In order to establish a design methodology for such limited potential systems, a trial has recently been carried out using different types of SBR in a number of configurations.   This presentation will give details of the trial, and the results obtained.  Sufficient information is now available to allow the design of limited potential cathodic protection systems operating within a closely controlled potential range.

Both the extent and intensity of corrosion processes initiated by seawater are often underestimated.  One UK operator reported that 1/3 of all failures reported on platforms were caused by corrosion. Thus tough demands are made on components exposed to seawater, in particular to piping systems. However besides good resistance to corrosion additional properties are required.

The most important ones are:

- good shock resistance and good ductility, also at low temperatures

- ease of welding and cold bending

-  suitability of safe alteration during installation phase

-  availability ex stock

-  resistance to marine growth

-  cost effectiveness

These requirements can be met by CuNi 90/10.

The alloy containing 10 % Ni and 1,5 % Fe. has a homogenious, single phase structure throughout the temperature below solidus. To sustain the desired properties it is mandatory to keep the Fe in solid solution and to avoid Fe-rich precipitates. Compared to other pipe materials CuNi 90/10 offers a number of significant advantages. Due to its high copper content of nearly 90 %, settlement and growth of marine organism are discouraged, the bores of seawater pipes remain clean, there is no pressure increase and nozzle blockage is no problem with CuNi 90/10. Other materials need a treatment with aggressive chlorine and are therefore often the subject of corrosion failures. Thus CuNi 90/10 can be categorized as environmentally-friendly.

Due to improvenments in the composition and the production process it was necessary to determine the actual limits of CuNi 90/10 concerning the erosion and corrosion resistance. Therefore 3 different longtime tests were carried out in natural North Sea water.

In the first test the resistance to stagnant seawater was investigated. After 8 months of exposure to untreated seawater no corrosion at the pipe surface or at the circumferrential welds could be seen.

For the second test a pipe system consisting of 5, 6 and 7 " pipes with the corresponding fittings and flanges was installed in the powerstation at Wilhelmshaven and operated for nearly 2 years. The total length of the pipe system was 22 m. Three different material versions were investigated: Version 1 was the standard CuNi 90/10. Version 2 had the same composition, however was specially heat-treated, and Version 3 was a modified composition containing 15 % Ni and 0,3 % Cr.

 The 5" pipes exposed to a velocity up to 8 m/s exhibited a decrease in wall-thickness of 0,2 mm over the whole testing period. Version 3 was obvious more resistant than version 1 and 2. The 6" and 7" pipes exposed to velocities up to 5 m/s showed no visible corrosion.

A third test program was carried out with 66 sheet specimen. These specimen had the same composition and heat treatment as used for pipe investigation. They were attached to steel racks and exposed to German North Sea water to 3 different conditions: spray water (splash zone), alternate immersion (tidal zone) and total immersion zone.

After 2.5 years of exposure the specimen were taken out and evaluated. Besides minor changes in colour and a thin accumulation of sediments, which could be easily removed, the specimen revealed neither any corrosive attack nor growth of algea. The metallographic investigation showed for the most severe condition corrosion rates of below 0.02 mm which can be neglected.  Specimen from FRP and galvanized steel, exposed to same conditions were fully covered with marine organism.

 Concerning recyclability it is known that in the next 30 years on the North European Shelf 600 odd platforms have to be de-commissioned. A part of them will be reused after modification. However most piping materials are difficult to reuse. The reasons are excessive corrosion rates and the difficulty of any alteration, which leads to serious quality problems. CuNi 90/10 can be reused again or can be remelted and processed into new pipes.

Typically corrosion research is conducted in quiescent solutions.  Thus, hydrodynamic factors are frequently ignored in the analysis of corrosion kinetics, or at best, a hydrodynamic regime is employed that does not effectively simulate that which occurs in the industrial environment of interest.  Few systematic studies have been reported on the effect of fluid velocity on the corrosion of metals.  However, a considerable body of empirical information has been developed on the effect of seawater velocity on corrosion rate of various metals.  It is generally considered that austenitic stainless steels, such as 304 and 316, undergo less pitting attack at increased velocities and a critical velocity of greater than approximately 1.5 ms^-1 is recommended to avoid stable pit growth.

The purpose of this present work is to develop a quantitative framework for relating metastable pitting and stable pit propagation of austenitic stainless steels to fluid pipe flow.  Hydrodynamic and electrochemical noise measurements were made in a circular flow test section of 28 mm inside diameter for a range of flow regimes from laminar to turbulent.  Flow velocities through the test section were controlled at 0.04, 0.07 0.11 0.36 1.8 and 2.7 ms^-1, equivalent to Reynolds numbers (Re) of 1000, 2000, 3000, 10000, 50000 and 75000, respectively.  Analysis procedures employed standard hydrodynamic parameters to characterise and evaluate the complex interrelationship between the mass transport of oxygen and momentum transfer through turbulence to the metal/solution interface.  Metastable pitting was evident under all flow regimes.  Fluid flow, whether laminar or turbulent, appeared to have little overall effect on the nucleation rates of metastable pitting events.  Conversely, stable pit growth was most evident at Reynolds numbers immediately before the transition to turbulent flow and below the critical velocity, at Re numbers of approximately 2000 and 50000 respectively.  For a superduplex stainless steel little or no metastable pit activity was observed when subjected to similar hydrodynamic flow regimes.

8.4 ‘Corrosion Fatigue - From Pits to Cracks’, Bob Akid (Sheffield Hallam University)

Fatigue cracking of engineering materials is almost always accentuated when  cyclic stressing occurs in the presence of an ‘aggressive’ environment. The difference between air fatigue and  corrosion fatigue is the additional ‘chemical’ driving force resulting from electrochemical reactions on the surface and at the crack tip.

Localised corrosion in the form of pitting is frequently associated with corrosion fatigue failures and is often a pre-cursor for crack development. The presentation will focus on the early stages of corrosion fatigue damage, notably pitting and the pit-crack transition regime. Initiation and growth from corrosion pits has been analysed for several steels ranging in yield strength from around 300 MPa to over 1200 MPa.  Fatigue damage maps, given in terms of pitting, pit-crack transition and cracking have been constructed from tests conducted under different loading modes and different test frequencies. The most outstanding observation made from these maps is that there is a significant contribution arising from corrosion (pitting) in these early stages of life, namely that up to 60% of life is taken up in the development of a crack from a pit. Furthermore this consumption of life (up to 60%) occurs while the size of the defect is little more than 10% of the final failure defect size.

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