Industrial Corrosion and Protection--11 Selective Corrosion

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Immerse yourself in reading in the novel reader. The basic concept of selective corrosion: When a certain structure or component within an alloy corrodes preferentially, while another structure or component does not corrode or corrodes only to a minor extent, this phenomenon is known as selective corrosion. As a result of selective corrosion, the alloy may lose strength in mild cases, while in severe cases it can suffer from perforations and damage, leading to serious accidents. Therefore, the issue of selective corrosion is receiving increasing attention. Conditions for selective corrosion: In terms of the medium, selective corrosion occurs most often in aqueous solutions; however, certain materials can also experience selective corrosion in molten salts or high-temperature gas environments. Apart from the medium conditions, a necessary condition for selective corrosion to occur is that there must be a significant difference in the stability of the various alloy components in the medium. Furthermore, one of the components should have a relatively high equilibrium potential or be a metal prone to passivation. If there is no such significant difference, the corrosion rates of the two components are similar, making selective corrosion difficult to occur. Therefore, selective corrosion can be further divided into two categories: tissue-selective corrosion and composition-selective corrosion. (1) Microstructural selective corrosion: For multiphase or microstructurally transforming alloys, their microstructure has a significant impact on selective corrosion. Generally, when there are significant differences in the electrochemical stability of the various phases in a multiphase alloy, corrosion tends to start first in the phase with lower stability, that is, selective corrosion of the microstructure occurs. If this type of corrosion also exhibits the characteristic of component selection as mentioned below, then what occurs is both tissue-selective corrosion and component-selective corrosion. After the less stable phase begins to corrode, the numerous pores formed allow the medium to penetrate deeper into the material, and the corrosion front continues to advance into the interior of the alloy. If the other phase is completely stable in the system, after the positively charged phase has been eroded, only a loose framework of the negatively charged phase remains; if the other phase also exhibits a tendency to corrode selectively, then as the medium penetrates, this phase begins to corrode as well, leaving behind a loose layer of the negatively charged components. Organized selective corrosion, from an electrochemical perspective, is caused by a significant difference in the dissolution currents of the two phases when an alloy corrodes under a constant potential. The electrochemical corrosion of multiphase alloys falls under the category of short-circuited battery corrosion and can be regarded as a fully polarized system. During corrosion, all phases are polarized to the same potential, so it can be considered a corrosion process at a constant potential. Austenite-ferrite duplex stainless steels also suffer from selective corrosion of their microstructure under certain medium conditions. For example, 0Cr18Ni5Mo3Si2 duplex stainless steel (equivalent to Swedish 3RE60 steel) was subjected to a constant-load stress corrosion test in a 30% MgCl2 solution at 117°C. The results showed that selective corrosion occurred in the specimens, with the α-ferrite dissolving preferentially and the γ-phase remaining. In the results of constant-load stress corrosion tests conducted on 1Cr20Mn13NB duplex stainless steel in boiling 45% MgCl2 solution, it was found that the austenite (γ phase) is preferentially corroded. More examples of selective corrosion in alloy microstructures can be cited, such as the corrosion of graphite in gray cast iron. In soil or water, in gray cast iron the graphite in the cast iron acts as the cathode, while the ferritic matrix serves as the anode; as a result of corrosion, only graphite and rust remain. (2) Component-selective corrosion: When a single-phase alloy corrodes, the various components in the solid solution do not dissolve in proportion to their composition in the alloy; instead, certain components (those that are relatively less resistant to corrosion at that corrosion potential) dissolve preferentially, a phenomenon known as component-selective corrosion. Selective corrosion of alloys under high-temperature conditions. Selective corrosion of alloys occurs not only in aqueous media but also in high-temperature media such as molten salts, liquid metals, and high-temperature oxidative atmospheres. Austenitic steel can be denickelized under the action of liquid sodium. And thus, a ferrite layer is formed on the surface of the steel. Some alloys also exhibit selective dissolution of their components in molten fluorides, chlorides, nitrates, sulfates, carbonates, and hydroxides. Moreover, it can occur even at low concentrations of alloying elements. For example. Inconel undergoes selective corrosion of Cr and Fe in a potassium-sodium-fluoride-chloride salt bath at 800°C, resulting in a residue composed of sponge-like nickel. Factors affecting selective corrosion (1) Influence of alloy composition 1. The sensitivity of an alloy to selective corrosion is related to the content of active elements in the alloy. Generally speaking, the higher the content of active components, the greater its tendency for selective corrosion. Red and yellow brasses with less than 15% zinc content have very low sensitivity to dezincification corrosion, and it can be considered that dezincification does not occur in practice. The higher the zinc content in brass, the lower its critical potential for dezincification; in other words, the greater its tendency to dezincify. The decoppering of the aforementioned Cu-Au alloy is also closely related to the gold content in the alloy. In fact, Cu-Au alloys with an Au content higher than 35% (molar fraction) are insensitive to decoppering. In general, the higher the content of active components, the greater the tendency for dealloying; this is a universal rule. 2. In addition to the main alloying elements, the addition of small amounts of secondary alloying elements also has a significant impact on the tendency toward selective corrosion. For example, adding small amounts of elements such as tin, phosphorus, arsenic, and antimony to brass can all inhibit this. The dezinching of brass, which has now become an important method for preventing corrosion in brass, has led to the development of some new alloys. 3. Influence of the medium: The selective corrosion of alloys is closely related to the medium. Alloys with certain compositions often exhibit sensitivity to selective corrosion only in the presence of certain media, and are not sensitive to other media. Furthermore, different alloying elements are sensitive to different media that cause selective corrosion of their components. Selective corrosion prevention measures: From the perspective of environmental conditions, a stagnant solution, porous scale or deposits on the alloy surface (which can create gaps), and rising temperatures all contribute to the selective corrosion of the alloy. To this end, reasonable structural design, proper material selection, control of temperature and harmful substances during equipment operation, management of liquid flow rates, use of corrosion inhibitors, and regular cleaning of scale or deposits are all effective measures to prevent selective corrosion.