Galvanic Corrosion: Mechanism, Examples, and Protection Methods

Galvanic corrosion is one of the most significant forms of electrochemical corrosion that affects metals in electrical and industrial applications. Understanding its mechanism, real-world examples, and effective protection methods is crucial for engineers and professionals working with metal structures and electrical systems.

What is Galvanic Corrosion?

Galvanic corrosion, also known as bimetallic corrosion or dissimilar metal corrosion, occurs when two different metals are electrically connected in the presence of an electrolyte. This electrochemical process results in accelerated corrosion of the more active (anodic) metal while the less active (cathodic) metal remains protected.

Mechanism of Galvanic Corrosion

Electrochemical Principles

The galvanic corrosion mechanism is based on fundamental electrochemical principles:

Formation of Galvanic Cell

When two dissimilar metals are connected in an electrolyte solution, they form a galvanic cell. The metal with higher electrode potential becomes the cathode, while the metal with lower electrode potential becomes the anode.

Electron Transfer Process

• Anodic reaction: Metal oxidation occurs at the anode (M → M²⁺ + 2e⁻)
• Cathodic reaction: Reduction reactions occur at the cathode (O₂ + 4H⁺ + 4e⁻ → 2H₂O)
• Current flow: Electrons flow from anode to cathode through the metallic connection

Factors Affecting Galvanic Corrosion Rate

Electrode Potential Difference

The greater the difference in electrode potentials between the two metals, the higher the galvanic corrosion rate. The galvanic series helps predict which metal will act as anode or cathode.

Area Ratio Effect

The ratio of cathode to anode surface areas significantly impacts corrosion rate:

• Large cathode/small anode: Accelerated corrosion
• Small cathode/large anode: Reduced corrosion rate

Environmental Conditions

• Electrolyte conductivity: Higher conductivity increases corrosion rate
• Temperature: Elevated temperatures accelerate electrochemical reactions
• pH levels: Acidic conditions typically increase corrosion rates
• Oxygen availability: Affects cathodic reactions

Examples of Galvanic Corrosion

Marine Applications

Ship Hull and Propeller Systems

Steel hulls connected to bronze propellers experience significant galvanic corrosion in seawater. The steel acts as anode and corrodes rapidly while the bronze propeller remains protected.

Offshore Structures

Steel platforms with aluminum components in saltwater environments demonstrate classic galvanic corrosion, where aluminum serves as sacrificial anode.

Construction Industry

Steel and Aluminum Connections

Direct contact between steel bolts and aluminum structural members in the presence of moisture leads to aluminum corrosion.

Copper-Steel Systems

Copper pipes connected to steel fittings in plumbing systems can cause steel corrosion due to copper’s higher electrode potential.

Automotive Applications

Battery Terminal Corrosion

Lead-acid batteries with copper terminals demonstrate galvanic corrosion where different metal components interact with electrolyte.

Body Panel Connections

Steel body panels connected with aluminum fasteners can experience galvanic corrosion in humid conditions.

Electronic and Electrical Systems

Circuit Board Connections

Dissimilar metal connections on PCBs in humid environments can suffer from galvanic corrosion affecting electrical conductivity.

Power Transmission Lines

Aluminum conductors connected to steel towers require special consideration for galvanic compatibility.

Protection Methods Against Galvanic Corrosion

Design-Based Prevention

Material Selection

• Similar metals: Use metals close in the galvanic series
• Galvanic compatibility: Ensure electrode potential difference <0.15V
• Noble metal avoidance: Avoid large cathode/small anode configurations

Geometric Design Considerations

• Surface area ratios: Design for large anode/small cathode ratios
• Joint design: Minimize crevices and water retention areas
• Drainage: Ensure proper water drainage from joint areas

Barrier Protection Methods

Insulation Techniques

• Electrical isolation: Use non-conductive gaskets and washers
• Dielectric unions: Install insulating joints in piping systems
• Plastic bushings: Separate dissimilar metals with insulating materials

Coating Systems

• Protective coatings: Apply barriers like paint, powder coating, or anodizing
• Galvanizing: Zinc coating provides sacrificial protection
• Organic coatings: Epoxy, polyurethane, and other polymer coatings

Electrochemical Protection

Cathodic Protection

• Sacrificial anodes: Install more active metals (zinc, magnesium) as sacrificial anodes
• Impressed current: Apply external DC current to make structure cathodic
• Reference electrodes: Monitor protection levels using reference electrodes

Anodic Protection

• Passive film formation: Maintain protective oxide films on metal surfaces
• Controlled potential: Apply anodic current to maintain passive state

Environmental Control

Electrolyte Management

• Corrosion inhibitors: Add chemicals to reduce electrolyte conductivity
• pH control: Maintain optimal pH levels to minimize corrosion
• Water treatment: Remove aggressive ions from electrolyte solutions

Atmospheric Control

• Humidity control: Maintain low humidity in enclosed spaces
• Ventilation: Ensure adequate air circulation to prevent condensation
• Dehumidification: Use desiccants in sensitive environments

Monitoring and Maintenance

Inspection Techniques

Visual Inspection

Regular visual examination for signs of corrosion, discoloration, or material degradation.

Electrochemical Testing

• Potential measurements: Monitor electrode potentials
• Corrosion rate monitoring: Use linear polarization resistance techniques
• Impedance spectroscopy: Analyze coating integrity and corrosion behavior

Preventive Maintenance

Coating Maintenance

• Regular inspection: Check for coating damage or degradation
• Touch-up repairs: Promptly repair damaged coating areas
• Recoating schedules: Establish systematic recoating programs

Sacrificial Anode Replacement

• Consumption monitoring: Track anode consumption rates
• Replacement scheduling: Replace anodes before complete consumption
• Performance verification: Verify protection levels after anode replacement

Conclusion

Galvanic corrosion represents a significant challenge in engineering applications where dissimilar metals must coexist. Understanding the electrochemical mechanisms, recognizing vulnerable applications, and implementing appropriate protection strategies are essential for preventing costly material failures.

Effective galvanic corrosion prevention requires a comprehensive approach combining proper material selection, protective barriers, electrochemical protection, and environmental control. Regular monitoring and maintenance ensure long-term protection effectiveness.

Engineers and designers must consider galvanic compatibility during the design phase, as prevention is always more cost-effective than remediation. With proper understanding and application of protection methods, galvanic corrosion can be effectively controlled in most industrial applications.

References

1. NACE International – Corrosion Basics: An Introduction
2. ASM International – Corrosion: Understanding the Basics
3. ASTM International – Standard Guide for Determining the Effects of Environmental Conditions on Protective Coating Systems
4. Corrosion Doctors – Galvanic Corrosion
5. National Physical Laboratory (NPL) – A Review of Galvanic Corrosion
6. Materials Performance Magazine – Galvanic Corrosion Prevention in Practice
7. Electrochemical Society – Fundamentals of Electrochemical Corrosion
8. Corrosion Engineering Science and Technology – Recent Advances in Galvanic Corrosion Research