Concrete and steel are in fact the standard materials of construction because they are plentiful, and relatively cheap. An application is considered to be a corrosion situation if a standard steel or concrete design will not give the best life cycle costs. All too often, when one runs into a corrosive application, the standard practice tends to be 'build it out of steel, then later let's figure out how to protect it from the corrosive environment'. The steel structure is then either cathodicly protected, or coated. The net result of this thought process is an application that would have been better designed with an alternate material (such as an FRP composite).

Fortunately, this scenario is not always practiced. High levels of maintenance betray the fact that steel and concrete are not the materials of choice for a given application. This realization gives rise to a process that identifies a superior material of construction. This replacement material becomes the original material of construction when the user is in charge of new construction. If the user is not involved in designing the part, composites are not usually considered. The reason for this is that formal courses on designing with composites are a rarity. Most FRP composite engineers are largely self taught with help from composite industry groups, such as the CFA. The CFA can be one of your sources if you choose to develop the special and unique capability of engineered corrosion resistant FRP composite design, manufacturing, or use. Currently, FRP composites are considered a primary corrosion resistant (CR) material of construction in the following markets: Chemical Processing, Mineral Processing, Food Processing, Pulp and Paper, Wastewater/Odor Control, Electronic Mfg., Power, Coatings and Linings, Electrical Insulating, and segments of Infrastructure.

When should an FRP composite be considered for corrosive applications?

The simple answer is when an FRP part gives either a lower life cycle, or a lower initial cost, it should be used. The initial cost takes into account the cost to buy the part, install it, insulate it (thermal or electrical), and the application of any corrosion resistant requirements (cathodic protection, rust proofing, etc.). Typically, FRP parts are much lighter than steel or steel reinforced concrete yielding lower cost installations. 1) The lower cost installation can come from lower cost pads, hangers and other support structures. Lighter duty cranes and longer reaches are also important considerations. 2) Installation of an FRP composite into an existing site or building can be made much easier. FRP composites have intrinsically good insulating characteristics. They are both good thermal and electrical insulating materials. While a steel duct, for example may need to be insulated to stop condensate from forming, an FRP composite may need little, no, or a cheaper type of insulation. 3) Corrosion resistant properties are an integral part of the composite. The resin and glass reinforcement provides the corrosion resistance. This corrosion resistance can be tailored for the situation. It can even provide a fully corrosion resistant exterior that would be analogous to cladding both sides of a steel vessel with two different alloys. 4) The add-on costs are much lower for FRP parts.

In order to determine the life cycle costs of a part, one needs to know the maintenance schedule, and the life expectancy. Normal considerations for maintenance schedule are repainting, relining (if coated), inspections, and maintaining the cathodic protection. Since the color of an FRP composite is an integral part, and cathodic protection is not needed, they generate no maintenance costs. The corrosion barrier is also an integral part of the composite, and often requires no maintenance. However, in some markets with very aggressive environments, it has become accepted practice to rebuild part of the corrosion barrier every so many years. Whereas this practice does add to the maintenance costs, it establishes the lifetime of the part as being virtually unlimited.

Why are more parts being built out of FRP?

One of the advantages of using FRP in CR applications is the great flexibility it offers the designer. This flexibility should not be intimidating to the designer even with no experience in using FRP. This flexibility available to a designer is best displayed by the common practice of fabricating a part with a resin rich surface next to the corrosive environment, and a glass rich section behind the 'corrosion barrier'. Resin rich laminates show improved corrosion properties in most environments, while higher glass ratios give improved physical properties. The industry has set standards around these corrosion barriers; they are described in ASTM C-582 and ASME RTP-1.

Additional examples of design flexibility are available through resin selection. This allows the designer to change the thermal, flame or corrosion resistant properties with minimal effect on the physical properties. Alternatively, the strength of the part can be modified by altering the direction, or amount of glass fiber reinforcement in the laminate. These modifications are facilitated by the wide variety of fabrication processes commonly available.

Handling Corrosive Environments.

Corrosive environments fall into one or more of the following categories: acidic, basic, salt, organic, or oxidizing media. Of these, solutions of salt are not considered to be corrosive towards FRP, and organics are not considered to be corrosive towards steel. This leads to an interesting result. Since salt water is not corrosive towards FRP, almost any polyester resin performs well in the ocean (hence the rapid rise of fiberglass boats). This leaves price and esthetics, rather than CR performance as the selection criteria for marine applications.

However, in industrial applications such as underground gasoline storage tanks, a CR resistant resin system must be chosen to handle not only gasoline but also moisture inside the tank and the outside ground water. Gasoline, by itself, is not corrosive towards steel. A steel gasoline tank is attacked by moisture, either from the surrounding ground, or from condensation in partially empty tanks. This water is of no concern to an FRP structure. The approach to handling this environment is very different for steel versus FRP. Steel requires outside help in the form of cathodic protection and/or coatings (happily to include FRP). FRP tanks have an inherent answer in their ability to be designed with the appropriate interior and exterior resins and reinforcement to be inherently capable of handling the environment.