The mechanism of cavity formation during high temperature oxidation of nickel

Abstract

Three mechanisms have been advanced to account for the grain boundary voids observed in nickel following high temperature oxidation: vacancy injection, oxide growth stresses, and internal oxidation of carbon. A critical test to distinguish between the two former and the latter is developed by comparing the cavitation response of three grades of nickel to exposure at 1000°C in air and in a controlled, low oxygen partial pressure. The observation of similar cavitation at oxygen partial pressures below those required to form an external oxide discounts both vacancy injection and creep voiding in response to oxide growth stresses as possible mechanisms. However, this result supports the third possibility, that involving gas-bubble formation due to the internal oxidation of carbon. This is proved in three ways: firstly, by demonstrating the crucial role of carbon in promoting cavitation by systematically removing and then replacing this element before oxidation; secondly by producing cavitation when carbon was diffused into oxygen-containing nickel; and thirdly, by using mass spectrometry to identify the carbon dioxide, and possibly carbon monoxide, contained within the grain boundary pores. Consideration of these results leads to the conclusion that the reported cases of grain boundary voiding on oxidation are explained by gas forming reactions and are not evidence for vacancy injection or the existence of oxide growth stresses.