Review of Five-Year Exposure Data for CuNi-Sheathed Steel Pilings
Authors: Melton D.G. 
Published: October 2008

Alloy C70600 (90/10 copper-nickel) has been used recently as sheathing on steel members of offshore oil and gas platforms for protection against corrosion and biofouling. To obtain long term technical data on this application, a number of piles with various constructions were exposed in a seawater channel. This paper describes the test program and gives results of removals after two and five years. These results demonstrate reduced anode consumption and biofouling accumulation as well as complete protection to the sheathed areas of steel piling.

BACKGROUND

The use of 90/10 copper-nickel (C70600) alloy sheathing on offshore structures in the tidal and splash zones for corrosion and biofouling protection offers many advantages. These areas are exposed to a very aggressive environment and are outside the reach of cathodic protection systems. Under present standard practice, a 0.5 inch (13 mm) corrosion allowance is considered typical in the construction of tubular steel supports on offshore structures designed for a 20 year life. The possibility of reducing this wastage allowance, in itself, is economically significant. This has been demonstrated by previous uses of sheathing materials such as Monel alloy 400, high build polymeric coatings and vulcanized neoprene sheathing, which do not offer any biofouling protection.

The 90/10 copper-nickel alloy has a long and successful history of corrosion resistance in marine environments with long term corrosion rates generally accepted to be less than 0.05 mils/yr (13µm/yr). It also offers the benefit of reduced loading of support members by reduction of biofouling.

To confirm these advantages, long term tests were initiated at the LaQue Center for Corrosion Technology, Inc.

EXPERIMENTAL PROCEDURE

The test matrix, as shown in Table 1, includes a) copper-nickel alloy C70600 sheathed piling attached by two techniques, b) bare steel and c) Monel alloy 400 sheathed controls. All except c) were exposed with and without cathodic protection. (Both of the Monel alloy piles were catholically protected.)

Materials

Steel Preparation

The steel material used in these test piles was selected to be representative of that used in the construction of offshore structures. A total of 26 pipes, measuring 9.1 meters in length by 17 cm in outside diameter and with a wall thickness of 0.7 cm. The steel was certified as meeting the chemical and physical requirements of ASTM A-36 specifications. The compositional limits for ASTM A-36 are shown in Table 2.

Sheet steel of this same composition was cut to form caps for both ends of each of the pipe lengths and welded to keep the inside dry for the duration of their exposure.

After welding the end caps, all of the piles were blasted to a commercial blast standard to provide a uniform base condition prior to application of the sheathing. This blasted condition, while not necessarily found in normal construction, did completely remove oils and inhibitor coatings applied by the manufacturer.

C70600 Sheathing Preparation

Concurrent with the steel piles preparation, the 4.6 mm thick C70600 copper-nickel alloy sheet was formed into the appropriate diameters for attachment as sheathing.