Industrial Corrosion and Protection--09 Pitting Corrosion

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Industrial Corrosion and Protection–Part 9: Pitting. Immersive reading in a novel reader. Basic concepts of pitting: In certain environmental conditions, metal materials do not experience corrosion or only suffer slight corrosion after a certain period of time; however, at specific points or small areas, pits or pockmarks appear. Over time, these pits continue to deepen, resulting in small-scale corrosive defects. This type of corrosion that occurs on a metal surface over small areas such as pores or spots is called pitting corrosion, also known as spot corrosion. Pitting is a type of localized corrosion that is highly insidious and extremely dangerous. Conditions for pitting corrosion: 1. Pitting corrosion typically occurs in metals or alloys that are prone to passivation, and it often happens in the presence of aggressive anions (such as Cl-) along with oxidizing agents. For example, stainless steel, aluminum and aluminum alloys, titanium and titanium alloys may undergo local dissolution to form pitting in nearly neutral chloride-containing aqueous solutions or other specific media. 2. Pitting is a form of localized corrosion that occurs in the anodic region as a result of a small anodic-cathodic corrosion cell. Here, five types of pitting are summarized, with their characteristics outlined as follows: A. The most common and important type of pitting occurs on passivated iron-based alloys in contact with chloride solutions having a moderate pH value (around 3–10). At a certain critical potential in the passivation region (which, in the absence of CI-, can extend up to the oxygen evolution potential), curve 1 deviates from its previously nearly horizontal portion, showing a sharp increase in the anodic current density. At this potential, pitting nucleation occurs and begins to grow. A classic example of this type of pitting corrosion is the pitting that occurs in stainless steel in sodium chloride solutions. B. Type II pitting begins at potentials near the activation-passivation transition region, and occurs in a more positive potential range up to the oxygen evolution potential. Its practical significance is that, in this case, there is actually no passivation region. For example: stainless steel in highly corrosive media, such as concentrated HCl at room temperature and high-concentration MgCl2 solutions above 100°C. C. In some cases, pitting occurs in the over-passivation potential region, where oxidative degradation takes place and the metal is locally eroded. For example: iron in ClO4- solution. D. Pitting also occurs in the activation-passivation transition region. At the passivation potential (the critical potential for passivation), the pore density is highest; however, if polarization continues under these conditions for a period of time, the corrosion pores can connect together, leading to general corrosion. At higher anode potentials, pitting stops because the metal surface is completely passivated. For example: iron in a sulfate solution. E. Pitting may occur in the activation potential region when the metal surface is uneven and contains areas that are more active than the surrounding surface. For example, when carbon steel (mild steel) containing sulfide inclusions comes into contact with a nearly neutral chloride solution, if the carbon steel is initially covered by an oxide film, corrosion occurs at the sites on the surface where sulfide inclusions are present (or where the oxide layer is incomplete); pitting nucleates at these locations, but overall corrosion usually follows thereafter. 3. Most pitting phenomena are related to halide ions, but it has been found that pitting can also occur in the presence of other substances or at other potential ranges. For example, in some cases, pure water and non-aqueous solutions can also cause pitting: in many metal-based systems, a specific pitting potential may not be available. Pitting can also occur in acidified alcohol solutions, and in most cases, the polarization curves also show the presence of a pitting potential. The presence of acid is a necessary condition for the occurrence of this type of pitting. Carbon steel and stainless steel can also suffer from pitting corrosion in pure water containing dissolved oxygen at temperatures ranging from 50 to 200°C. Characteristics of pitting morphology: Visually, there are open-type pits as well as closed-type ones; in the latter case, the surface is covered with corrosion products or a thin layer of metal remaining on the surface showing indentations, while severe pits hide inside. The cross-sectional morphologies of pitting pits obtained from experiments and analysis of actual field failures basically exhibit 7 different characteristic shapes.