Non-chromate chemical treatments used on magnesium alloys

Formulations and a process are disclosed for chemically treating a surface of a product made from magnesium alloys. One formulation comprises an acid pickle comprising hydrofluoric acid and a binary alcohol and a second formulation comprises a conversion solution comprising nitric acid, permanganate, and ammonium acid difluoride. The process comprises degreasing the product with alkaline solution, rinsing the product with water, pickling the product with the acid pickle, rinsing the product with water, modifying the product using the modifying solution, rinsing the product with purified water, and drying the product by heating.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to finishing operations for products made
 from magnesium alloys, and particularly to non-chromate chemical
 treatments which form a protective film to protect the product from
 surface corrosion.
 2. Description of the Prior Art
 Magnesium alloys have many advantages, such as low density, high thermal
 conductivity and high conductivity of the surface thereof. Because
 magnesium alloys can comply with the trend forward lighter, thinner,
 shorter, and smaller fittings having better heat dissipation and improved
 EMI shielding characteristics for electronic apparatus, magnesium alloys
 is replacing plastics and become the newest popular material of mobile
 computer, consumer and mobile communication electronic products. However,
 magnesium alloys are vulnerable to oxidation in air, so the surfaces of
 the alloys need a protective film for inhibiting corrosion.
 Chromate is normally used to chemically modify surfaces of products made
 from magnesium alloys. Such as the chemical treatment operations for
 magnesium alloys proposed by Dow Chemical Company and U.S. Pat. No.
 4,676,842 disclose the use of chromate to chemically modify metallic
 materials, which forms a compact protective film inhibiting the corrosion
 on surfaces of magnesium alloys. Chromic ions (Cr.sup.3+) in the film and
 the acid pickle are poisonous and dangerous pollutant to the environment
 and result in recycling difficulties of magnesium alloys and increased
 cost of manufacture and waste handling. Since chromic ions (Cr.sup.3+) are
 to be banned from use in the near future, a reliable operation without
 using chromate is desired.
 SUMMARY OF THE INVENTION
 An object of the present invention is to overcome the shortcomings of the
 prior art by providing a non-chromate chemical conversion coating to the
 surfaces of magnesium alloys to inhibit corrosion on the surfaces.
 Another object of the present invention is to provide a non-chromate
 operation to reduce environmental pollution.
 Another object of the present invention is to provide a process using
 non-chromate formulations, which operates at a lower temperature and
 reduces cost.
 Two formulations in accordance with the present invention comprise an acid
 pickle prepared mainly from hydrofluoric acid and a binary alcohol, and a
 conversion solution prepared mainly from nitric acid, permanganate, and
 ammonium acid difluoride.
 A process in accordance with the present invention which uses the new
 formulations follows. A specimen composed of a magnesium alloy is
 initially immersed into an alkaline solution at 333.15.about.343.15
 degrees Kelvin for 3.about.5 minutes for degreasing, and is then rinsed
 twice with purified water at ambient temperature. The rinsed specimen is
 then dipped in the prepared acid pickle at ambient temperature for
 0.5.about.1.5 minutes. After acid pickling, the specimen is rinsed twice
 with purified water at ambient temperature. Following the fourth rinsing
 the specimen is immersed into the prepared conversion solution at
 293.15.about.333.15 degrees Kelvin for 0.5.about.1.5 minutes to form a
 compact protective film on the surface of the specimen. The modified
 specimen is rinsed for a fifth and sixth times using purified water at
 ambient temperature. Following the final rinse, the specimen is dried at
 333.15.about.343.15 degrees Kelvin for 50 minutes. Except for the drying
 step, at least one of the above steps includes an environment with
 ultrasonic waves. Each of the rinsing steps consumes 10.about.20 seconds.
 Other objects, advantages and novel features of the invention will become
 more apparent from the following detailed description of the preferred
 embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 In a chemical treatment operation in accordance with the present invention,
 an acid pickle solution and a conversion solution are used in the process.
 The below two tabulations are listings of those substances (together with
 their percentage by weight) that make up formulations of the acid pickle
 and the conversion solutions in accordance with the present invention.

Approximate
 Substance Percentage by Weight
 Acid Pickle Solution Formulation
 Deionized water Q.S. %
 Hydrofluoric Acid 1.6%
 Ethylene Glycol 3%
 Conversion Solution Formulation
 Deionized water Q.S. %
 Nitric Acid 0.325%
 Potassium Permanganate 3%
 Dibasic
 Ammonium Hydrogen 0.5%
 Difluoride
 In the preferred embodiment, a concentration by weight of the hydrofluoric
 acid is 40% and a concentration by weight of the nitric acid is 65%. Weigh
 40 grams of hydrofluoric acid (40% HF) and 30 grams of ethylene glycol
 (C.sub.2 H.sub.4 O.sub.2) at ambient temperature using an electronic scale
 and mix the two in a container, adding an appropriate quantity of purified
 water to the mixture to just adequately inter-dissolve the components
 while stirring. Dilute the solution with an appropriate quantity of
 purified water to get 1 liter of acid pickle. Also weigh precisely 5 grams
 of nitric acid (65% HNO.sub.3), 30 grams of potassium permanganate dibasic
 (KMnO.sub.4), and 5 grams of ammonium acid difluoride
 (NH.sub.4.multidot.HF.sub.2) at ambient temperature using an electronic
 scale and mix them with a proper quantity of purified water for just
 adequately inter-dissolving upon being stirred. Dilute the solution using
 an appropriate quantity of purified water to get 1 liter of conversion
 solution. Percentages by weight of the components of the above solutions
 may vary in a range of -5%.about.+5%.
 A specimen composed of a magnesium alloy is initially immersed into a
 pre-cleaning solution comprising alkaline solution and surfactants at
 333.15.about.343.15 degrees Kelvin for 3.about.5 minutes to remove forming
 lubricant, fingerprints, and other organic deposits on the surface of the
 specimen. Then the specimen is rinsed twice with purified water at ambient
 temperature to eliminate residual pre-cleaning solution and molecular
 grease on the surface thereof. The rinsed specimen is immersed into the
 prepared acid pickle at ambient temperature for 0.5.about.1.5 minutes to
 slightly etch the surface of the specimen and get rid of oxides on the
 surface thereby exposing the surface for direct contact with the modifying
 solution in a following step. The acid pickle can also neutralize the
 possibly residual alkali materials of the pre-cleaning solution. The
 specimen is rinsed twice with purified water at ambient temperature after
 acid pickling to eliminate the remainder of particulate oxides, resultants
 and residual acid pickle on the surface thereof. The fourth rinsed
 specimen is immersed into the prepared conversion solution at
 293.about.333 degrees Kelvin for 0.5.about.1.5 minutes to form a compact
 protective film on the surface of the specimen. The modified specimen is
 rinsed twice using purified water at ambient temperature for eliminating
 residual test solutions. Finally the specimen is dried at a temperature of
 333.about.343 degrees Kelvin for 50 minutes. Except for the drying step,
 all of the above steps include an environment with ultrasonic waves. The
 frequency of the ultrasonic waves is about 28 K Hz. All of the rinsing
 steps are conducted for 10.about.20 seconds. The environment with
 ultrasonic waves can be obtained by using an ultrasonic cleaner which is
 well known by one skilled in the art of cleaning of mechanical components.
 The applied duration of the ultrasonic waves is equal to the duration of
 the corresponding step.
 The present invention does not use the conventional chromate, so it reduces
 pollution, facilitates recycling of the magnesium alloy, and decreases
 handling costs. Additionally, as can be seen from the description above,
 operating temperatures of the processing steps of the present invention
 are far below those of the conventional art thereby further reducing cost.
 It is to be understood, however, that even though advantages of the present
 invention have been set forth in the foregoing description, together with
 details of the invention, the disclosure is illustrative only, and changes
 may be made in detail within the principles of the invention to the full
 extent indicated by the broad general meaning of the terms in which the
 appended claims are expressed.