Development of Nondestructive On-Site Measurement Techniques by Means, of an Electrochemical Method for Material Degradation of S/H SUS Steel Tubes of Fossil Boilers.

Abstract

The change in electrochemical properties caused by microstructural changes was utilized for the nondestructive detection of material degradation. S/H tubes of fossil boilers were studied where carbide, Laves and sigma phase formation were detected by the anodic current peak IP in an anodic polarization measurement of 316H stainless steel in lM KOH electrolytes. For practical uses, a small and light-weight electrochemical probe and a portable anodic polarization measuring system were developed; their effectiveness was shown through on-site measurements in many actual plants. Using measured IP, the equivalent operational temperature of tubes after long-term service operation was estimated with reference to the IP-t exp (-Q/RT) curve of aged materials. It is suggested that their metallographic changes can be evaluated by a parameter of IP/IPC, where Ipc is estimated as to be the threshold value of IP. It is also indicated that their mechanical damage markedly increases by nucleation and growth of Laves phases at IP/IPC value over 0.7.