Crud and Galvanic Corrosion Summary (Cont.)

CRUD AND GALVANIC CORROSION DOE-HDBK-1015/1-93 Corrosion CH-02 Rev. 0 Page 26 Crud and Galvanic Corrosion Summary (Cont.) The causes of a crud burst in the reactor coolant are: Increased oxygen concentration Reduced (or significantly changed) pH Large temperature change Physical shock (for example, starting and stopping pumps, changing speeds of pumps, reactor scram, or relief valve lift) Galvanic corrosion functions on the principle of the electrochemical cell, and occurs when two electrochemically dissimilar metals are joined together in a conducting medium. The two dissimilar metals generate an electrical potential, and this electrical potential serves as the driving force for the electrical current flow through the corrodant or electrolyte. The less resistant metal, called the active metal, becomes anodic. The other metal, called the noble metal, becomes cathodic. The two locations susceptible to galvanic corrosion are piping transitions between two dissimilar metals and at sacrificial anodes. Measures used to control galvanic corrosion include: Cathodic protection by introducing a third metal (sacrificial anode, normally zinc) to the metals being protected or using only metals that are close on the activity series. Choosing relative surface areas such that the material to be protected has a larger surface area than the active metal. Separating dissimilar metals with a non-conducting material Separating the metals from a conductive environment Use of poorly conducting electrolytes (pure water)