Plastics are often used to solve corrosion problems of metallics; so, one may be led to believe that they cannot be attacked by corrosion. But this is not true. It is well known in a pulp mill or a chemical plant, particularly by the maintenance staff; that, in time, corrosion damage may appear on plastic structures and components.

In many instances, the lifetime or the need for repair of the structures is determined by the corrosion attack. Depending on the type of plastic, fabrication method, type of fiber, temperature and other factors, the corrosion rate or changes in mechanical properties may be negligible over a period of 20 years or more, or may lead to a failure in a couple of weeks. From that point of view, metals and plastics or other materials are equal. We can find numerous corrosion-related failures, as well as successful applications.

Compared with metals, a shortcoming for plastics is that the corrosion science of the former is more developed. One reason is certainly that plastic materials are younger than metals/ alloys, for example, steels.

From metal corrosion, we can foresee the necessary future developments in polymer corrosion science. The confidence and the general status of plastic materials would probably be significantly increased if their socalled "chemical resistance" could be presented in technical corrosion terms analogous to metals. An increased knowledge of the corrosion properties of plastic materials is also necessary to master existing instances of corrosion damage and is key to the future development of improved materials and products.

Selecting plastics
Practical experience shows that the reasons and causes for selecting plastics in process equipment include:

• Corrosion damage has occurred repeatedly with various steels that have worked well previously, and the corrosion resistance of still more-highly alloyed steels is uncertain or they have already failed.
• Unexpected rapid corrosion has occurred on new construction of stainless steel, e.g., SS 2343 (Type 316, UNS S31600) or on more-highly alloyed steels.
• Unexpected rapid corrosion has occurred with expensive materials, such as titanium and nickel-based alloys.
• Damage in older constructions made of wood, concrete, or lead- or brick-lined steel has become difficult to repair.
• No other material was considered with regard either to the corrosion stresses or mechanical load.
• The corrosive environment may vary inside or outside the component. Unexpected external corrosion damage, sometimes under insulation, has occurred due to spills or from leaking aggressive gases.
• A plastic (solid or lining) is considered the best selection from a technical/economical viewpoint.

Plastics such as FRP are "traditionally" considered only where the environment is strongly corrosive to Type 316 stainless steel, for example, and their use in other cases is often overlooked.

Plastics vs. metals
Corrosion of metals is mostly determined by electrochemical processes, while that of FRP or other plastics is the result of organochemical ones. A vital difference between metals and plastics is that the structure of the former is 100% crystalline and the structure of the latter is semicrystalline or amorphous. Therefore, also physical processes like diffusion, osmosis and swelling may play an important role in the corrosion of plastics. However, there are still certain similarities. Most types of corrosion found in metals may also be found in plastics, for instance, uniform corrosion, selective corrosion, stress corrosion, layer corrosion (delamination for FRP) and localized corrosion (pitting). Some types of metal corrosion, such as crevice corrosion, may possibly not be found in plastics and vice-versa (e.g., swelling and osmosis blistering).

Corrosion data
For metals, there are well-developed testing methods for each type of corrosion and the data achieved for specific materials can easily be compared and be of use in materials selection and design. For plastics, the situation is not as clearcut. In manufacturers' data sheets, corrosion data are never or seldom presented. Instead, the sheets list the so-called "chemical resistance" of the material in different environments as a recommendation for use up to a certain maximum temperature, or, more generally, as "resistant," "limited resistance" and "not resistant." Consequently, there is no information given about the types of possible attack, how fast attack proceeds into the wall, and what damage it may cause. It is therefore difficult to take any possible corrosion into account when designing a product for a certain lifetime. The service reliability cannot be assessed and one cannot compare the resistance of different materials or materials of the same type, but only of different brands or grades.

From- http://findarticles.com/