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may occur at stagnant conditions.

If, however, there is a possibility of H2S contamination, Cu-Ni is not

recommended. As a follow-up, exotic materials such as 22 Cr and 25 Cr duplex

and 6 Mo stainless steels, Ni base austenitic alloys and titanium alloys began

to be introduced. These new alloys offered excellent corrosion resistance and

are weldable.

Materials for Seawater Systems

* The recommended materials for seawater service below 60°F

(15°C) are 6 Mo and 25 Cr duplex stainless steels, and for service greater than

60°F (15°C) Ti or fibre-reinforced polymer (FRP). Some problems were

encountered with early application of FRP that were attributed to improper design,

handling and installation. For stainless steels, a minimum PRE value (PRE = % Cr

+ 3.3 x % Mo + 16 x % N) of 40 must be specified.

* The 6 Mo or 25 Cr duplex stainless steel piping can be used in

higher temperature service up to 95°F (35°C), if crevices are avoided or they

are overlaid by Inconel 625. Flanges and threaded connections must also be

manufactured from crevice corrosion-resistant alloys such as the superaustenitic

material (654 SMO or UNS S34565).

* A small amount of oxygen in the water causes pitting and crevice

corrosion in 22 Cr, while 25 Cr and 6 Mo will stand higher oxygen content.

Materials for Process Piping and Equipment

* Produced fluids are generally corrosive due to the presence of water,

CO 2, H 2S and chlorides. Assessment of the corrosivity of the produced

fluids is established using prediction models. The most widely used

models are for predicting CO 2 corrosion. The common practice is to use

carbon steel if the predicted corrosion rates can be accounted for by the

addition of less than 0.25" (6 mm) as a corrosion allowance. If a higher

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