US3470073A - Method of electroplating aluminum - Google Patents

Description

United States Patent METHOD OF ELECTROPLATING ALUMINUM Akira Miyata, Hideyo Okubo, Chickayoshi Tomita, and Akio Suzuki, Kawasaki-shi, Japan, assignors to Nippon Kokan Kabushiki Kaisha, Tokyo, Japan No Drawing. Filed Dec. 26, 1967, Ser. No. 693,093 Claims priority, application Japan, Oct. 25, 1967, 42/ 68,434 Int. C1. C23!) 9/02; C22d 3/12 US. Cl. 20439 5 Claims ABSTRACT OF THE DISCLOSURE Aluminum is electroplated from a fused salt bath containing dehydrating substance.

This invention relates to a method of electroplating aluminum by using a fused salt and has as an object to provide aluminum plated metal sheets having a coating which is dense, adherent and resistant to corrosion by incorporating dehydrating substance on the surface of or in the plating bath whereby aluminum is plated in a moisture-free state.

With the remarkable advances of metallurgy various new techniques have been developed which are gradually being applied to commercial production. It has been highly desired in the art to develop a new method capable of applying aluminum on to the surface of other metals. Electroplating of aluminum by utilizing a fused salt bath is one new approach, but this method is not yet widely applied because of its various technical problems. Especially control of the electrolytic bath is extremely diflicult, so that many proposals have been abandoned for this reason.

Electroplating of aluminum by means of a fused salt bath is usually performed by employing a bath essentially consisting of an aluminum halide incorporating a few other compounds for decreasing its melting point and satisfying other objects. However, aluminum halide is not entirely suitable because not only does it readily sublimate but it is also highly hygroscopic so that an ordinary fused salt bath absorbs moisture and oxygen from the surrounding air to be degraded rapidly and become usuitable for plating. Therefore, in order to obtain satisfactory plating with fused salts it is necessary to control the plating bath so that it can be used over a long period.

It has been impossible to regenerate a fused salt which has absorbed moisture in the air to restore it to a condition which permits satisfactory plating. One has thus been obliged to discard a bath which has been used for a certain period. One reason for this is that when the bath absorbs moisture aluminum halide undergoes hydrolysis to yield a hydrogen halide and aluminum hydroxide, the latter yielding negative hydroxyl ions or oxygen ions. Inasmuch as aluminum has a strong aflinity towards these ions, aluminum deposited in plating reacts with these ions and thus can not form an aluminum layer on the surface of an article to be plated. Under this condition a plated surface becomes irregular and is liable to form a dendritic structure. In addition, current efi'iciency is lowered so that it is impossible to obtain satisfactory plating. None of the prior chemical or electrical methods have succeeded in removing hydroxyl ions from the electrolytic plating bath. Thus, although aluminum halide can produce satisfactory plating in the initial stage of its use, it has not been used commercially because of its short life. Many previous attempts to improve the bath did not solve these problems. In other words, the bath deteriorates with time, lowers current efficiency, and increases the quantity of deposit of "Ice dendritic structure which readily falls off and provides a plating of irregular thickness, thus leaving some portions of the article unplated. The life of a known bath is therefore extremely short, and there has been no effective method of regenerating it.

We have carefully investigated the reason for the above described deterioration of the plating bath, and after confirming the reason, have found an effective method capable not only of effectively and economically preventing the deterioration of the bath but also of regenerating a deteriorated bath to a usable condition.

The most important basic factor that determines the degree of deterioration was found to be due to the introduction of moisture into the bath from surrounding air or from air and moisture absorbed by the articles to be plated. Even if the plating operation is performed in an ambient absolutely dried air for the purposes of preventing this, it is impossible to prevent introduction of moisture into the bath since the breathing of the workmen and the articles to be plated constitute sources of moisture, unless the latter is absolutely dried.

As stated above an aluminum halide comprising the essential component of the bath for electroplating aluminum is strongly hygroscopic, so that it is inevitable to introduce into the plating bath absorbed moisture caused by the pretreatment of the articles to be plated or the moisture in the atmosphere. Such moisture acts as the source of oxygen ions, prevents satisfactory precipitation of aluminum, and results in the precipitation of dendritic aluminum which is liable to fall off. Thus, if some substance which can prevent introduction of moisture into the plating bath or which can collect the moisture introduced in the bath were used the above described difficulties could be avoided. We have found that by utilizing a suitable moisture absorbing substance it is possible to increase current efficiency, to prevent precipitation of dendritic aluminum and provide aluminum plating of excellent quality over a long period.

Previously, we have invented a method of regenerating a deteriorated bath by passing hydrogen chloride gas through it. This method is considerably effective in converting aluminum hydroxide formed by deterioration back to aluminum chloride. However, to carry out this method effectively it is necessary to constantly introduce a very large quantity of hydrogen chloride. The reason for this is that when current is passed, at the anode surface tension of the electrolyte decreases to send hydrogen chloride to be lost in the form of gas bubbles and as the quantity of hydrogen chloride is reduced, the reaction proceeds in the opposite direction thus yielding oxygen.

An object of the present invention is to provide a method of electroplating aluminum in which deterioration of a fused salt bath is minimised.

A further object of the present invention is to provide a method of electroplating aluminum in which a deten'orated bath may be regenerated.

Another object of the present invention is to provide a method of electroplating aluminum to obtain an improved quality of plated aluminum.

Other objects and advantages of the invention will be apparent from the following description.

According to this invention a suitable dehydrating substance such as silica gel particles, quartz sand and minute pieces of porous ceramic and the like is added to the surface of or in the plating bath essentially consisting of aluminum chloride. The plating operation is then performed in the absence of moisture to provide dense aluminum plating.

The dehydrating substance utilized in this invention must withstand the high melting temperature of the fused salt, not react with highly reactive fused salt, have a strong aflinity for water, and not affect the plating operation, or the quality of the plating. In addition, it should not be expensive and should readily be regenerated.

Such dehydrating substance is selected singly or in combination from the group consisting of silica gel particles, granules of silica, quartz sand, minute pieces of porous ceramics, adsol (a kind of sintered activated clay), diatomaceous earth, charcoals, graphite, carbon black, alumina gel particles, glass particles, glass fibers, asbestos, powder of anorthite, calcium sulphate, fluorspar, zeolite, acid clay, pumice stone and alumina particles.

The following specific examples are given by way of illustration, and are not to be construed as limiting in any way the scope and spirit of the invention.

EXAMPLE I 30 g. of quartz sand prepared by heating it in vacuum by electron bombardment and made sufiiciently dehydrating were added to 1 liter (1.7 kg.) of a fused plating bath having a composition of 60 mol percent of aluminum chloride and 40 mol percent of sodium chloride. This had been before-hand left to stand in the atmosphere for such a long period that it caused to precipitate dendritic aluminum and make no use for an ordinary aluminum plating process. After adjusting the mixture, plating operation was again commenced. Plating ability was gradually recovered with time until finally a steel plate having a dense aluminum plating was obtained at a high current efficiency of more than 80%. The latter value is comparable to that of a fresh bath.

EXAMPLE II 30 g. of heated and dehydrated silica gel particles were floated on the surface of one liter (1.7 kg.) of an electro'lytic bath consisting of 60 mol percent of aluminum chloride and 40 mol percent of sodium chloride and this was sealed off from the atmosphere. An aluminum plate was dipped in this bath and the bath was repeatedly used to plate aluminum on steel plates. With this bath, dense aluminum platings were obtained at high current densities of more than 80%, as in the initial stage of a plating operation even after one week.

In the case of a plating bath of the same quantity and composition but not containing the moisture absorbing material, the current efiiciency was decreased to less than 50% after one week and it was noted that precipitated aluminum had dendritic structure that tended to fall off.

EXAMPLE III One liter of fused salt bath consisting of 60 mol percent of A101 and 40 mol percent of NaCl was prepared and placed in a beaker. A 50 x 100 mm. aluminum plate acting as the anode electrode was placed at the middle of the beaker and a 50 x 100 x 0.18 mm. steel plate to be plated was suspended in the bath opposite the anode electrode at a distance of 48 mm. Plating operations of ten minutes each were continuously repeated for four days at a current of 2 amps and voltages from 280 to 320 mv. During this period there were some rainy days so that the bath greatly deteriorated. Continuous plating under these conditions resulted in the following current efliciencies (in percent) of deposition: 39.4, 39.3, 51.2, 58.2, 60.6, 63.4, 62.0, 56.3, 47.5.

The color of the plated surface was grey and the surface appearance was coarse, which showed that the quality of the plating had degraded owing to a large increase of oxygen ions.

30 gm. of silica gel particles were incorporated in this bath and in addition hydrogen chloride gas was blown in for 24 hours at a rate of 5060 milliliters per minute. Oxygen ions were thus combined with hydrogen ions to form water which was removed by being absorbed by the silica gel. On the other hand aluminum ions were restored to aluminum chloride thus giving a clear and transparent bath. When resuming plating under these conditions, the following current efliciencies of precipitation (in percent) were obtained: 60.0, 68.7, 72.0, 71.8, 69.3, 71.2, 71.2, 68.4.

Although this bath had been dehydrated, the quantity of aluminum chloride decreased due to sublimation during the continuous operation of four days. To make up for this loss g. of aluminum chloride were added. After this the current efficiencies of deposition (in percent) were restored to original values as follows: 81.7, 83.5, 83.0, 85.8, 85.6, 85.2, 86.2, 84.4, 86.2.

The resulted plated surface was white, flat and dense. These properties were identical to those of platings obtained from a fresh plating bath.

EXAMPLE IV Heretofore in order to obtain platings thicker than 50 microns it has been necessary to prepare a large bath (to decrease consumption and deterioration thereof) and to use various troublesome means such as agitation of the bath, vibration of an article to be plated, superposition of alternating current and the like. However, under the same conditions as in the preceding example, by merely floating 30 g. silica gel particles upon the bath and without relying upon agitation, vibration or superposition of alternating current a plated film of 56 microns thick was obtained after a long plating period of 5 hours. Whereas, in the absence of the silica gel particles, the thickness of the plating was about 30 microns at the best. This is because the spacing between electrodes was bridged by the dendrite. On the contrary no dendrite was formed by the method of the invention.

EXAMPLE V With the same conditions as in the preceding example the quantity of aluminum chloride lost by sublimation was measured. The area of the surface contacted by air in the cross section of a beaker was 184 cm The measured quantities of Sublimated aluminum chloride condensed on a lid after 17 hours were: With 30 g. of silica ge1-82 g.; 89 g. No silica gel.106 g.; 127 g.

It was found that the quantity of sublimated aluminum chloride could be further decreased by increasing the quantity of silica gel.

Thus according to this invention, it is not only possible to prevent deterioration of a fused salt bath but also one can readily regenerate a deteriorated bath. More specifically, these advantages are:

(1) To be able to maintain high current efiiciencies, to regenerate deteriorated bath whose current efficiency has been decreased, thus prolonging the life of the bath and permitting the economical use thereof.

(2) To be able to regenerate the moisture absorbing substance because silica gel particles or quartz sand which does not react with the bath composition and has a high dehydrating ability is utilized. These substances are not expensive so that the cost of plating is not increased.

(3) Certain moisture absorbing substances float on the surface of the bath and thus are effective to shield the bath from the atmosphere, preventing loss by evaporation of the bath component and dissipation of heat.

As stated herein above, the subject matter of this invention lies in the discovery that the basic reason for the deterioration of the plating bath and for inferior plating is the moisture absorbed or introduced into the bath and in a novel method of plating in the absence of moisture.

It is to be understood that according to the novel method aluminum can be plated to iron, copper, titanium, or any other metals which can be electroplated with aluminum and that the composition of the bath is not limited to those illustrated hereinabove. Further, in addition to silica gel and quartz any other suitable dehydrating or moisture absorbing substances such as porous ceramic can be used as well.

Thus this invention provides a novel method of aluminum plating capable of producing electroplated articles of good quality at low cost.

We claim:

1. In a method of electroplating aluminum using a bath of fused salt comprising aluminum chloride, the improvement including the steps of adding a dehydrating substance to said bath, said dehydrating substance remaining solid at bath temperatures and being capable of removing moisture from said bath, and performing the plating operation in the substantial absence of moisture.

2. The method according to claim 1 wherein said moisture absorbing substance is a. member or some combination of members selected from the group consisting of silica gel particles, granulates of silica, quartz sand, minute pieces of porous ceramics, adsol (a kind of sintered activated clay), diatomaceous earth, charcoals, graphite, carbon black, alumina gel particles, alumina particles, glass particles, glss fibers, asbestos, powder of 6 anorthite, calcium sulphate, fluorspar, zeolite, acid clay and pumice stone.

3. The method according to claim 1 wherein said bath consists essentially of aluminum chloride, and sodium chloride, with or without potassium chloride.

4. The method according to claim 1, wherein said dehydrating substance is added to the surface of said fused salt bath.

5. The method according to claim 1, wherein said dehydrating substance is added to the interior of said fused salt bath.

References Cited UNITED STATES PATENTS 9/1931 Drigs et al. 204-64 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner