Abstract: The corrosion inhibitor of the present invention comprises a mixture containing (a) the reaction product of a nitrogen containing compound containing at least one active hydrogen attached to the nitrogen atom per molecule; a ketone having at least one hydrogen atom attached to the carbon atom alpha to the carbonyl group; an aldehyde; a fatty acid, and a quaternary ammonium compound and (b) a secondary quaternary compound which is different from the quaternary compound employed to prepare the reaction product. The inhibitor is added to acidic media which comes into contact with a metal to be protected.