Events at a Scratch-Line
Ulick Richardson Evans · Transactions of the IMF · 1956
SynopsisThe surface defects on Bare metal often constitute starting points of corrosion. Where protective coatings have been applied, such defects sometimes contribute to imperfections in the coating. A scratch-line engraved under a definite load represents a “repro-ducable” type of defect and has been used at Cambridge University in several pieces of pure-science research into the mechanism of corrosion. The paper brings together those features of the researches likely to possess interest for the metal finisher.Statistical experiments have shown that the probability of corrosion at a scratch-line wetted with a liquid standing on the border-line between the corrosive and the passivating classes decreases steadily with the period of air-exposure before wetting; also, corrosion at one scratch-line depresses the probability of corrosion starting at a neighbouring scratch-line. Corrosion is found to be quantitatively related to electric currents flowing between the scratch as anode and the surrounding region as cathode.The combination of internal stresses left after scratching may be favourable to attack, which often tends to start at the margins of the scratch. The phenomenon of crack-heal (alternate cracking of an invisible film followed by repair through exposure to air) has been experimentally demonstrated. Heating in air, which restores the oxide film broken by scratching, does not necessarily eliminate the special liability to corrode at the spot if the stresses remain unrelieved.