Galvanic reaction: Fundamental principles and their impact on the corrosion of metals

Introduction

Galvanic reaction is an electrochemical process that occurs when two metals with different electrochemical potentials come into contact within a conductive medium, generating a flow of electrons that favors the corrosion of the less noble metal. This process is significant in materials engineering and corrosion protection because it considerably affects the lifetime and safety of metallic structures exposed to aggressive environments.

In industrial environments, the presence of uncontrolled galvanic cells represents a problem, especially in piping systems, marine structures, petrochemical facilities, among others, where premature degradation of metallic components can lead to structural failures with considerable economic and operational consequences. The formation of galvanic couples increases the corrosion rate of the anodic metal, and can induce localized corrosion, such as pitting and crevices, increasing the susceptibility to structural damage.

Throughout this article, the electrochemical processes involved in the galvanic reaction, the factors that cause this type of corrosion, and the prevention methods used in the industry, such as the correct selection of materials, the use of protective coatings. The implementation of cathodic protection by sacrificial anodes and the proper structural design will be analyzed. Effective mitigation of galvanic corrosion is key to ensuring the reliability and sustainability of metallic systems in various industrial applications.

Galvanic reaction, how it occurs and consequences in metallic materials

What is a galvanic reaction?

The galvanic reaction between two metals is a redox reaction, representing the principle on which galvanic corrosion of metallic materials is based.

Galvanic reaction in redox processes

Redox reactions are chemical reactions in which an electronic transfer occurs between the atoms or molecules involved. This exchange is reflected in the change of oxidation state of the reactants, the reactant that gives up electrons undergoes oxidation and the reactant that receives them undergoes reduction¹.

Oxidation-reduction (redox) reactions are the basis of the electrochemical processes responsible for the degradation of metals in aggressive environments. In particular, the galvanic reaction is an electrochemical phenomenon that occurs when two metals of different reduction potential come into electrical contact within an electrolyte, generating a transfer of electrons that accelerates the corrosion of the less noble metal (anode).

Electrochemistry of the galvanic reaction²

The electrochemistry of this type of reaction is based on oxidation-reduction processes. Every redox reaction involves an oxidation and a reduction reaction, i.e., it is composed of two half-reactions:

• Oxidation reaction: where the loss of electrons occurs, which is observed by an increase in the oxidation number of the element: M → M⁺ⁿ + ne^–
• Reduction reaction: Where there is a gain of electrons, indicated by a decrease in the element’s oxidation number: R⁺ⁿ + ne^– → R

Oxidation and reduction occur simultaneously. They cannot occur one without the other; every electron gained by one species is given up by the other.

An example of a spontaneous redox reaction is the one that takes place when a piece of Zn metal is introduced into a solution of copper sulfate. As the reaction proceeds, the blue color of the Cu²⁺ ion solution fades, due to the Cu²⁺ ions being deposited as Cu on the Zn film. At the same time, the Zn dissolves and is transformed into Zn²⁺. The half-reactions that take place are:

1. Oxidation reaction: Zn(s) → Zn⁺⁺(ac) + 2e^–
2. Reduction reaction: Cu⁺⁺(ac) + 2e^– → Cu(s)
3. The overall reaction is: Zn(s) + Cu⁺⁺(ac) → Zn⁺⁺(ac) + Cu(s)

Each of the half-reactions occurs in different compartments called half-cells. One consists of a Cu film immersed in a CuSO₄ solution and the other of a Zn film immersed in a ZnSO₄ solution. The Zn and Cu sheets are called electrodes and are connected by an electrical conductor, forming a galvanic couple, as shown in figure 1.

The half-reactions as presented in the picture above (figure 1) occur spontaneously. On the contrary, if metallic Cu is added to a zinc sulfate solution, no reaction occurs because the metallic Cu is not oxidized by the Zn⁺⁺. The ease of oxidation of metals depends on their standard potential, according to the galvanic series of standard reduction or oxidation potentials.

Implications of the galvanic reaction on metallic materials

This interaction has negative implications, such as accelerated corrosion known as galvanic corrosion, and positive implications when applied to the protection of metallic structures. In this context, its study is fundamental in materials engineering and corrosion protection in industrial applications.

1. Negative implications: Galvanic corrosion

Galvanic reaction can have a significant impact on material and structural integrity. The occurrence of this reaction between dissimilar metallic material systems in an electrolyte medium accelerates the corrosion process of one of the metals, known as galvanic corrosion, due to the electronic transfer (electric current) between the two metals; leading to structural failure and degradation of materials, functional damage to metallic components, weakening materials, affecting the integrity of structures and equipment; it can also compromise the safety and durability of materials in construction, industry, and transport. Preventing this phenomenon is key to preserving the efficiency and lifetime of metallic materials.

2. Positive implications: Cathodic protection by sacrificial anodes

Although it seems contradictory, the galvanic reaction can contribute to the preservation of materials. This electrochemical mechanism is used in cathodic protection by sacrificial anodes, where highly active metals (with a greater tendency to corrode), such as zinc, aluminum or magnesium, are coupled to steel structures or other less reactive (more noble) metals. In this process, in this case the more active material acts as an anode and corrodes instead of the metal to be protected, mitigating corrosion and extending the life of the structure. This system is applied on ships, offshore platforms, storage tanks and buried pipelines, providing an effective defense against metal degradation.

Fundamentals of galvanic corrosion

What is galvanic corrosion?

Galvanic corrosion, also called bimetallic corrosion, is an electrochemical phenomenon that occurs when two metals of different electrochemical potential are in direct contact within a conductive medium or electrolyte. The potential difference between the two metals generates a transfer of electrons, thus establishing a galvanic cell in which the less noble metal acts as the anode and undergoes an accelerated dissolution process. The more noble metal becomes the cathode, favoring reduction reactions that limit its degradation. In the following image (Figure 2), a graphic representation of the electrochemical process of this type of corrosion is shown.

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.

Galvanic series or electromotive force series

The galvanic series is a classification of metals and alloys according to their electrochemical potential in a specific medium (Table 1), and is a fundamental concept for understanding galvanic corrosion. Metals at the bottom of the series, such as magnesium and zinc, are more anodic and prone to corrosion, while those at the top, such as gold and platinum, are more cathodic and corrosion resistant.

How is the galvanic series related to the corrosion of metals?

The galvanic series helps to predict the behavior of metals at different galvanic potentials according to the position they occupy in the galvanic table or electromotive force series, when they are in contact in the presence of an electrolyte. The potential difference between the metals in the galvanic series determines the speed and severity of the corrosive attack, the greater the separation in the series, the more intense the corrosion of the anode.

Figure 3, shows a representative image of the galvanic corrosion process between steel and copper³. In this interaction, the less noble metal (anode) corrodes faster, while the more noble metal (cathode) is protected.

This principle is significant in the design of metallic structures, selection of compatible materials and application of protection methods, such as cathodic protection with sacrificial anodes. Understanding this electrochemical mechanism in corrosion engineering is important to avoid this type of corrosion by means of the most appropriate control methods.

Factors affecting the severity of galvanic corrosion

• The voltage difference between the two metals in the galvanic series: This is the main factor with the greatest impact on the acceleration of the corrosion rate; a greater difference generally leads to faster 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.

How can we avoid galvanic corrosion?

Galvanic corrosion is a phenomenon that can be avoided or mitigated by taking appropriate measures to prevent interaction between dissimilar metals and exposure to an electrolyte.

1. Selection of compatible materials: Choose metals that are close in the galvanic series or that have a low electrochemical potential.
2. Protective coatings: Applying protective coatings, such as paints, varnishes or zinc coatings, to metals can provide a barrier between the metals and the electrolyte, thus reducing the possibility of galvanic corrosion.
3. Electrical insulation: If possible, non-conductive materials, such as plastics or rubbers, can be used to electrically insulate metals that could come into contact. This prevents electron transfer and reduces corrosion.
4. Corrosion inhibitors: Chemical corrosion inhibitors can be used to reduce the rate of galvanic corrosion. These chemicals are applied to the surface of metals and form a protective layer.
5. Use sacrificial metals: In some cases, sacrificial metals that are more active than the parent metal can be used. These metals will preferentially corrode instead of the parent metal. For example, in marine applications, zinc or aluminum anodes are used to protect steel structures.
6. Physical separation: Physically separating metals with insulators or non-conductive gaskets can prevent direct contact between them and thus reduce galvanic corrosion. Washers of other material, in joints of various metals, is a clear example of physical separation to prevent galvanic corrosion.
7. Engineering design: Designing components and structures so that water and moisture do not accumulate or become trapped between metals in contact can help reduce galvanic corrosion.
8. Monitoring and maintenance: Performing regular monitoring and preventive maintenance can help detect and address corrosion problems. It is important to keep in mind that prevention of galvanic corrosion is critical in applications where corrosion can have serious consequences, such as in the marine industry or in metallic structures exposed to the environment. The appropriate choice depends on the specific application and the nature of the materials involved.

Conclusions

Understanding the fundamental principles of the galvanic reaction is essential for the proper selection of materials and the design of corrosion mitigation systems. Factors such as the potential difference between metals, the electrolyte composition, the ratio of anodic and cathodic areas, and environmental conditions determine the severity of the corrosive process; thus the absence of preventive measures, the combination of dissimilar materials in a conductive environment can accelerate the degradation of infrastructures.

Galvanic corrosion represents a significant problem in materials engineering and structural integrity of metallic components. Its proper analysis and mitigation are critical to prolong the life of structures and avoid premature failure. Detailed knowledge of the galvanic series, the factors that influence the process and the protection methods allows an efficient control of this type of corrosion in industrial applications.

References

1. https://en.wikipedia.org/wiki/Redox
2. Bockris, J. O’M., & Reddy, A. K. N. (1970). Modern Electrochemistry. Springer.
3. Introduction to corrosion and corrosion inhibitors”; Materials Science, Engineering, 2015.