Chloride stress corrosion

Chloride stress corrosion is an important form of material degradation, especially for austenitic stainless steels. Chloride ions can cause localized corrosion on metal surfaces, leading to the consumption of metal and material failure. At the microscopic level, chloride ions destroy the microstructure of austenitic stainless steel through several steps. First, chloride ions adsorb onto the metal surface and penetrate into the grains and grain boundaries. Secondly, chloride ions react with iron atoms on the metal surface to form complexes such as ferric chloride and iron oxide. These products will further adsorb onto the metal surface, leading to its roughening and flaking off. Energy spectra can be used to analyze the compounds formed in chloride stress corrosion, such as ferric chloride and iron oxide. Techniques such as X-ray diffraction and scanning electron microscopy can be used to further examine the microstructure and compound composition of the samples, in order to gain a deeper understanding of the mechanism of chloride stress corrosion. .

Let me share my own understanding; if there are any mistakes, I hope experts can point them out and even help me correct them. Cl- is small in size; it can penetrate directly through chromium oxide and into the shallow surface layer of the metal, reaching the grain boundaries and even inside the grains. It accumulates at the defects within the crystal structure. Cl- accelerates the electrochemical reactions that occur in the metal, leading to increased electrochemical diffusion within the crystal. The Cl- ions do not remain attached to the surface as precipitates, nor do they form a dense oxide film on the surface. Cl- does not disappear during these reactions; it merely participates in intermediate reactions, acting as a catalyst. In reality, it is the galvanic cell formed between the metal and carbon elements that causes electrochemical corrosion. As a result, tiny pores form on the surface and in the shallow layers of the metal, increasing surface roughness and surface energy. Under the combined effects of erosion by the surrounding medium and other stresses, the passivation film is damaged more rapidly. The passivation rate of Cr is slower than that of Cl-, leading to pitting that causes damage to the surface and shallow layers of the metal. The cross-section at the pitting site undergoes a sudden change, resulting in structural discontinuity; the fluidity at the pitting site decreases, and Cl- accumulates there, intensifying the reaction. Chloride stress corrosion is a self-accelerating electrochemical reaction; as time passes, the corrosion rate increases, which is why two perforations are generally not observed. Adding Mo to metals can increase the passivation rate of Cr; it can also reduce the corrosion rate by lowering the carbon content and thereby raising the potential of C in the metal-C galvanic cell.

In my opinion, dissolved oxygen does not cause too much damage; low concentrations of dissolved oxygen do not result in any harmful effects. Chloride-induced stress corrosion cracking is mainly due to the small size of Cl- atoms, which allow some of them to penetrate deep into the surface layer. Together with stress, this leads to the destruction of the passivation layer on the surface. Additionally, the presence of Cl- alters the environment within the crystal, increasing the number of defects in the alloy, and these defects interact with each other. One thing is certain: Cl- significantly accelerates electrochemical corrosion, expands its scope, and increases the potential difference between the positive and negative electrodes, thereby triggering intense electrochemical reactions. Cl- remains in pitting pits for longer periods of time; in some cases, it even accumulates directly at the defects within the crystal. This leads to an increase in Cl- concentration in the pitting areas and the surrounding matrix, with the affected area continuing to expand. Stress plays a secondary role; the flow of the medium erodes the passivation layer on the surface, and there are also defect areas within the crystal. These defect areas have a higher free energy, and electrochemical corrosion leads to the formation of ultramicroscopic voids in these defects. As the density of these ultramicroscopic voids increases, microcracks are formed under the effect of residual stress. Such a self-accelerating stress corrosion cracking that repeats in this cycle.