Figure 9 Effect of pH on Aluminum Corrosion and Oxide Solubility

Summarv

The major points of this chapter are summarized below.

General Corrosion Summary

The two conditions that contribute to general corrosion are:

Metal and water in the same environment

Chemical reaction between the metal and water to form an oxide

The corrosion rate is affected by the following:

A temperature rise in the range of 20F to 50F doubles the corrosion rate until the formation of the protective oxide film is complete.

An extremely high water velocity, 30 to 40 ft per second, tends to remove the oxide film allowing the corrosion rate to increase.

The presence of oxygen in water to which iron is exposed increases the corrosion rate. The reason for the corrosion rate increase is due to the rapid reaction between the oxygen and the polarizing layer of hydrogen absorbed on the oxide layer.

A pH between 4 and 10 results in minimal corrosion rate. If the pH falls below or above this range, the corrosion will normally increase.

The condition and composition of the metal surfaces affects the corrosion rate. Deposits, scale, or irregular surfaces create areas on the metal where local corrosion can initiate and proceed at a faster rate than normal.

Dissolved solids tend to make it easier for current to flow, which results in a higher corrosion rate.

The three products formed from general corrosion of iron are FeO, Fe₂0₃, and Fe₃0_a.

During the initial fill of a reactor system, great care is taken to promote the formation of the best possible protective corrosion film. The protective film acts as a barrier and stops further uncontrolled corrosion from taking place. This process, called pretreatment or pickling, involves careful control of reactor coolant water chemistry and temperature during the pretreatment period.

Four methods used to chemically control general plant corrosion are:

Passivators (inhibitors) consist of a naturally active metal that corrodes at a very low rate. A common passivator is potassium chromate.

Cathodic protection is the external application of an external electric current to the iron so that it acts as a cathode and has no anodic areas. This is accomplished by using either an external electrical source or the use of a sacrificial anode to provide the electrical current.

Removing corrodants to reduce the dissolved oxygen and lesser extent carbon dioxide. Treating the water in this manner reduces the dissolved solids and reduces the conductivity.

Chemical addition to alter the chemical reaction or tie up a particular corrodant. Filming amines, phosphates, and sodium hydroxide are used to provide organic films and adjust the system pH.

The six water chemistry conditions that limit corrosion of aluminum are:

Maintaining pH slightly acidic with the value of the pH depending largely on operating temperature.

Elimination of dissolved oxygen Elimination of soluble and solid impurities Prevention of the introduction of organic impurities Pretreatment (or pickling)

Maintaining water purity