Understanding and Preventing Galvanic Corrosion

Galvanic corrosion, also known as bimetallic or dissimilar metal corrosion, is an electrochemical process that occurs when two different metals are in contact with each other in the presence of an electrolyte, such as seawater. In the context of boats, galvanic corrosion commonly occurs in underwater components and metal fittings due to the saltwater environment. Here’s how galvanic corrosion works and its implications for boats:

Electrochemical Process:

– Galvanic corrosion involves the transfer of electrons between two different metals, leading to the deterioration of one metal (anode) while the other metal (cathode) remains relatively unaffected.

– An electrochemical cell is formed when two dissimilar metals are in contact with each other and immersed in an electrolyte (such as seawater). The metal with a higher electrical potential (more reactive) becomes the anode and undergoes corrosion, while the metal with a lower electrical potential acts as the cathode and is protected from corrosion.

Factors Influencing Galvanic Corrosion:

– Electrode Potential: The relative electrical potential (electrode potential) of the metals involved determines which metal acts as the anode and cathode in the galvanic couple. Metals with higher electrode potentials (e.g., magnesium, zinc) are more likely to corrode, while metals with lower electrode potentials (e.g., stainless steel, bronze) are more corrosion-resistant.

– Electrolyte Conductivity: An electrolyte, such as seawater, accelerates galvanic corrosion by providing a medium for electron transfer between the metals. Seawater, with its high conductivity and salt content, is particularly corrosive and can exacerbate galvanic corrosion.

– Surface Area Ratio: The surface area ratio between the anode and cathode affects the corrosion rate. A larger surface area of the anode relative to the cathode results in accelerated anode corrosion.

– Distance Between Metals: The proximity of the dissimilar metals to each other affects the severity of galvanic corrosion. Closer proximity allows for more efficient electron transfer between the metals and accelerates corrosion.

Implications for Boats:

– In the marine environment, boats are particularly susceptible to galvanic corrosion due to various metals (e.g., aluminum, stainless steel, bronze) in contact with each other and with seawater.

Underwater components such as hulls, keels, propellers, shafts, and through-hull fittings are particularly vulnerable to galvanic corrosion. If not properly protected, these components can suffer corrosion damage, leading to reduced performance, structural integrity, and safety.

– Galvanic corrosion can also affect metal fixtures, fasteners, and electrical components on board, compromising their functionality and longevity.

– To mitigate galvanic corrosion, boats may employ sacrificial anodes (also known as zincs) made of highly reactive metals such as zinc, magnesium, or aluminum. Sacrificial anodes are strategically placed on the boat’s hull and underwater components to attract corrosion away from critical metal components. Regular inspection and replacement of sacrificial anodes are necessary to ensure effective corrosion protection.

– Additionally, galvanic isolation systems, bonding systems, and impressed current cathodic protection (ICCP) systems may be used to further protect against galvanic corrosion and maintain the integrity of underwater metals.

Conclusion

In summary, galvanic corrosion is a significant concern for boats operating in saltwater environments. Understanding the principles of galvanic corrosion and implementing appropriate corrosion protection measures are essential for preserving the structural integrity, performance, and safety of boats and their underwater components. Regular inspection, maintenance, and proactive corrosion management practices are essential to minimize the risk of galvanic corrosion and ensure the longevity of marine vessels.

For more information, contact Veteran Yacht Services or ABYC.