Method of Forming Chromate Conversion Coatings on Aluminum Surfaces and the Coating Formed Thereby

For many years, chromate conversion coatings have been applied to aluminum surfaces and surfaces of aluminum alloys to prevent corrosion. The compositions for forming these coatings contain, in addition to hexavalent chromium and activators such as fluorine, compounds to accelerate the reaction rate for chromate conversion coating on a volume basis. Due to the dramatic accelerating effect of potassium ferricyanide, it is the most common accelerator employed. Other accelerators enjoying limited use include molybdenum, vanadium and tungsten salts.

However, these accelerators, especially potassium ferricyanide, are toxic and they are environmentally harmful. These accelerators form toxic waste products of the conversion coating process and, due to the possible serious environmental consequences, become a serious disposal problem. The liability which may arise from utilizing these toxic chemicals as accelerators continues after disposal of the chemicals and cannot be avoided. There are other disadvantages to using such accelerators. Compounds such as potassium ferricyanide decompose during the conversion coating process, requiring repeated replenishment. Futhermore, ferricyanide compositions are also known to decompose into highly toxic hydrogen cyanide gas, thereby endangering workers. As a result, conversion coating processes have been sought which could, without the use of harmful accelerators, achieve effective coatings at a rate high enough to coat aluminum surfaces on a volume basis. Until the present invention, the results of this search have been largely negative.

The process of the present invention alleviates these problems by reducing the need for these environmentally harmful accelerators, while achieving effective chromate conversion coatings at acceptable coating rates.

The process of the present invention consists broadly of immersing an aluminum or aluminum alloy surface in a chromate conversion coating bath while exposing the immersed surface to ultrasonic energy and controlling the ultrasonic power environment to which the aluminum surface is exposed.

We have discovered that when ultrasonic energy is applied within a limited effective range to an aluminum or aluminum alloy surface immersed in an accelerator-free aqueous hexavalent chromium coating bath, satisfactory conversion coating action is achieved. This conversion coating reaction produces effective chromate coatings at coating rates high enough to coat aluminum surfaces on a volume basis. Therefore, the problem-laden accelerators such as potassium ferricyanide previously included in the coating solutions may now be eliminated through the practice of this invention.

The ultrasonic energy effective range may be controlled by varying the energy produced by ultrasonic transducers or by varying the positions of such transducers and the aluminum or aluminum alloy surface relative to each other so that the metallic surface passes through zones of varying energy intensities inherent to energy produced by ultrasonic transducers. The preferred frequency range for such ultrasonic energy lies in the range from about 18 kilohertz to about 60 kilohertz.

The method of this invention comprises immersing an aluminum or aluminum alloy surface in a hexavalent chromium conversion coating bath and exposing the bath and aluminum surface to ultrasonic energy. The standard chromate conversion coating solutions without the previously employed environmentally harmful accelerators, such as potassium ferricyanide, is used. Even without these accelerators, effective coatings and acceptable coating rates are obtained through the practice of the present invention.

In the preferred embodiment, the chromate conversion coating solution consists essentially of an aqueous solution containing hexavalent chromium ions in a concentration in the range of from about 0.5 g/l to about 10 g/l and fluorine ions in a concentration in the range of about 0.2 g/l to about 4 g/l. The solution is applied at a temperature within the range of from about 60.degree. F. to about 160.degree. F., and at a pH within the range of from about 0.8 to about 2.2. An alternative preferred embodiment of the solution consists essentially of an aqueous solution containing hexavalent chromium ions, fluorine ions and phosphate ions in concentrations in the ranges of about 1 g/l to about 10 g/l, about 0.2 g/l to about 4 g/l, and about 0.5 g/l, to about 60 g/l, respectively.

The acidity of the solution must be maintained in the stated pH range in part to inhibit the reduction of Cr(VI) to Cr(III). Chromic acid may be used as a source of hexavalent chromium. Other sources of Cr(VI) include sodium chromate, sodium dichromate and potassium chromate and potassium dichromate. Fluorine sources include hydrofluoric acid, potassium titanium fluoride, nickel fluoride, calcium fluoride, ammonium silco fluoride, sodium fluoroborate, sodium fluoride, sodium bifluoride, potassium fluoride and potassium bifluoride and ammonium bifluoride. The proper acidic environment may be maintained through addition of an acidic chromium source, such as chromic acid, and an acidic fluorine source, such as hydrofluoric acid. The acidic environment may also be achieved by adding an acid, such as nitric acid. If the solution is too acidic, such as occurs upon addition of large amounts of chromic acid, the excess acid may be neutralized by addition of an alkali such as ammonium hydroxide.

The process of the present invention is applicable to aluminum and aluminum alloys such as 2024 aluminum, 3003 aluminum, 1100 aluminum, 7075 aluminum and 6061 aluminum.

The following examples illustrate that optimum results are obtained when reduced ultrasonic energy is applied. The preferred frequency range for such ultrasonic energy lies in the range from about 18 kilohertz to about 60 kilohertz.

Aluminum panels, 4".times.12" panels of 2024 aluminum, were first thoroughly cleaned with an alkaline cleaner and then rinsed by dipping for one minute in a warm water rinse. An aqueous solution was prepared including the following ingredients in the following concentrations:

Chromium ion as Cr(VI)--5.3 g/l

Fluorine ion as F.sup.- --1.7 g/l

Nitrate ion as NO.sub.3.sup.- --1.3 g/l

Water--Balance to 1 liter.

Sixty-four gallons of the aqueous solution at 140.degree. F. were put into a stainless steel container measuring two feet square and two feet deep. Within the container and attached to the bottom thereof were two stainless steel containers containing piezoelectric type transducers generating ultrasonic energy at about 26.5 kilohertz. Each of the two transducers was rated at 350 watts at full power with the power being variable. The distance from the top of the transducer containers to the top of the solution was 18 inches. The top edges of the aluminum panels were placed one inch below the surface of the solution at various ultrasonic power settings and held stationary in the solution for various lengths of time as shown in the table below. The aluminum panels were then rinsed by two one-minute dips in cold water and a passivating rinse. Next, the aluminum panels were spray tested for corrosion resistance. The results are presented in the table below:

The process of Example 1 was followed except that the solution contained additionally 1.3 g/l of phosphate ion as PO.sub.4.sup.-.

The following results were obtained:

Although, in the examples, 2024 aluminum was coated, it should be understood that aluminum or other alloys of aluminum may be coated in this manner. It is further understood that even though the aluminum panels were held stationary while the ultrasonic power was varied, the process of this invention may be carried out by varying the distance between the aluminum surface and the power source.