US2111377A - Art of coating aluminum - Google Patents

Description

March 15, 1938. F, A, WALES 2,111,377

ART oF GOATING ALUMINUM Filed Nov. 14, 1935 2 Sheets-Sheet. l

A TTORNEY.

Filed Nov. 14, 1935 2 Sheets-Sheet. 2

Inn un "uuu INVENITOR. ai /Z Mles.

ATTORNEY.

Patentedl Mar. 15, 1938 UNITED STATES PATENT OFFICE ART, oF ooA'rlNG ALUMINUM Fred A. Wales, Detroit, Mich. Application November-14, 1935, serial No. 49,647

3 Claims.

'Ihe present invention relates to the art of coating aluminum or aluminum alloys or articles formed therefrom, and particularly to improvements in the method of and apparatus for 5 forming such coatings, particularly coatings of the so-called "oxide type.

Such coatings on aluminum or aluminum alloys are commonly recognized as being formed chiefly of A1203, S03, and H2O, and are formed as a 1n plurality of laminated layers which vary. as to thickness, hardness and porosity. Such coatings have been formed by processes heretofore known to the art by making the aluminum or aluminum alloy an anode in an electrolytic cell having sulphuric acid asthe electrolyte. The coatings are formed upon the passage of direct current electrical energies through the cell and are formed on the aluminum or the aluminum alloys which form the anode in the cell. The direct current used in such processesI is of the order of twelve to eighteen volts and of from six to twenty amperes per square foot of material undergoing treatment. A cathode,` electrically connected in the circuit, forms the other contact with the electrical circuit and is placed in the electrolyte. In certain embodiments, such cathode frequently was the tank, and the current owed between the cathode and the anode through the electrolyte and caused the growth of the sor-called oxide'coating at the anode. In using such processes for the formation of the oxide coatings on aluminum or aluminum alloy articles, 'such for example as pistons for use in internal combustion engines, I have discovered that the coating formed by the processes of the prior art was not'satisfactory in such fields due particularly to the large variations in neness of grain structure, the irregularity asto size and distribution of p'ores, the extreme variations in hardness between the re- 0 spective layers, and the relatively high` cost of `processing and of the apparatus for use therewith.

The process and apparatus of the present invention has as its principal object to provide a coating for aluminum or aluminum alloys, or articles formed therefrom, the coating being superior to coatings previously known and characterized by having a very dense ne grained laminated structure. The various layers exhibit substantially uniform hardness characteristics and are permeated with very small and substantially uniform pores between the particles of the coat- 'It is a'further object of the present invention to provide a process and apparatus for producing a protective coating on aluminum or aluminum alloys, or articles formed therefrom, which is particularly, but not exclusively, adapted to pro-` duce the improved coatings on. pistons formed of aluminum or aluminum alloys -for use in in- 5 ternal combustion engines or similar purposes.

A `further object of the present invention is to provide a process for the production of protective coatings on aluminum or aluminumalloys, or articles formed therefrom, in which the cost 10 of operation and of the equipment has been reduced over similar costs of prior art processes and equipment, and which, while being more economical than processes and equipment of the prior art, produces coatings which are superior 15 A to the coatings produced by the prior art.

It is a further object of the present invention to provide an apparatus for use in forming protective coatings on aluminum or aluminum alloys, or articles formed therefrom, which is a portable self-contained unit.

Other objects of the present invention will appear 'in the following description and appended claims. One preferred form of apparatus em. bodying the present invention is shown by Way of example Vin the accompanying drawings forming a part ofthis specication wherein like reference characters designate corresponding Darts in the several views, and in which Fig. 1 is a view in perspective of an apparatus 30 which is adapted to be used in connection with the present process and embodied in the form of a self-contained portable unit.

Fig. 2 is a front view in elevation of the apparatus of Fig. 1 showing the enclosing cover in 35 closed position.

Fig. 3 is an end view in elevation of the apparatus shown in Fig. 2.

. Fig. 4 is a sectional view taken along the line 4-4 of Fig. 2 in the direction of the arrows.

Before explaining in detail the present invention it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various Ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of descriptionv and not of limitation, and it 50 is not intended to limit the invention claimed herein beyond the requirements of the prior art.

The apparatus, shown in the drawings com- `prises a casing I0 which is mounted upon a supporting rack Il, which may be provided with a plurality of casters I2 to permit moving the en.-

tire unit from place to place about a plant. The

casing I0 is divided interiorly to form a rinse tank I3 anda tank I4, which is provided with a lead or other non-corrosive lining I5 and is adapted to contain the electrolytic bath.

Mounted on one end of the framework II is a bracket I6 on which is mounted an electric motor I1 which drives afan I 8 of the blower type. An air pump IB, preferably of the pulsating type, is also driven by the motor Il and is connected by means of the conduit 2l) with a series of perforated pipes 2I yextending across they bottom of the lead-lined tank I4. Air is supplied to the pipes 2| and is discharged therethrough to keep the electrolyte agitated during operation of the apparatus. The fan or blower I8 is connected by means of a duct 22 with the hooded upper part of the tank I4 (Fig. 4) and serves to ventilate the apparatus while in operation.

During the operation of the apparatus, a hood or cover 23 isplaced in the closed position, as shown in Fig. 4. 'I'he cover 23 prevents fumes given olf during the carrying out of the process from escaping into the atmosphere, thus functioning as a collection hood from which the' collected fumes are drawn through the duct 22l and are thereafter discharged through a stack (not shown), which is connected with a discharge opening 24 on the blower I8. i

As shown in Fig. 4, the lead lining I5 forming the tank I4 is spaced apart from the casing IU in such a manner as to provide a duct 25 which extends about all sides of the lead lining I5 inside the casing I0. The duct 25 communicates with the interior of the tank I3, into which may be placed a suitable temperature controlling fluid mass which will flow through the chamber I3 and around the duct 25 surrounding the lead,` lining I5 -of the tank I4. By varying the teme,

perature of thefluid medium in the tank I3, the temperatures within the tank I4 may b e controlled within definite predetermined limits.

'Ihe uid is fed into the tank I3 through a suitable conduit 26 which is operatively connected with the source of uid supply and the level is maintained in the tank I3 by means of a waste or outlet pipe 21 which acts as an overiiow pipe when the fluid reaches the top thereof. If water is used as the fluid temperature control medium in the tank I3, and its temperature is not sufficiently low to control the temperature of the tank I4, a cake of ice or other refrigerant may be placed therein and so refrigerate the iiuid which runs through the tank and around `the chamber 25. During the operation of the apparatus, the tank I3 functions also as a rinse tank and the aluminum or aluminum alloys or other articles which have been treated in the treating tank may be rinsed in the tank I3 after the coating has been formed thereon.

Electrical energy is supplied to the tank I3 by means of a transformer 3D which is suitably mounted on the framework II, preferably at a point adjacentthe bottom of the tank I3. The transformer operates on alternating electrical current and in a preferred-embodiment has a rated capacity of 200 amperes and 24 volts. The transformer is of a conventional design adapted for use either on 110 volts or 220 volts, and is provided with a series of taps through which the A suitable control panel 3l is electrically connected with the transformer 3i) and actsto provide the connections through which the current flows tothe electrolytic bath. Electrical connections are provided between the vtransformer 3i! through the control panel 3| with bus bars 32 which extend crosswise of the tank M. The ar' ticlers to be treated, such for example as aluminum or aluminum alloy pistons 33, are provided with a suitable clamp 34 which is electrically connected with and engages one of the bus bars .32.110 suspend the article being treated into the .electrolyte in the tankV I4. In this manner the articles to be treated 'are connected through the electrolytic bath in a parallel series so that the flow of electricity is through the bus bars 32, the clamps 34, the article 33, through the electrolytic bath to the article 33, its clamp 34, and the bus bar 32. This method of connection is contrary tovconventional methods ln that the tank or bath remains neutral, whereas in prior known processes the, bath or tank forms the other connection with the electrical current and only the article being treated forms the anode thereof. In the present apparatus, with the use of alternating current, the articles 33 alternately form the anode and the cathode of the current flow and on the 11i) volt, 60 cycle, line this alternation occurs sixty times each second, but will vary; however, with variations in the cycles of the a1ter,

nating current.

A valve 35 is provided in the casing I El and communicates with the duct 25 to permit withdrawal of fluid temperature control mediums therefrom.

rThe foregoing described apparatuswhile being particularly adapted to the process of the present invention, is also adaptable (with slight changes, particularly as to its electrical circuit) for -use in conventional processes known to the art, and as such, by reason of the temperature control provided in the electrolytic bath and in the portability of the unit, produces results which are desirable and which are superior to the apparatus previously known to the art, although lt is my belief that using such apparatus with conventional processes will not produce the satisfactory results produced by the practice of my improved process therein.

'I'he improved process of the present invention proceeds in part upon my discovery that a superior type of coating may be formed either on aluminum or aluminum alloys or articles formed therefrom, and can be provided by using the aluminum or aluminum alloy or article formed therefrom as both the anode and the cathode of the electrical circuit flowing through the electrolytic bath. The change from anode to cath-,

ode is preferably a successively occurring change which will be brought about in relatively rapidly alternating sequence. The process also proceeds upon my discovery that such successively and rapidly alternating electrical changes in the character of the aluminum or aluminum alloy during the coating operation can best be effected by utilizing various voltages of 60 cycle alternatihg current, beginning at approximately 12.5

volts and increasing'the voltage of the electrical flow dring the coating operation to 24 volts at a time when the temperature of the bath is deflnitely controlled between limits of approximately 69 to '74 F.

In carrying. out the present process, I provide an electrolytic bath in the tank by using commercial grades of sulphuric acid in concentrations of 13% by weight in aqueous solution. At least y two pieces of the aluminum or aluminum alloy or article formed therefrom are electrically connected, one with each side of the line supplying the electric current .to the electrolytic bath and from the anode and cathode respectively of the cell resulting from the immersion of the articles in the electrolyte.

The operation is started at a voltage of the lower order, preferably from 9 to 121/2 volts, which is increased as the operationprcceeds, to the higher range `of voltages, preferably from 20 to 24 volts. The temperature of the bath is /preferably maintained from approximately 69 F. to approximately 73 F. by the use of a suitable circulating temperature control medium surrounding the tank in which the operation is being carried on. The treatment and processing is complete Within time intervals of about 20 to 40 minutes, the

operation being carried on'approximately threequarters of the time at the higher range of voltages, i. e. voltages in excess of 12.5 volts.

In'coatings which are formed on the aluminum or aluminum alloy, or the articles formed therefrom, I have found that there is relatively no precipitation of the metal into the electrolytic bath such as occurs in forming coatings by means of conventional processes utilizing direct current electricity. This feature of the present process permits the use of the electrolytic bath for a very much longer period of time, for once the acid concentration ofthe bath has-been normalized it appears to continue at the normalized acidconcentration during the entire operation for a considerable period of time. ,Y Utilizing direct current, the metal decomposes yand reacts with the bath to form sulphates.

When the bathl contains 20 grams per litre, the bath must be completely changed because 4of the resultant decrease in electrical conductivity of the bath. The present process is characterized by the fact that little or none of the metal-is decomposed by the electrolyte and put into the bath in the form of va, sulphate. This is due to the use of the relatively dilute bath which is possible by the use of the alternating current electrical energy in the' process. This particular feature is one of theoutstax'iding defects of the direct current method now being used, as this sulphate is insoluble in the bath and readily affects the conductivity of the bath, and when it reaches the concentration of 20 grams per litre of electrolyte, the bath must be changed. V 'I'he efliciency of such processes of theart is progressively lessened by the rapidly diminishing conductivity value in this bath up to the ultimate point of 20 grams per litre.

The coatings formed by the process of the present invention as above described, will be found.

to have a substantially increased uniformity of hardness throughout the successive layers, which,

when investigated by means of photomicrographs, will be shown to possess a very ne grained crystalline structure and very small and uniformly distributed pores extending therethrough. -In an average'4 case, the variation between the softest layer and the hardest layer is inthe neighborhood of approximately 20%, Whereas in a characteristic coating formed by the processes of the prior art, utilizing direct current electrical energy and baths of the specied concentrations of acid, there is a variation of from 140 microcharacter to approximately 5,000 microcharacter between the hardest layer and the softest layer formed.

Also, there is a considerably thicker coating formed by means of the process herein disclosed than that of processes utilizing a ilow of direct current electrical energy. In a typical example, the coating formed by the process herein described is approximately twice as thick as the coating formed by the conventional direct current processes treated for like periods of time. I believe this is due to the fact that the metal is not decomposed and changed to the sulphate as is the case in all prior operations.

A particular field in which the present invention has great utility is in the iield of formed protective and oil bearing coatings onaluminum or aluminum alloy pistons for use in internal com-- bustion engines. In this field the coatings of the present invention are -superiorto coatings formed by prior methods in that the coating is very much more closely knit, being formed of very much sions, other circumstances being equal, are con-v versely proportional to the diameters of the capillary tubes so that, considering the pores of the coating as capillary tubes having an ainity for the oil or other uid medium, it will be seen -that the smaller size of the pores and the greater uniformity of placement of the pores in the coating resulting from the present invention, will account for the greatly increased capillary values of such coating.

Excellent oxide coatings are produced under the following conditions on pistons produced from a low-aluminum high-silicon alloy which contains approximately 12% of silicon. An equal number of pistons are placed in the electrolyte and act"4 as the electrodes. The electrolyte is composed of approximately 13% by weight of commercial sulphuric acid in aqueous solution. A l2 volt alternating current` is used, one side of the line being connected to each electrode.and the .12..volt alternating current is impressed'bn the "cell for a period ofV 3 minutes. The voltage is then increased to 24 volts and the treatment is conto note that similar coatings formeduby conyentional methods utilizing direct current and a stronger electrolyte, where the article is treated for the same length of time, produces coatings having an average thickness of .000327 inch as against the average thickness of the coatings produced by the use of the alternating` current of .000746 inch; land the abrasimeter values will be less than 30. y

In treating the aluminum o r aluminum alloy articles by the4 present process, I nd that it is possible to dispense with -the preliminary cleaning and Washing processes which are common to the prior art, itbeing sufficient with the improved process of the present invention to merely dip thealuminum or 'aluminum alloy article in a hot water bath. This is possible, in my opinion,

conventional processes.

because of the use of the alternating current in the present process which, in conjunction with the electrolyte, reacts in such a way as to in eiiect clean the articles more completely and thoroughly than it would be possible to do by previously suggested methods. This, I believe., is due to some extent at least to the action oi the alternating current in expelling the adhered surface oxygen.

A characteristic of the coating produced is that it is very smooth on the exterior surface so as to make possible the use of the article without using a buiiing machine for the purpose of removing the chalk-like soft coating which is formed by the treatment of aluminum or its alloys by The soft outer coating formed by conventional processes is a coating which consists, to a large extent, of the material which is precipitated into the bath as the sulphate which has heretofore been described. The coating formed by the present process has no such soft outer coating and tests to date indicate that there is not more than approximately 20% variation in the microcharacter val ues from the outside skin of the coating to its base where it forms on the aluminum or the aluminum alloy.

Due to the action of the alternatingcurrent, less metal is taken away from the material undergoing treatment and thus the coating is oi greater strength than coatings produced by prior methods wherein more of the metal is taken away during the processing operation.

While particular uses have been mentioned in the foregoing discussion of the apparatus and process of the present invention, it is to be understood that both the apparatus and the process are adapted to other uses than those herein particularly mentioned and'may be used successfully to form protective coatings on articles which may bejformed of some material other than aluminum or aluminum alloy but which may be coated with aluminum or aluminum alloy by such processes as calorizing, metal spraying, etc.

I claim:

1 A process of producing a protective coating having a thickness ln excess of approximately .0004 inch and a hardness in excess of approximately'30 gramsabraslmeter value, on an alux'ninum-'siliconalloy which contains approximately 12% of lsilicon having substantially increased uniformity of hardness throughout the successive layers, which comprises immersing the farticles'formed of vsaid alloy in an electrolytic bath 'containing 13% by weight of sulphuric 'acid in aqueous solution, electrically connecting the immersed articles in parallel with 'a source of alternating current electrical energy and. utilizing the immersed articles as both the anode and cathode of the circuit, and subjecting said articles to a flow of alternating current elec trical energy in successive stages beginning at approximately 9 to 12.5 volts intensity for a period of approximately 3 minutes duration and increasing through successive stages to approxi-r mately 24 volts for a period of approximately7 20 minutes duration at the conclusion of the process.

2. A process of producing a protective coatingv on an aluminum-silicon alloy containing approximately 12% silicon having substantially increased uniformity of hardness throughout the successive layers, which comprises immersing the aluminum alloy in an electrolytic bath containing 13% by weight of sulphuric acid in aqueom solution, electricallyconnecting the immersed alloy in parallel with a source of alternating current electrical energy and utilizing the immersed alloy as both the anode and cathode of the circuit through the bath, and subjecting the said alloy to a flow of alternating current electrical energyin successive stages beginning at approximately9 lto 12.5 'volts for a period of approximately 3 minutes and thereafter increasing to approximately 20 to 24 volts for a period of approximately 20 minutes while agitating the bath to maintain a substantially constant circu-l lation thereof and maintaining a substantially constant temperature of the bath between approximately 70 and 74 Fahrenheit.

. 3. A process for forming a nished and smooth unbuffed protective coating having a thickness in excess of .0004 inch and a, hardness inl excess of approximately 30 grams abrasimeter value on an aluminum-silicon alloy containing approximately 12% silicon having substantially 1ncreased uniformity of hardness throughout the successive layers, which comprises immersing said alloy in an-oxidizing bath containing 13% by weight of sulphuric acid in an aqueous solution and passing an electrical current therethrough for a period of approximately 3 minutes duration at a potential of approximately 9 to 12.5 volts and thereafter for a period of approximately 20 minutes duration at a potential of approximately 24 volts, utilizing the said alloy in the bath as both the anode and cathode of the circuit through the bath, while agitating the bath and maintaining therein a substantially constant temperature of between approximately 70 a'ndf'l" Fahrenheit.

' FRED A. WALES.

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