A designer has to make a decision on the choice of an appropriate material for seawater service prior to the specification of the system since a broad range of conditions will usually be imposed on the piping material, the condition of the material, system design, fabrication procedure, various seawater temperatures and flow regimes, biological activity, and presence of oxidizing compounds determine the impact of seawater on material performance. Further factors that are relevant in choosing a material for a seawater piping system are physical and mechanical properties, availability, material costs, ease of fabrication and maintenance, anticipated design life, and previous design experience.
Over several decades, many thousands of tons of copper-nickel alloys UNS C71500 and UNS C70600 have been installed in different marine engineering structures for the shipbuilding, offshore, power and desalination industries. Various standards have adopted these alloys which have been applied for seawater piping and heat exchangers. UNS C71500 is predominantly used for military submarine service due to its higher strength and maximum allowable flow rate, as well as low magnetic permeability.
However, the wider commercial application of this alloy is limited to a certain extent because of its higher material cost. The work-horse, therefore, is the UNS C70600 (CuNi 90/10, cupronickel). This alloy reveals a well-balanced combination of characteristics allowing its widespread and economical use.
To ensure the further reliable application of the material, there is a need for a detailed discussion on its properties. In particular, we shall pay attention to the quality of CuNi 90/10 products, the performance in waters containing hydrogen sulfide, and the prevention of erosion as well as galvanic corrosion.
Comparison of chemical composition between various specifications for cupronickel 90/10 used as tubing material:
|Ref. No.||2.0872/CW352H||UNS C70600||UNS 7060X|
|Single values represent the maximum content.|
The merits of the UNS C70600 as an appropriate alloy for seawater pipework can be attributed to various aspects. First of all, it is a simple alloying system with a single-phase face-centered cubic structure providing excellent hot and cold workability. The absence of phase transformations during welding contributes to its easy weldability with no need for extensive post-weld treatments. However, the chemical composition and the manufacturing of cupronickel products must comply with international standards.
Low uniform corrosion rates of UNS C70600 allow the specification of thinner walled piping and, therefore, provide weight savings. Cathodic protection is not usual for UNS C70600 piping. The alloy is resistant to biofouling and does not reveal sensitive corrosion potential variations under different seawater conditions. Such a combination of features leads to the improved resistance to localized corrosion and the elimination of extensive monitoring procedures associated with chlorination and higher seawater temperatures which other alloy systems might require. The alloy has high resistance to crevice corrosion and is resistant to stress corrosion cracking under marine conditions.
For lower seawater temperatures, protective surface films may take up to 3 months to fully mature. For this reason, hydro testing and commissioning recommendations have been given. In addition, precautionary measures are to avoid premature failures in the presence of hydrogen sulfide. We should follow the given practical recommendations to avoid corrosion problems. Recommended design considerations can eliminate the susceptibility to erosion corrosion and galvanic corrosion.