Sixty years ago, Carl Kiekhaefer purchased a defunct outboard motor manufacturing company in Cedarburg, Wisconsin. Although he intended to make magnetic separators for the dairy industry, the place came with 300 rejected outboard motors for boats. Rather than lose out on an opportunity, Mr. Kiekhaefer rebuilt the engines and sold them. The rebuilt engines worked so well, the buyer ordered more. Thus began what is now the largest boat motor manufacturing company in the world, Mercury Marine.
This track record of leadership and innovation started with Mr. Kiekhaefer and continues today. Mercury continuously develops new and improves existing systems, manufacturing methods and finishing processes. Recently, Mercury Marine started producing aluminum-plated steel components for three families of outboard engines. The aluminum-plated steel is designed to help these engines perform better and last longer under highly corrosive conditions.
"Corrosion of key parts plays a critical role in determining the longevity of the motors," said Chris Misorski, Mercury Marine Metallurgical and Advanced Material Group. "We found that plating parts with aluminum provides the best corrosion resistance without sacrificing high performance and value."
Engineers at Mercury were looking for a cadmium plating replacement and a way to extend product life cycles. AlumiPlate™ aluminum-plating technology provided Mercury with a lightweight, non-toxic, corrosion resistant and environmentally acceptable cadmium-plating alternative.
Mr. Misorski remarked, "Mercury Marine is pleased to find an alternative plating that not only ensures product life, but significantly reduces hazardous environmental effects."
Three mechanisms work to provide corrosion protection in salt and acidic environments. The aluminum layer is pore free and provides a physical barrier. Pure (99.9+%) aluminum forms a thin, tenacious, non-degenerating oxide almost immediately upon contact with the air. The aluminum layer also acts as a sacrificial anode to the steel. If a minor abrasion or scratch exposes the steel base metal, the aluminum plating surrounding the scratch will oxidize more readily and provide sacrificial protection.
Typically, a 0.5-mil aluminum coating with no chromate conversion coating provides more than 1,200 hrs of ASTM B117 salt-spray resistance. Chromated aluminum plating provides 6,000+ hrs of protection.
Galvanic corrosion resistance
One property Mercury is especially interested in is galvanic corrosion protection. For example, in applications where steel fasteners are used in conjunction with aluminum structures, aluminum-plated fasteners protect the aluminum structures from galvanic corrosion.
Considering the nature of the product, Mercury's use of a non-aqueous aluminum plating process is ironic. The actual aluminum plating process occurs in an aprotic (proton-free), non-aqueous electrolyte that does not evolve hydrogen or expose the parts to free hydrogen during plating.
The coating's non-porous nature is also important to Mercury Marine. The amorphous, pore-free coating structure is one of several differences between the electrodeposited AlumiPlate layer and vapor-deposited aluminum. A 0.3-mil layer ensures a pore-free aluminum coating. This property allows Mercury to provide its customers outstanding corrosion resistance.
The process is able to plate parts with complex geometries. Also, some parts with inside diameters can be plated with an even layer of aluminum. The throwing power is similar to that of electrolytic cadmium. Using supplemental anode racking, Mercury can plate into some holes where the depth exceeds two diameters.
The process can uniformly plate parts 0.1 to 15 mils thick with aluminum, and it functionally conforms to the part's surface.
The prime benefit Mercury realizes from this aluminum plating system is corrosion resistance. Preventing rust helps engines last longer. Engine and engine parts that last longer do not need to be replaced as often, reducing environmental impact. Mercury also benefits because the coating process takes place in a fully closed system, keeping air emissions, chemical consumption and disposal costs low.