The corrosion of a metal in an acid bath is inhibited by providing the bath with a corrosion inhibitor obtained by reacting a heterocyclic nitrogen base, the heterocyclic rings of which consist of carbon and nitrogen atoms with an organic sulphonic acid ester containing an aryl group or an organic halide containing an aryl group. 1 to 3 mols. of base may be reacted with one mol. of the halide or the sulphonic acid ester. The reaction probably produces a quaternary ammonium compound. The nitrogen bases referred to are polyatomic-carbon heterocyclic bases containing one or more trivalent nitrogen atoms in the heterocyclic nucleus, e.g. pyridines, quinolines, acridines, or their derivatives, or commercial materials containing such compounds such as denaturing or flotation pyridines, bone oils or the like. Crude pyridine consisting of a coal tar base distillate boiling between 217 and 346 DEG C. at atmospheric pressure may be employed or bases derived therefrom by fractional distillation under reduced pressure. Methyl-para-toluene sulphonate is given as a suitable sulphonic ester and benzyl chloride, benzal chloride, benzotrichloride, benzyl bromide, phenyl ethyl chloride, phenyl ethyl bromide, orthochlorbenzyl chloride, paranitrobenzyl bromide, benzoyl chloride, benzoyl bromide, m-nitrobenzoyl chloride, para-nitrobenzoyl chloride and chloroacetophenone are given as suitable halides. Any halide may be employed having an aromatic nucleus consisting of one or more carbocyclic or heterocyclic rings, with or without substituents such as --NH2, --NO2, --HSO3, --Cl, --Br, --I, --CN, or --OH and whose aliphatic side chain contains one or more carbon atoms to which one or more halogen atoms are attached, which aliphatic chains may be partially or completely saturated with hydrogen and oxygen atoms. According to an example, a mixture of benzyl chloride and high boiling pyridine bases is heated under a reflux condenser to 130 DEG C. the reaction subsequently raising the temperature to 160--170 DEG C. After cooling, the product or the other inhibitors according to the invention may be diluted with one of the following solvents, namely, denatured alcohol, high-boiling pyridine bases, aqueous hydrochloric acid, aqueous sulphuric acid, a mixture of alcohol and aqueous hydrochloric acid or a mixture of pyridine bases and aqueous hydrochloric acid. The resulting solutions may be diluted with water to precipitate tarry matter and then decanted. A pickling bath for iron and steel may contain 2--15 per cent mineral, e.g. sulphuric acid, and about 0,1 per cent of the above reaction product added either alone or in the form of one of the above solutions. A pickling temperature of 50--95 DEG C. may be employed. The Specification as open to inspection under Sect. 91 describes also the production of inhibitors by reacting organic nitrogen bases with alkyl halides or alkyl sulphonates. This subject-matter does not appear in the Specification as accepted.ALSO:Quaternary ammonium compounds, for use in reducing the corrosion of a metal in an acid bath, are obtained by reacting a heterocyclic nitrogen base with an organic sulphonic acid ester containing an aryl group or with an organic halide containing an aryl group. From 1 to 3 mols. of base may be reacted with one mol. of the halide or the sulphonic acid ester. The nitrogen bases specified are pyridines, quinolines, and acridines or their derivatives, or commercial materials containing such compounds such as denaturing or flotation pyridines or bone oils. Crude pyridine consisting of a coal tar base distillate boiling between 217 and 346 DEG C. at atmospheric pressure may be employed or bases derived therefrom by fractional distillation under reduced pressure. Methyl-p-toluene sulphonate is given as a suitable sulphonic ester and benzyl chloride, benzal chloride, benzotrichloride, benzyl bromide, phenyl ethyl chloride, phenyl ethyl bromide, o-chlorbenzyl chloride, p-nitrobenzyl bromide, benzoyl chloride, benzoyl bromide, m-nitrobenzoyl chloride, p-nitrobenzoyl chloride and chloracetophenone are given as suitable halides. Any halide may be employed having an aromatic nucleus consisting of one or more carbocyclic or heterocyclic rings, with or without substituents such as --NH2, --NO2, --HSO3, --C1, --Br, --I, --CN, or --OH, and an aliphatic side chain containing one or more carbon atoms to which one or more halogen atoms may be attached. In an example, a mixture of benzyl chloride and high boiling pyridine bases is heated under a reflux condenser to 130 DEG C. the reaction subsequently raising the temperature to 160--170 DEG C. The Specification as open to inspection under Sect. 91 describes p also the production of inhibitors by reacting organic nitrogen bases with alkyl halides or alkyl sulphonates. This subject-matter does not appear in the Specification as accepted.ALSO:The corrosion of a metal in an acid bath is inhibited by providing the bath with a corrosion inhibitor obtained by reacting a heterocyclic nitrogen base, the heterocyclic rings of which S consist of carbon and nitrogen atoms, with an organic sulphonic acid ester containing an aryl group or an organic halide containing an aryl group. 1 to 3 mols. of base may be reacted with one mol. of the halide or the sulphonic acid ester. The reaction probably produces a quaternary ammonium compound. The nitrogen bases referred to are polyatomic-carbon heterocyclic bases containing one or more tri-valent nitrogen atoms in the heterocyclic nucleus, e.g. pyridines, quinolines, acridines, or their derivatives, or commercial materials containing such compounds such as denaturing or flotation pyridines, bone oils or the like. Crude pyridine consisting of a coal tar base distillate boiling between 217 and 346 DEG C. at atmospheric pressure may be employed or bases derived therefrom by fractional distillation under reduced pressure. Methyl-paratoluene sulphonate is given as a suitable sulphonic ester and benzyl chloride, benzal chloride, benzotrichloride, benzyl bromide, phenyl ethyl chloride, phenyl ethyl bromide, orthochlorbenzyl chloride, para-nitrobenzyl bromide, benzoyl chloride, benzoyl bromide, m - nitrobenzoyl chloride, para - nitrobenzoyl chloride and chloroacetophenone are given as suitable halides. Any halide may be employed having an aromatic nucleus consisting of one or more carbocyclic or heterocyclic rings, with or without substituents such as -NH2, -NO2, -HSO3, -Cl, -Br, -I, -CN, or -OH and whose aliphatic side chain contains one or more carbon atoms to which one or more halogen atoms are attached, which aliphatic chains may be partially or completely saturated with hydrogen and oxygen atoms. According to an example, a mixture of benzyl chloride and high boiling pyridine bases is heated under a reflux condenser to 130 DEG C. the reaction subsequently raising the temperature to 160-170 DEG C. After cooling, the product or the other inhibitors according to the invention may be diluted with one of the following solvents, namely, denatured alcohol, high-boiling pyridine bases, aqueous hydrochloric acid, aqueous sulphuric acid, a mixture of alcohol and aqueous hydrochloric acid or a mixture of pyridine bases and aqueous hydrochloric acid. The resulting solutions may be diluted with water to precipitate tarry matter and then decanted. A pickling bath for iron and steel may contain 2-15 per cent mineral, e.g. sulphuric, acid and about 0,1 per cent of the above reaction product added either alone or in the form of the above solutions. A pickling temperature of 50-95 DEG C. may be employed. The Specification as open to inspection under Sect. 91 describes also the production of inhibitors by reacting organic nitrogen bases with alkyl halides or alkyl sulphonates. This subject-matter does not appear in the Specification as accepted.