Coating processes – Direct application of electrical – magnetic – wave – or... – Sonic or ultrasonic
Reexamination Certificate
2002-03-26
2003-12-30
Barr, Michael (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Sonic or ultrasonic
C427S600000, C427S405000, C427S435000, C427S436000, C427S437000, C427S438000, C427S429000, C427S190000, C427S191000, C205S188000
Reexamination Certificate
active
06669997
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a chemical process for coating magnesium and its alloys, and to a coating so formed.
BACKGROUND OF THE INVENTION
With the increasing awareness of fuel consumption and human ecology, a global commitment has been made to reduce vehicle mass through application of lightweight materials. Magnesium is the lightest structural metal with the highest specific strength and is the eighth most abundant element on the earth. Many researchers and developers have looked to magnesium to provide a solution for vehicular mass reduction for the automotive, aircraft and aerospace industries. However, challenges exist owing to its low corrosion and wear resistance. To achieve the necessary mass reductions, various coating technologies have been applied to enhance the corrosion and wear resistance of magnesium alloys. To date, no coating technology provides a solution that satisfies the combination of functionality, cost, scalability and environmental concerns. Development of a high volume, environmentally friendly, low cost and mass production scaleable coating process to increase the corrosion and wear resistance of magnesium remains a challenge. Conventional coating technologies are briefly summarized below.
Conversion coatings, the most commonly used type of coatings, contain hexavalent chromium, a highly toxic carcinogen. Conversion coatings alone do not provide sufficient corrosion and wear protection for magnesium alloys in harsh service conditions. Conversion coatings are generally used as an undercoat.
Anodizing is a process that does not provide sufficient corrosion resistance without further sealing because the coatings produced are comprised of a thick porous layer over a thin continuous barrier layer. The coatings produced are brittle insulating ceramic materials, which limits their use in applications where electrical conductivity or load-bearing properties are necessary. High energy consumption is another drawback to this process.
Gas-phase deposition processes require large capital investment and cannot uniformly coat complex shapes due to their line of sight nature. The corrosion, adhesion and wear properties of these coatings on magnesium alloys have not been well documented.
Organic coatings alone do not have sufficient corrosion and wear resistance to protect magnesium for use in harsh service conditions. They are typically used as top-coats and must be applied in multiple layers due to difficulties in achieving uniform pore-free coatings.
Electrochemical coating processes are available for plating of magnesium alloys. These processes are alloy specific and do not work well on alloys with high aluminum content. Direct electroless nickel plating and zinc immersion are two types of electrochemical coating processes.
Direct electroless nickelplating is limited by the short lifetime of the plating baths, the toxicity of chemicals used in the pretreatment process and the narrow operating window required for optimum coatings.
Direct electroless nickel plating comprises a pretreatment process in which electroless nickel is plated directly onto magnesium alloy AZ91 die castings, developed by Sakata et al.(
1
). In general the pretreatment is as follows:
Pretreat→Degrease→Alkaline Etch→Acid Activation→Alkaline Activation→Alkaline Electroless Nickel Strike→Acid Electroless Nickel Plating.
This process has been criticized (
2
) for using an acid electroless nickel treatment that can result in corrosion of the underlying magnesium if any pores are present in the nickel strike layer. A simpler process has been developed by PMD (U. K.) Limited (see references
3
,
4
,
5
). The basic sequence of this pretreatment is as follows:
Pretreat→Alkaline Clean→Acid Pickle→Fluoride Activation→Electroless Nickel Plating.
The authors determined that the etching, conditioning and plating conditions had a large effect on the adhesion obtained. An insufficient etch or fluoride conditioning resulted in poor adhesion. It was also determined that using hydrofluoric acid for conditioning led to a wide plating window while ammonium bifluoride resulted in a much narrower (pH 5.8-6.0 and temperature=75-77° C.) window for acceptable adhesion. The chromic acid treatment was found to heavily etch the surface and leave behind a layer of reduced chromium. The fluoride conditioning was found to remove chromium and control the deposition rate by passivating the surface. The passivating effect of fluoride was also exploited in the plating of magnesium alloy MA-8 (
6
). In this case the nickel plating bath contained fluoride to inhibit corrosion of the substrate during plating. The authors report strong adhesion of the nickel film however, the bath life is too short to be industrially applicable. The addition of a complexing agent, glycine, was shown to improve the stability of the plating bath. Another proposed process (
7
) involves treatment of the sample with a chemical etching solution containing pyrophosphate, nitrate and sulfate, avoiding the use of toxic chromium ions. The process sequence is as follows:
Chemical Etching→Fluoride Treatment→Neutralization→Electroless Nickel Plating.
The electroless nickel plating bath does not contain any chloride or sulfate. The plated samples achieved have high adhesion and corrosion resistance. One obstacle to coating magnesium with nickel is that most conventional nickel plating baths are acidic and can attack or corrode the magnesium surface. This problem has been addressed by the development of an aqueous acidulated nickel bifluoride electroplating bath that contains a polybasic acid (
8
). This bath has been shown to not corrode magnesium.
Zinc Immersion Processes (see references
9
a
and
9
b
) are limited by the poor uniformity of the zinc undercoating produced as well as the need for a copper cyanide strike prior to any further plating. The chemicals required for zinc immersion processes are extremely toxic. The zinc immersion pretreatment process has been criticized for the precise control that is required to ensure adequate adhesion. In many cases non-uniform coverage of the surface is seen with spongy non-adherent zinc deposits on the intermetallic phase of the base alloys (
1
). The copper cyanide strike that must follow has also been criticized for a number of reasons (
1
). The first is that it is an electroplating process, which means that it is more difficult to coat complex shapes. Copper deposits slowly in the low current density areas, which allows attack of the zinc by the plating solution. This in turn allows attack on magnesium by the plating solution resulting in non-adherent copper depositing by displacement directly on the magnesium surface. The deposits in these areas are porous and have poor corrosion resistance. The second criticism levelled at the copper cyanide plating process is the high cost treatment of waste generated by the use of a cyanide containing bath. A patented methodology (
10
) attempts to improve this process by eliminating the copper cyanide step from the pretreatment process. The copper cyanide electroplating is replaced by a zinc electroplating step followed by copper deposition from a pyrophosphate bath after the zinc immersion. This patent claims that by creating a uniform zinc film of at least 0.6 micrometers in thickness, adherent plating films can be obtained on any magnesium alloy using the disclosed process. The zinc electroplating step can occur simultaneously with the zinc immersion process or in a separate step. The process is as follows:
Degrease→Alkaline Clean→Acid Clean→Activation→Zinc Immersion→Zinc Electroplate→Copper Plating.
A number of processes based on the zinc immersion pretreatment process have been developed. The three main processes are the Dow Process, the Norsk-Hydro process and the WCM Canning Process (
1
,
11
). One criticism of all of these processes is that they do not produce good deposits on magnesium alloys with an aluminum content greater t
Gray Joy Elizabeth
Luan Ben Li
Barr Michael
Borden Ladner Gervais LLP
Marsman Kathleen E.
National Research Council of Canada
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