Brass is an alloy primarily composed of copper and zinc, while aluminum is a lightweight, silvery metal. When these two metals are physically connected in an environment with moisture, they are not a stable pairing. They react when coupled together through a destructive process called galvanic corrosion. This electrochemical reaction is possible because they occupy different positions on the galvanic series, which dictates their relative reactivity when placed in contact.
The Mechanism of Galvanic Corrosion
The reaction between aluminum and brass is driven by galvanic corrosion, which is essentially the creation of a spontaneous electrical cell. For this process to occur, three components must be present simultaneously: two dissimilar metals, direct electrical contact between them, and an electrolyte. The electrolyte is any conductive liquid, such as water, that allows ions to move between the metals, completing the circuit.
The relative activity of the two metals is determined by their position on the Galvanic Series. The metal higher on this series is considered more active or “less noble,” and it becomes the anode. The anode is the metal that sacrifices itself by dissolving, while the metal lower on the series, the cathode, is protected from corrosion. This causes a flow of electrons from the anodic metal to the cathodic metal through the physical connection.
Specific Interaction: Aluminum and Brass
When aluminum and brass are coupled, aluminum is more active than brass and acts as the anode, while brass functions as the cathode. The aluminum component will corrode preferentially and rapidly in the presence of an electrolyte. This corrosive attack manifests as pitting and material loss on the aluminum surface, which can lead to the structural failure of the component.
The corrosion is often visibly marked by a white, powdery buildup of aluminum oxide and hydroxide. This expanding corrosion product can seize threaded connections, making the joint impossible to separate without damage. A particularly damaging scenario occurs when a small aluminum part is connected to a much larger brass component. In this situation, the large surface area of the noble cathode (brass) accelerates the dissolution of the small anodic area (aluminum) drastically.
Environmental Factors Influencing Reaction Rate
The rate at which aluminum corrodes when coupled with brass is controlled by the electrolyte and its conductivity. Environments with high moisture, condensation, or humidity provide the necessary conductive medium to initiate and sustain the electrochemical reaction. The presence of dissolved salts, such as in seawater or de-icing spray, dramatically increases the electrolyte’s conductivity and accelerates the corrosion rate.
Acidic or alkaline environments also act as aggressive electrolytes, intensifying the reaction. Conversely, the galvanic reaction is effectively halted when the assembly is kept in a dry environment where no conductive liquid is present. Higher temperatures can also increase the rate of the chemical reaction once the electrolyte is established.
Methods for Preventing Corrosion
The primary strategy for using aluminum and brass components together is to break the electrical connection between the metals. This can be accomplished by physically isolating the two surfaces using non-conductive materials. Insulating barriers such as gaskets, non-metallic washers, or spacers made of plastic or rubber prevent electron flow and stop the galvanic process.
Applying surface treatments is another effective preventive measure. Painting or coating the junction with epoxy or a similar non-conductive barrier material shields the metals from the electrolyte. Furthermore, using a third, compatible metal as a transition fitting, like a stainless steel alloy, can bridge the gap between the two metals on the galvanic scale. For the aluminum component, anodizing can create a protective, non-conductive oxide layer that reduces its reactivity.