Copper is normally present in stainless steel only as a residual element. In selected alloys, however, it is added deliberately to enhance corrosion resistance, particularly in seawater and sulphuric acid solutions.

Copper's mixed reputation in steelmaking

For many years, copper was seen as an undesirable element in steel production. It has been associated with hot shortness, where it is molten at the temperatures used for forging and rolling, causing items to break during processing. Great lengths are taken to segregate copper-containing parts from scrap steel consumed in volume steel production, to avoid picking it up as a tramp element.

Despite these historical concerns, copper has been widely used in stainless steel castings, partly because it was associated with good castability - the ability of molten metal to flow through the mould and achieve a sound casting. The case for copper being good for casting but poor for hot working is far less conclusive than the anecdotes suggest, given that it has been used as a deliberate alloying addition in larger amounts. Its positive impact on corrosion resistance in selected environments is much clearer.

How copper improves corrosion resistance

Copper has been shown to significantly improve the corrosion resistance of stainless steels, particularly in seawater and sulphuric acid environments.

The corrosion resistance of stainless steels is due to the highly stable passive film that forms on the surface when exposed to an oxygen-containing environment. Pitting corrosion, however, often initiates on or close to sulphide inclusions that happen to exist near the surface.

Sulphur is not added to steels deliberately (except in selected free-cutting steels), as it can be problematic during processing; in large quantities it results in hot shortness. Manganese is normally added to combine with sulphur to form manganese sulphide inclusions. As long as they are present in small quantities and finely dispersed, the effect on mechanical properties is limited. In larger quantities, they can align in the rolling or forging direction and result in poor impact toughness in the transverse direction.

Any inclusion present at the exposed surface of a stainless steel component will not allow the stable chromium oxide layer to form there, making it an ideal site for pitting corrosion to initiate. Ferralium® 255, Langley Alloys' trademarked super duplex stainless steel, seeks to limit this by specifying sulphur content below 0.005%, far less than the typical limit of below 0.01% for S32760 and below 0.02% for S32750.

Adding copper has a further beneficial effect around these inclusion-initiated pitting sites. Dissolution of copper at the surface results in the precipitation of an insoluble copper sulphide (Cu2S) compound. This prevents sulphur species from adsorbing onto the surface of the steel, inhibiting further pit growth. In some ways copper provides a self-healing effect: it will not prevent pitting from initiating at points of weakness in the passive layer, but it slows or halts further corrosion at these sites, preventing more harmful corrosion.

Copper is also thought to form an extra protective (passive) layer across the surface of the stainless steel in these solutions. Copper sulphide can exist stably compared to either iron sulphide or nickel sulphide, so a layer of copper sulphide is proposed to complement the main chromium oxide passive layer.

Alloys that use copper additions

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 highly alloyed nickel grade and not currently part of the Langley Alloys stock programme. It is referenced here as another alloy that uses copper additions - between 3.0 and 4.0% - specifically to enhance corrosion resistance in sulphuric acid.

Copper has also been included in lesser-alloyed grades of stainless steel, such as 316Cu (a variant of Alloy 316, UNS S31603) and 204Cu (a lower-cost variant on Alloy 304L), with improvements in corrosion resistance claimed.