parallel rovings tied with knitted or stitched organic fiber yarns. (Often called "uni-fabric" for its ability to provide unidirectional strands either parallel or perpendicular to the warp direction).
- Surface mat ("veil") made from fine continuous glass fibers in a very thin product (normally 0.01 inch) for providing a high resin content surface for improved corrosion resistance. A competing product is made from thermoplastic polyester fibers.
Thermosetting Resins for Corrosion Applications
There are numerous thermosetting resins that are used to produce corrosion-resistant fiberglass laminates. One of the earliest was a variation from general-purpose polyester resin called isophthalic polyester. Numerous tests have indicated it to be satisfactory for many mildly acidic conditions, such as those found in many soils or waste water. A widely observed application lies in the many hundreds of thousands of ribbed-wall fuel storage tanks that have been seen traveling on highways throughout the nation. The durability of these laminates has been shown in 40-year-old buried tanks that were removed and tested to find virtually no degradation of properties.
For more severe acidic conditions, vinyl esters (a combination product of epoxy and polyester) have been found to have improved resistance, especially at elevated temperatures.
The fabrication of various products takes numerous forms, depending on the shape and the quantity of items to be produced. For one-of-a-kind or small-quantity runs, contact molding (hand layup or sprayup) is undoubtedly the least costly method of production. This is evident in numerous applications, such as replica automobile bodies, shower stalls, architectural shapes, racing cars, boats, truck cabs or other low production rate items.
In this method, laminates are placed on a mold and built up by layers of mat, chopped fibers (sprayup) or combinations with woven roving where higher strength is required (boat hulls are an example). Cylindrical or similar shapes can be made by winding resin-impregnated rovings on to a mandrel in a process known as filament winding. This has been cited as providing the highest strength product with the lowest cost materials, and it is favored for making pipe or tanks.
Other fabrication processes, not often seen in corrosion-resistance applications are compression molding, resin transfer molding or injection molding. A unique technology, similar to extrusion, is called "pultrusion." This process combines resin-impregnated rovings (with layers of mat) and draws them through a heated metal tool that cures the resin. The cured laminate is then grasped by a pulling mechanism that produces a structural shape such as a bar, angle, channel, I-beam or hollow member. This process, being continuous, has a very low labor content and produces a product with extraordinary strength and stiffness. The principal limitation is in size and lack of variable cross-section. Pultruded members have found many applications, in combination with fiberglass gratings, for stairs, cat-walks and platforms.
In water and wastewater treatment applications, numerous examples of fiberglass structures can be found. Ducts for handling corrosive gasses abound; in some instances these may be placed underground to reduce the possibility of damage from vandalism or graffiti.
Many of the solutions used in water treatment are corrosive, requiring corrosion-resistant tanks. A special example is ferric chloride, which can dissolve stainless steel or almost any iron-containing alloy. Corrosion tables rate ferric chloride as about the same as sodium chloride for fiberglass laminates.
Fume scrubbers are usually made like fiberglass tanks, but with a thermoplastic packing material resting on fiberglass grating. In turn, the grating may be supported by fiberglass beams. In some instances the support beams may be pultruded shapes, or they may be contact-molded hollow beams. Ductwork, valves and blowers for handling the gasses often may be constructed of fiberglass; there are a number of producers of these elements over a wide range of sizes.
In many cases, large covers over waste treatment ponds are made from fiberglass; the author had an opportunity for the design of such a cover, which was 40 feet in diameter. The volutes were a subject of concern, so it was decided to make a finite element analysis for the expected maximum wind loads. This analysis showed that high-stress areas of concern would occur in peripheral areas, and thickness adjustments should be made in those areas, but thinner walls could be used in other areas. Overall, the analysis gave the design much greater assurance. Figure 1 shows the mesh layout for the finite elements.
Figure 1. Finite element analysis of dome cover.
Demonstrating the durability of fiberglass laminates, Figure 2 shows a duct and cover for a paper mill waste treatment unit constructed by Corrosion Controllers, Washougal, Wash., in 1965. The cover has an approximate diameter of 30 feet, the crossover elbow, or "goose-neck," has a 6-foot diameter. After nearly 40 years, this unit is still in service. Durability such as this, coupled with economic benefits, has made such composites continue to be the material of choice in many water and wastewater treatment applications.
Figure 2. 82-foot fiberglass cover for clarifier.
1. W. Gerstacker, "Applications for Fiberglass in Corrosion," Unpublished paper at NACE Regional Conference, Niagara Falls, circa 1978.
2. Meyers, L., "A Progress Report of the Cooperative Effort Between Fort Belvoir And SPI," 1952 SPI Conference, SECT 03.
3. Reinsmith, G., and Pebly, H., "Glass Reinforced Plastics in Ordnance Corps Applications,"1953 SPI Conference, SECT 22.
This article originally appeared in the 03/01/2004 issue of Environmental Protection.