Corrosion is a major factor in the selection of material for a reactor plant. The material selected must resist the various types of corrosion discussed in the Chemistry Fundamentals Handbook.

EO 1.20 STATE the reason that galvanic corrosion is a concern in design and material selection.

Corrosion

Corrosion is the deterioration of a material due to interaction with its environment. It is the process in which metallic atoms leave the metal or form compounds in the presence of water and gases. Metal atoms are removed from a structural element until it fails, or oxides build up inside a pipe until it is plugged. All metals and alloys are subject to corrosion. Even the noble metals, such as gold, are subject to corrosive attack in some environments.

The corrosion of metals is a natural process. Most metals are not thermodynamically stable in the metallic form; they want to corrode and revert to the more stable forms that are normally found in ores, such as oxides. Corrosion is of primary concern in nuclear reactor plants. Corrosion occurs continuously throughout the reactor plant, and every metal is subject to it. Even though this corrosion cannot be eliminated, it can be controlled.

General Corrosion

General corrosion involving water and steel generally results from chemical action where the steel surface oxidizes, forming iron oxide (rust). Many of the systems and components in the plant are made from iron.

Some standard methods associated with material selection that protect against general corrosion include:

The use of corrosion-resistant materials such as stainless steel and nickel, chromium, and molybdenum alloys. (Keep in mind that the corrosion is electrochemical by nature, and the corrosion resistance of the stainless steels results from surface oxide films that interfere with the electrochemical process.)

The use of protective coatings such as paints and epoxies.

The application of metallic and nonmetallic coatings or linings to the surface which protects against corrosion, but allows the material to retain its structural strength (for example, a carbon steel pressure vessel with stainless steel cladding as a liner).

Galvanic Corrosion

Galvanic corrosion occurs when two dissimilar metals with different potentials are placed in electrical contact in an electrolyte. It may also take place with one metal with heterogeneities (dissimilarities) (for example, impurity inclusions, grains of different sizes, difference in composition of grains, or differences in mechanical stress). A difference in electrical potential exists between the different metals and serves as the driving force for electrical current flow through the corrodant or electrolyte. This current results in corrosion of one of the metals. The larger the potential difference, the greater the probability of galvanic corrosion. Galvanic corrosion only causes deterioration of one of the metals. The less resistant, more active one becomes the anodic (negative) corrosion site. The stronger, more noble one is cathodic (positive) and protected. If there were no electrical contact, the two metals would be uniformly attacked by the corrosive medium. This would then be called general corrosion.

For any particular medium, a list can be made arranging metals sequentially from most active, or least noble, to passive, or most noble. The galvanic series for sea water is discussed in the Chemistry Fundamentals Handbook.

Galvanic corrosion is of particular concern in design and material selection. Material selection is important because different metals come into contact with each other and may form galvanic cells. Design is important to minimize differing flow conditions and resultant areas of corrosion buildup. Loose corrosion products are important because they can be transported to the reactor core and irradiated.

In some instances, galvanic corrosion can be helpful in the plant. For example, if pieces of zinc are attached to the bottom of a steel water tank, the zinc will become the anode, and it will corrode. The steel in the tank becomes the cathode, and it will not be effected by the corrosion. This technique is known as cathodic protection. The metal to be protected is forced to become a cathode, and it will corrode at a much slower rate than the other metal, which is used as a sacrificial anode.

Localized Corrosion

Localized corrosion is defined as the selective removal of metal by corrosion at small areas or zones on a metal surface in contact with a corrosive environment, usually a liquid. It usually takes place when small local sites are attacked at a much higher rate than the rest of the original surface. Localized corrosion takes place when corrosion works with other destructive processes such as stress, fatigue, erosion, and other forms of chemical attack. Localized corrosion mechanisms can cause more damage than any one of those destructive processes individually. There are many different types of localized corrosion. Pitting, stress corrosion cracking, chloride stress corrosion, caustic stress corrosion, primary side stress corrosion, heat exchanger tube denting, wastage, and intergranular attack corrosion are discussed in detail in the Chemistry Fundamentals Handbook.