Hydrolyzable cations contained in crude oil are removed from crude oil by treatment of the crude oil with an aqueous solution containing 100 to 5000 ppm of a water soluble anionic polymer containing at least 20 mole percent mer groups from the group consisting of acrylic acid, methacrylic acid, sulfomethylated polyacrylamide, aminomethanephosphonic acid modified acrylic acid and their water soluble alkali metal and ammonium salts. By removing the hydrolyzable cations, corrosion occurring on metal surfaces in contact with the treated oil during subsequent refining is reduced.