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concentrate by evaporation on a hot metal surface; this can include conditions found in hot climates. Chloride SCC in stainless steel is essentially transgranular, frequently propagating from pits but is unlikely to occur at temperatures below about 50-60°C (Figure 6).


Chloride SCC is primarily related to nickel content, type 300 alloys being the most susceptible. Higher and lower nickel contents have significant improvements. Super- austenitic alloys with 18-25% nickel therefore are significantly more resistant than alloy 316 with 10-12%. Duplex stainless steels, with mixed ferritic and austenitic structures and lower nickel, have a higher resistance to stress corrosion cracking. Ferritic grades with little or no nickel have very high resistance to stress corrosion cracking.


Copper-zinc alloys are susceptible to ammonia stress cracking which may occur in polluted seawaters, whereas copper-tin, copper-nickel and copper-aluminium alloys have high resistance. There have been instances of stress corrosion cracking on austenitic cast irons in warm seawater (>35°C); the solution in this case has been to stress relieve at 650°C. Copper- nickel, high nickel and titanium alloys are essentially immune to chloride SCC in seawater.


Figure 6 - Stress corrosion cracking of 316 stainless steel


Galvanic Corrosion(1,5) It is often necessary to use a number of different materials to construct a seawater system and the galvanic compatibility of these materials must be considered. Galvanic corrosion involves corrosion of the least noble (anodic) alloy within a mixed system, in electrical contact and exposed to an electrolyte - the seawater. The galvanic series (Figure 7) can be used to predict which alloy may corrode when connected to other alloys in seawater. The further apart the alloy potentials are in the series, the more likely the less noble one is to corrode. However, corrosion rate is often more dependent on the galvanic current which can be influenced by relative surface areas. From the galvanic series it will be seen that copper base alloys have similar potentials whereas steel is appreciably anodic. Passive stainless steels are towards the more noble (cathodic) end of the galvanic series and are slightly


more noble than copper alloys and rather more noble than aluminium and steel. Carbon steel and aluminium thus provide cathodic protection to type 316 but copper alloys will not provide significant protection. 316 stainless steel is less noble than the high nickel alloys, titanium and graphite.


Graphite containing gaskets, packing and lubricants have all been responsible for serious galvanic corrosion of stainless steel and other alloys in seawater. Should localised corrosion initiate in a stainless steel, the alloy becomes more active and if the contact metal is then more noble, the local corrosion rate may increase further.


Galvanic corrosion of less resistant metals coupled to titanium may be harmful if conditions lead to the uptake of hydrogen and formation hydrides and subsequent embrittlement. Limiting cathodic potentials in CP systems may be necessary(1)


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The Report • June 2018 • Issue 84 | 39


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