Plating Aluminum – Cast and Wrought Aluminum Alloys
Advanced Plating Technologies offers plating aluminum alloys in various finishes including electroless nickel plating, tin plating, silver plating and gold plating. Barrel, vibratory and rack methods are employed in plating aluminum based upon the geometry and specific application of the component. APT’s process for plating aluminum has been refined over sixty years in business for applications within the power transmission/distribution, energy, defense, telecommunications and aerospace markets. Plating aluminum of all grades – both cast and wrought – is offered with specific pretreatment cycles tailored to the specific aluminum grade to ensure consistent adhesion and cosmetics of the plated component.
Plating Aluminum – Zincate and Electroless Nickel Strike
Plating aluminum is a specific subset of plating that is complicated by the fact that aluminum rapidly oxidizes when exposed oxygen to present in the atmosphere. When plating on any metallic substrate, two basic steps must occur: 1) clean the oils, grease and nascent shop dirt from the surface of the product, and 2) remove the metal oxides present on the surface of the parts. Most metals will not reform oxides on the surface immediately after step 2 above. However, aluminum reoxidizes almost immediately after deoxidizing which requires a unique pretreatment protocol to address this metallurgical characteristic.
To counter the rapid oxidation of aluminum, a specific zincate process is employed by APT, which provides a thin zinc immersion deposit on the surface of the aluminum. Zincate is a specially formulated alkaline zinc immersion deposit that has the ability to strip any residual aluminum oxide from the surface while depositing a thin zinc immersion deposit. A double zincate application is most commonly utilized at APT when plating aluminum to provide a very thin, dense and uniform zinc deposit. The zinc on the surface acts as a “metallic shrink wrap” to seal the aluminum from oxidation until the subsequent plating process.
After the zincate process, an electroless nickel strike is employed which fully removes the immersion zinc from the surface of the component to reveal the pristine aluminum surface below. Electroless nickel is autocatalytic on aluminum and rapidly deposits on the surface while stripping the zincate to form an extremely adherent and tight bond to the aluminum surface. Once an electroless nickel strike is deposited, subsequent plating processes such as tin, silver or gold can be applied over the sound nickel strike.
This method of aluminum plating has proven superior to alternative pretreatment methods such as a copper strike. When a copper strike is applied, the adhesion of the copper is dependent upon the strength of the zincate bond to the aluminum since a copper strike will not remove the zincate from the surface. Although a copper strike is a less expensive method of plating aluminum, the consistent results and superior adhesion of an electroless nickel strike are unmatched by any other pretreatment method.
Plating Aluminum – Specific Pretreatment Cycles Based on Grade
To further complicate plating aluminum, various grades of aluminum utilized in manufacturing have wide ranges of alloying metals and constituents. 1000-series wrought aluminum grades are nearly pure, with very little alloying metals. Common grades in the 6000-series are alloyed with copper, magnesium, chrome and silicon. Grades such as 2000 and 7000 series have very high copper and 7000-series is further alloyed with the addition of zinc. Cast grades such as A380 and A356 are laden with silicon and copper to facilitate the flow of the metal during the casting process.
In order to achieve a proper bond when plating aluminum, the alloying constituents must be removed from the surface of the part prior to the zincate application. If this does not occur, a “bridged” deposit over the alloying material will occur which commonly results in blisters during field use. Due to the wide range of alloying constituents found in aluminum alloys, the pretreatment acids must be specially formulated to address the metals on the surface of the aluminum.
Many job shops will try to use a one-size-fits-all approach when plating aluminum with a pretreatment cycle that fits “most” alloys. The is that compromises must be made which often manifest in the final deposit in the form of blisters, pitting or etching of the aluminum surface. Over many years of plating aluminum, Advanced Plating Technologies has specific pretreatment acids that are customized to the alloying constituents within each grade. Setup of the line is made based upon the alloy to be processed as required. This method is more expensive and slower than the alternative, but the results are evident in consistent adhesion and cosmetics in aluminum plating.
Plating Aluminum – Diversity of Finishes and Applications
Aluminum is an excellent engineering material that is growing in use due to weight and cost savings that can be realized. However, aluminum is a reactive metal that is amphoteric in nature, or is corrosively attacked in both alkaline and acidic environments. Aluminum is also a relatively soft metal subject to galling and affords minimal wear resistance. In addition, aluminum rapidly forms an oxide layer that impedes the flow of current and is a barrier to soldering and brazing operations.
As such, the selection of aluminum in the design of products must often first be paired with a consideration of a proper coating for corrosion protection, hardness, and/or conductivity of subsequent joining operations. Advanced Plating Technologies offers a wealth of metallic deposits for plating of aluminum to meet all of these design considerations. A brief summary of various coatings and their benefits for plating aluminum is listed below:
Electroless Nickel Plating Aluminum: Electroless nickel provides excellent hardness, lubricity and wear resistance. Corrosion resistance over 500-hours of salt spray is possible.
Sulfamate Nickel Plating Aluminum: Sulfamate nickel plating provides good corrosion resistance and improved hardness of aluminum. Sulfamate nickel excels in applications where brazing is required, offering excellent wetting of the nickel surface.
Tin Plating Aluminum: Tin plating of aluminum offers good surface conductivity and corrosion resistance for applications within the electronics and power markets. Bright tin plating can be provided for interconnect, butt or lap-joint applications whereas matte tin plating of aluminum can be provided for applications requiring soldering.
Silver Plating Aluminum: Silver plating of aluminum offers outstanding surface conductivity and corrosion resistance for critical applications within the power transmission/distribution and energy markets. Silver offers good solderability and excellent lubricity and anti-galling in high temperature applications. Silver over electroless nickel on aluminum is an excellent choice for switchgear applications where repeated cycling is a concern.
Gold Plating Aluminum: Gold is a fully noble metal which does not form an oxide on other compounds under most conditions. As such, gold plating of aluminum is the best choice for low voltage current carrying applications where even a thin oxide or sulfide compound can interfere with the design. Gold plating of aluminum over a sulfamate nickel underplate offers unmatched consistency in solderability. Due to its unreactive nature, gold offers excellent biocompatibility for critical medical applications.