Reasonable care corrosion

Intake water tunnels are generally made from concrete, and absorption of water by concrete is the main reason for corrosion in reinforcement. In intake structures the problems are due to concrete failure from salts penetrating into the concrete and corroding the rebar. Hard, dense concrete with ample cover to reinforcement and without cracks and porosity has good resistance to corrosion against seawater. In Indian nuclear power plants, the experience with concrete intake tunnels with respect to corrosion behavior is not bad except that special care is required for protection against algae, clams, mussels, etc. which attach to the tunnel surface. 

Most plastics materials could be used to provide corrosion resistance. Given reasonable care in selection of grades, the mechanical requirement could also be quite easily met, with good design. The chief problem surrounds the high-temperature tolerance. 

Some of the reasons are aesthetic, and on many larger articles, such as motorcars, an attractive appearance is achieved by a careful visual balance between parts which are finished and protected by electroplating, and parts which are protected by organic finishes. In many other instances the manufacture of parts from corrosion-resistant materials is ruled out because of the relatively high cost of such materials, and in some cases the physical properties of the appropriate corrosion-resistant materials may render them either unsuitable for economic production or unable to perform the function for which the article is required. 

Although the initial choice of coating material applied for reasons (b) or (c) may be dictated by the particular properties required, the corrosion behaviour of the composite metal coating/metal substrate system must also be taken into consideration in so far as it may affect the maintenance of the desired properties. Consequently, in all cases where protective metal coatings are used the corrosion performance of both coating and substrate require careful consideration. 

Certain impurities on zinc can act as catalysts for the generation of hydrogen, thereby greatly increasing the corrosion rates. For this reason, zinc in alkaline cells must be of high purity, and careful control exercised over the level of the harmful impurities. Moreover, other components of the cell must not contain harmful levels of these impurities that might dissolve and migrate to the zinc anode. 

Careful application of linings is required to provide a reasonably uniform thickness and to give complete coverage. Substantial variations in thickness will affect the local heat transfer and may affect the overall performance of the heat exchanger. Incomplete coverage may not substantially affect fouling reduction, but where corrosion resistance is required, breaks in the lining membrane can lead to severe localised corrosion. 

Because of these relationships, we need to be very careful when using Equation (7.8) on combined erosion and corrosion. To make such calculations reasonably accurate they must be eombined with and based upon several experiments under relevant conditions. 

The careful reader has probably noticed that we implied that corrosion is linked to metals. This is an important matter as it is always prudent to use corrosion for metals and words such as deterioration or degradation for nonmetals. The main reason is that the mechanisms of corrosion and degradation are essentially different. However, bearing in mind that even among professionals, such distinctions are not always observed, we have used these words interchangeably throughout this book. 

---