Industrial Corrosion and Protection--04 Electrochemical Corrosion

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Electrochemical corrosion of metals, in a broad sense, refers to the damage caused to metals and conductive media as a result of electrochemical reactions. It can also be considered a corrosion process that relies on the anodic and cathodic reactions of a corrosion cell. In corrosion electrochemistry, it is an electrochemical discipline that typically takes corroding metal electrodes as its subject of study. For example, in the absence of an external current, two or more electrode reactions occur simultaneously on the surface of the corroding metal electrode, and the corrosion potential is the non-equilibrium potential resulting from the coupling of these two or more electrode reactions. The kinetic curves measured from the corroding metal electrode represent the combined kinetic behavior of those two or more electrode reactions, namely the polarization curves. Factors such as the formation and distribution of multi-electrode corrosion systems and their influence on the corrosion rate, as well as the role of the formation and breakdown of passive films on metal surfaces in corrosion electrochemical behavior, are all governed by the mechanisms of electrochemical corrosion. Therefore, electrochemical corrosion is the result of the electrode reactions in a corrosion cell. Examples of electrochemical corrosion of metal materials can be found everywhere: the corrosion of metal equipment left outdoors in a humid atmosphere; the corrosion of underground pipes in soil; the corrosion of boiler tubes and heat exchange equipment caused by boiler water, acids, and water-cooling media; and the corrosion of metals in molten salts or under high-temperature gases when a molten salt coating is formed. These corrosion phenomena occur as a result of the metal coming into contact with a certain electrolyte or electrolyte solution. However, any corrosion reaction that occurs according to electrochemical mechanisms involves at least one anodic reaction and one cathodic reaction, and is connected by the flow of electrons within the metal and the flow of ions in the medium. Among them, the anodic reaction is the process in which metal ions transfer from the metal surface to the interstitials while electrons are released, that is, the anodization process. The corresponding cathodic reaction is the process in which the oxidizing component in the meson absorbs electrons from the anode, that is, the cathodic reduction process. For example, when carbon steel corrodes in acid, the anodic reaction is the oxidation of iron to Fe2+, and the electrons released flow from the anode (Fe) to the cathode (Fe3C) within the steel, where they are absorbed by H in the acid and reduced to hydrogen gas. Furthermore, if the acid contains dissolved oxygen or is under a gas-filled condition, the cathodic reaction may also involve the reduction process of O2 + 4H+ + 4e- → 2H2O simultaneously. Generally speaking, the former is known as the hydrogen evolution reaction or hydrogen release reaction, while the latter is known as the oxygen reduction reaction or oxygen absorption reaction. When the overall reaction rate increases, the dissolution rate of carbon steel also increases, as this is determined by the conservation principle that during electrochemical corrosion of a metal, the overall oxidation rate must equal the overall reduction rate; reducing either of these reaction rates can decrease corrosion. Therefore, the characteristic of electrochemical corrosion is that its corrosion process can be divided into two (or more) relatively independent and simultaneous oxidation and reduction reactions; the metal surface under corrosion has separate anodic and cathodic regions, and an electrolyte solution is present on the metal surface, allowing electrons to flow from the anodic region to the cathodic region through the metal, thereby generating electrons. In short, the only reason why electrochemical corrosion can occur is the presence in the solution of a depolarizer, that is, an oxidizing agent, which can oxidize the metal into ions or compounds. If there is no suitable depolarizer in the solution, an electrochemical corrosion process cannot occur ； In simple terms, any metal or alloy is \"impure\"; there are always certain impurity phases present, which can also be attributed to the chemical heterogeneity of the metal itself. When metal comes into contact with an electrolyte solution, these impurities form numerous microcells together with the base metal, creating a system of many short-circuited microcells. The driving force behind this microbattery effect remains the potential difference between the two electrodes, with the anode and cathode both located on the same metal surface. Therefore, it can be regarded as a micro-corrosion cell in which two microscopic regions with different electrode potentials in the metal are in direct electrical contact. An important reason for the formation of micro-corrosion cells is determined by the electrochemical heterogeneity on the metal surface. In addition to uneven chemical composition of the metal, there are also issues such as uneven microstructure, uneven physical state or stress distribution, and imperfections in the metal surface coating. These factors cause certain areas to have a more negative potential and thus act as an anode, while other areas have a more positive potential and thus function as a cathode. Each local cathodic area can form a corrosion galvanic cell with the adjacent local anodic area, and the combined effect of numerous such corrosion galvanic cells causes the metal in the anodic areas to be continuously corroded. In addition, there is another category of macroscopic corrosion cells that are well-known, such as galvanic corrosion cells, concentration cells, and thermocouples – all of which are common types of corrosion in actual production. In particular, galvanic corrosion (sometimes also referred to as corrosion due to contact between different metals) can cause sudden corrosion-related damage to steel structural components as a result of the use of different metal combinations. But sometimes it can also be exploited by people; cathodic protection using the sacrificial anode method in underground pipelines, chemical processing equipment, large storage tanks, etc., is implemented based on this principle.