CRUD AND GALVANIC CORROSION
Crud and Galvanic Corrosion Summary
Crud is corrosion products in the form of finely divided, insoluble oxide particles
suspended in the reactor coolant or loosely adhered to metal surfaces or activated
corrosion and wear products.
Scale is the deposition on the surfaces of the piping from the formation of insoluble
compounds from normally soluble salts. Most common are calcium or magnesium
Galvanic corrosion is the corrosion that results when two dissimilar metals with
different potentials are placed in electrical contact in an electrolyte.
The problems of crud in reactor plants are:
Fouling of coolant flow paths
Fouling of heat transfer surfaces
High general background (ambient) radiation levels
Radiation hot spots
Radioactive waste disposal
Galvanic corrosion can also be limited by: 1) using only metals that are close on the activity
series (discussed in the chapter on Corrosion Theory), 2) electrical insulation of dissimilar metals,
and 3) using poorly-conducting electrolytes (very pure water).
The relative surface areas of the two metals are also important. A much larger surface area of
the non-active metal, compared to the active metal, will accelerate the attack. It has been
determined that the relative surface area is the determining factor in the corrosion rates.
The required electrical current for galvanic corrosion will be stopped if the dissimilar metals are:
separated by a non-conducting junction,
separated from a conductive environment, and
located in a poorly conducting electrolyte (pure water).
The important information of this chapter is summarized below.