Understanding and preventing galvanic corrosion

Introduction

Corrosion is a natural process that degrades materials, especially metals, due to chemical reactions with their environment. This phenomenon compromises the structural and functional integrity of materials and causes significant economic losses in various industries. Among the different types of corrosion, galvanic corrosion occurs when two dissimilar metals come into electrical contact in the presence of an electrolyte. This specific type of corrosion accelerates the degradation process, often causing unexpected failures in critical applications.

Understanding the mechanisms and principles of this type of corrosion is essential for selecting the most appropriate control methods, thus ensuring the service life of industrial structures in sectors such as aerospace, marine, construction and energy. Proper application of this knowledge significantly improves the durability and reliability of projects.

What is galvanic corrosion?

It is an electrochemical process that occurs when two dissimilar metals come into electrical contact in the presence of an electrolyte, such as water, generating a galvanic cell, where the more active metal acts as the anode and the more noble one as the cathode. The anode oxidizes and corrodes, while the cathode remains protected.

This type of corrosion consists of an electrochemical process that occurs when two dissimilar metals come into electrical contact in the presence of an electrolyte, forming a galvanic cell, where the more active metal acts as the anode and the nobler one as the cathode. The anode oxidizes and corrodes, while the cathode remains protected.

The electrochemical process involved is driven by the difference in electrode potentials between the two metals, where the metal of higher potential represents the cathode, and the metal of lower potential is the anode. Electrons flow from the anode to the cathode through the electrical connection, while ions move through the electrolyte, closing the electrical circuit.

It generally occurs in marine environments where dissimilar metal components are exposed to seawater, plumbing systems with copper and steel piping, and architectural structures that combine aluminum and steel. For example, joining a steel bolt to an aluminum panel in a wet environment can lead to accelerated corrosion of the steel due to galvanic action.

For more information on galvanic corrosion the following video is presented, courtesy of: Samarbeid forSikkerhet.

Fundamentals of galvanic corrosion 

Galvanic series or electromotive force series

The galvanic series is a fundamental concept in understanding galvanic corrosion. It is a list of metals and alloys organized according to their electrode potentials in a specific environment, typically seawater. Metals at the top of the series, such as magnesium and zinc, are more anodic and corrosion prone, while those at the bottom, such as gold and platinum, are more cathodic and corrosion resistant. The position of metals in the galvanic series helps predict the behavior when two dissimilar metals are paired.

Factors affecting galvanic corrosion severity

The most influential factors in severity are:

The voltage difference between the two metals in the galvanic series: This is the main factor of major incidence in the acceleration of the corrosion rateb; a greater difference generally leads to the celerity of corrosion.

The nature and aggressiveness of the medium: The environment also plays a significant role; the presence of the electrolyte is to facilitate the flow of ions between the anode and cathode, completing the electrochemical circuit. The conductivity of the electrolyte, influenced by its composition and temperature, affects the corrosion rate, for example, the presence of chloride ions in seawater can accelerate galvanic corrosion.

Anode/Cathode area ratio: An important consideration is the area ratio of the contact zone between the materials, which is another critical factor; a small anode area in relation to the cathode can lead to severe corrosion. The higher the ratio of anode to cathode, the faster the corrosion process occurs.

Dissimilar metal corrosion

According to the above, this type of corrosion occurs prominently in dissimilar materials when they are electrically connected in the presence of an electrolyte. This interaction causes one metal (the anode) to corrode faster than it would in isolation, while the other metal (the cathode) experiences reduced corrosion. For example, connecting two copper and iron pipes as shown in the figure below, it can be observed that the copper is protected while the iron is oxidized because copper is a more noble metal with respect to iron. There is a large potential difference between the two metals according to the series of standard reduction potentials.

The phenomenon is driven by the electrochemical potential difference between the metals. In a galvanic couple, the metal with a lower potential becomes the anode and is sacrificed, protecting the nobler metal, which becomes the cathode. Understanding these interactions is critical for engineers to prevent premature failure of materials in various applications.

How to identify galvanic corrosion?

Physically it can be detected by exhibiting forms of rust coloration and material degradation around the contact points of dissimilar metals. For example, corroded areas on anodized aluminum structures joined with stainless steel fasteners are common indicators. Chemical and microstructural characterization techniques such as Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX) are also used.

Early detection of galvanic corrosion is significant in preventing severe structural damage. Regular inspections, especially in marine and industrial environments where this corrosion is prevalent, help to identify early signs. Engineers should also use potential measurements to monitor the electrochemical behavior of metals in service. Implementing preventive measures based on early detection can save significant repair and replacement costs.

Prevention of galvanic corrosion on dissimilar materials

Strategic planning during the design and maintenance phases is important. Key strategies include:

1. Material selection: Choosing metals close to each other in the galvanic series reduces potential differences. For example, using similar stainless steel alloys for fasteners and structural components can minimize galvanic effects.
2. Electrically insulating materials: Using dielectric bonds, insulating gaskets or non-conductive coatings can physically separate dissimilar metals, preventing electrical contact. For example, applying a rubber or plastic gasket between a steel pipe and a brass valve can effectively prevent electrical contact.
3. Protective coatings: Applying coatings to the nobler metal can prevent it from acting as a cathode. Techniques such as painting, anodizing or sacrificial coatings (e.g. zinc plating) can significantly reduce galvanic interactions.
4. Cathodic protection: Sacrificial anodes made of zinc, magnesium or aluminum can be used to protect more valuable metals. These anodes preferentially corrode, preserving the structural integrity of the protected metal.
5. Environmental control: Reduce the presence of electrolytes, such as controlling humidity or using dehumidifiers, minimizing the conditions that produce them. In marine environments, periodic washing of structures to remove salt deposits can also help.
6. Design considerations: Designing for easy maintenance and inspection, such as incorporating accessible inspection points and using modular components, can facilitate early detection and control.

Best practices for material selection and design

When selecting materials and designing systems to prevent galvanic corrosion, consider the following best practices:

1. Material compatibility: Ensure that the materials selected have similar electrochemical potentials to minimize risk. Refer to the galvanic series to make informed decisions.
2. Environmental suitability: Choose materials that can withstand specific environmental conditions, such as humidity, salinity and temperature variations. For example, in marine environments, opt for marine-grade stainless steel or coated metals.
3. Proper design techniques: Design joints and connections to minimize direct contact between dissimilar metals. Use insulating materials and ensure that the design allows for easy application of protective coatings.
4. Maintenance Friendly Design: Incorporate features that allow for regular inspections and maintenance. Design components so that sacrificial Anodes can be easily replaced and coatings can be reapplied without disassembling large structures.
5. Documentation and Standards: Comply with industry standards and guidelines for material selection and corrosion prevention. Document all design decisions and maintenance plans to ensure consistent application and future reference.

Implementing these best practices helps to create durable and reliable systems that can effectively resist galvanic corrosion.

Case studies and real-world applications

Case 1. Statue of Liberty: The Statue of Liberty experienced significant galvanic corrosion due to contact between its copper surface and its iron structure. Restoration efforts included the installation of insulating materials and the application of protective coatings to prevent further corrosion.

Case 2. Marine infrastructure: The use of sacrificial anodes and the selection of appropriate materials have proven to be effective in extending the life of these structures. For example, zinc anodes are commonly used to protect steel components in seawater.

Case 3. HVAC systems: HVAC systems often involve the use of copper and aluminum components. Engineers have successfully reduced galvanic corrosion through the use of dielectric bonds and coatings, ensuring long-term reliability and efficiency of the systems.

Future trends in the prevention of galvanic corrosion

An important factor of consideration is the area ratio of the contact zone between the materials:

1. Advanced materials: New alloys and composite materials with superior corrosion resistance will be developed, reducing the need for dissimilar metals and minimizing galvanic risks.
2. Self-repairing coatings: Intelligent coatings with self-repairing capabilities will emerge that will automatically repair micro-scratches that can initiate corrosion.
3. Microstructure engineering: Tailoring the microstructure of metals to improve their inherent corrosion resistance will become a powerful tool.
4. AI-driven predictive maintenance: Machine learning algorithms will analyze sensor data to predict corrosion risks before they become critical, enabling preventive measures.

5. Bio-inspired techniques: Mimicking nature’s corrosion resistance mechanisms, such as those found in seashells, could lead to the development of more sustainable and effective corrosion inhibitors.

These developments mark a remarkable progress in the management and prevention of galvanic corrosion, thus ensuring the durability and reliability of critical infrastructure in various industries.

Conclusions

Galvanic corrosion is a critical challenge to maintaining the integrity and longevity of metallic structures, hence the importance of understanding the electrochemical principles and factors that influence this type of corrosion is critical for engineers and practitioners. By implementing strategic prevention methods, such as careful selection of materials, protective coatings and cathodic protection, the adverse effects of this type of corrosion can be significantly controlled.

The future of corrosion prevention looks promising with advances in materials science, monitoring technologies and predictive analytics. These innovations will enable more effective and efficient management of galvanic corrosion, ensuring the durability and safety of infrastructures in various industries.

References

1. Corrosion Engineering: Principles and Practice” by Pierre R. Roberge. This book provides a comprehensive overview of the principles of corrosion and engineering practices for its prevention.
2. “Uhlig’s Corrosion Handbook” edited by R. Winston Revie. An extensive and up-to-date resource on various aspects of corrosion.
3. NACE International: Publishes standards and technical guides on corrosion prevention, including galvanic corrosion.
4. ASTM International: Offers standards such as ASTM G71-81(2014) “Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes”.