Investigation of corrosion products formed on C35–AlMgSi0·5 friction welds during natural weathering in marine climate

AlMgSi0·5–C35 friction welds were exposed to marine climate in Helgoland for 2 years. The corrosion products formed on the samples were investigated in view of their electrochemical reactivity, morphology as well as chemical and phase compositions. Various types of corrosion attack (atmospheric corrosion, bimetallic corrosion and pitting corrosion) were verified. Generally, the corrosion attack of C35 is stronger than that of aluminium. Aluminium serves as a sacrificial electrode only close to the welding plane. The rust scale formed on the steel side of the friction weld mainly consists of γ-FeOOH and α-FeOOH. Despite the significant chloride concentration inside the rust layer, no β-FeOOH was found. As it is well known, only the γ-FeOOH phase is electrochemically highly reactive and can be reduced to Fe₃O₄, depending on the polarisation potential. The reduction of γ-FeOOH is associated with an increase of oxygen reduction.