IMPORTANCE OF SURFACE PREPARATION FOR CORROSION CONTROL IN NUCLEAR-POWER STATIONS

To enable nondestructive inspection, construction rules generally spec ify the final surface condition of components only in terms of limitin g roughness values. However, the microgeometry is only one of many imp ortant surface characteristics. Failure to ensure overall cleanness of circuits (i.e., of all exposed surfaces) by taking certain precaution s can have serious consequences. Further more, the presence of a heavi ly cold-worked surface layer, from machining or other causes, can comp letely modify its corrosion behavior in service. It has long been know n that the oxidation resistance of stainless steels in steam or cal bo n dioxide (CO2) at temperatures in the vicinity of 500 degrees C is co nsiderably improved by a cold-worked surface layer, which enhances Cr diffusion and promotes the formation of a tenacious al rd protective o xide scale. More recently, the effect of cold-worked surface layers on the susceptibility to stress corrosion clacking has been clearly reve aled in components that have been subjected to strain during manufactu re by tube bending, expansion operations, or even welding. Very high r esidual stresses can be induced in the skin region, and if the cold-wo rked layer is sufficiently thick, cracks can be initialed ill a very s hort time. Based on the difference ill the depth of cold work induced by various mechanical treatments, it has been possible to model this e ffect and to explain the wide scatter in fire service behavior of cert ain pressurized water reactor components. It is essential to take thes e factors into account to predict correctly the service behavior of th e components concerned, without which satisfactory plant maintenance p lanning is impossible.