Industrial Corrosion and Protection--10 Intergranular Corrosion

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Industrial Corrosion and Protection–Part 10: Intergranular Corrosion. Immersive reading in a novel reader. Basic concepts: In certain corrosive environments, the grain boundaries may be corroded first. This type of localized damage that occurs along the metal grain boundaries is known as intergranular corrosion. This type of corrosion often occurs on stainless steel, nickel, aluminum, and copper alloys. Grain boundaries are regions in metals where various solute elements tend to concentrate, or where metal compounds (such as carbides and σ-equivalents) precipitate out. In such a system, intergranular corrosion occurs if the dissolution rate in the grain boundary regions is much greater than that in the grain itself. According to relevant statistics, intergranular corrosion accounts for about 9.5% of the cases of stainless steel failure due to wet corrosion. Conditions for occurrence: For stainless steel, intergranular corrosion usually occurs as a result of sensitization or improper composition. For chromium-nickel austenitic stainless steels, the sensitization temperature range is 400–900°C; for chromium-based ferritic stainless steels, the sensitization temperature is above 925°C. For chromium-manganese-nitrogen austenitic-ferritic duplex stainless steels, if the ferrite is present in a continuous network structure, they are susceptible to intergranular corrosion. The heat-affected zone of a welded joint, located 3 to 5 mm away from the weld line, has a very narrow area that is susceptible to sensitization; if corrosion occurs, it appears as a groove running along the junction where the weld meets the base metal. This type of intergranular corrosion is known as \"knife-edge corrosion\". Morphological characteristics: When intergranular corrosion occurs in metal materials, it is characterized by the fact that the macroscopic dimensions of the metal remain almost unchanged; sometimes the surface still retains its metallic luster, but its strength and ductility decrease. Under exposure to cold bending or severe impact from process fluids (especially those containing solid particles), or as a result of intense mechanical collisions, cracks appear on the surface; in severe cases, the surface becomes brittle, and with even slight external force, the grains break off, resulting in the loss of the metallic sound. Upon microscopic cross-sectional metallographic examination, local corrosion can be observed at the grain boundaries or in their adjacent areas; in some cases, grains even detach, with the corrosion progressing uniformly along the grain boundaries. Mechanism of intergranular corrosion: Intergranular corrosion, in terms of its electrochemical nature, can be considered to result from uneven dissolution between the alloy’s grain boundaries and the grain interiors in the corrosive medium, at the corrosion potential. Intergranular corrosion occurs as long as the dissolution rate of the material in the alloy’s grain boundary region is much higher than that in the grain itself. The main reason is the significant difference in electrochemical properties between a certain substance in the grain boundary region and that in the grain itself. Secondly, this difference can be observed only under appropriate medium conditions (which thus determine the appropriate corrosion potential). The presence of impurities or stress in the grain boundary region relates to two aspects, and modern theories of intergranular corrosion can be summarized as follows: one is the depletion theory; the other is the theory of selective dissolution of impurities or second phases in the intergranular region. (1) Depreciation theory: This is a general term. For stainless steel, it is the chromium-deficient theory ； For nickel-chromium-molybdenum alloys, it is the low-molybdenum theory ； For aluminum-copper alloys, it is the copper-poor theory ； Taking austenitic stainless steel as an example, the solubility of carbon in austenite decreases as the temperature drops. For example, in Cr18Ni9, the equilibrium carbon content at 500–700°C does not exceed 0.02%. When austenitic stainless steel is heated to 500–850°C (for example, in the heat-affected zone during welding), the supersaturated carbon is completely or partially precipitated from the austenite to form chromium carbides, primarily of the (Cr,Fe)23C6 type, which are distributed at the grain boundaries. The chromium content of chromium carbide is much higher than that of the austenitic matrix. The precipitation of chromium carbide results in a lack of chromium in the grain boundary regions surrounding it, thereby forming chromium-deficient grain boundary zones. The Cr content in the chromium-deficient regions is below the critical concentration (12%) required for passivation. As a result, chromium-deficient grain boundaries in an activated state coexist with chromium-rich grain regions in a passive state, creating an activation-passivation cell with a large potential difference. This leads to the erosion of the chromium-deficient grain boundary areas, resulting in intergranular corrosion. (2) Theory of selective dissolution of impurities or second phases in the intergranular region: The mechanisms of intergranular corrosion are applicable to specific alloy microstructures, particularly under certain environmental conditions; they are not mutually exclusive but rather complement each other. It should be noted, however, that the most common intergranular corrosion occurs in weakly oxidizing (or oxidizing) media; therefore, the vast majority of intergranular corrosion phenomena can be explained by the depletion theory, especially in the case of stainless steels. This is because materials that rely on passivation for corrosion resistance, such as stainless steel, are primarily used in oxidizing or slightly oxidizing media; in reducing or highly oxidizing media, the vast majority of stainless steels suffer from severe general corrosion and cannot be used. Factors affecting corrosion (1) Corrosive medium: The type and composition of the corrosive medium not only determine whether intergranular corrosion will occur, but also affect the degree of such corrosion. Generally, stainless steel suffers from severe intergranular corrosion in acidic media. Adding oxidizing cations such as Cu, Hg2+, and Cr6+ to sulfuric acid or nitric acid will accelerate the rate of anodic dissolution at the grain boundaries, thereby accelerating intergranular corrosion. (2) Chemical composition: The element C is a key element that plays an important role in sensitizing stainless steel; it has a significant impact on intergranular corrosion. Moreover, as the carbon content increases, the tendency for intergranular corrosion as well as the corrosion rate also increase. Adding Ti and Nb to stainless steel can narrow the temperature and time range at which intergranular corrosion occurs, and may even eliminate its susceptibility to such corrosion. This is because their affinity for carbon is greater than that of Cr for carbon, which results in fewer or no chromium carbides forming in the steel when it is heated at sensitization temperatures, thereby preventing chromium depletion. (3) Heat treatment process: The intergranular corrosion tendency of austenitic stainless steels is mostly caused by the precipitation of chromium carbide, which leads to chromium depletion in the grain boundary regions. The precipitation of chromium carbide and the tendency for intergranular corrosion are both related to the heating temperature and time. Both the relationship between heating temperature and time for the precipitation of carbides, and the relationship between heating temperature and time that leads to intergranular corrosion, follow a \"C\"-shaped curve. At higher temperatures, carbides do precipitate, but there is no tendency for intergranular corrosion, as the high temperature facilitates significant chromium diffusion, thereby eliminating chromium-deficient regions. The temperature at which a tendency for intergranular corrosion occurs is called the sensitization temperature. For austenitic stainless steels, this temperature range lies between 500 and 850°C; generally, 650–700°C is the most sensitive range, as it is at these temperatures that the time required to develop intergranular corrosion is shortest. (4) Cold deformation: Generally, cold deformation reduces the sensitivity of steel to intergranular corrosion. It is because cold deformation provides nuclei for carbide formation within the deformed grains, thereby minimizing the amount of carbides that precipitate along the grain boundaries during sensitization treatment. Of course, cold deformation can also lead to the formation of strained martensite, resulting in complex corrosion phenomena.