Corrosion Comparison between cast Nickel Aluminium Bronze and CNC

The use of aluminium bronze in the building of sub sea platforms has been well documented over a long history of refits.

Numerous problems with cast aluminium bronze have been encountered relating to leaking valves and other components. On routine inspection the full extent of corrosion penetration can be masked by surface corrosion product. This can hide selective phase corrosion which can penetrate much deeper into the part.

The understanding of selective phase corrosion in cast Nickel Aluminium Bronze, although known for many years, has been helped more recently by the use of scanning electro-microscopes and electrochemical analysis. Research conducted recently has given a clearer understanding of the complex electrochemical mechanisms that take place in pitting and crevice corrosion. This suggests that selective phase corrosion can initiate crevice corrosion by a very complex electrochemical cell. The κ phase can exist in many forms) and SEM examination has enabled specific analysis of each phase.

The micro structure of Nickel Aluminium Bronze is quite complex with up to 6 separate phases present, as shown below.

Micrographic phase structure of nickel aluminium bronze X500 magnification.

Energy Dispersive X-Ray Spectroscopy Analysis of the Phases Present in Cast Nickel Aluminium Bronze

The studies suggest initial selective phase corrosion of α, which changes as the chemistry of the corrosion site evolves. Surface corrosion was initially confined to the eutectoid region with a slight attack of the copper-rich α within the α +κIII eutectoid area. Whilst the eutectoid α phase was preferentially attacked, the primary α grains exhibited very little corrosion, which indicates a form of selective phase corrosion.. The accumulation of Cu2O deposits at these locations will limit the diffusion of other ions, copper, iron, chloride and dissolved oxygen away and into the deposit.

This creates a micro-environment below the deposit, which causes the base of the pit to drift towards the acidic range i.e. below pH 4.0, which alters the complete electrochemical equilibrium of the cell.

The κIII phase then becomes anodic to the α phase and corrodes preferentially changing the selective phase corrosion.

If the κIII is laminar or a continuous grain a boundary network as commonly seen in un-heat treated castings then deeper penetration into the substructure may occur.

Corrosion attack of κIII (Ref 30)

It is well known that the corrosion resistance of cast Nickel Aluminium Bronze can be improved at heat treating above 650ºc for 6hrs minimum. Even so, it is still found lacking compared to Wrought Nickel Aluminium Bronze.

The next generation of high-strength marine-resistant material

Copper-Nickel-Chrome to CAL CNC-1 (and soon-to-be-published Def Stan 02 886)

The micro structure of CNC is mono phase and has no discernible precipitates. The hardening mechanism of CNC is based on a spinodal decomposition through a chemical order /disorder transformation which takes place over a long period.

Micro structure of CNC (magnified 500x)

The reaction takes place at an atomic level and can only be observed at a transmission electron microscope level. Hence the alloy copper-nickel-chrome is not subjected to selective phase corrosion.

Corrosion rates of CNC compared with 70/30 Cupro Nickel

The alloy has been in service in sub-sea platforms for over 25 years and regular refits has revealed low corrosion rates and has surpassed all expectations.

The development of the wrought form opens up exciting opportunities for design engineers in the production of a high strength alloy with combined fracture toughness and excellent corrosion resistance and potential resistance to higher sea water flow rates.

Graph of Corrosion rates of CNC compared with 70/30 Cupro Nickel

Historic Development

• In 1979 Mr David Taylor and Robert Ferrara presented a paper at AMPTIAC,on work conducted on behalf of the Naval Ship Research and Development Centre based in Annapolis USA
• The project was based on finding an alternative piping alloy to 70/30 Cupro Nickel for naval applications.
• The alloy under test was CA 719 CuNi30Cr2 extruded pipe and plate
• Tests were conducted in quiet and flowing sea water over a 2 and 4year period
• These included impingement and erosion tests conducted at sea water velocities of 15-45fps and 4.5-13m/s

Conclusion

• CuNi30Cr2 could withstand impingement up to 50fps(15m/s) where as 70/30 CuNi sustained damage at 15fps (4.5m/s) and this also compares with a recommended maximum of 4.3m/sec for Nickel Aluminium Bronze
• General corrosion on alloy CuNi30Cr (CAL CNC-1) was less than 0.025mm/y, a tenth that of wrought nickel-aluminium bronze and comparable to Titanium.