US2918390A - Corrosion inhibition - Google Patents

Description

United States Patent CORROSION INHIBITION Lewis J. Brown, North Wales, Pa., and Samuel Spring,

Van Nuys, Califi, assignors to .Pennsalt Chemicals Cor- This invention relates to the corrosion protection of metal surfaces, and more particularly to the application of corrosion-inhibiting films to surfaces composed of zinc, cadmium, iron, steel, aluminum, etc.

:Users of metal articles will, for their various and particular purposes, apply different types of protective finishes to the metal when they wish to provide it with relatively permanent resistance to corrosion. There still remains the problem of supplying such articles to the ulti mate user in fresh and uncorroded condition, even though they might have to be' handled, shipped and stored in the interim period between fabrication and delivery. Some kind of interim protection must be provided for this period, since practically all metals are quickly attacked when exposed to even ordinary atmospheric conditions.

Manytypes of treatment to provide this protection have been proposed, as for example the process described in US. Patent 2,333,206, but in general these are all unsatisfactory for at least one of the reasons, that they provide protection at the sacrifice of appearance, i.e., that they dull, discolor, streak or stain the metal surface; that they dont provide adequate corrosion protection, particularly under humid or actually wet conditions; or that they are too expensive. Of these drawbacks, the loss of good appearance is from the-practical standpoint by'far the most serious, since it has a controlling effect on the marketability of the metal article, and customer appeal is greatly reduced in articles which dont appear to have a fresh, lustrous metallic surface. Appearance is equated in the customers mind with essential quality, and this conclusion is more often than not justified when the damage to appearance is of a permanent nature. The conventional chromate and phosphate conversion coatings, though usually affording adequate corrosion protection, have this drawback, that they permanently dull or stain the metal surface.

Accordingly, the primary object of the present invention is to provide metal surfaces with a corrosion-inhibiting film which does not impair or in any way alter the metallic appearance of the surface. Another obiect is to provide metal surfaces with a coating which will protect the surface adequately from the corrosive conditions that may be encountered during the period between fabrication and ultimate use. A further object is to provide such interim protection at a reasonable cost.

These and other objects have been achieved by applying to a substantially clean metal surface two suecessive coating solutions, the first of whichcontains a hydrophobic film-forming material, hereinafter defined,

and the second containing hexavalent chromium. The

order in which these solutions are applied is critical,

2,918,390 Ce .Palt,ented Dec. 22, 1959,

in that corrosion resistance of the resulting film is practically non-existant when the order given above is reversed. The solutions employed are extremely dilute, and deposit on the metal a vanishingly thin film, believed to approach a monomolecular film in thickness, which nonetheless is su'ificiently'aclh'erent, water-repellent and continuous to confer a high degree of corrosion resistance'to the metal surface. The most significant'feature of this film, however, is that it in no way changes the appearance of the metal: it is essentially colorless, clear, transparent and for all practical purposes invisible, so that the metal appears through it as fresh and lustrous as before treatment.

With regard to the hydrophobic film-forming material hereinabove referred to, this consists of a salt of a fatty acid amine; more precisely, it has the general structure wherein R is an aliphatic hydrocarbon group containing at least 8 carbon atoms and X is an anion, preferably of the group consisting of acetate, lactate, and chloride. Examples of typical compounds having this structure include octylamine acetate, dodecylamine acetate, octadecenylamine acetate, octadecylamine lactate, decylarnine chloride, and the like. Preferably the aliphatic hydrocarbon group is an alkyl group of from 8 to 18 carbon atoms (corresponding roughly to the length obtainable from the common fatty acid residues). The acetate is the anion of choice, being the least expensive and most readily available. A particularly good compound forlthe purpose of this invention is a commercial product of Armour and Company, going by the trade name Armac HT, which is a hydrogenated tallow amine acetate, i.e., a mixture comprising primarily hexadecyl and octadecylamine acetates with a small amount of octadecenylamine acetate also present.

In practicing the present invention, the metal should, to begin with, be substantially clean; that is, it should be free from gross contamination by oil, shop dirt, rust, etc. In general, the cleanerthemetal is prior to treatment, the more effective the treatment will be. Conventional cleaning methods, 'such as alkaline spray or immersion cleanin vapor'degreasing, etc. may be used if needed. The metal may be passed directly from the cleaning operation, if any, to the treatmentwith the fatty amine salt without intermediate drying, although it is immaterial whether the work piece is wet or dry. It is only advantageous that the surface be free from contamination, such as entrained alkali from the cleaner, so that the amine salt solution is not contaminated. The amine salts defined above are all either soluble or dispersible in water, and in practicing this invention they are applied to the metal in the form of a very dilute aqueous solution. The concentration of the amine salt in the treating solution should be about 0.01 to 1.0% by weight, and preferably about 0.05 to 0.20%. The solution is applied to the metal most conveniently by immersion for a few seconds, followed by a short draining period, but other conventional methods of application such as spraying or brushing may also be used. Longer periods of immersion, etc. are not harmful; the temperature of the solution is not critical.

Following treatment with the amine salt, the metal may be immediately passed to the solution containing hexavalent chromium without intermediate drying, though it is preferred to rinse the surface first, again to minimize drag-out and contamination of the chromium solution. The surface may be dried, but this is quite unnecessary and only adds an extra time-consuming step. For the chromium solution, compounds of hexavalent chromium which possess some water solubility are used, for example chromic acid or sodium or potassium dichromate, etc. The concentration should be within the range of 0.05 to 0.5% by weight (calculated as CrO and preferably between about 0.1 and 0.2%. Again, simple immersion in the solution is the easiest method of application, though other methods may be employed. The duration of application and the solution temperature are not critical.

After application of the two solutions, in the order given, the surface may be force dried or simply permitted to air dry. The metal now has a film on it that has a weight of less than 5 milligrams per square foot, and probably on the order of 1 to 3 mg./sq. ft. This film is visually imperceptible, yet without any further treatment, the metal is equipped to withstand the rigors of handling and storage, even when the latter is without benefit of protection from the elements.

It has further been discovered that the remarkable resistance to corrosion conferred by this extremely thin film is surprisingly even further enhanced when the metal is given a pretreatment prior to application of the amine salt and chromium solutions, this pretreatment consisting of cathodic cleaning in an alkaline solution. The reason for the increased corrosion resistance resulting from this added process step is still obscure, but the practical advantages are unequivocal.

Accordingly, in the preferred practice of the present invention, the metal surface, even though it may be quite clean and fresh, as for example metal that is just freshly machined or galvanized, etc., is immersed in an alkaline cleaner in an electrolytic cleaning bath wherein the metal to be cleaned is made the cathode. With regard to this cleaning step, it follows the conventional practice for this type of cleaning. The solution may be any of the usual alkalies, such as caustic soda, alkali silicates, phosphates, carbonates, etc., and may contain other conventional cleaning additives such as sequestrants or surfactants. The strength of the cleaner can vary within wide limits, as can its temperaature and total alkalinity. The time of immersion can be a few seconds or a few minutes, as described, and the current density will vary with the type of metal being treated; for example a current of 10 to 5 amperes per square foot may be used for zinc surfaces, while up to 100 amps/sq. ft. may be appropriate for steel. The metal surface should be carefully rinsed following this treatment and prior to the amine salt and chromate treatments. Again, the order of steps of the preferred process is critical, and should be followed as given herein to procure adequate corrosion protection.

-A further preferred aspect of the present invention is that it has been found advantageous to prepare both the fatty amine salt solution and the chromium solution with water which is substantially free from a certain group of anions which either tend to initiate corrosive processes or which form insoluble fatty amine salts, notably chlorides, sulfates, phosphates and silicates. Any rinse water that may be employed in the process should also be preferably free of these contaminants. The source of water to be used, if contaminated with such anions, can be treated with anion exchange media, or completely deionized water can be used, or, if a convenient source is present, it may be expedient to use distilled water or steam condensate. In general, the undesired anions should not be present in amounts in excess of about parts per million for best results, though the use of reasonably s'oftwater may be sufficient protec- -ftic an against their harmful effects in many applications. Where the cost of supplying suitable water must be considered, its use may be restricted just to the make-up of the chromium bath, where the absence particularly of chlorides and sulfates is most beneficial, since these anions tend either to etch the metal or to deposit unsightly stains, or both.

The following examples will help to illustrate the practice of this invention and the practical advantages derived therefrom; it should be understood that their purpose is only illustrative and not limiting.

EXAMPLE I Test panels cut from commercial untreated galvanized steel were cathodically electrocleaned in a 2 oz./gal. solution of a proprietary alkaline cleaner, rinsed, and then immersed for 15 seconds in a distilled water solution of Armac HT, the solution having a concentration of 1 gram per liter (0.1%) of the amine salt and a temperature of 60 C. They were then immersed for 15 seconds in a distilled water solution of chromic acid, the solution having a concentration of 1 g./l. (0.1%) and a temperature of 70 C., and air blast dried to remove the remaining beads of water. The resulting surfaces had an unchanged, metallic appearance and showed strong hydrophobicity.

After treatment, these panels were subjected to a battery of corrosion tests, designed to simulate severely corrosive storage conditions, the procedures of which are as follows:

Test 1 Water film.Panels are stacked with a few milliliters of water placed between each panel; the stacks are clamped and held for 24 hours at room temperature EXAMPLE II Galvanized steel panels were cleaned for 15 seconds at 60 C. in a 2 oz./ gal. alkaline cleaner. These panels were divided into two groups; the first group was simply immersed in the cleaner, while the second group were cathodically cleaned in the same cleaner at 25 amps/sq. ft. After cleaning all the panels were rinsed and treated with the amine salt and chromic acid solutions in the same manner as described in Example I.

EXAMPLE III EXAMPLE IV Panels of 52 S aluminum alloy were cleaned by immersion in an alkaline cleaner for 15 seconds at a temperature of C. They were then immersed for 15 seconds in a l g./l. solution of Armac HT in distilled water at 60 C., and this treatment was followed by immersion in a l g./l. chromic acid solution in distilled water, at 70 C., also for 15 seconds. The coating thus applied was not visible.

EXAMPLE V Galvanized steel panels were cleaned for 30 seconds by simple immersion in alkaline cleaner, rinsed, and treated for 15 seconds in-a 1.2 g./l. solution of Armac HT in distilled water at 60 C. The panels were then divided into two groups. "The first group was immersed in 0.1% chromic acid and the second in 0.1% potassium dichromate (both solutions made with distilled water and havinga temperatureof 70' C.).

EXAMPLE VI Four sets of galvanized steel panels, cathodically cleaned in alkali, were treated in solutions of Armac HT ranging in concentration from 0.1% to 1% (15 second immersion, distilled make-up water and 60 C.bath temperature), followed by treatment in 0.1% chrornic acid as described. At 0.1% and 0.2% amine salt the films depositedwere undetectable; at 0.5 and 1%. they appeared as .a veryslightcloudiness on the metalsurface. This cloudiness, it was found in subsequent tests," could be reduced by thorough rinsing prior to the chromate treatment.

EXAMPLE VII Four sets of galvanized steel panels, cathodically cleaned in alkali, rinsed, and treated in 0.1% Armac HT solution, were then treated in chromic acid solutions (in distilled water for 15 seconds at 70 C.) ranging in concentration from 0.1% to 2%. At 0.1% CrO the resulting film was invisible; at 0.5% the panels were slightly stained with yellow streaks; at 1% and 2% this staining became progressively more pronounced, and at 1% was too severe to be commercially acceptable. Subsequent rinsing proved to be slightly helpful in reducing this stain, but insuflicient to overcome it adequately.

EXAMPLE VIII Panels of galvanized steel I except that the amine salt were treated as in Example used was Armac 18, which is esentially octadecylamine acetate, also made by Armour and Company. Corrosion tests on these panels compared favorably with those of Example I.

EXAMPLE IX Three groups of galvanized steel panels were treated as described in Example I, with the exception that the second and third groups were immersed in chromic acid solutions that contained 75 p.p.m. sodium sulfate and 75 p.p.m. NaCl respectively. The first group was bright and unstained; the second group was objectionably discolored, and the third group exhibited some cloudiness.

The results of corrosion tests, described in Example I, applied to all the panels prepared in the above examples, are given in the following table:

Table Water Film Test Condensation Test Panels Example it No. of Degree of No. of Degree of Cycles Corrosion Cycles Corrosion IUntreated I IV Untreated 000000000 0 O H- H 1 Scoring: 0=none, =l:=traoe, l+=sllght, 2+=moderate, 3+=severe. i Galvanized steel panel, used as control.

1 16 hours instead of 3.

4 82 5 Aluminum alloy panel, used as control.

The process of this invention is treating zinc surfaces (i.e., principally galvanized iron or steel), wherein it is an exceptionally effective, inexpensive and convenient way to prevent the formation of white rust, a disfiguring type of corrosion that plagues the manufacturers of galvanized stock through its strong predilection for stacks and coils of galvanized stock in shipment or storage. It has, however, been found that the advantages conferred on zinc surfaces by this treatment are also obtainable and often as badly needed on other types of metal surfaces, such as those of steel, aluminum, cadmium, etc.

.Having thus described our invention, we claim: .1. The method of protecting metal surfaces selected from the class consisting of zinc, cadmium, iron, steel, and aluminum surfaces from corrosion comprising first contacting such surface with a solution of a fatty amine salt which is at least water dispersible and which has the formula R-NH X where R is an alkyl group having at least 8 carbon atoms and X is an anion, thereby depositing a hydrophobic fatty amine salt layer on the bare metal, and thereafter bringing said amine salt layer into direct contact with a solution of a hexavalent chromium compound.

2. The method of claim 1 in which the fatty amine salt is in water.

3. The method of claim salt is in aqueous solution.

4. The method of claim 3 in which the said anion X is selected from the group consisting of acetate, lactate and chloride.

5. Method of claim 4 in which the fatty amine salt is aqueous solution at a concentration of 0.01 to 1%. 6. The method of claim 1 wherein said solution of salt and said solution of chromium compound contain less than 5 p.p.m. of any of the anions chloride, sulfate, phosphate and silicate.

7. The method of protecting metal surfaces selected from the class consisting of zinc, cadmium, iron, steel and aluminum surfaces from corrosion comprising first contacting said surface with a solution containing a fatty amine salt which is at least water dispersilble having the formula RNH X wherein R is an alkyl group having at least 8 carbon atoms and X is an anion of the group consisting of acetate, lactate and chloride, said salt being present in said solution in an amount of 0.01 to 1.0%, whereby a hydrophobic fatty amine salt layer is depositcd on the bare metal, and thereafter bringing said amine salt layer into direct contact with a solution containing 0.05 to 0.5 of a hexavalent chromium compound, calculated as CrO 8. The method of claim 7 wherein said alkyl group contains between 8 and 18 carbon atoms and said anion is acetate.

9. The method of claim 8 wherein said salt is pres ent in an amount of 0.05 to 0.20% and said chromium compound is present in an amount of 0.1 to 0.2%.

10. The method of preventing white rust on galvanized stock comprising cathodically cleaning the stock in alkali, rinsing off the entrained alkali, applying a 0.01 to 1% solution of a fatty amine salt thereby depositing a hydrophobic fatty amine salt layer on the bare metal, and then applying a 0.05 to 0.5% solution of a hexavalent chromium compound directly to said fatty amine salt layer, said salt having the formula RNH X wherein R is an alkyl group having at least 8 carbon atoms and X is an anion of the group consisting of acetate, lactate and chloride.

11. The method of inhibiting corrosion of metal surfaces selected from the class consisting of zinc, cadmium, iron, steel and aluminum surfaces comprising cathodically cleaning the metal surface in an alkaline solution, applying to said surface a solution of a fatty amine salt, thereby to deposit a hydrophobic fatty amine salt layer on the bare metal and then applying directly to said amine particularly. useful in 1 in which the fatty amine salt layer a solution of a hexavalent chromium compound, said amine salt being at least water dispersible and having the formula R-NH X herein R is an alkyl grouphaving 8 to 18 carbon atoms and X is an acetate ion, said salt being present in an amount of 0.01 to 1%, ,said chromium compound being present in an amount of 0.05 to 0.5%, calculated as CrO 12. An article having a surface of a metal selected from the class consisting of zinc, cadmium, iron, steel, and aluminum provided with a corrosion-inhibiting film prepared by deposting a hydrophobic, fatty amine salt layer on the bare metal and then contacting saidsalt layer directly'with a solution of a hexavalent chromium compound, said .salt being at least Water-dispersible and havdrocarbon group having at least 8 carbon atoms and X is an anion.

References Cited in the file of this patent UNITED STATES PATENTS 2,206,064- Thompson et a1. July 2, 1940 2,400,784 Rust May 21, 1946 2,460,259 Kahler Ian. 25, 1949 10 2,793,932 Kahler et al. May 28, 1957 OTHER REFERENCES Protective Coatings for Metals (Burns and Bradley), published by Reinhold, New York, N.Y., 1955 ing the formula R-NH X, wherein R is an aliphatic hy- 15 (pages 552454).

Claims (1)

10. THE METHOD OF PREVENTING WHITE RUST ON GALVANIZED STOCK COMPRISING CATHODICALLY CLEANING THE STOCK IN ALKALI, RINSING OFF THE ENTRAINED ALKALI, APPLYING A 0.01 TO 1% SOLUTION OF A FATTY AMINE SALT THEREBY DEPOSITING A HYDROPHOBIC FATTY AMINE SALT LAYER ON THE BARE META, AND THEN APPLYING A 0.05 TO 0.5% SOLUTION OF A HEXAVALENT CHROMIUM COMPOUND DIRECTLY TO SAID FATTY AMINE SALT LAYER, SALT HAVING THE FORMULAR-NH3X WHEREIN R IS AN ALKYL GROUP HAVING AT LEAST 8 CARBON ATOMS AND X IS AN ANION OF THE GROUP CONSISTING OF ACETATE, LACTATE AND CHLORIDE.