DE1009881B - Process and bath for chrome plating aluminum - Google Patents

Description

GERMAN

The invention relates to chrome plating of aluminum, especially the production of chrome-plated aluminum that is suitable easy to take on a permanent ornamental luster. Aluminum comes in various forms Applied for many uses that require a shiny or polished surface for decorative purposes is desirable.

The subject of the invention is a chromium plating process, which in a very short time creates a firmly adhering, Extensible chrome coating that does not crack when the coated objects are bent and is easy to polish. In addition, the electrolytic bath inclination method works against becoming cloudy with prolonged use. Otherwise, this deterioration resulted from the formation of Trivalent chromium as well as accumulating impurities through which the continuous Use of the bathroom was impaired.

It has been found that in the manufacture of chrome-plated aluminum by the continuous Maintaining a low temperature within the electrolyte improves results significantly can be obtained .. According to German Patent 841095, it is known in the electrolytic Separation of chrome coatings temperatures below 30 ° to be used. It has, however found that the desired benefits can only be achieved are, if essentially composed of chromic acid and sulfate radical, treatment baths are used in which temperatures of up to 20 °, preferably from 2 up to 10 °. The chrome coating then adheres well to the aluminum and does not break if it is bent cracked. It's proportionate, soft and ductile and can be easily polished. In addition, only a minor one is necessary for the production of an effective chrome coating Current density and: a shorter electrolysis time required.

When carrying out: the invention, the temperature of the chromium plating bath is kept between 0 and 20 °. The preferred working temperature is between 4 and. 10 °. As the temperature approaches 20 °, the coating becomes more difficult to polish and a longer chrome plating time is required. The temperature can be kept at the desired low level using water or ice-cooled cooling coils. It is particularly expedient to use conventional cooling systems in which a flow of coolant is passed through one or more coils located inside the bath. It is also advantageous to thermostatically control the cooling in order to maintain the temperature within the desired low temperature range, as the process and bath for chrome plating
of aluminum

Applicant:

William Donald MacLean, Bronte,
Ontario (Canada)

Representative: Dr.-Ing. A. von Kreisler,

Dr.-Ing. K. Schönwald,

Dipl.-Chem. Dr. phil. H. Siebeneicher

and Dr.-Ing. Th. Meyer, patent attorneys, Cologne 1,

Deichmannhaus

William Donald MacLean, Bronte, Ontario (Canada),
has been named as the inventor

naturally - the temperature tends to increase during the deposition. The current yield is on greatest at low temperatures. In general, normal continuous operation already get favorable results when the temperature is maintained within the preferred temperature range.

The normally used chrome plating bath contains chromic acid (Cr O ₃ ) and sulfate radicars (SO ₄ ) in a ratio of 100: 1, with 250 or 400 g CrO ₃ and 2.5 or 4 g SO ₄ / liter in the solutions used. At the low temperature of the present procedure, a CrO ₃ : SO ₄ ratio of 115: 1 is preferred. However, it is also possible to work with a ratio between 80: 1 and 140: 1. In order to make the advantages of the process clear after registration, the following comparative examples are given.

B e i s ρ ie 1 1

Aluminum strips or aluminram chips were chrome-plated with a solution containing 250 g of chromic acid and 2.5 g of sulfate / liter. 12 Amp./dm ² were applied at temperatures of 77 ° and a chrome coating of a thickness of 0.025 mm was achieved in 1 hour 45 minutes. When using the same bath, the same current density and a temperature of

709 547/360

2 ° a coating 0.025 mm thick was already scored in 35 minutes.

Example 2

Using the same chrome plating bath as in Example 1, a coating 0.025 mm thick was deposited in 1 hour ^{at a current density of 32 amps / dm 2 and a temperature of 55 °.}

When using the same bath and an amperage of 8 amps / dm ² and maintaining temperatures of up to 10 °, a coating 0.025 mm thick was produced within a period of 5 minutes.

If temperatures of up to 20 ° and a current strength of 6 to 14 amps / dm ² are maintained, no more than 5 minutes are required for the formation of a normal decorative chrome coating. The lower the temperature, the lower the required current density. At 7 °, with a simultaneous current of 8 amps / dm ^2, a chrome coating about 0.051 mm thick is deposited, which is firmly adherent, flexible and very easy to polish. If the low temperature is maintained, the bath can be used for almost any length of time, with only the chromic acid and the sulfate radical having to be renewed. As conductivity increases with chromic acid concentration, current efficiency decreases, so it is best to maintain a Cr O ₃ : SO ₄ ratio of about 115: 1.

With the previous normal process without cooling, a comparable coating can be formed in 7 minutes, but the chrome-plating time increases very quickly with the temperature. At temperatures of 22 ° the coating is too hard to allow economical polishing for decorative purposes. In addition, it is necessary to _{frequently remove Cr 2} O ₃ from the chrome-plating bath, at least once a week and _{to adjust the CrO 3} : SO ₄ ratio at least daily if the operation is continuous. When using a chrome-plating container with 1550 liters of cooled electrolyte solution and a power source of 5000 Amp. 28 m ^{2 of} aluminum can be chrome-plated every hour with continuous operation, while without cooling the production is only 10 m ² per hour and the electrolyte solution can only be used for 4 to 6 hours can.

Since, in maintaining low temperatures lead to lower current densities are needed, it is \ ^r erständlich that characterized thinner electric wires can be used, thereby reducing the cost of the apparatus.

For the favorable effect of the temperature effect on the hardness of the chrome coating, a series of tests serve that with a group of aluminum batches at various temperatures had been coated, were executed. The hardness measurement was made with a Tukon hardness testing machine carried out with a 136 ° Vickers diamond cutter with a load of 100 g worked. The measurements of the notches were made using a micrometer screw eyepiece at a magnification of 1: 500. The data obtained clearly demonstrated that with increasing temperature of the electrolytic bath was accompanied by an increase in hardness. t

Gases are generated during the chrome plating process released, which at the same time discharge some chromic acid, causing losses. At the low one The temperature of the present process, the gas evolution is less violent, which at the same time a less swirling of the sludge takes place, which is during the continuous operation in the Treatment bath tends to pile up. This avoids the formation of cracks on the coating.

When cleaning the aluminum items to be chrome-plated, it was previously common practice to remove the oxide skin remove and dip the items in a solution of zinc oxide in caustic soda to bond to support between aluminum and chrome plating. The method of the invention is completely independent of whether such a pretreatment is carried out or not. The lower one Temperature of the present process reduces the tendency to deteriorate adhesion and for blistering.

The low chrome plating temperature makes it unnecessary to have a nickel or copper base coat to get a bright final shine. In addition, the corrosion resistance is lower Temperatures formed coating better than that with the previously common chrome plating process chrome plating obtained from a copper base plating.

If the aluminum objects have been chrome-plated at the low temperature of about 2 °, you can they can be polished to a shine by simply rubbing them by hand. At bath temperatures of 20 ° the polishing can be achieved with one pass through a normal polishing roller, the forward movement of the chrome-plated object and another passage of the object through the role, but this rotates in the same sense. With the chrome plating processes that have been used up to now, tinter Using temperatures of about 40 to 41 ° it is practically impossible in an economical way to get the desired shine.

Claims (5)

Claims 1. Process for chrome-plating aluminum in an essentially composed of chromic acid and Treatment bath composed of sulfate radicals, characterized in that by cooling at Temperatures up to 20 °, preferably at temperatures from 2 to 10 °, is carried out. 2. The method according to claim 1, characterized in that a current density of 6 to 14 Amp./dm ² is used and the electrolysis time is not more than 5 minutes. 3. Bath for carrying out the method according to claim 1 and 2, characterized in that a ratio of chromic acid: sulfate radical (Cr O ₃ : SO ₄ ) within the limits of 80 to 140: 1, preferably 100 to 115: 1, in the treatment bath, is present. 4. Bath according to claim 3, characterized in that the CrO ₃ : SO ₄ ratio is not significantly below 115: 1. Considered publications:
Pfanhauser, "Galvanotechnik", 1941, p. 524;
Metalloberfläche 1951, pp. B 33 to 42;
German Patent No. 841095;
U.S. Patent No. 2,541,083. © 709 5W360 5. CT