Fe(III)-Fe(II) Redox Kinetics and their Effects on 2205 Duplex Stainless Steel Corrosion

The kinetics of the Fe(III)/Fe(II) redox couple on 2205 duplex stainless steel (DSS) were studied in 0.5 M H₂SO₄ at 25, 60, and 95°C, across Fe(III)/Fe(II) ratios of 1:10, 1:1, and 1000:1. A novel method was developed to isolate the mixed potential, applied to DSS for the first time. The reversible potential (Erev) was maintained within the passive range (0–1 V vs SHE) to minimize corrosion. PDP and 60-day immersion tests confirmed passivity with <0.1 mm/y corrosion rate and no pitting. Potentiostatic tests showed consistent Tafel regions (R² > 0.95). At a 1:1 Fe(III)/Fe(II) ratio, temperature variation from 25 to 95°C has little impact on E₍rev₎, which remains stable at 0.65–0.68 V vs SHE. In contrast, increasing the Fe(III)/Fe(II) ratio markedly raises E₍rev₎ from 0.60 V to 0.86 V. Despite these shifts, E₍rev₎ stays within the passive region, providing anodic protection and minimizing crevice and pitting risks.