<p>Frequent discoloration of security filter cartridge in the cooling systems of high-voltage direct current converter valves during operation affects equipment reliability. Taking a converter station in the Sichuan power grid as a case study, this research systematically investigated the causes of discoloration and the corrosion mechanisms of 316L stainless steel during the circulating cooling process through on-site water quality testing, simulated filtration test, and characterization of phase composition and microstructure. It was confirmed that chloride ions (Cl^–) in the cooling water are the key factors inducing localized corrosion of stainless steel and destabilizing the passivation film. Furthermore, the uneven distribution of the passivation film thickness resulting in interference effects and the presence of iron oxides was identified as the primary cause of discoloration. Additionally, it was found that sulfate ions (SO₄^2–) significantly inhibit the corrosive action of Cl^– by reducing its adsorption through competitive adsorption. It also synergistically enhances the stability of the passivation film with the molybdenum (Mo) element in 316L stainless steel, thereby suppressing discoloration and corrosion. The research results provide a critical theoretical basis and engineering reference for the precise control of water quality and the design of anti-corrosion protection in cooling systems for similar ultra-high-voltage direct current transmission projects.</p>