US3620940A

US3620940A - Method of inducing polarization of active magnesium surfaces

Info

Publication number: US3620940A
Application number: US36665A
Authority: US
Inventors: Reyburn Wick
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1970-05-12
Filing date: 1970-05-12
Publication date: 1971-11-16
Anticipated expiration: 1988-11-16

Abstract

Methods for surface polarizing magnesium and its alloys; thus permitting large pieces to be readily anodized using only nominal electric power. Low adhesion, easily strippable film strips are adhered to the pieces, the strips providing areas of exposed metal which allow a high current density to be built up thereat which produces vigorous electrolytic action for progressively peeling off the films for subsequent anodization of the pieces.

Description

United States Patent Reyburn Wick Southampton, Pa.

May 12, 1970 Nov. 16, 1971 The United States of America as represented by the Secretary of the Army Inventor Appl. No. Filed Patented Assignee METHOD OF INDUCING POLARIZATION OF ACTIVE MAGNESIUM SURFACES 8 Claims, No Drawings US. Cl 204/56 M, 204/ l 5 Int. Cl C23b 9/00, C23b 5/48 Field of Search 204/15, 56 M [56] References Cited UNITED STATES PATENTS 2,214,876 9/l940 Clark 2,348,826 5/l944 Krause et al.

2,880,148 3/l959 Evangelides Primary Examiner-John H. Mack Assistant Examiner-T. Tufariello 204/56 M 204/56 M 204/56 M Attorneys-Harry M. Saragovitz, Edward J. Kelly, Herbert Berl and S. Dubroff sequent anodization of the pieces.

METHOD OF INDUCING POLARIZATION OF ACTIVE MAGNESIUM SURFACFS The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

This invention relates to the anodization of magnesium and magnesium alloy pieces, and more particularly concerns improved methods for their anodization by inducing polarization at newly exposed specified surfaces thereof.

A difficulty sometimes encountered in anodizing magnesium and its alloys in the past resided in their inability to polarize. With high-voltage alkaline processed, required current density may readily be attained, but voltage will fail to rise to proper operational range, i.e., from about 60 to 100 volts. lfnot corrected, operation under low voltage conditions will result in formation of an undesirable thin film, rather than a normal thick coating.

Another practice employed was to apply a high source of current to the magnesium work pieces such that current density is momentarily increased approximately three times the nonnal value. In most instances, anodization would proceed normally thereafter. In a few instances, a repetition of the surge would be necessary. If the amount of the work being processed however required the entire capability of the power supply, there would be no reserve capacity to supply the needed surge of current.

In such cases, it was necessary to reduce the size of the load in the tank such that present current capacity would yield the desired surge load. Such reductions obviously lengthen process time with obvious undesirable effects. An alternative would be to divide the work lots into sufficiently small groups partially anodize each, then rejoin them for the final portion of anodizing. In the case of very large pieces, it is apparent these procedures described above would not successfully be utilized.

Such large parts may be allowed to oxidize in the air for several days, after which time they may be reanodized with an increased chance of proper polarization occurring.

It is therefore an object of this invention to provide improved methods for anodizing magnesium and its alloys.

Another object of the invention is to provide methods whereby polarization if newly exposed magnesium surfaces is readily induced.

Still another object of the invention is to provide methods whereby larger magnesium or magnesium alloy pieces may be efficiently and rapidly anodized with a nominal supply of electric power.

These and other objects of the invention will be apparent to those skilled in the art upon studying this disclosure.

ln accordance with the aforementioned objects, I have discovered that a high current density will result at predetermined portions of a magnesium piece to thereby efiect immediate polarization at those portions.

More specifically, l have discovered that selected areas of magnesium pieces may be masked off with a strippable film having poor stopoff qualities or a low adhesion to the magnesium or magnesium alloy, to thereby effect polarization of those surfaces not so masked. The anodizing process may then be started and the current adjusted to that required for the entire area of the workpieces, and resulting in a high current density begins applied to the exposed magnesium surfaces which then become polarized. Within a few seconds, the vigorous electrolytic action occurring adjacent the strippable film edges will roll the film back to expose more bare metal. Because of the low resistance of the freshly exposed metal, a larger amount of current passes thereat, resulting in a higher density. As the film peels progressively, this high current density area follows until the entire film drops off. Anodizing then proceeds normally over the entire piece.

For maximum effectiveness, the film should be applied in bands or strips around the workpiece, about an inch or so wide, the bands being approximately one-sixteenth to onefourth inch from each other. The film material is preferably a plastic coating compound, strippable, having poor stopoff properties, and is described in Military Specification MlL-P-4 502"), Mar. 18, I969. It comprises:

Cellulose ucetate butyrute'. (maximum free acidity, ul acetic acid 0.02%) 56 g. Chlorinated biphenyl 52 g. Polyethylene glycol di-2-ethyl-hexunoate l2 Mineral oil 8 g. Xylol I67 ml. lmpropunol 82.3 ml. Phenylsulicylate l g. Tn'lauryl phosphate 1 g. Di-2-ethyl hexylamine or methyl morpholine 1 ml. Solubilized l0% copper-B-quinolinolate 0.75 ml.

The preservatives and fungicides called for in the Military Specification need not be included for these purposes.

My invention is not limited to the above strippable film compound, any plastic film compounds having poor stopoff 20 properties on magnesium or its alloys being generally satisfactory.

The thickness of the film will generally be at least about 0.005 inches. Preferably, the film should be of sufficient thickness to mask the metal surface such that when progressively peeled by the polarizing action, it will remain as a continuous piece and not become flaky. The invention therefore contemplates thicknesses up to about one-eighth inch thick.

The film will normally be placed along or around the piece in strips about 1 inch wide and will provide spaces therein- 3 between ranging from about one-sixteenth to one-fourth inches wherein the magnesium or magnesium alloy piece remains exposed. Since the normal range of current densities is about 15-30 amperes per square foot (hereinafter referred to as a.s.f.), it will become necessary for larger pieces to be masked off to a proportionately greater extent, or to reduce the area of the exposed metal, in order to achieve desired current densities. The power source cannot and need not differentiate between one large piece or several smaller pieces having surface areas.

The strips or bands may be applied to the metal piece by brushing, dipping or spraying techniques. When brushing the strippable film material onto the workpiece in accordance with my invention, the material can ordinarily be used as is. If dipping is to be employed, it is desirable that the film material be heated to about l001 10 F. When spraying is contemplated, the viscosity of the film material should be lowered with solvents such as xylol, isopropyl and isoamyl alcohols. When dipping or spraying techniques are used, it will be 50 necessary to remove portions of the film by a knife or razor to expose the bare metal. Regardless of the means of application, the film should be allowed to dry for about 24 hours at room temperature.

The upper limit of the size of the workpiece to be anodized is governed by the electric power available. The workpiece contemplated for use with my invention comprises magnesium or magnesium alloys such as AZ3l, A291, and the like, wherein magnesium will usually comprise at least about 90 percent of the alloy, although alloys having less than this amount have been induced to polarize readily. The shapes and configurations of the workpieces are necessarily limited to those which lend themselves to the orderly reception of the strippable film thus eliminating from consideration very intricate and hollow shapes.

In order to more clearly define my invention, the following examples are herein presented for illustrative purposes:

EXAMPLE 1 Two 4X6XV4-inch plates of AZ9l were prepared for anodization in accordance with Military Specification MlL-M-45202, dated 20 Nov. 1959, paragraph 5. The plates were then brushed with the aforedescribed strippable film material along its length in 4 strips of equal width such that approximately percent of the piece was masked, or 15 percent of the surface area was exposed. The film thickness was approximately 0.04 inches. A variable AC transformer having a l ampere maximum output was employed. The initial cur rent density was 135 a.s.f. and the voltage ranged from 0-80 AC After the strippable film was progressively peeled and removed, the current density was 15 a.s.f. An anodic film 15 mils thick resulted after 90 minutes of operation. The bath composition used for this and succeeding examples is specified in Military Specification MIL-M45202, dated 20 Nov. 1959, Type ll, class A (High-Voltage Alkaline Process), as well as the tank equipment.

EXAMPLE ll Two identical plates as described under ex. 1 were sprayed with the aforedescribed strippable film and allowed to dry over night. One pint of the strippable film material was thinned with 20 ml Xylol, 5 ml isopropyl alcohol and ml. isoamyl alcohol. The film was carefully removed by razor blade from selected portions of the workpiece such that 5 spaced-parallel areas of exposed metal resulted each being approximately inch wide. The film thickness was approximately 0.025 inches. A variable AC transformer having a 10 ampere maximum output was employed, the initial current density being approximately 130 a.s.f. and the voltage ranging between about 0-80 AC After the strippable film was removed, the current density was 18 a.s.f. An anodic film of 16 mils thick resulted after 87 minutes of operation.

EXAMPLE [I] Two l2Xl2X /4-inch AZ3I plates were prepared as described under ex. 1. The plates were brushed with the strippable film compound aforedescribed such that l l spacedparallel stn'ps covered about 90 percent of the entire workpiece surfaces. The film thickness was approximately 0.045 inches. A variable transformer having a maximum output of 30 amperes was employed. The initial current density was around 145 a.s.f. and the voltage ranged between 0-85 AC After the strippable film was removed, the current density was a.s.f. An anodic film 1.5 mils thick resulted after 85 minutes of operation.

lclaim:

1. A method for inducing polarization of newly exposed specified surfaces of magnesium and magnesium alloy pieces prefatory to anodization thereof comprising the steps of adhering a poor stopoff strippable plastic film along selected portions of said pieces, said film providing narrow-spaced sections wherein surfaces of said pieces are exposed to form treated pieces having substantially alternating masked and unmasked surfaces,

electrically connecting said treated pieces in an anodizing tank filled with anodic bath composition used in highvoltage alkaline processes, adjusting the current to that required to anodize the entire surface area of the pieces, said adjusted current forming high current density at the exposed surfaces of said treated pieces,

maintaining said current until vigorous electrolytic action occurs at the exposed portions of said treated pieces and edges of said strippable film to continuously progressively roll back those portions of the film adjacent the electrolytic action and until all film is stripped from said pieces,

further maintaining said current until said pieces are anodized.

2. The method as described in claim 1 further characterized by cleaning said pieces prefatory to adhering said strippable films thereto.

3. The method as described in claim 1 wherein adhering said strippable film comprises the step of brushing spaced bands of film material onto said pieces.

4. The method as described in claim I wherein adhering said strippable film comprises the steps of dipping said pieces into strigpable film material and then provi mg spaced areas of exposed metal on said pieces by removing selected portions of said dipped film.

5. The method as described in claim I wherein adhering said strippable film comprises the steps of spraying said pieces with strippable film material and then providing spaced areas of exposed metal on said pieces by removing selected portions of said sprayed film 6. The method as described in claim 1 wherein said strippable film comprises:

Cellulose acetate butyrate; (maximum free acidity. us acetic acid 0.02%) 56 g. Chlorinated biphenyl 52 g. Polyethylene glycol di-2-ethyl-hexanoate l2 g. Mineral oil 8 g. Xylol I67 ml. lsopropanol 82.3 ml. Phenylsalicylate l g. Trilauryl phosphate I g. DiZ-ethyl hexylamine or methyl morpholine l ml. Solubilized 10% copper-8-quinolinolate 0.75 ml.

7. The method as described in claim 1 wherein said magnesium alloy is selected from the group consisting of A231 and A291.

8. The method as described in claim 1 wherein said masked surfaces comprises about to percent of the total surface area of said pieces.

* t i III

Claims

2. The method as described in claim 1 further characterized by cleaning said pieces prefatory to adhering said strippable films thereto.
3. The method as described in claim 1 wherein adhering said strippable film comprises the step of brushing spaced bands of film material onto said pieces.
4. The method as described in claim 1 wherein adhering said strippable film comprises the steps of dipping said pieces into strippable film material and then providing spaced areas of exposed metal on said pieces by removing selected portions of said dipped film.
5. The method as described in claim 1 wherein adhering said strippable film comprises the steps of spraying said pieces with strippable film material and then providing spaced areas of exposed metal on said pieces by removing selected portions of said sprayed film.
6. The method as described in claim 1 wherein said strippable film comprises: Cellulose acetate butyrate; (maximum free acidity, as acetic acid 0.02%) 56 g. Chlorinated biphenyl 52 g. Polyethylene glycol di-2-ethyl-hexanoate 12 g. Mineral oil 8 g. Xylol 167 ml. Isopropanol 82.3 ml. Phenylsalicylate 1 g. Trilauryl phosphate1 g. Di2-ethyl hexylamine or methyl morpholine 1 ml. Solubilized 10% copper-8-quinolinolate 0.75 ml.
7. The method as described in claim 1 wherein said magnesium alloy is selected from the group consisting of AZ31 and AZ91.
8. The method as described in claim 1 wherein said masked surfaces comprises about 80 to 90 percent of the total surface area of said pieces.