Austenitic Cast Irons
The main alloying addition to cast iron to improve corrosion resistance is nickel, often in conjunction with chromium and/or copper. If sufficient nickel (15-25%(1)
) is
Figure 2 - Crevice corrosion under a washer on 316 stainless steel immersed in sea water for 3 months
The conventional basic austenitic grade used in seawater is type 316 which contains 17%Cr, 12%Ni and 2%Mo. This has a limited resistance to localised corrosion in the presence of crevices. In order to counteract this tendency, higher alloying is necessary. Resistance to both pitting and crevice corrosion can be improved by increasing the chromium, molybdenum, tungsten and nitrogen contents. Development over the last 30 years has seen the emergence of a range of super-stainless steels of austenitic and duplex (austenitic/ ferritic) metal structures designed to have much higher resistance to localised corrosion.
A ranking can be made by calculating the Pitting Resistance Equivalent Number (PREN) according to the formula:
The higher the PREN value, the greater the resistance to pitting and crevice corrosion (Table 1). A PREN above 40 is normally considered necessary for resistance to aerated ambient temperature seawater. Alloys with a low PREN, e.g. alloys 316 and duplex alloy 2205, may require cathodic protection if not galvanically protected by other metals in the system.
Where metal and environmental conditions are such that crevice corrosion is a possibility, it is always good practice with stainless steels to design out crevices e.g. by avoiding threaded connections, sealing tight crevices with welds and making full penetration pipe welds. Alternatively local improvements may be achieved at areas such as flange faces by weld overlaying with more resistant high nickel alloys.
PREN = %Cr + 3.3 x (%Mo + 0.5%W) + 16%N(1)
PREN (%Cr + 3.3 x (%Mo + 0.5%W) + 16%N) 316L
Duplex 2205 Super-duplex
Super-austenitic Table 1 - PREN values for various stainless steels 36 | The Report • June 2018 • Issue 84
24 35
>41 43
added, then cast irons may become austenitic and their corrosion resistance in seawater is significantly increased over the grey iron alloys. They have general corrosion rates of about 0.02 mm/yr in low flow conditions but this rate increases as the water velocity increases.
The alloys are used for pump casings, often with stainless impellers. Being somewhat more anodic to the stainless gives a beneficial galvanic effect which lessens the pitting in the impeller when the pump is stationary. Austenitic cast iron valves can be used in copper alloy and ferrous seawater systems although the latter is more common.
Copper-based Alloys
Copper alloys have long been used in marine engineering where good resistance to seawater corrosion and biofouling (see later) is required. The alloys can be grouped into coppers, copper-zinc (brasses), copper-tin (bronzes), copper-aluminium (aluminium bronzes), copper-nickels and copper-beryllium. There are important differences between these groups but their general behaviour can be summarised as: low general corrosion rate in quiet seawater i.e. about 0.02 mm/year, with little tendency to pit. They also have useful resistance to flowing seawater, even at moderately high velocities, and are commonly used for piping, heat-exchangers, pumps, valves and propellers.
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