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Corrosion Fundamentals: Understanding the Science Behind the Damage

Kaoutar Zaidi1, Walid Daoudi2*, Selma Lamghafri3, Omar Dagdag4, Abdelmalik El Aatiaoui2 and Abdelouahad Aouinti1

1University Mohammed Premier, Faculty of Sciences, Laboratory: Applied Chemistry and Environment (LCAE), Department of Chemistry, Oujda, Morocco

2Laboratory of Molecular Chemistry, Materials and Environment (LCM2E), Department of chemistry, Multidisciplinary Faculty of Nador, University Mohamed I, Nador, Morocco

3Laboratory of Applied Sciences, National School of Applied Sciences Al-Hoceima, Abdelmalek Essaadi University, Tetouan, Morocco

4Department of Mechanical Engineering, Gachon University, Seongnam, Republic of Korea

Abstract

This chapter explores the basics of corrosion, an electrochemical process that degrades materials, especially metals, on contact with the environment. It explains that corrosion involves redox reactions and describes the different types of corrosion, such as uniform, galvanic, and pitting. Accelerating factors such as humidity, chloride ions, and acidity are also discussed etc. The concepts of anode, cathode, electrochemical potential, and corrosion cell are explained in detail, providing a better understanding of the mechanisms involved. The role of electrolytes in corrosion cell formation is highlighted.

Keywords: Corrosion, electrochemical process, redox reactions, corrosion types, environmental factors

1.1 Introduction

Metals and alloys are often used in service conditions that expose them to aggressive environments, making them vulnerable to corrosion in a variety of forms [1]. Knowledge on the fundamentals of corrosion is relevant to many industries from engineering to materials science, where it is well recognized that corrosion can have an unfavorable effect on structures and equipment. Corrosion can be defined broadly as the deterioration of materials caused by chemical and/or electrochemical reactions with the surrounding environment, which can generate safety and economic issues worldwide. There are many types of corrosive conditions that can develop based on the environmental conditions and the properties of the materials that are involved [2]. Corrosion can be in the form of uniform, galvanic, localized, and others, which can be affected by environmental conditions, materials, and surface morphology. The electrochemical reactions that are involved in the deterioration of metals underpin the understanding of corrosion, where metals degrade due to oxidation and reduction reactions taking place in the presence of moisture, oxygen, and other elements [3].

It is important to include the mechanical, physical, and chemical properties of metals. Mechanical and physical properties can be defined through constants; however, the chemical properties of a particular or specific metal are entirely dependent on the exact environmental conditions that the metal encounters while in use. The importance of mechanical, physical, and chemical properties is determined by the use of the metal. For example, railway tracks place importance on mechanical properties such as elasticity, tensile strength, hardness, and wear resistance, while electrical transmission relies primarily on electrical conductivity. Although a metal or alloy might often be chosen based on its mechanical or physical characteristics, in most applications, the interaction between the metal and its environment cannot be entirely disregarded [4]. However, the impact of this interaction differs in importance depending on the specific conditions. The impact of the interaction between a metal and its environment on the environment itself can be more critical than the metal's own degradation. For example, lead pipes are unsuitable for transporting plumbo-solvent waters, as lead concentrations above 0.1 ppm are toxic. Similarly, galvanized steel is not appropriate for certain food products due to the toxicity of zinc salts. In numerous chemical processes, the choice of metal is often driven by the need to prevent environmental contamination from metallic impurities that could alter the color or taste of products or trigger unwanted reactions. For instance, copper and its alloys are unsuitable for soap production because trace amounts of copper ions can cause discoloration and rancidity in the soap. In such cases, it becomes necessary to use unreactive and relatively costly metals, even if the environment would not cause significant deterioration of less expensive options like mild steel [5].

1.2 Basic Chemistry of Corrosion

1.2.1 Electrochemical Nature of Corrosion

Aqueous corrosion is fundamentally electrochemical, rooted in Michael Faraday's nineteenth-century principles of electrochemistry. Understanding corrosion and its prevention hinges on these principles. Each electrochemical corrosion cell necessitates four components: an anode (the corroding metal), a cathode (another conductor where environmental reactions occur), an electrolyte (aqueous environment facilitating ionic conduction between the anode and cathode), and an electrical connection (enabling electron flow between the anode and cathode). Typically, anodes and cathodes are close together, often on the same metal piece. Without any of these components, electrochemical corrosion cannot occur, so analyzing the corrosion cell is crucial for mitigation. Corrosion reactions can be divided into anode and cathode half-cell reactions for clearer understanding; the anode reaction involves the metal corroding and releasing ions into the electrolyte [6].

1.2.2 Chemical Reaction of Corrosion

The principles of corrosion as a heterogeneous chemical reaction refer to understanding how metals react with nonmetal environments (the reactants) at their interface, leading to the degradation of the metal. This approach simplifies the complex details of metal structures by focusing on the overall chemical reaction where the metal (A) and the nonmetal (B) reactants produce new compounds (C and D). The reaction can be represented by a simple chemical equation, highlighting that while many environmental factors may be present, only a few significantly influence the corrosion process. The nonmetallic reactants are often called the environment, but it is important to note that in a complex environment, the primary constituents might play a minor role in the reaction. For instance, in the atmospheric corrosion of steel, nitrogen makes up 75% of the atmosphere, yet its impact is negligible compared to that of moisture, oxygen, sulfur dioxide, solid particles, etc. However, in the high-temperature reaction of titanium with the atmosphere, nitrogen becomes a significant factor [7].

Electrochemical corrosion reaction equations contain symbols for electrons, reacting elements (ions and/or molecules), and ions or molecules produced by the corrosion reaction [8]. For example, the corrosion of iron is represented by the anodic electrochemical equation

where Fe° represents iron atoms at the metal surface, Fe+2 represents iron ions, and 2e-represents the two electrons produced by the anodic reaction. Equation 1.1 is referred to as an anodic half reaction because free electrons are produced.

The electrode at which chemical reduction occurs (or + current enters the electrode from the electrolyte) is called the cathode. Examples of cathodic reactions are:

All of them represent reduction in the chemical sense.

The electrode at which chemical oxidation occurs (or + electricity leaves the electrode and enters the electrolyte) is called the anode. Examples of anodic reactions are:

These equations represent oxidation in the chemical sense. Corrosion of metals usually occurs at the anode. Nevertheless, alkaline reaction products forming at the cathode can sometimes cause secondary corrosion of amphoteric metals, such as Al, Zn, Pb, and Sn, which corrode rapidly on exposure to either acids or alkalines [9].

1.3 Types of Corrosion

Corrosion can affect the metal in a variety of ways, which depend on its nature and the precise environmental conditions prevailing, and a broad classification of the various forms of corrosion. However, corrosion damage occurs in other ways as well, resulting, for example, in failure by cracking or in loss of strength or ductility. The five primary categories of corrosion are categorized based on their visual characteristics or changes in physical properties.

1.3.1 Uniform Corrosion

This is the most prevalent form of corrosion, characterized by uniform deterioration across the metal surface. It generally leads to a consistent reduction in material thickness [10]. When iron corrodes in acidic environments or aluminum in alkaline conditions, the entire surface can be compromised. The formation of protective oxide films on specific areas is not feasible, as these films cannot maintain stability in such liquids. Examples include oxidation and tarnishing, active dissolution in acids, anodic oxidation and passivation, as well as chemical and electrochemical polishing. Additionally, corrosion can occur both in atmospheric conditions and when immersed in certain environments. One approach to prevent this type of corrosion is to choose a material that forms a passive layer in the specific environment. Alternatively, conducting...