In this work, we investigated the effects of Sn ions and the co-existing anions on the formation of beta-FeOOH from an Fe3+ solution under atmospheric corrosion conditions and a high chloride ion concentration. By adding a small amount of Sn ions into an FeCl3 acidic solution, alpha-FeOOH instead of beta-FeOOH was formed in the precipitate, regardless of the coordination and valence of the Sn ion. The added Sn2+ affected the coordination of Fe3+ and presumably formed a dinuclear complex with Fe. As a result, Sn2+ inhibited the polymerisation of beta-FeOOH, and played a role in catalysing the ligand exchange of Fe3+ and the formation of alpha-FeOOH.