When different metals are placed in contact with moisture, galvanic corrosion can occur. This process impacts system longevity and functionality. Understanding this dictates which metals can be safely combined without risking premature degradation. Metal compatibility involves complex electrochemical interactions, leading to accelerated corrosion. Avoiding these reactions ensures durability and reliability in structures, plumbing, and electrical connections.
The Science Behind Metal Compatibility
Metal compatibility is governed by galvanic corrosion, also known as bimetallic or dissimilar metal corrosion. This electrochemical reaction takes place when two different metals are in electrical contact and exposed to an electrolyte, such as water or moisture. In this setup, one metal acts as an anode and the other as a cathode, similar to a simple battery. The difference in electrochemical potential between the two metals drives electrons from the more reactive metal (the anode) to the less reactive metal (the cathode), causing the anode to corrode at an accelerated rate.
The galvanic series, or electrochemical series, ranks metals based on their tendency to corrode in a given electrolyte. Metals listed higher in the series are more “active” or anodic and are more likely to corrode, while those lower down are more “noble” or cathodic and tend to be protected. The greater the separation between two metals on this series, the higher the risk and rate of galvanic corrosion when they are in contact. The presence of an electrolyte is essential, providing a medium for ion migration and completing the circuit for corrosion.
Metals That Pair Well With Copper
Certain metals exhibit good compatibility with copper due to their similar electrochemical potentials, which minimizes the driving force for galvanic corrosion. Brass, an alloy of copper and zinc, and bronze, an alloy of copper and tin, are generally compatible with copper. Their close positions in the galvanic series mean that when in contact with copper, they experience minimal accelerated corrosion, making them suitable for plumbing fittings and decorative applications.
Tin is another metal that pairs well with copper, often used as a coating on copper wires or components to improve solderability and provide some corrosion resistance. Stainless steel, particularly certain grades, can also be used in contact with copper, though moisture can influence the interaction. In many plumbing systems, stainless steel and copper can be connected with minimal issues if proper installation techniques are followed.
Metals That Clash With Copper
Conversely, some metals are highly incompatible with copper and should be avoided in direct contact, especially with an electrolyte. Aluminum, for instance, is significantly more anodic than copper. When aluminum and copper are in electrical contact and exposed to moisture, the aluminum will corrode rapidly to protect the copper, potentially leading to structural failure. This accelerated corrosion can be particularly aggressive if copper ions leach onto its surface.
Zinc, commonly found as a coating on galvanized steel, is another metal that reacts unfavorably with copper. Galvanized steel in contact with copper, especially in humid environments, can lead to rapid corrosion of the zinc coating. The zinc sacrificially corrodes to protect the copper, quickly compromising the integrity of the galvanized steel component. Similarly, iron and mild steel are less noble than copper and will corrode significantly faster when directly connected and exposed to an electrolyte. This can result in severe degradation of iron or steel components, such as pipes or fasteners, leading to leaks or structural weakening.
Avoiding Compatibility Issues
Preventing galvanic corrosion when dissimilar metals must be used together involves several strategies to interrupt the electrochemical reaction. One common method is using dielectric unions or insulators, specialized fittings that physically separate the two metals with a non-conductive material like plastic or rubber. These unions prevent direct electrical contact, effectively breaking the circuit necessary for electron flow and stopping the corrosion process. Dielectric unions are frequently installed in plumbing systems where copper pipes connect to other metal components like galvanized steel.
Applying protective coatings or paints to one or both metal surfaces can also create a barrier against the electrolyte, preventing the corrosion reaction. It is often recommended to coat the more noble metal (the cathode) to avoid concentrating corrosion if the coating is damaged. In certain situations, sacrificial anodes, typically made of a more active metal like zinc or magnesium, can be intentionally introduced into the system. These anodes corrode preferentially, protecting the copper and other less active metals by acting as the primary site for the electrochemical reaction.