In broad terms the object of every engineer operating oil burning plant has always been to obtain the maximum amount of effective B.Th.U.s for every pound of oil burned. Recently, however, it has been established that as the thermal efficiency of modern high pressure, high temperature oil burning plant has increased there has been a similar increase in corrosion. The extent of this problem is due to the fact that currently available residual fuel oils contain inorganic substances, amongst them vanadium, sodium and sulphur. If these substances were not present hydrocarbon petroleum oils would merely be burned to yield carbon dioxide and water.