Corrosion testing and appearance of grease coated galvanised steel

Abstract

The effectiveness of grease, used as a corrosion preventative in galvanised steel reinforced overhead electrical power conductors, was evaluated by a variety of methods. The ability of ions to penetrate grease was assessed by an electrochemical technique and by solution analysis. The structure of free grease films was observed by scanning laser optical microscopy to give insight into the nature of ion motion. Changes in the grease both after exposure to the outdoor environment and after laboratory testing were evaluated by optical microscopy and infrared spectroscopy. In addition, the corrosion performance of grease coated galvanised steel wire in a mixed salts spray, wet-dry cycle test was evaluated by electrochemical potential measurements. Morphological examination of free grease films indicated the presence of features at the surface and within the body of the films. These appeared to be microscopic ‘defects’, which could allow the penetration of aggressive species to the substrate metal. ‘Degraded’ grease, observed in conductors after extended periods of natural weathering, could be reproduced in the test and was found to be primarily a physical mixture of grease and corrosion product. Solutions of low pH, containing AlCl₃, penetrated grease more rapidly than near neutral solutions of NaCl. The corrosion morphology of grease· coated wires exposed in the accelerated test resembled that of corroded galvanised steel core wires in naturally weathered conductors. Corrosion was found to commence under thin grease films which were apparently free from macroscopic defects as well as beneath thicker grease films which had had defects introduced using a hot needle. Grease was thus found to play a major role in restricting the initiation of corrosion in conductors, eventual attack probably initiating at ‘holidays’ in the coating.