US2215167A - Process of protecting metal surfaces by electrodeposition - Google Patents

Description

Patented Sept. 17, 1940 PROCESS OF PROTECTING METAL SUR- FACES BY ELECTKODEPOSITION Cyril Gordon Sumner,

Robert Ian Johnson, and

William Clayton, London, England,; assignors to Crosse & Blackwell Limited, London, England,aBritish company -No Drawing. Application March 29, 1938, Serial No. 198,682. In Great Britain March so, 1931 9 Claims. (01. 204-38) This invention relates to improved means of coating metal or otherconductive surfaces by electrodeposition. It is well known that a film of aluminium oxide may be formed electrolytically on an aluminium surface by anodic oxidation, and that such a film has a protective value in itself, while it forms a basis for the adhesion of a subsequently applied film of lacquer or other coating material. A further distinctive property of the alumina film is that when freshly formed it is capable of being coloured by suitable dyestuffs, so that pleasing decorative finishes may be obtained.

It is also known that conductive surfaces, without restriction to aluminium, may be coated with a film consisting of or containing aluminium oxide, by making such a surface the cathode in electrolysis of an aqueous solution containing basic aluminium salts, and a process of this kind has been proposed for obtaining insulating coatings on copper conductors. In such processes the constituent of the electrolyte which contains aluminium is associated with a positive electric charge.

15 According to the present invention, alumina is deposited on the'conductive surface as anode, by use of an aqueous medium such that the constituent containing aluminium is associated with a negative electric charge. This has the advang9 tage that other effects can also be obtained on themetal surface very readily, as hereafter explained. In a preferred form of the invention we have found that when there is used as electrolyte a solution containing an aluminate, for example sodium aluminate, in which aluminium occurs in the form of a negatively charged ion (or anion), under suitableconditions the passage of an electric current results in the precipitation on the anode of a film consisting of or containing 40 aluminium oxide or hydroxide. This film may be washed and dried, when it may be coated if desired with a lacquer or other material, for which it affords an excellent basis, or it may be dyed (preferably beforedrying), in the wellknown manner of anodically oxidised aluminium. A valuable application of this invention is to the coating of metals such as' tinplate, which are used to make containers for foods'tuffs,.as the non-toxic character of aluminium 'oxide makes it 5i) a suitable. compound either for direct contact with the foodstuff, in cases where a lacquer is unnecessary, or as a basis for application of a lacquer where this is desirable. The anodic deposition of alumina has in the case 'of tinplate as the advantage that flnodlc oxidation ofthe tin surface occurs at the same time, and we have already disclosed in our U. S. Patent application Serial No. 146,691 filed June 5, 1937, that anodically oxidised tinplate is resistant to discolouration bysulphur-bearing foodstuffs. Our experi- 6 ments have shown that the additional cover provided by an aluminous'film is a useful supplement in that the resistance to discolouration is increased.

A further application of the invention which 10 again is of interest in connection with tinplate, is the use of dyed alumina films for producing decorative external coatings, comparable with those obtained from anodically oxidised aluminium.

Deposition of the aluminous film at the anode has an important advantage in that we have found it possible to emulsify or disperse in the alkaline solution substances such as lacquers or waxy materials, and passage of an electric current then results in the simultaneous deposition of an aluminous compound and of the dispersed material, so that composite coatings may be obtained in a single operation.

In the deposition of a simple aluminous coat- 5 ing, we have found that a solution of, for example, sodium aluminate, as commercially supplied, may not give rise to the best results, since there may be sufficient free alkali in the solution to keep the alumina from precipitation unless the current density is relatively high. Under these conditions there is a strong evolution of gas at the anode which impairs the continuity and adhesion of the deposit, and'superior results may be obtained if the solutioncontains a depolarizing agent, for example a reducing agent. Furthermore, even without the use of a depolarizing agent the effect of gas evolution may be reduced by adjusting the alkalinity of the solution to such a value that aluminium oxide or hydroxide is 40 only just held in solution in absence of electrolysis. Deposition then occurs at a low current density, at which gas evolution is relatively small.

Reduction of the alkalinity may be effected by addition to the solution of an acid, or of a salt having a reaction more acid than sodium aluminate containing alumina only just held in solution, with the limitation that the anion of the added acid or salt should not form with the metal of the' electrodea soluble salt or a bulky precip- 5o itate which might interfere with the formation of a coherent alumina film. For example, hydrochloric acid is not suitable. Preferably the anion of the added acid -or salt is such as will cause any film formed by direct anodic attack as on the metal, as apart from decomposition of the aluminate, to be compact and adherent. Suitable compounds for use with tinplate are, for' example, sodium bicarbonate or sodium dihydrogen phosphate.

The aluminous film arises from decomposition of the aluminate ions as a result of changes in the composition of the electrolyte in the vicinity of the anode during electrolysis, and the invention is therefore not limited to the use of a particular metal or other conductive surface as anode. In the examples tinplate is mentioned as an illustration of an application of the invention to the art of tin box making, but no restriction on the field of the invention is thereby implied.

As an example of the production of a solution modified in this way, sodium bicarbonate solution is gradually added to a solution of sodium aluminate in amount just insufficient to cause precipitation' of aluminium oxide or hydroxide in the absence of electrclysis, or addition may be continued until precipitation just sets in, the solution being then filtered, and further diluted if desired. This de-stabilised solution may be used without addition of a depolarizing agent. Alternatively, sodium bisulphite maybe used instead of sodium bicarbonate in the above preparation, in which case the sodium bisulphite itself provides a depolarizing agent as well as adjusting the alkalinity. If additional depolarizer is desired, excess of'sodium bisulphite may be used, and caustic soda then added so as just to redissolve the precipitate.

We have further found that improved deposits may be obtained if the solution contains also colloidal material, or a substance or substances capable of giving rise to colloidal material, such as will migrate towards the anode during electrolysis. In certain cases, particularly when relatively thick films are deposited, or when deposition is effected at a relatively high current density, the dry film may be superficially somewhat powdery when an aluminate is used without colloidal material, whereas under the same conditions a smooth coherent film is obtained when colloidal material is present.

In an example of such a solution, a 2 per cent. solution of sodium aluminate may be destabilised with sodium bicarbonate as already described, and gelatin dissolved in a small amount of water may be added, the solution being then diluted to bring-the concentration of sodium aluminate to 1 per cent. The concentration of gelatin in the final solution may be of the order per cent. Using this solution a smooth, opalescent thin film may be obtained on tinplate by electrolysis for 612 seconds at a current density of 2 to 4 amps. per square foot.

As a further example, 90 parts of a 1 per cent. solution of sodium aluminate maybe mixed with 10 parts or a solution containing 1 per cent. (reckoned as solid) of a sodium silicate having a NazOZSiOa ratio of 1:3.8. The solution is then destabilised with sodium bicarbonate as before. Tinplate or tinplate containers may be coated with a smooth, coherent film of opaque appearance by electrolysis for 10 seconds at a. current density of 10 amps. per square foot.

According to a further modification of the invention thedeposited film may undergo further treatment, preferably after washing but before drying. For example, we have found that if the film in this condition is treated with an emulsion or dispersion coagulable by aluminium compounds, coagulation of the dispersed particles occurs at the surface. In an example of this process, a tinplate can may be coated internally with an aluminous film by anodic deposition as de-,'

scribed above, and after washing the deposit the latter may be treated with an emulsion of lacquer dispersed in dilute ammonia. Under such conditions a certain amount of thelacquer becomes fixed to the surface and is not removable by washing, and it may be stoved in the usual way.

Alternatively, in conjunction with this coagulation the surface may be further coated with lacquer by electrodeposition from a lacquer-inwater emulsion, the aluminous deposit being a conductor while still wet. As an alternative, the dried film may be coated with lacquer, wax, cellulose or other materials from solutions in organic solvents or by other means.

In a further modification of the invention composite coatings may be obtained on the anode by use of an aqueous medium containing an alkalimetal aluminate in solution and a lacquer or waxy material in dispersion. We have found that the simultaneous deposition of alumina (which term is used to describe any aluminium compounds in the deposit, their exactnature be-' ing unknown) greatly improves the characteristics of an electrodeposited lacquer or similar film even when the proportion of alumina is not large. For example, the deposit is more water repellent before drying, so that it may be freed of emulsion.

and of drops of wash liquor more readily. There is also-less tendency for the material to flow during subsequent drying and stoving or melting, so

that the surface remains uniformly covered and recession from slightly greasy areas of'the metal does not readily occur. Tinplate cans internally lacquered by this method after fabrication have been found to be almost entirely free from discontinuities in the lacquer coating.

We have'found that although addition of sodium aluminate solution to .an already formed' adjustment of the composition of the emulsion,

so that this process is capable of yielding a wide variety of finishes. For example, using a given lacquer and given conditions of electrodeposition, coatings may be obtained which after stoving are pale, matt and substantially opaque, or which are deep in colour, bright and substantially transparent, or coatings may be obtained having intermediate characteristics in respect of colour,

brightness, or both. These differences are obtained by varying the proportions in which alumina and lacquer are co-deposited, as well as of the actual amounts deposited.

Our experiments indicate that the type of finish obtained is dependent on the relative concentrations of sodium aluminate and of emulsifying agent'in the emulsion. When .a lacquer of the resin/drying-oil type is emulsified in a solution of sodium aluminate alone, or of sodium aluminate together with free alkali, the emulsifying agent is formed by interaction of the alkaline solution with acidic constituents of the lacquer to yield soaps or soap-like compounds. If the aqueous phase of the emulsion is kept constant in amount and initial composition, the ratio 75 emulsified in a given quantity of the solution is increased, the depth of colour and the brightness of the him under constant conditions of electrodeposition also increase. Alternatively, keeping the ratio of lacquer to aqueous solution the same,

the depth of colour and brightness may be enhanced by diminishing the concentration of aluminate.v

Since the same driers were used throughout, these have been omitted from the formulae.

Lacquer A Parts Linseed stand n r 1'75 Oil-soluble modified phenol-formaldehyde resin 100 Estimated acid value: 12-14.

Lacquer B Linseed stand oil 1'15 Oil-soluble modified phenol-formaldehyde resin 100 Rosin 11 Estimated acid value: 18-20.

' Lacquer C Linseed stand 011"--- 175 Copal ester. 100 Estimated acid value: 10-12.

. Lacquer D Linseed stand oil 175 Copal 100 Estimated acid value: 28-32.

' Lacquer E Linseed stand oil 175 Rosin 2% Estimated acid value: 12.

Lacquer F V Linseed stand n 1'15 Rosin Y 25 When different lacquers are compared, in

emulsions having the same ratio of lacquer to aqueous phase and the same initial composition of the aqueous phase, it is found that to a certain extent the brightness of the finish increases with the proportion of acidic constituents in the lacquer. We have found however, that this efiect'does not depend simply on the acid value of the lacquer (by which term is meant the number of milligrams of potassium hydroxide required to neutralise the acids contained in 1 gram of lacquer), but also on the nature of the acids present. The lacquer acids are in general contributed partly by the drying oil and partly by the resin, and we have found that diiferent results are obtainable according as the oil is kept constant and the type of resin is varied or the resin is kept constant andthekind of oil is varied, in each case without alteration of the ratio of resin to oil, or alternatively, without significant alteration of the acid value of the lacquer as a whole.

These differences may be illustrated by reference to results obtained with the following lacquer formulae, which were devised to distinguish the efiects of total acid value, ratio of resin acids to drying oil acids, and the strength of the acids. The compositions given refer to parts by weight.

Estimated acid value: 30

; the aqueous phase.

Lacquer G Linseed stand o'L 175 (Coumarone resi 100- Estimated acid value: 6-7.

Lacquer H w Tung stand oil- 175 Oil-soluble modified phenol-formaldehyde resin 100 Estimated acid value: 6-7.

Here lacquer B was lacquer Awith its acid value increased by addition of rosin,-a nearly completely saponifiable resin. Lacquers C and D were identical in resin/oil ratio and both contained the same kinds of acids, but with diflerent proportions of resin acids. Lacquers E and F had practically the same acid values as C and 1), respectively, but the resin used was different. Lacquers G and H had both low acid values and contained the same ratio of oil to resin, but in the former case the acidity was derived almost entirely from the oil and in the latter case almost entirely from the resin.

Before emulsiflcation, each lacquer was mixed with a thinner in the ratio parts of lacquer to 1 part of thinner. The thinner consisted of 85 per cent (by weight) white spirit and 15% turpentine. In each case emulsions were made of 1, 2 and 3. parts respectively of thinned lacquer in 4 parts of 1 per cent sodium aluminate solution, and in the 'case of the less acid lacquers, also of 4 parts of lacquer in 4 parts of solution.

It was found that under constant conditions of electrodeposition there was in all cases an increase in depth of colour and in brightness as the proportion of lacquer emulsified was increased, but that when emulsions of the same lacquer concentration were compared, the finish an increase in the concentration of saponifiable acids enhanced the brightness. On the other hand, comparing lacquers of similar acid value, lacquer A and lacquer E both gave brighter finishes than lacquer C, and lacquer F than lacquer D. This might be assigned .to the weaker character of the acids derived from copal. Lacquer H gave darker and brighter finishes than anyof the lacquers using linseed stand oil. 0n the whole, the depth of colour was greatest in the lacquers of low acid value, but no uniform correlation with acid value was found.

Using a given lacquer and given phase ratio in the emulsion, the character of the film may be modified by variations in the composition of The effect of reducing the concentration of sodium aluminate has already been mentioned. We have found that if the aluminate concentration is unaltered, the addition of free alkali, such as sodium hydroxide or ammonia, may decrease the depth of colour and brightness, but that this effect is more marked in some cases than in others, for example with lacquer A more than with lacquer B.

Furthermore, addition to the solution of alkali metal salts of certain organic acids in which alumina is soluble, has been found not to increase but to diminish the colour and brightness of the film. In an example, emulsions were made of 100 parts of lacquer A in 200 parts of a solution containing 1 per cent sodium aluminate and 0, 0.14, 0.35 and 0.70 per cent respectively of was found that as the concentration of tartrate was increased, the film became progressively paler and more matt.

An alternative method of obtaining a similar result is by emulsification in a solution containing sodium silicate in addition to sodium aluminate. In an example, a mixture of 75 partsof 1 per cent sodium aluminate solution and 125 parts of a solution containing 1 per cent (as solid) of sodium silicate having a NaaO, SiOa ratio of 1:3.8 may be used.

On the other hand, the brightness of the finish may be increased by including in the aqueous phase an emulsifying agent of the same type as is obtained by saponification of the lacquer acids. In an example, emulsions were made of 100 parts of lacquer A in 200 parts of a solution containing 1 per cent of sodium aluminate and 9i and per cent respectively of rosin soap. With increase in rosin soap content the films were progressively brighter and also progressively paler.

It is to be clearly understood that where it has been stated that an increase in brightness or in mattness is accompanied by a diminution in depth of colour, this refers to constant electrical conditions, and it is not to be inferred that the change in brightness cannot be obtained without loss of colour. The brightness (for a given depth of colour), depends on the proportions of alumina present in the film, whereas the depth of colour (for a given lacquer stoved under constant conditions) increases with the Weight of the film. The weight of the film may be increased by suitable alteration in the electrical treatment, for example by an increase in initial current density, in the duration of treatment or by decrease in the rate of decline of the current. Where a thin deposit is obtained, this is often partially due to the fact that the film builds up a high resistance by deposition of a relatively small amount of material, so that the current drops rapidly to a low value. The rate of fall in current depends on the ratio of the film resistance to the total resistance in circuit, .so that if the initial current is obtained by application of a large E. M. F. in conjunction with a high series resistance, the current will diminish more slowly than if a lower E. M. F. and lower series resistance are used.

For example, in most of the experiments described above an E, M. F. of 36 volts was used,

with resistance adjusted to give an initial current density of about 8 amps per square foot and the current was passed for 6 seconds. Under these conditionsan emulsion of parts of lacquer A in. 200 parts of a solution containing 1 per cent. of sodium aluminate and 1% per cent. of rosin soap gave a film which was bright but very pale. By using an E. M. F. of 240 volts and the same initial current density and duration of treatment,a film was obtained which was also bright, but of a golden brown colour.

It has been found, that the ease of removal of the final film by the usual organic solvents, such as chloroform, progressively diminishes as the proportion of alumina in the film increases.

While all the lacquer mentioned-in the examples gave good emulsions and satisfactory films when the emulsions were freshly made, it was found that the stability with time differed considerably. In general, where the emulsifying agent was derived entirely from saponification of the acids in the lacquer, stability diminished as the proportion of lacquer in the emulsion increased. There was no general correlation bein the proportion of soap-forming acids.

tween the acid value-of the lacquer and the stability of an emulsion of given lacquer concentration, but stability was improved by an increase On the other hand, thestronger the acids the less stable was the emulsion for a given acid value. Stability was much greater in the case of lacquer H than with lacquer G.

'It has been found that the inclusion of a soaplike emulsifying agent in the aqueous phase, for

example rosin soap, improves the stability of the emulsion. On the other hand, addition of free sodium hydroxide may lead to instability if the total concentration of sodium ion is increased too far. Our results point to the importanceof using lacquer constituents in which the proportion of acids whose alkali salts are not emulsii'ying agents, is kept to a minimum. Thus it is known that in the preparation of linseed stand oil, fatty acids of low molecular weight are formed, but that these may be largely removed by suitable means.

By utilisation of these principles, it is possible to choose a lacquer composition and concentration such that a desired type of coating may be obtained from an emulsion in a given aqueous phase, or alternatively when the lacquer is given it is possible by choosing the aqueous phase and lacquer concentration suitably to modify the ratio of alumina to lacquer in the coating. It is to be understood when the term alumina is used in connection with these composite coatings that this is for descriptive purposes, and does not imlacquer, waxy materials may be dispersed in so-- lutions containing aluminates either alone or in conjunction with other materials, and that coherent deposits are then obtained more readily by electrodeposltion than when such substances are not present. The electrodeposition of waxy materials presents special characteristics which are disclosed in our co-pending application Serial No. 189,588 filed February 9, 1938, which describes and claims a process for the electrodeposltion of wax with or without the presence of sodium aluminate and/or silicate.

By the term wax or lacquer in liquid form as used in the claims, it is meant wax or lacquer dispersed in solution or dissolved in organic solvent.

We declare that what we claim is:

1. A method of coating a conductive surface which consists in making said surface the anode,

in an aqueous solution containing a soluble aluminate which has been reduced in alkalinity to such an extent that aluminium hydroxide is only just held in solution in the absence of electrolysis, and passing an electric current through said solution at a low current density at which gas evolution is relatively small whereby an aluminous compound is deposited as a film on said surface.

2. A method of coating a conductive surface, which consists in making said surface the anode in an' aqueous solution containing a soluble aluminate, and also a polarizing reducing agent and passing an electric current through said solution whereby an aluminous compound is deposited as a film on said surface.

3. A method of coating a conductive surface which consists in making said surface the anode in an aqueous solution containing a soluble aluminate and sodium bisulphite, and passing an electric current through said solution, whereby an aluminous compound is deposited as a film on said surface.

4. A method of coating a conductive surface, which consists in electrcdepositing an aluminous compound on the said surface as anode in an aqueous solution containing a soluble alumlnate and a depolarizing reducing agent and a coating selected from the group consisting of lacquer and wax by electrodeposition from an aqueous dispersion.

5. A method of coating a conductive surface, which consists in electrodepositing an aluminous compound on the said surface as anode in an aqueous solution containing a soluble aluminate and a depolarizing reducing agent and a coating selected from the group consisting of lacquer and wax by application in liquid form.

6. A method of' coating a conductive surface which consists in making said surface the anode in an aqueous medium containing an alkali metal aluminate and also lacquer indispersion and passing an electric current through said medium, whereby an aluminous compound and lacquer are dep'osited'together as a film on said surface.

7. A method of coating a conductive surface,

which consists in makingsaid surface the anode in an aqueous medium containing an alkali metal aluminate, an additional compound of alkaline reaction and also lacquer in dispersion and passing an electric current through said medium whereby an aluminous compound and lacquer are deposited together as a film'on said surface.

8. A method of'coating a conductive surface,.

which consists in making said surface the anode in an aqueous medium containing an alkali .metal aluminate, alkali metal tartrate, and also lacquer in dispersion and passingv an electric' compound and lacquer are deposited together as a film on said surface.

' CYRIL GORDON SUMNER.

ROBERT IAN JOHNSON.

WILLIAM CLAYTON.