Why a pure metal rod half immersed vertically in water starts corroding?

A pure metal rod half immersed in water starts corroding due to electrochemical reactions occurring at the interface between the metal and the water. The submerged portion of the rod is exposed to moisture, which facilitates oxidation, while the air-exposed portion may remain relatively dry. This creates a galvanic cell, causing the submerged part to lose electrons and corrode faster than the exposed part, leading to uneven corrosion and potential structural degradation over time. Additionally, impurities and dissolved salts in the water can accelerate the corrosion process.