Other Metals and Alloys Zinc
The principal use for zinc is to sacrificially corrode whilst cathodically protecting ship hulls. Galvanising steel offers little advantage immersed in seawater because the zinc products are soluble(1)
. Titanium
Titanium and many titanium alloys have very low corrosion rates in static and flowing seawater and are immune to crevice corrosion below at least 70°C. Seawater velocities in excess of 36 m/s can be handled (1)
. Aluminium (1)
Alloys for seawater service contain either magnesium on its own or as the main alloying element (5000 series), or magnesium and silicon (6000 series). The latter group are slightly less corrosion resistant, but they can be heat treated to improve mechanical properties. The corrosion resistance of aluminium and its alloys is due to their ability to form a thin but protective oxide layer in the presence of oxygen and/or water. General corrosion rates are overall less than 0.005 mm/yr in seawater but like stainless steels in an aggressive environment, localised corrosion may occur often being associated with the chloride ion.
Special Corrosion Effects
Apart from the effects of flow and general corrosion, there are other corrosion mechanisms that can occur and need consideration. Two more common types are selective phase corrosion and stress corrosion cracking.
Selective Phase Corrosion
Some two phase copper-zinc alloys are prone to selective corrosion in seawater but this is well understood and usually avoidable.
Figure 4 - Dezincification in 60-40 brass valve stem
Such corrosion of the high zinc component in the alloy leaves a network of copper. The rate of attack can be severe, for example 20 mm/yr in 60-40 brass, and as the remaining copper deposit is porous, leakage may occur. This type of corrosion is called dezincification(2)
,
Figure 4, and a similar effect is found in aluminium bronze (dealuminification)(4)
.
Naval brass, a duplex alloy, containing 1% tin, was specifically developed to improve resistance to dezincification and is often used for tube plates. The addition of about 5% each of nickel and iron renders the duplex 10% aluminium bronze alloy more resistant to dealuminification, although the heat affected zone of welds may be susceptible and post weld heat treatment is necessary.
Single phase brass alloys such as aluminium brass, can be rendered immune to dezincification by the addition of a small amount of arsenic (0.02%) and this alloy has been widely used in heat exchangers.
The high density of inclusions in 303 and 303Se, free machining grades of stainless steel, create numerous galvanic cells in the material(1) and these grades (Figure 5) should not be used in seawater. They can fail rapidly even in contact with aluminium or steel (see galvanic section).
Figure 5 - Corrosion of type 303 stainless steel along manganese sulphide inclusions (Courtesy Rolled Alloys)
For welded components in 316 stainless steel, only low carbon (<0.03%) or stabilised grades should be used to avoid intergranular corrosion at chromium depleted zones in the heat affected zones caused by the precipitation of chromium carbides.
Graphitisation has already been discussed for cast irons. The austenitic cast irons have a much higher resistance to this type of attack(1)
. Stress Corrosion Cracking (SCC)(1)
Most alloys, when stressed and subjected to corrosion in certain specific environments, can fail through SCC. For this to happen, three conditions have to be fulfilled:
• a susceptible alloy • a tensile stress of sufficient magnitude
• a specific corrodent.
The elimination of one of these factors will prevent cracking, for example by changing the alloy or removing the stress by a stress relief anneal.
Stainless steel is resistant to chloride SCC in seawater at normal temperatures and cracking is rare. If it occurs, it is usually in areas of high applied or residual stress such as expansion joints, bolting or circumferential welds and/or areas where chlorides can
38 | The Report • June 2018 • Issue 84
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