US2887418A

US2887418A - Composition for and method of chrome pickling of magnesium shapes

Info

Publication number: US2887418A
Application number: US630049A
Authority: US
Inventors: Whitby Lawrence
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1956-12-24
Filing date: 1956-12-24
Publication date: 1959-05-19
Anticipated expiration: 1976-05-19

Description

water rinse. very pronounced at 30 seconds or longer. .thus formed adheres to the chromate film and resists and often escapes removal by the water rinse and remains as United States Patent l COMPOSITION FOR AND METHOD OF CHROME PICKLING OF MAGNESIUM SHAPES Lawrence Whitby, Bay City, Mich, assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Application December 24, 1956 a Serial No. 630,049

1 Claims. (Cl. 148-62) This invention concerns an improved chrome pickle solution for forming protective chromate coatings on magnesium articles, and especially concerns the method of forming such coatings which aresubstantially free from powdery deposits and which provides an improved etched surface for the formation thereon of additional protective coatings.

The term magnesium, as used herein, refers to magnesium; and alloys of magnesium of which magnesium comprises at least 85 percent by weight. The term chromate .coatings refers to coatings which contain a high percentage of a chromium compound.

Magnesium articles, such as shapes formed by extruding, forging, casting, and molding operations and articles fabricated of such shapes, have been found to have increased resistance to the attack of deteriorating substances Such a process of forming in a bath of the solution known as chrome pickle.

A problem has long persisted in the art of chrome picklingrdue to the attack upon the surface of the magnesium .a rticle. being treated by a thin film of residual chrome pickle, known as drag-out, during the interim of its transfer from the chrome pickle bath to the rinse water when this interim exceeds about thirty seconds. This attack is at least partially due to the continued action of the drag-out causing the pH value to rise to 5 and above during the transfer time. A delay in the transfer time .of about 5 seconds, after its removal from the pickling bath and prior to the Water rinse, is practiced in the art since such delay increases the uniformity ,of the coating and thereby makes an improved base for the application of coatings of paint, plastic and the like. Delays of up to 15 or 20 seconds may be tolerated but the reaction during the delay is so rapid that a delay of over 30 seconds has heretofore been avoided Whenever possible. The reason for the criticality of the period of transfer is due to the formation of a particulate material on the chromate coating after a delay of about 30 seconds which heretofore has not been adequately removable by the The formation of the particulate material is The material a powder on the dry finished magnesiumarticle. The transfer must, therefore, be rapid if an excessive residue of particulatematerial remaining after the water rinse, is to be avoided.

In commercial practice, the limitation in the permissible transfer time between bath and rinse which is ,inherent in known practice is undesirable both from the l stan'dpoint of operation and of quality of etch. As a re- "sult, therefore, the particulate deposit thus often 2,887,418 Patented May 19, 1959 present in excessive amounts on the surface of the treated article after the water rinse. Such deposit is objectionable because it is a serious impediment to the adhesion to such surfaces of paint and plastic which are commonly applied as further coatings and is a fire hazard during grinding and milling of magnesium articles thus affected.

A number of unsuccessful, orb-ut partially successful, attempts have been made to lessen the tendency of such a particulate deposit, which is formed by the dragout chrome pickle during contact with air, to resist its removal therefrom by the water rinse. Among these attempts are the inclusion of buffering agents in the pickbe present in relatively large amounts. The expense of such treatment has proved to be well nigh prohibitive in practical operations. Another attempt to lessen. the amount of the particulate material adhering to the rinsed shapeis the addition to the chrome bath of an inhibitor such as an acid fluoride of sodium, potassium, or ammonium which inhibits the etching effect of the chrome pickle solution. This clearly is disadvantageous because theetching effect of the bath is necessaryfor good paint and plastic adhesion. As a matter of fact, when: an inhibitor is added in sufiicient amounts to sufiiciently dispel the residual particulate material adhering to the rinsed shape and prevent its remaining thereon as a troublesome powder causing interference with subsepowder on the finished magnesium articles formed by the interaction of the magnesium and the dragout atan .in-

creasing pH value and yet obtaining the benefits of such action in bringing about animproved etch, by employing a suitably stable surfactant as an additament in the chrome pickling bath.

By surfactant is meant herein a material composed of molecules having a hydrophobic group and hydrophilic group which, when added to a chrome pickle solution, reduces the surface tension thereof. The chrome pickle solution containing a surfactant Within the scope of the invention has a surface tension of less than 70 dynes per centimeter in the chrome pickle bath.

As used herein, a surfactant is considered stable if it does not lose its effectiveness by decomposition or otherwise within twenty-four hours after its addition to the chrome pickle. However, surfactants which. are stable for relatively long periods of time, say for at least 20 days, are preferred. For example, a surfactant which is not stable for as long as about twenty-four hours is not recommended because the bath containing it must be revivified by additions of the surfactant to the bath more frequently than is consonant with good operation. Al-

though such additions are relatively inexpensive, they introduce a control problem which makes the use of the the structure: R--Ar-SO M where Ris: a C toC alkyl radical, AI is a naphthalene or benzene group, and Mis an alkali metal. Sodium alkylnaphthalene isulfonate illustrates this type.

(2) Alkyl sulphates, such as RC H -0SO Na where R is a C toC alkyl radical. An example of thisfis sodium-lauryl sulfate.

(3) Sulfonated esters, e.g.,

(4) Alkali salts of alkylaryl polyether sulfonates. (5) Alkanesulfonates such as those represented by where R is an alkyl chain of C to C but is preferably a C alkyl radical.

(6) Alkylphenyl polyethylene glycol ethers.

(7) Alkali salts of fluoro-substituted alkanesulfonates represented by R SO M where R is a perfluorinated alkyl radical having from 5 to 18 carbon atoms and M an alkali metal or ammonium. Examples of this type of surfactant are CF (CF SO K and CF3(CF2)7SO3K.

(8) Those in which the hydrophilic fraction of the molecule is provided by ethylene oxide and the hydrophobic fraction is provided by, a carboxylic acid, RCOOH; by an aliphatic alcohol, ROH; a glycol, HOR'OH; a polyoxyalkylene glycol, HOROROH; by an alkylphenol, ArROH; a phenol, ArOH; or by a mercaptan, RSH. R is an alkyl chain consisting of 8 to 18 carbon atoms and R' is an alkyl chain of 2 to 4 carbon atoms. An example of a reaction product of ethylene oxide and an alkylphenol is one having the general formula; n being an integer oar-[110w omormnon (9.) Alkylphosphates as represented by the expression:

general OR R-OP O OR where R is an alkyl group of 8 to 16 carbon atoms and R is a solubilizing group.

(10) Substituted amides formed by the reaction of a carboxylic acid and ethanolamine, e.g.

(12) The condensation products of a carboxylic acid and an amine, e.g.

where .R is a C to C alkyl radical and R is an alkyl "radical of 2 to 4 carbon atoms.

(13) Monoethers having the generic formula R(OR'),,(OR"),,,H

where R is an alkylchain of C or over, R is a C to C alkylene group, R" is the C alkylene group, n is 6 or more, and m has a value such that (OR) constitutes at least 25 percent of the total weight of the molecule.

In practicing the invention, the magnesium shape or article to be coated is cleaned, if necessary, by immersing it in an aqueous alkaline solution or by scrubbing it with such a solution in the customary manner employed in preparing magnesium shapes for chromate coating. Usually the temperature is between 190 to 212 F. and the period of cleaning from 3 to 10 minutes. The solution may be prepared by dissolving about 3 oz. by weight of sodium carbonate and about 2 oz. by weight of sodium hydroxide in enough water to make a gallon of solution. After dipping or scrubbing the magnesium, it is rinsed well with cool water. It is then treated by subjecting it to the novel aqueous chrome pickle solution of the invention at 70 to F. for 0.5 to 2 minutes. This solution of the invention is prepared by dissolving in water from 9 to 40 percent by weight, but preferably about 18 percent by weight, of an alkali metal chromate, e.g., sodium dichromate, and from 4.5 to 60 percent by Weight, but preferably about 26 percent by weight, of 70 percent aqueous nitric acid (sp. gr. of about 1.41), and between 0.01 to 0.5 percent by weight of a surfactant as described hereinabove. The treated magnesium is removed from the chrome pickle, exposed to air for from 15 to seconds, and rinsed thoroughly with water. The recommended procedure is to rinse first with cool water at a temperature from 40 F. to 70 F. and thereafter rinse with hot water at a temperature from about to F.

An improved chromate coating is thus formed on the surface of the magnesium shape by the action of the improved chrome pickle of the invention. The chrome pickle containing a surfactant as defined herein attacks the magnesium surface, and thereby produces a more uniform etch as shown by microscopic examination than do chrome pickles wherein no surfactant is used. The coatings thus formed are readily rinsed substantially free from discrete particles on the surface. Furthermore, the coated magnesium piece need not be unduly hurried in its transfer from the pickle to the rinse. A time well beyond 30 seconds may now be conveniently employed to produce a superior etch according to my invention. A period of 0.5 to 1.0 minute is usually employed, but a period up to 2 minutes may be employed, to give excellent results.

Examples of the practice of the invention with various surfactants are described below, pertinent data as to the surfactants used and results obtained being set forth in the accompanying table.

For each of the examples, prior to treatment according to the invention, a 3" x 6" x 0.064" test piece of a magnesium alloy, consisting essentially by weight of 96 percent magnesium, 3 percent aluminum, and 1 percent zinc, was prepared by dipping it for 5 minutes in an alkaline bath at 200 F., the bath being made according to the following formulation:

Sodium carbonate (Na CO .l0H O), 3 oz. by weight; Sodium hydroxide (NaOH), 2 oz. by weight; Water sufficient to make 1 gallon of solution.

Sodium dichromate (NaCr O .2H O), 710 ,g.;

70 percent nitric acid, sp. gr. 1.4, 1065 g.;

A surfactant as tabulated and sufficient water to make about 4 liters of solution at room temperature.

The pieces were removed from the solution, allowed to drain vertically for 60 seconds and then rinsed for .30

seconds in cool Water at about 55 F., thereafter immersed in hot water at 170 F. for 5 seconds and dried at room temperature for one hour. As a result, on the surface of each test piece, a chromate coating was formed which was substantially uniform in thickness and coloring and substantially free from powdery residue due to adhering particulate material. To determine the amount of powdery residue adhering thereon, if any, each test piece was thereafter weighed and vigorously rubbed with :clean dry' cotton and then reweighed to determine the loss of weight due to the removal of any powdery material which had formed on the surface. The losses so .de- Rtermined in repeat runs were averaged.

As a blank or control, two repeat runs were made in which like pieces of magnesium alloy were used except no surfactant was present in the pickle. The weight of powder removed inthe two repeat runs was averaged. The average weight of powder removed from the control samples is presented in the table as the blank.

Various surfactants were selected from different types and tested according to the above described procedure. The type and amount of surfactant used and the average weight of powder removed from 1 to 4 samples of 3" x 6" x 0.064" pieces of the magnesium alloy treated according to the invention are set out as Examples 1 to 30 in the table.

TABLE Efiects on chromate coating of difierent surfactants in varying amounts Gone. in No. 01' Avg. Wt.

Example Surfactant in Pickling Bath Wt. Per- Samples of Powder cent Run in Grams None 2 0519 Potassium perfiuoro n 0. 01 2 0186 oetanesulfonate. 100% N a lauryl sulfate 0. 025 4 0058 (Stepanol ME).

do 0.05 4 0087 do 0.1 4 0057 -do 0. 5 4 0076 100% solubllized alkyl plans 0. 1 1 0262 phate (Victawet 12). 25% Na-ethylhexene sulfon- 0.025 4 0181 ate (Tergitol EH).

0.1 4 0050 n 0.2 2 i 0139 n 0. 5 2 .0135 40% Na 2eth lhexyl sulfate 0.025 3 .0151

(Tergitol 08 do 0.05 3 0165 d 0. 1 3 0137 100% polyoxyalkylene glycol 0. 01 4 0168 ether (Tergitol XD) do 0. 025 4 0065 ---do 0. 4 0114 d0 0.1 4 0025 100% allk lvl plienyll t liloly- 0.05 4 0085 0 et ene co e er. jtio .z %Z H-i 0.1 i 100% alkyl polyoxyet y ene 0. 01

glycol ether (Tergitol TD).

21 do 0.025 4 0204 do 0.05 4 .0266 .....(lo 1 0.1 gl

100% alkylaryl o yether 0. 025

alcohol (Triton 102). do 0.05 4 0218 do 1 0.1 4 100% alkylaryl p0 yether 0. O5 4 alcohol (Triton X-100). d0 0.1 4 .0081 28% N a alkylaryl polyether 0. 05 4 0214 sulfonate (Triton X-200). do 0. 1 4 0146 In the above table, the examples of the invention show that the presence of the surfactant employed in the chrome pickling bath definitely reduces the amount of powdery material retained on the surface of magnesium articles which have been treated in the chrome pickle bath, trans ferred through air to a Water rinse, and rinsed in the water, even though the transfer time is as much as ninety seconds. This period of time is over 3 times as long as the maximum time usually permissible for such transfer when a conventional chrome pickling bath is used.

losing its effectiveness after the bath containing it had been usedfor over days.

EXAMPLE 31 Magnesium alloy pieces which had been treated in the chrome pickle containing 0.1 weight percent of the 25 percent strength Na 2-ethylhexene sulfonate according to Example 8 of the table and those which were treated in the same chrome pickle except it did not contain a surfactant, i.e., prepared according with thefprocedure followed in the blank of the table, where subjected topaint durability tests. In these tests, a primer coating which was prepared according to United States Government specifications No. MIL6889A, Type 1, was applied to the magnesium alloy pieces and top-coated with an aluminumpigmented nitrocellulose lacquer. The zinc chromate primer is known to possess only fair adhesion to magnesium alloys after aging. All pieces were exposed to like natural outdoor weathering conditions. After 12 months of such exposure, the adhesion of the paint over the straight chromate coating was only fair whereas that over the chromate coating formed by the chrome-pickling solution as modified by addition of the surfactant according to the invention was excellent. Exposure was continued for a total of 18 months. A scratch test was then used to evaluate the degree of adhesion or strength of the bond formed between the paint and the magnesium metal. The degree of adhesion was ascertained according to a cross-hatch scratch test. This test consisted of making, by means of a steel needle, a series of scratches through the paint at approximately /2 millimeter intervals and repeating such scratches at right angles to and over those previously made, thus preparing a number of squares of painted surface of /2 millimeter sides bounded by scratch lines. The scratches were made while the panel was under observation with a low-power binocular microscope. Complete flaking of the paint over the conventional chromate coating took place when it was sub jected to the scratch test, whereas such test caused no flaking of the paint over the chromate coating formed by treatment with the chrome solution modified by the surfactant according to the invention.

The paint durability as shown by the scratch test demonstrates that the reduction in powdering and improved etching of magnesium alloys treated with a chrome pickle containing a surfactant in accordance with the invention results in a marked improvement in the firm retention of paints applied thereover, particularly when the primer paint is one which manifests increasingly poor adhesion to magnesium surfaces with aging upon exposure to natural outdoor conditions.

From the foregoing examples of the invention, it is not to be inferred that the invention is to be limited to the chrome pickling solutions containing surfactants named therein, as the invention contemplates broadly the employment of a surfactant which is soluble and stable in a chrome pickling solution, which permits a longer period of exposure to air during transfer of a magnesium article from the solution of a water rinse and substantially decreases the particulate material adhering to the article.

Having described the invention, what is claimed and desired to be protected by Letters Patent is:

1. In a process of chromate coating of a magnesium article by subjecting the article to the action of a chrome pickle solution consisting essentially of an aqueous solution of nitric acid and an alkali metal dichromate the step of admixing in said chrome pickle solution between 0.01 and 0.5 percent by weight of a surfactant selected from the class consisting of alkylaryl sulfonates, alkyl sulfates, alkanesulfonates, halo-substituted alkane sulfonates, sulfonated esters, and alkylaryl polyether sulfonates.

-2. The processes according to claim 1 wherein said alkali metal salts of a halo-substituted alkanesulfonates is an alkali metal salt of a perfluoroalkanesulfonate having from 5 to 18 carbon atoms.

3. The process according to claim 2 wherein said alkali salt of a halo-substituted alkanesulfonate is the potassium salt of perfluoro-n-octanesulfonic acid.

4. An aqueous chrome pickling solution for treating articles of magnesium and its alloys, said solution containing 3 to 40 percent by weight of nitric acid and from 9 to 40 percent by weight of a dichromate selected from the group consisting of ammonium and alkali metal dichromates, and from 0.1 to 0.5 percent by weight of a surfactant selected from the class consisting of the alkali metal salts of alkylaryl sulfonates, alkyl sulfates, al-

kanesulfonates, halo-substituted alkanesulfonates, sulfonated esters, and alkylaryl polyether sulfonates.

5. The pickling solution according to claim 4 wherein the halo-subsituted alkanesulfonate is an alkali metal pe'rfluoroalkanesulfonate having from 5 to 18 carbon atoms in the alkane group.

8 6. The pickling solution according to claim 5 wherein the alkali metal is potassium.

7. The pickling solution according toclaim 6 wherein the potassium perfluoroalkanesulfonate is potassium perfiuoro-n-octanesulfonate having the formula:

CF (CF SO K References Cited in the file of this patent UNITED STATES PATENTS 2,301,983 Tanner Nov. 17, 1942 2,321,948 Shawcross June 15, 1943 2,548,420 Chester Apr. 10, 1951 2,610,133 Thomson Sept. 9, 1952 FOREIGN PATENTS 287,450 Great Britain Oct. 18, 1928 921,686 France Jan. 13, 1947 OTHER REFERENCES Metals and Alloys, February 1945, pages 417 -434. Page 423 relied on.