The role of copper in superduplex stainless steels has been the subject of previous investigations and it is generally recognised to be an alloying addition which enhances the corrosion resistance of these materials in acid environments, although the effect of copper on their corrosion properties in chloride media has been the source of some controversy. This study seeks to determine the influence of copper on corrosion resistance of superduplex stainless steels by comparing the behaviour of the copper-bearing alloy FERRALIUM 255 [SD40] (UNS S32550) with the copper-free alloy, UNS S32750. Experiments to determine critical pitting temperature (CPT) and polarisation behaviour of these alloys in 3.5% NaCl, HCl, and H₂SO₄ were carried out and the results assessed electrochemically as well as through observation by optical and electron microscopy.

Although the CPT of the copper-bearing superduplex alloys were not observed to be significantly different, their pitting potentials (as measured in NaCl and HCl by potentiodynamic methods) were found to increase with higher copper content. A reduction of corrosion current density could be produced in the copper-free superduplex alloy when copper ions were added to an acid corrosion media. This strongly suggests that copper can accumulate on the corroding surface either as metal atoms or as a chloride complex and that, in this form, it acts to effectively protect the surface from further corrosion. This mechanism, which explains the benefits of copper additions in superduplex stainless steels, would also relate to chloride media, as it is well known that the pit environment in neutral aqueous solutions containing chloride ions rapidly becomes acidic through local corrosion cell activity.

Direct ex-situ observation of the pitting process indicated that corrosion initiation was preferentially in the ferrite phase, with the occurrence of metastable pitting occurring within certain ranges of potentials and temperatures. The significance of the CPT appears to be the point at which the current required to passivate metastable pits is greater than that required to maintain a stable salt film cover over active sites. Large grain size material produces larger metastable pits, although grain size does not appear to influence the onset of stable pitting.