Corrosion
DOE-HDBK-1015/1-93
CRUD AND GALVANIC CORROSION
Rev. 0
CH-02
Page 23
The crud release can result from an increased oxygen concentration, a reduced (or significantly
changed) pH, a large temperature change (heatup or cooldown), or a physical shock to the
system. Physical shocks include starting, stopping, or changing pump speeds, or other
evolutions like a reactor scram or a relief valve lift. The result is a sudden increase in reactor
coolant activity. The release of crud in this fashion is termed a crud burst. Crud bursts often
lead to the removal of protective corrosion films and make the freshly exposed metal more
susceptible to additional corrosion. In addition to the corrosion film and crud, some of the
corrosion products are soluble and are easily transported throughout the system.
High crud concentrations in the system can also complicate disposal of primary coolant. Many
of the corrosion products have relatively long half-lives and represent significant biological
hazards. If, therefore, primary coolant is drained or leaks from the plant shortly after a crud
burst, additional procedures may need to be utilized to minimize the effects of this condition.
Therefore, if the conditions mentioned previously (O , pH) are changed, the solubility of these
2
corrosion products will change, and they can then be transported to and deposited anywhere in
the reactor coolant system.
Another corrosion byproduct is scale, which is made up of deposits on surfaces from the
formation of insoluble compounds from normally soluble salts. Most common are calcium or
magnesium carbonates (CaCO or MgCO ).
3
3
Galvanic Corrosion
Galvanic corrosion is the corrosion that results when two dissimilar metals with different
potentials are placed in electrical contact in an electrolyte.
Of all the different types of corrosion, galvanic corrosion corresponds most closely to the
electrochemical cells described previously in this module because galvanic corrosion occurs when
two electrochemically dissimilar metals are joined together (in electrical contact) in a conducting
medium (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,
differences in mechanical stress); abnormal levels of pH; and high temperatures. 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, active metal
becomes the anodic corrosion site. The stronger, more noble metal is cathodic and protected.
If there were no electrical contact, the two metals would be uniformly attacked by the corrosive
medium as if the other metal were absent. Two locations susceptible to galvanic corrosion is a
piping transition from one metal to another and a sacrificial anode (such as zinc).
