The influence of acid strength on the corrosive wear of grey cast irons in oil-sulphuric acid mixtures

Abstract

The sliding wear of grey cast iron against similar material in base lubricating oil-sulphuric acid mixtures as a function of acid strength in the aqueous phase and of volume concentration of aqueous phase in the oil has been investigated. For all acid strengths (10%, 20%, 30% and 40%), the amount of wear increases with increasing volume of the acid phase in the oil mixture because of increasing disruption of the hydrodynamic oil film, leading to metal-metal contact and corrosion of the metal surfaces by the acid. In mixtures containing low percentages of acid in oil (5 and 10 vol.%), the corrosive-wear process is very dependent on the acid concentration. Here, the presence of 10% acid concentration in water is effective in partially disrupting the film, leading to significant damage as mechanical wear is able to take place. As the acid concentration is increased progressively to 40%, the formation of adherent protective corrosion product films reduces metal-metal contact and decreases the wear rate. In mixtures containing high percentages of acid in oil (20–40 vol.%), the amount of aqueous phase is sufficient to give long periods of metal-metal contact. In addition, the more continuous nature of this phase and the vigorous stirring motion during sliding prevents formation of adherent wear-protective films. The products of corrosion are removed rapidly away from the metal surfaces, resulting in a much enhanced wear rate with a considerable contribution from the corrosion processes.