Technical Presentations at the July 2007 Meeting

2.1  ‘An Overview of Recent Copper-Nickel Sheathing Studies’, Carol Powell (Nickel Institute)

The presentation described 3 exposure studies: two in the USA and a third at Langstone Harbour, Portsmouth.

In 1984, 90-10 Cu-Ni (C70600) was installed for splash zone protection on the legs of Stage 1 of the Morecambe Gas Field platforms in the Irish Sea. This was the first major use of the alloy as a sheathing material and to examine the performance of Cu-Ni in close detail, two studies were initiated by the LaQue Center for Corrosion Technology in North Carolina, USA. 

The first commenced in November 1983 with a series of 26 test pilings exposed at Wrightsville Beach. Ten had Cu-Ni welded directly to steel, seven had the sheathing insulated from the steel with concrete and seven were bare steel. Some possessed cathodic protection and others did not. Two pilings were clad with 65%Ni-Cu alloy 400(N04400) and cathodically protected. Shortly afterwards, the programme was extended to include 14 pilings with a layer of butyl rubber acting as insulator between the steel and Cu-Ni sheathing. The pilings were systematically removed and examined over the next ten years. 

The second set of trials began in 1987 and involved six pilings, slightly offshore from the breaker line at Kure Beach fishing pier. Three were positioned on the north side of the pier and three on the south. Each side involved a plain steel control, Cu-Ni sheathing welded to the steel and Cu-Ni sheathing insulated from the steel. All had cathodic protection applied on the steel below the water line. 

Both sites suffered the ravages of extreme weather including hurricanes to which the area is prone, but the piles survived. After 20 and 16 years exposure respectively, the test sites were abandoned and final long-term investigations carried out.

The Wrightsville Beach Trails aimed to examine the viability of 90-10CuNi for corrosion and biofouling protection in the splash, spray and tidal zones. The test programme compared levels of fouling on electrically insulated vs non insulated Cu-Ni sheathing, anode consumptions with and without sheathing, corrosion rates of steel behind the sheathing and the sheathing itself, galvanic attack at the top and bottom steel/sheathing junctions when welded in position.

Physical measurements of the depth of attack for the two year removals and extensive sectioning and thickness measurements for the 5 and 10 year removals were performed. The results indicated complete protection of the steel behind the sheathing in the splash and spray zones. There was no measurable loss of thickness of the Cu-Ni sheathing itself in the case of the directly welded and insulated pilings. Post exposure measurements on submerged areas of steel on the cathodically protected sheathed piles showed that corrosion losses were comparable to those at the corresponding areas on the cathodically protected bare steel controls. The mass of biofouling accumulation on the sheathing that was electrically insulated from the steel was 1-4% of that present on the bare steel after 10 years exposure and low levels of fouling continued through 20 years. Pilings with the directly welded sheathing accumulated less than 40% of the biofouling mass compared to the bare steel controls.

The sheathing also reduced the anode consumption rates particularly for 5 and 10 year removals. The most significant reduction was for the insulated sheathing, presumably because the concrete reduced the area of metal requiring cathodic protection. The anode consumption associated with the directly welded sheathing was lower than for the bare steel and the corresponding anode output was lower. This is considered to be due to the favourable polarisation behaviour of the 90-10 Cu-Ni alloy. The potentials indicated satisfactory levels of cathodic protection throughout the test.

Corrosion at the sheathing/steel atmospheric junction eventually (15 years) perforated the steel pipe in both the alloy 400 sheathed and direct welded 90-10 Cu-Ni sheathed pilings indicating it is important to maintain coatings at the steel/sheathing interface at the top of the sheathing. Cathodic protection will eliminate any galvanic effects at the bottom interface although the data from these exposure results do not seem to indicate that this is at all pronounced.

The Kure Beach trials were less detailed and have produced more limited results. The biofouling mass measurements were taken after 2 hurricanes had damaged the pilings. However, they have served to reinforce the general trends found in the Wrightsville Beach trials; namely, insulated Cu-Ni sheathing provides the best resistance to biofouling and even directly welded sheathing with cathodic protection on the steel can achieve reduced fouling levels to bare steel pilings.

The third trial initiated by the Nickel Institute involved an evaluation of composite products involving 90-10 Cu-Ni. A 7 and 8 year raft exposure trial study in Langstone Harbour, UK, by Portsmouth University evaluated the corrosion and biofouling behaviour of adhesive backed foil and granules embedded in polychloroprene rubber. Removals of panels for destructive assessment after 1,4 or 5 and 7 or 8 years were made. In addition, a third product under development at the start of the study, involving expanded mesh with a neoprene backing, and a single sample of hot rolled plate were included in this study.

The results showed that all products showed restricted colonization of fouling species and remained largely free of macro fouling even though the Cu-Ni coverage varied between 30-100%. Where present it could be wiped away fairly readily. The foil product had thinned 5.5 μm per annum when averaged over a 7 year period.

[CDA Inc is acknowledged for the use of slides and information associated with the LaQue studies].