Figure 3 - Erosion corrosion of 90-10 copper-nickel when exposed to excessive flow rates
On copper alloys, protective surface films form naturally by interaction with the seawater and are more complex than many other alloy systems. Above a certain flow called the breakaway velocity, the films can become damaged leading to a type of corrosion called erosion corrosion (Figure 3). This varies with alloy and there are recognised maximum design flow ranges which are important to operate within. In general, the nickel aluminium bronzes and copper-nickels have the highest velocity tolerances and are somewhat better than the gun metals and tin bronzes which, in turn, are better than the brasses.
The wrought copper-nickel 90-10 and 70-30 alloys are commonly used for seawater condensers, heat exchangers and pipework in naval vessels and merchant shipping. Recommended maximum velocities vary for seawater pipework and depend on diameter. Typically they are around 3.5 m/s for the 90-10 alloy
and 4 m/s for the 70-30 alloy at 100 mm diameter and above (Table 2).
Heat exchangers generally operate to around 2 m/s flow velocity which is normally readily acceptable for both these alloys. High turbulence areas such as tight radius bends, partially throttled valves and localised obstructions need to be avoided. Both alloys have a very high resistance to chloride crevice corrosion, even at high temperatures.
Gun metals (copper-tin-zinc) and aluminium bronzes also have good resistance to erosion corrosion but these alloys are not normally used for heat exchanger tubing. Both groups are however used for components such as pumps and valve bodies. The nickel aluminium bronze alloy is used for propellers as it has excellent resistance to cavitation compared to many other copper- based alloys(4)
.
Nickel Alloys: Nickel-copper Alloys
The main nickel-copper alloys are Alloy 400 (about 65% Ni, 30% Cu) and Alloy K500 which is similar but contains small additions of aluminium and titanium. Alloy K500 can be heat treated to double the strength of Alloy 400 while at the same time retaining the same corrosion resistance. These alloys tend to behave in a similar manner to type 316 stainless steel in terms of localised corrosion although pits, when they do form, tend to be flat and broad unlike the undercut pitting typical of stainless steels. Cathodic protection offered by contact with less noble metals enhances the performance of these alloys. At velocities greater than 1 m/s, the surface remains passive and good resistance continues up to high flow velocities. The alloys are used for fasteners, shafting and pump and valve components(1)
.
Nickel Alloys: Nickel-chromium- molybdenum Alloys
Piping >DN80 (NPS 3) Heat exchanger tube 90/10 CuNi 70/30 CuNi 3.0-3.5 3.5-4.5 2.0-3.0 2.5-3.0 Table 2 - Typical safe velocities for copper-nickel tube and pipe(3)
Alloys 625, C 276, 22, 686 and 59 have a very high resistance to chloride pitting and crevice corrosion in seawater and this together with high resistance to flowing seawater to over 40 m/s makes them suitable for many applications. They form a very resilient oxide film in much the same way as stainless steels and have a high PREN value. However, they are relatively expensive and are normally restricted to critical applications(1)
. The Report • June 2018 • Issue 84 | 37
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