Next generation marine alloy – corrosion analysis

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.

Corrosion attack0.1

Micrographic phase structure of nickel aluminium bronze X500 magnification

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

Phase Alloy Component (wt.%)
Al Mn Fe Ni Cu
Α 7.90 0.20 2.58 2.91 86.41
Β 8.51 0.52 2.20 2.58 86.19
κI 17.35 1.25 35.69 18.07 27.64
κII 19.09 0.93 26.60 26.04 27.34
κIII 18.87 0.45 12.86 26.80 41.03
κIV 8.12 0.84 42.70 35.32 13.01

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.

Selective Phase

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

Corrosion attack of κIII (Ref 30)

Corrosion attack2

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.

Microstructure

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.

CNCvs. 7030

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.

© Copper Alloys

 

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The British Corrosion Research Project – update July 2014

Now 8 months in, our exciting corrosion research project is starting to illustrate very clearly just how well copper alloys withstand marine environment conditions over all other metals commonly used in marine environments

1) After 8 months of submersion, the rack at Shotley Marina (slow moving sea water) weighed over 50 kg and proved difficult to haul out of the water due to a colony of Sea Squirts latching on to the majority of the sample bars. It is apparent (see image below) that the copper alloy samples are the only exception, creating a window effect among the overgrown colony of Sea Squirts.

Shotley Marina-july-2014

Clear evidence – copper alloys repelling Sea Squirt growth and creating a window effect – bottom left and bottom right.

2) The rack at Levington Marina (fast moving sea water) is showing similar growth patterns to the one pictured above, the only considerable difference being in the diversity of marine life present. With 8 months elapsed since initial immersion, the Aluminium Bronze and Cupro-Nickels are still performing very well in this aggressive environment showing almost no sea growth compared to the rest of the heavily affected samples.Thewindow effect is clearly visible in the pictures below (bottom left toward the middle and bottom right), leaving no room for doubt when it comes to the impressive anti-corrosive properties of the cooper alloy specimens.

Our suite of Elite Marine Alloys is faring exceptionally well, demonstrating high anti bio-fouling properties: CAL CNC-1 (third from the left), CAL T-1000 (fourth from the left) andCAL T-800 (fifth from the right)

corrosion-rack-annotated-categories

Marine life growth on a range of metal bars over 8 months of submersion. Note copper alloy samples on bottom left and bottom right showing virtually no sea growth.

It is interesting to note that Inconel 625 and the Monel alloys are showing significant levels of biofouling compared to the Elite Marine Alloys and copper-based alloys. In the picture below, it is clear to see the increasing marine growth from the copper alloys on the left to the nickel alloys and Inconels on the right.

dscn7972

Close-up of window phenomenon created by the anti-biofouling performance of the Elite Marine Alloys in a sea water environment

This is the only project that is giving a balanced appraisal through primary research of metals for the marine environment. It is important that engineers take note of the new information to ensure that the optimal material is specified at design stage for equipment expected to operate offshore. The racks will be reviewed monthly, with a full analysis to be conducted after 12 months at sea.

If you are interested in further information, you can talk directly to the material engineers managing this project on +44 (0) 1782 816 888, or email ben.turner@copperalloys.net.

The British Corrosion Research Project – update May 2014

The UK Corrosion Project update – surprising findings already May 2014

Six months ago, after months of scouring the Earth to collect all of the materials that can be used in marine environments, three test racks were installed in three increasingly aggressive environments. The corrosion racks have now been in situ for six months, and the two salt water sites are showing completely different growths of sea weeds and gel-like organisms at the two locations even though they are only about five miles apart. Please read on for a brief update on the findings, which are already showing signs of useful information after six months.


Rack 1 –  Levington Marina, Suffolk Coast End view of small rack showing growth of kelp which has appeared within a month. Water temperatures had risen from 6°c at the end of March to 13°c at the end of April.Rack 1 – Levington Marina, Suffolk Coast

The rack below contains 27 different crevice corrosion samples and impact charpy test pieces. The growth of weed is strange as it is mainly one sided even though the rack is suspended in open water underneath a walk way. However the water flow may greater on the far side of the rack where it is outside the influence of the walk way support posts. This marina is open to the main river and will have fresh nutrients on every tide change.

Rack 1 – Levington Marina, Suffolk Coast1

Rack 2 – Location Shotley Marina, Suffolk Coast

This marina is located closer to the open sea but the water is only refreshed when the  lock gates are opened to allow movement of yachts.The weed growth on this rack is completely different with a fine red coloured sea weed. We would welcome the views of any Marine Biologists as to the difference in growth rates and species of weed that has appeared.

Rack 2 – Location Shotley Marina, Suffolk Coast

More updates to follow! If you would like more specific information, please contact us on +44 (0) 1782 816 888.

The British Corrosion Research Project – update March & February 2014

Corrosion racks from salt water marina after 2 months

Corrosion racks from salt water 2months

Racks after two months in polluted sea water.

The rack contains a range of copper alloys, stainless steels, Monels, super duplex stainless , nickel alloys and titanium as well as stainless steel with special coatings.

All of the alloys are being compared with CuNi30Cr2 wrought alloy in a series of galvanic, crevice corrosion and mechanical tests specimens.

In total there are over 400 samples in three locations two in sea water and a third in a neat sewerage channel with high hydrogen sulphide levels.

At the end of the two year project the results will be a valuable guide to design engineers. It is believed that this project is a first, for so many sea water alloys being tested in one environment at the same time.

Crevice Corrosion Samples after 3 months
Crevice corrosion samples third from left 304 stainless
Crevice Corrosion Samples after 3 months

Galvanic Corrosion Samples after 3 Months
Corrosion product growing from (19) 316 stainless (20) 304 stainless
and (21) 17/4 Super duplex
Galvanic Corrosion Samples after 3 Months

Extreme engineering demand extreme materials

Geometric design can only go so far in addressing the requirements of increasingly demanding engineering applications. Copper Alloys is committed to working with leading engineering organisations to supply significantly improved copper-based alloys that can replace materials such as Inconel, beryllium copper and Toughmet.

Along with the ease of sourcing (everything supplied by Copper Alloys is made by Copper Alloys, England) this new material technology offers greater cost effectiveness and intrinsic property advantages such as being anti-biofoulling and extreme corrosion resistance.

The best of these alloys for marine applications can be found at copperalloys.net/elite-marine.

Or if you are interested in an alloy modified specifically to your application, speak directly to one of our engineers on +44 (0) 1782 816 888.
Elite Marine Alloys