US3278343A - Conversion coating of magnesium alloy surfaces - Google Patents

Description

United States Patent 3,278,343 CUNVERSION COATING 0F MAGNESIUM ALLOY SURFACES John M. Leuzinger, Detroit, Mich., assignor to Amchem Products, llnc., Ambler, Pa., a corporation of Delaware No Drawing. Filed Mar. 12, 1963, Ser. No. 264,660 8 Claims. (Cl. 1486.16)

This invention relates to the art of coating magnesium and is more particularly concerned with the application of chemical conversion coatings on magnesium alloy surfaces. It provides a new method for forming such coatings, which method is especially useful in the coating of certain magnesium alloys which are unusually diflicult to coat. For instance, alloys of magnesium which contain from about 6 to about 9% of aluminum are the alloys to which the invention is particularly applicable. Examples of alloys of this kind are the so-called diecasting alloys and those types which are referred to as sand mold'castings and extrusions. These alloys, in addition to their 69% of aluminum content, also contain from about 0.5 to about 3% zinc and from about 0.13 to about 0.3% manganese, the balance being magnesium. Typical, but non-limiting, examples of alloys of this type are those which are designated by the American Society of Testing Materials as AZ-63, AZ-91 and AZ92.

Before proceeding to a detailed description of the present invention it is desired to refer briefly to certain facts which are well known to those skilled in this art. Magnesium, of course, is relatively reactive and comparatively easy to coat but despite this face very few, if any, of the coating procedures heretofore advanced for the purpose have been completely acceptable, especially from a practical commercial standpoint. This applies particularly to the alloys above referred to. A few processes have been adopted for commercial use but they involve rather long, laborious treatments which are expensive to operate and are wholly inconsistent with the rapid, present day production techniques which are demanded by industry.

Among prior processes for the coating of magnesium surfaces are those which combine dichromates with either nitrates or fluorides; or chromates in combination with chlorides or mixtures of chlorides and nitrates; or mixtures of chromates and phosphates. Furthermore, the coating processes which utilize halides, particularly chlorides, have not proven satisfactory in view of the corrosive action which is imparted to the surface by the halide ions occluded in the coatings.

With all of the foregoing in mind, the principal object of the present invention is to provide an improved method of forming chemical conversion coatings on the surface of magnesium alloys, which method completely overcomes the objections of former practices, which yields coatings having excellent paint bonding and corrosion resistant properties, which can be applied in very short periods of time, which can be employed at very much less cost and which is particularly useful in the coating of the alloys mentioned above.

How the foregoing objects and advantages together with such others as are incident to my invention or which may appear hereinafter are attained will now be described in detail.

Broadly speaking, I have discovered that if clean magnesium alloy surfaces, and especially the surfaces of alloys of the type described above, are subjected first to the action of an aqueous activating solution consisting essentially of a phosphate having a molar ratio of cationic oxides to anionic oxides of from 1 to 2 plus fluoride ions in amounts as hereinafter specified and, following water rinsing, are thereafter subjected to the action of a halidefree coating solution containing, as its principal and es sential coating producing ingredients, the ions of both 3,278,343 Patented Oct. 11, 1966 trivalent and hexavalent chromium plus a phosphate, in the amounts and ratios as specified hereinafter, there is produced on the surface a chemical conversion coating which has unusually excellent paint bonding and corrosion resistant properties.

As noted above, the aqueous activating solution must contain a phosphate having a molar ratio of cationic oxides to anionic oxides of from 1 to 2. Such phosphates as defined in the Encyclopedia of Chemical Technology, volume X, pages 403-405 (1953), include metaphosphates, wherein the cationic to anionic oxide ratio is 1, polyph'osphates, having ratios between 1 and 2, and pyrophosphates, wherein the ratio is 2. While all of these phosphates have been found to perform satisfactorily in the activator solutions of this invention, the preferred phosphates have been found to be the pyrophosphates inasmuch as these provide greater heat stability with less reversion to the ortho form than either the meta or polyphosphates.

The amount of phosphate, from the class described, which must be employed in the aqueous activator solutions of this invention has been found to be from 4 to 70 grams/liter, calculated as P 0 So far as the fluoride ion content is concerned, it has been found that this constituent must be utilized in amounts of from 0.5 to 10 grams/liter, calculated as F, 1n order to insure obtaining the improved coatings on magnesium die-casting alloy surfaces.

In addition to maintaining the phosphate and fluoride ion content within the limits noted hereinabove, it is also essential that the solution pH be maintained between 7.0 and 11.0 in order to insure obtaining the required activation of magnesium alloy surfaces prior to coating formation. Where the activator solution pH is allowed to fall below the minimum pH value of 7.0, or to rise above the 11.0 maximum limit, the subsequently produced coat- 1ngs on magnesium surfaces will be found to be thin and uneven and to possess little or no corrosion resistance.

So far as introduction of the phosphate and fluoride ions are concerned it is preferred to use tetrasodium, potassium or ammonium salts of the phosphate acids, particularly of pyrophosphoric acid, and the sodium, potassium or ammonium fluoride salts. Use of these salts provides solution pHs within the range desired with no need for subsequent adjustment thereof. However, it is within the purview of this invention to employ any of the alkali metal salts of these phosphate acids, or to employ only phosphate acids and hydrofluoric acids in preparing the activator solutions of this invention, provided of course that the solution pH is adjusted to the range noted above. The solution pH is controlled by the use of alkali metal or ammonium hydroxides where less basic salts or the acids of the essential anions are employed.

In general, it has been found that where the amount of phosphate ion present is on the high side of the permitted range then the amount of fluoride ion should be maintained near the upper permissible limit. Conversely, where the phosphate ion concentration is more dilute, that is where it is present in the lower limit of concentration, then the amount of fluoride ion should also be kept on the low side of the permitted range.

The temperature at which the activator solution is operated is critical it completely satisfactory coatings are to be obtained. It has been found that the temperature range of to F. provides the required activation of magnesium alloy surfaces. Where temperatures of less than 130 F., or greater than 180 F. are employed the magnesium surfaces will not properly be conditioned for reception of a chemical conversion coating of the second stage, and subsequently produced coatings will not demonstrate the required paint bonding and/or corrosion resistant qualities.

The overall time of treatment is not critical so long as the activator solution temperature is maintained between l30180 F. as noted hereinabove. However, it is preferred to operate the activator stage for periods of time ranging from 30 seconds to about 5 minutes duration.

Insofar as the second major step is concerned, i.e. the coating step per se, the following factors are to be observed.

The coating solution must be prepared so as to contain, initially, at least 0.75 grams/liter of trivalent chromium ion calculated as Cr Where the coating solutions are initially prepared without the addition of this essential ingredient, notwithstanding the fact that all of the other ingredients may have been added thereto, the solution simply will not produce the required corrosion resistant and paint bonding coatings on magnesium die-casting alloy surfaces.

Use of large amounts of trivalent chromium, for example up to about 20 grams/liter does not appear to cause any deleterious effects upon either the coating formation or its quality. However, since no added benetfis are obtained from high concentrations of this constitutent it is preferred to add no more than about 1.5 grams/liter of trivalent chromium to a freshly prepared coating solution.

Introduction of the trivalent chromium ion may be by way of any salt which is soluble in the coating solution, just so long as the anion thereof is in no way detrimental to the coating reaction. A preferred salt is chromic nitrate since this is not only readily available, but it also aids in controlling the acidity of the coating solution.

Hexavalent chromium ion must be present in the coating solution in amounts ranging from 2 to 46 grams/liter (calculated as Cr). Where less than 2 grams/liter of hexavalent chromium are used, a soft, non-adherent, black coating is obtained; whereas use of more than 46 grams/ liter of hexavalent chromium results in little or no coating being formed upon magnesium die-casting alloy surfaces.

A preferred range of hexavalent chromium ion content has been found to be from about 4 to 30 grams/liter, since within this narrower range optimum coatings have been obtained consistent with desired quality.

The hexavalent chromium ion may be supplied by chromium trioxide, or by an alkali metal or ammonium chromate or dichromate salt. It is preferred to utilize chromium trioxide since this particular source of hexavalent chromium ion does not introduce alkaline cations which in turn require the addition of hydrogen ion in order to maintain the essential pH range as discussed hereinafter.

Phosphate ion (calculated as P is preferably introduced in the form of phosphoric acid, and this is particularly true where a salt of chromic acid has been employed. If desired, however, the phosphate may be added in the form of an alkali metal or ammonium salt, but such usage will necessitate pH adjustments to compensate for the alkalinity of the cations employed. The amount of phosphate ion, calculated as P0 which is essential to the successful operation of the process of this invention has been found to be from about 3 to about 52 grams/liter of the coating solution. Where less than 3 grams of phosphate ion are employed no useful coating is formed on the magnesium alloy surfaces, if indeed a coating is formed at all. Conversely, if more than about 52 grams of phosphate ion are utilized the coating produced will be non-adherent and completely unsuited to receive a siccative finish.

Experience with the process of this invention has shown that use of from 5 to about 44 grams/liter of phosphate ion, calculated as P0 provides excellent and completely acceptable coatings, so that this narrower range of phosphate ion is preferred in order to insure optimum coating results.

In addition to the requirements that the hexavalent chromium and phosphate ion concentrations (calculated as Cr and P0 respectively) be maintained within the limits indicated above, it is also essential that the ratio of hexavalent chromium to phosphate be maintained within the range of 1 part of hexavalent chromium to from 0.5 to 2.5 parts of P0 This range has been found to be absolutely essential for the production of corrosion resistant, .paint bonding coatings on magnesium die-casting alloys which contain from 6 to 9% aluminum, since coating solutions, even if prepared to contain the above indicated amounts of both hexavalent chromium ion and phosphate ion, will fail to produce the desired coating quality if the ratio of hexavalent chromium to P0 lies outside the range of 1 to from 0.5 to 2.5 as indicated above.

The acidity of the coating solution is critical, and if completely satisfactory results are to be obtained, such acidity, as measured by pH, must fall within the narrow range of 0.8 to 1.2. Where the solution acidity is permitted to fall below the 0.8 lower limit the resultant coatings will be soft and non-adherent. Conversely, use of a solution having an acidity, as measured by pH, above 1.2 will result in dark colored, soft and non-adherent coatings having essentially no paint bonding or corrosion resistant properties.

In the interests of insuring the desired high quality coatings which result from the process of this invention, it is preferred to operate the coating solution within the narrower pH range of 0.9 to 1.1. This preferred range provides a small working margin on either side but still within the broader critical pH range specified and serves to guard against obtaining poor, non-adherent coatings which will result, as noted hereinabove, from operation outside the indicated critical pH range.

In order to maintain the required pH range throughout operation of the coating process it is periodically necessary to add increments of nitric acid, since neither phosp'horic nor chromic acids when used alone will yield a pH sufficiently low to fall within the required range. Use of halide ions, particularly chloride ions, in any form must be strictly avoided in view of adverse corrosion results caused by halide ions occluded within the coatings produced upon magnesium alloy surfaces. Use of sulfuric acid for pH control must also be avoided since it has been found that the presence of sulfate ions leads to powdery coating formation, which coatings fail to provide either adequate corrosion resistance or the desired paint bonding properties. Should the coating solution pH be found to be lower than 0.8, the addition of ammonium or alkali metal hydroxides will serve to restore the pH to the required range.

The temperature at which the coating stage may be operated is not critical and may range from average living room temperature (i.e. 70 F.) to as high as 190 F. However, the preferred operating range has been found to be from about 70 to about F. using a contact time of as little as 20 seconds to several minutes. Generally, as is well known in the art, a lower coating temperature will require longer contact times, while the reverse is also true.

It is of course important that the magnesium die-casting alloy surfaces be degreased or otherwise cleaned and pickled according to well established prior art practices before treatment by the process of this invention. However, since neither the cleaning nor the pickling stages form any part of the present invention, they are not described in detail.

The magnesium articles may be immersed in the treating solutions of this invention or the treating solutions may be sprayed, flowed, brushed or otherwise brought into contact with the magnesium alloy surfaces in accordance with conventional practices in this art.

In order to demonstrate the process of the present invention, magnesium alloy AZ-91 (9.0% aluminum, 0.2% manganese, 1.0% zinc, balance magnesium) castings were cleaned and pickled according to well known prior art practices and then subjected to the action of an activator solution containing:

Example I Ingredient: Grams N84Pz07 NaF 3 4 Water, to make 1 liter.

Ingredient: Grams NaH2PO4 8 Nagcrzoq CrO 5 .4 Cr (N0 3 6 Water, to make 1 liter.

This solution was adjusted to a pH of 1.0 by adding thereto 14 grams of 42 B HN0 per liter of solution.

The activated magnesium alloy castings were immersed in this coating solution for a period of 1.5 minutes at 85 F., and were thereafter rinsed with fresh Water. F 01- lowing this coating treatment the castings were painted with a primer finish and baked at 250 F. for ten minutes. A baking enamel was then applied and cured at 250 F. for 20 minutes.

The treated and enameled castings were then subjected to standard salt spray corrosion testing (ASTM B-1l7-57T) and after 1047 hours these castings were found to be perfect in their resistance to corrosion as measured by this test method.

Control castings of the same magnesium alloy similarly treated in the activator and coating solutions, but unpainted, were found to show only very slight corrosion after 1047 hours exposure in the salt spray test.

Additional castings treated by the above process and including both painted and unpainted specimens were subjected to standard water soak corrosion testing in accordance with ASTM D-870-54T and after a period of 912 hours were found to be in perfect condition. Still other AZ-91 alloy magnesium die-castings, treated in accordance with the foregoing example, and then painted, were subjected to standard humidity testing in accordance with ASTM JAN H-792 and after 1000 hours of exposure were found to be in perfect condition.

Example II Magnesium AZ63 (6.0% aluminum, 0.25% manganese, 3.0% zinc, balance magnesium) die-casting alloys were immersed in a solution containing:

Ingredients: Grams NflzHzPzOq 1 HF 0.53 Non-ionic wetting agent 0.1 Water, to make 1 liter.

The pH of this solution was adjusted to with 50% NaOH solution. A contact time of 2 minutes at 175 F. was employed after which the castings were water rinsed and then subjected to the action of the following coating Water, to make 1 liter.

This solution was adjusted to a pH of 0.9 by adding thereto 42 B. HNO A contact time of 3 minutes at 75 F. was employed after which the castings were immediately water rinsed. Following rinsing, both primer and baking enamel finishes were applied to the castings as noted following Example I and various of the castings so treated were subjected to the standard salt spray, water soak and humidity tests. All of the corrosion test results showed perfect, unmarred surfaces after 1000 hours each in water soak and humidity tests and after 2117 hours in salt spray, all tests being identical to the ASTM test methods listed after Example I.

Example III Magnesium AZ-92 (9.0% aluminum, 0.15% magnesium, 2.0% zinc, balance magnesium) alloy die-castings, previously cleaned and pickled in accordance with the prior art procedures, were immersed in an activating solution containing the following constituents:

Ingredients: Grams N34P207 KF 30.5 Non-ionic wetting agent 0.1

Water, to make 1 liter.

The pH of this solution, as prepared, was 9.5. The alloy castings were immersed in this solution for 30 seconds at 180 F. After treatment the castings were water rinsed and immediately subjected to the action of the following coating solution:

Ingredients: Grams (NH4)2CI'O4 C1'O3 Cr(NO 7.5 H PO 45 Water, to make 1 liter.

The pH of this solution was 1.2 and the magnesium diecastings were immersed in the coating solution for a period of 25 seconds at 75 F. After coating the castings were water rinsed and then had applied thereto both primer and baking enamels as indicated following Example I above. Standard water soak and humidity testing, in accordance with the foregoing examples, showed that after 1000 hours there was no corrosion on any of the diecasting alloy surfaces. After 2117 hours in 5% salt spray testing, examination of the castings showed only very slight corrosion on the edges thereof. It will be noted that the activator solutions of Examples II and III incorporated therein a small quantity of a non-ionic wetting agent. It is within the purview of this invention to add from 0.01 to 0.5% of a non-ionic wetting agent in order to increase the wettability of the activator solution with respect to the magnesium alloy surfaces. Preferred nonionic Wetting agents include the polyethoxylated alkyl phenols having from 6 to 15 mols of ethylene oxide and from 8 to 9 carbon atoms in the alkyl chain.

The foregoing results clearly demonstrate the very high degree of corrosion protection afforded to magnesium alloys containing 6 to 9% aluminum as well as the excellent paint adhesion qualities resulting directly from the process of this invention.

I claim:

1. In the art of applying a chemical conversion coating to a magnesium alloy surface of the type which contains from approximately 6 to approximately 9% of aluminum, the method which comprises (A) subjecting the surface to the action of an aqueous activating solution consisting essentially of a phosphate having a molar ratio of cationic oxides to anionic oxides of from 1 to 2, the quantity of such phosphate being from 4 to 70 grams/ liter, calculated as P 0 and fluoride ions in an amount of from 0.5 to 10 grams/liter, calculated as F, the pH of said activating solution being between 7.0 and 11.0 and its temperature between and F.;

(B) rinsing the surface; and then (C) subjecting the surface to the action of a coating solution consisting essentially of trivalent chromium ion, hexavalent chromium ion and phosphate ion the quantity of trivalent chromium ion being at least 0.75 gram/liter, of hexavalent chromium ion from 2 to 46 grams/liter (calculated as Cr) and of phosphate ion (calculated as P from about 3 to about 5 52 grams/liter; maintaining the ratio of hexavalent chromium to phosphate within the range of 1 part of hexavalent chromium to from 0.5 to 2.5 parts of P0 and maintaining the acidity of said coating solu tion (as measured by pH) within the range of 0.8 to 1.2.

2. The method of claim 1 wherein the phosphate for the activating solution is chosen from the class which consists of the pyrophosphates.

3. The method of claim 1 wherein the phosphate for the activating solution is chosen from the class which consists of the tetrasodium, potassium and ammonium salts of the phosphate acids and, further, wherein the fluoride is chosen from the class which consists of the sodium, potassium and ammonium fluoride salts.

4. The method of claim 1 wherein the pH of the coating solution is maintained at from 0.9 to 1.1 by adding nitric acid as required.

5. The method of claim 1 wherein the temperature of the coating solution is maintained at from to F 6. The method of claim 1 wherein the trivalent chromium ion is supplied by employing chromic nitrate.

7. The method of claim 1 wherein the hexavalent chromium ion is supplied by employing chromium trioxide.

8. The method of claim 1 wherein the hexavalent chromium ion is supplied by employing chromium trioxide in an amount of from 4 to 30 grams/ liter.

References Cited by the Examiner UNITED STATES PATENTS 1/1959 Dell 148-6.l6 2/1964 De Long et a1 148-6.27 X

Claims (1)

1. IN THE ART OF APPLYING A CHEMICAL CONVERSION COATING TO A MAGNESIUM ALLOY SURFACE OF THE TYPE WHICH CONTAINS FROM APPROXIMATELY 6 TO APPROXIMATELY 9% OF ALUMINUM, THE METHOD WHICH COMPRISES (A) SUBJECTING THE SURFACE TO THE ACTION OF AN AQUEOUS ACTIVATING SOLUTION CONSISTING ESSENTIALLY OF A PHOSPHATE HAVING A MOLAR RATIO OF CATIONIC OXIDES TO ANIONIC OXIDES OF FROM 1 TO 2, THE QUANTITY OF SUCH PHOSPHATE BEING FROM 4 TO 70 GRAMS/LITER, CALCULATED AS P2O7, AND FLUORIDE IONS IN AN AMOUNT OF FROM 0.5 TO 10 GRAMS/LITER, CALCULATED AS F, THE PH OF SAID ACTIVATING SOLUTION BETWEEN 7.0 AND 11.0 AND ITS TEMPERATURE BETWEEN 130* AND 180* F.; (B) RINSING THE SURFACE; AND THEN (C) SUBJECTING THE SURFACE TO THE ACTION OF A COATING SOLUTION CONSISTING ESSENTIALLY OF TRIVALENT CHROMIUM ION, HEXAVALENT CHROMIUM ION AND PHOSPHATE ION THE QUANTITY OF TRIVALENT CHROMIUM ION BEING AT LEAST 0.75 GRAM/LITER, OF HEXAVALENT CHROMIUM ION FROM 2 TO 46 GRAMS/LITER (CALCULATED AS CR6+) AND OF PHOSPHATE ION (CALCULATED AS PO4) FROM ABOUT 3 TO ABOUT 52 GRAMS/LITER; MAINTAINING THE RATIO OF HEXAVELENT CHROMIUM TO PHOSPHATE WITHIN THE RANGE OF 1 PART OF HEXAVALENT CHROMIUM TO FROM 0.5 TO 2.5 PARTS OF PO4; AND MAINTAINING THE ACIDITY OF SAID COATING SOLUTION (AS MEASURED BY PH) WITHIN THE RANGE OF 0.8 TO 1.2.