Use of topographic polymeric replica to characterize electric corrosion failure

Abstract

Polymeric replica, as a non-destructive technique, is generally used for detecting service failures in mechanical components together with other complementary methods. The components more frequently inspected are boilers, streamers, heat exchangers, pipes. In most cases, this technique fulfills the requirement of non-surface modifications of the damaged zone, be it a fracture surface, a gear tooth, a bearing raceway or any other part. Fracture and corrosion are the type of failures usually studied with this technique. It allows the description of crack morphology and propagation type, and is also a tool to differentiate crevice corrosion, stress corrosion cracking, pitting corrosion, etc. As far as we know, this technique has not been used so far to detect wear failures, to measure scar size (volume or critical dimensions) or to determine the type of wear failure. In this study, polymeric replicas are used to find the root cause of electric corrosion failures or spark-erosion wear in different gear-tooth coupling of feedwater pumps that convey light water to the steam generators at a Nuclear Power Plant in Argentina (CNE NPP). By means of this “in situ” and non-destructive technique we have been able to describe failures, originated in engine electrostatic losses. It was determined that parasitic currents were distributed in the whole transmission line, and the sparks on the gear's teeth melted small hemispheric drops of metal producing micro voids or craters. In order to copy the wear scars, foils of cellulose acetate replicas were used on the surfaces and a special elastomer rubber was also applied, filling the space between the teeth. The scar copies were observed by means of optical microscopy. The micrographies have allowed the characterization of microscopic holes (of 100 μm diameter) produced by electric discharges.