Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2000-06-20
2001-09-18
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C428S689000, C428S699000
Reexamination Certificate
active
06291076
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the formation of a cathodic protective coating on magnesium or magnesium alloys and to the hydride or hydrogen-rich coating so formed. More specifically, such coating is produced by an electrochemical treatment in an alkaline bath containing hydroxide and supporting electrolytes with use of a source of cathodic current.
2. Brief Description of the Prior Art
Magnesium alloys have been increasingly utilized in structural applications. By minimizing metallic impurities and adding aluminum or rare-earth elements, the corrosion rates of magnesium alloys become comparable to those of carbon steels or A380 aluminum alloys in salt spray environment. Painting is a popular method to improve the corrosion resistance and to add decorative appearances. Chemical or electrochemical pretreatment is usually applied before painting to strengthen the adhesion between the paint film and Mg surface. These treatments also provide limited corrosion protection. Among them, chromium (VI) compound based chemical conversion coatings are known to offer a good paint base. However, because of its toxic nature, the handling of the solution and its disposal are of concern. As such, several non-chromium (VI) based coatings such as zirconium- or permanganate-based coatings have been developed (e.g. U.S. Pat. No. 5,380,374 of Jan. 10, 1995 entitled “CONVERSION COATINGS FOR METAL SURFACES”). These surface coatings, including chromium based coatings, usually require regular control of chemical composition, as chemicals are consumed during the operation.
Another electrochemical surface treatment of magnesium or its alloys is called “anodizing” or “anodization” and involves formation by anodic deposition of an oxide/hydroxide or similar protective film or coating on the magnesium article. Examples of such treatments are disclosed, for example, in U.S. Pat. Nos. 2,314,341 and 2,426,254. There are also two-step processes where the magnesium article is first pre-treated in a chemical or electrochemical solution, before being subjected to the anodic deposition of the protective coating. Examples of such two-step processes may be found in U.S. Pat. Nos. 5,240,589 and 5,264,113. These processes employ an anodic technique, i.e. the Mg substrate is polarized to a more positive voltage.
OBJECT AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a cathodic protective coating on magnesium or its alloys which has a number of significant advantages over the anodic coating and conversion coatings.
Another object is to provide a simple and efficient method for effecting such cathodic coating.
Other objects and advantages of the invention will be apparent from the following description thereof.
In essence, the Mg substrate is polarized according to the present invention to a more negative voltage so that the current direction and the nature of the formed film are completely different from the prior art.
The method of the present invention, therefore, comprises electrolytically forming a protective coating on a magnesium containing article by electrochemically treating the article, acting as a cathode, in an alkaline solution, preferably having a pH of between about 10 and 14, at a temperature of between 20 and 90° C., preferably between 40 and 80° C., using a cathodic current density of 5-200 mA/cm
2
, preferably 20-100 mA/cm
2
. A hydrogen rich protective layer of magnesium hydride is thereby formed on the magnesium article essentially without corroding the surface of the article. This can be done by imposing a cathodic DC current, but it is preferable to use a cathodically biased AC current to shorten the process time of hydride formation. In particular, the use of biased square wave current, or intermittent current with a frequency of up to 5 Hz, preferably 0.1-3 Hz is recommended for the ease of instrumentation. During the treatment, hydrogen gas evolution is observed on the Mg article and it is, therefore, advisable to operate under a good ventilation.
The alkaline solution in which the magnesium article is treated may be prepared by adding alkali metal hydroxide, ammonium salts or similar alkaline materials. The addition of NaOH or KOH to water provides the most convenient and economical solution. Some supporting electrolyte, such as KNO
3
or Na
2
SO
4
, may also be added to minimise the solution resistance and to assure uniform current distribution. There is no particular limitation for the choice of the supporting electrolyte, however the use of chlorides is not desirable as it would damage the anode materials during the operation. Also, although operating temperatures may range from room temperature (20° C.) up to close to the boiling temperature (90° C.), temperatures below 40° C. and above 80° C. would retard the reaction and lengthen the time of deposition of the protective coating. There is no particular limitation of the process time which can be as short as 5 or 10 minutes, although preferably it will be 20 minutes or longer. The treatment with longer periods, for example 2 hours, or even 8 to 16 hours, will be useful to obtain a stand-alone protective coating. However, if the coating is used as a paint base, a treatment for 20-40 minutes is usually sufficient. The time of treatment depends on the current density employed: the smaller the current density, the longer the treatment time. After the treatment, the colour of the Mg surface will change to light gray.
Since only water is consumed during the treatment, no complicated analytical procedure is required to maintain the concentration of the chemical compounds. However, it may be useful to control the conductivity and the pH within the desired ranges to ensure the quality of the coating and to avoid unnecessary anodic dissolution of anode materials during the process.
The obtained magnesium-containing article has a protective coating of magnesium hydride of predetermined thickness and a high count of hydrogen particles. The novel magnesium-containing article of the present invention shows a passivation phenomenon at anodic potentiodynamic curve in 5% NaCl solution saturated with Mg(OH)
2
which has a passivation current in the range of 0.1-100 &mgr;A/cm
2
.
REFERENCES:
patent: 2314341 (1943-03-01), Buzzard
patent: 2426254 (1947-08-01), Waterman
patent: 3762938 (1973-10-01), Ridenour et al.
patent: 5240589 (1993-08-01), Bartak et al.
patent: 5264113 (1993-11-01), Bartak et al.
patent: 5339675 (1994-08-01), DiLeo et al.
patent: 5380374 (1995-01-01), Tomlinson
patent: 07278839A (1995-10-01), None
Jones Deborah
McNeil Jennifer
Primak George J.
Technologies Intermag Inc.
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