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I. Core initiation mechanism: The core of under-scale corrosion is localized electrochemical corrosion induced by a blocked microenvironment. When the metal surface is covered with deposits such as scale and sediment, gaps form beneath these deposits; poor oxygen supply results in an oxygen-poor anodic region, while the exposed metal outside these gaps serves as an oxygen-rich cathodic region. These two regions form an oxygen concentration cell, which drives corrosion to occur specifically under the scale. II. Acidification and autocatalytic acceleration process: The scale prevents the exchange of substances between the interior and exterior of the material. During corrosion, the Fe²⁺ generated by anodic dissolution cannot diffuse outward; instead, it undergoes hydrolysis to produce H⁺, which causes the pH value beneath the scale to continue dropping, thereby creating an acidic environment ； At the same time, Cl⁻ continuously migrates and accumulates in the anode region, further accelerating metal dissolution and creating a self-catalytic corrosion cycle. III. Synergistic intensification effect: The anaerobic environment at the crack bottom fosters microorganisms such as sulfate-reducing bacteria, whose metabolic products further lower the local pH value. This, in combination with electrochemical corrosion, significantly accelerates the rate of pitting corrosion, ultimately resulting in deep, ulcer-like corrosion pits.