Industrial Corrosion and Protection--07 Local Corrosion, the Most Severe Type of Corrosion

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Immersing oneself in reading a novel is enjoyable; for chemical processing equipment, it is particularly important to prevent local corrosion. Since chemical processing units are often exposed to highly corrosive substances such as acid, alkali, and salt solutions, corrosion-resistant metal materials (such as stainless steel, titanium, nickel-chromium alloys, etc.) are commonly used to prevent corrosion. However, corrosion-resistant metal materials, especially those that rely on passivation for corrosion resistance, are most prone to local corrosion. Although general corrosion can cause significant loss of metal, its rate is easy to measure and detect; moreover, during engineering design, a margin for corrosion can be taken into account in advance, thereby preventing the equipment from being damaged by corrosion too early. Local corrosion often causes sudden metal failure without any prior signs, making it difficult to predict and prevent. In damage incidents involving chemical processing units, those caused by local corrosion are far more common than those caused by general corrosion, resulting in greater hazards. Taking the survey of 166 damage incidents that occurred during my tenure as an employee as an example, general corrosion accounted for 8.5%, high-temperature oxidation accounted for 4.9%, with the remainder being cases of local corrosion. Local corrosion can be divided into galvanic corrosion, pitting corrosion, crevice corrosion, intergranular corrosion, selective corrosion, stress corrosion cracking, hydrogen embrittlement, corrosion fatigue, and wear corrosion, among others. This chapter focuses on galvanic corrosion. 1. Basic concepts of galvanic corrosion: When different metals come into contact with each other and are both in the same or connected electrolyte solution, the presence of a potential difference (corrosion potential difference) between these metals causes the metal with the lower (more negative) potential to corrode more rapidly. This phenomenon is known as galvanic corrosion, or bimetallic corrosion, or contact corrosion. For example, when ordinary carbon steel (or high-strength steel) comes into contact with copper (either riveted or bolted together) and both are placed in an electrolyte solution, the corrosion that occurs is known as galvanic corrosion; the steel corrodes much more rapidly than it would if it were alone. 2. Electrode pair sequence: Electrode pair corrosion is related to the actual potentials of the metals in contact with each other in the solution, and it is this that forms a macroscopic corrosion cell. The driving force behind galvanic corrosion comes from the actual potential difference between two different metals in contact. Generally speaking, the greater the difference in electrode potentials between the two metals, the more severe the galvanic corrosion ； The couple sequence is a table of potential order that ranks various metals or alloys based on the levels of their stable potentials (non-equilibrium potentials or corrosion potentials) measured under certain conditions. When a metal or alloy A (with a more negative potential) and another alloy B (with a more positive potential) form a cell, alloy A will experience accelerated corrosion (acting as the anode), while alloy B will experience reduced corrosion or no corrosion at all (acting as the cathode). The greater the potential difference between them, the more severe the corrosion may be, all other conditions remaining constant. In the couple series, only the relative relationships of the stable potentials of metals are usually listed, rather than the actual values of those stable potentials. The main reason is the significant variation in the actual corrosive media. Couple sequence data can also serve as a reference for studying couplage effects in other environments. However, to better address some practical problems, it is best to actually measure the stable potential (self-corrosion potential) of the relevant metal or alloy in specific environmental media and conduct necessary coulomb cell tests to obtain reliable results. The actual potential difference between the electrodes is a necessary condition for the occurrence of galvanic corrosion; it indicates the thermodynamic possibility of such corrosion, but it does not determine the efficiency of galvanic corrosion, as information on polarization properties and the characteristics of the corrosion behavior is also required. 3. Mechanism of galvanic corrosion: According to the kinetics of electrochemical corrosion, the intensity of the corrosion current resulting from the coupling of two metals is related to the potential difference, polarization rate, and ohmic resistance. The greater the contact potential difference, the more severe the metal corrosion, as the driving force for galvanic corrosion increases. The galvanic corrosion rate is also proportional to the galvanic current.