Copper is normally present in stainless steel as a residual element only. However, it is added to selected alloys in order to enhance their corrosion resistance, particularly in seawater and sulphuric acid solutions.
The corrosion resistance of stainless steels is due to the highly-stable passive film that forms on its surface when exposed to an oxygen-containing environment. However, pitting corrosion resistance often initiates on or close to sulphide inclusions that happen to exist near the surface. The presence of such inclusions at the exposed surface prevent the formation of the chromium oxide stable layer. This becomes the ideal site for pitting corrosion to initiate, and a pit can be quickly formed where the inclusion once was.
Adding copper to stainless steels has an interesting beneficial effect around these inclusion-initiated pitting corrosion sites. Dissolution of copper at the surface results in the precipitation of an insoluble copper sulphide (Cu2S) compound. This, in turn, prevents sulphur species from adsorbing on to the surface of the steel, thereby inhibiting further pit growth. In some ways, copper provides a self-healing effect for stainless steels, which will not prevent pitting corrosion from initiating at points of weakness in the passive layer, but it will slow or halt further corrosion at these sites preventing more harmful corrosion.
As well as the above mechanism, copper is also thought to create form an extra protective layer (passive layer) across the surface of the stainless steel in the previously described solutions. Copper sulphide is able to exist stably compared to either iron sulphide or nickel sulphide. Therefore, a layer of copper sulfide is proposed to complement the main chromium oxide passive layer.
Ferralium 255 (F61, 1.4507, UNS S32550) is perhaps the best-known exponent of increased copper additions. As the original super duplex stainless steel, it has been providing enhanced corrosion resistance in the harshest of environments for more than 50 years. Developed from a cast grade that contained copper for reasons of castability, the novel wrought form maintained a copper content around 2.0% in order to achieve superior corrosion resistance in seawater and sulphuric acid.
Alloy 20 (2.4660, UNS N08220, AL20, C20, CN7M) is a high-alloyed nickel grade, and not currently part of Langley Alloys stock programme. However, it is referenced as another alloy that utilises copper additions specifically to enhance corrosion resistance in sulphuric acid. It contains between 3.0 – 4.0%.