Galvanic Corrosion 2

Seawater is a very corrosive liquid and there is an increasing trend to use high alloy stainless steels, nickel alloys or titanium, because of their high resistance to corrosion and the consequent reduction in maintenance costs, particularly offshore. The most common high alloy stainless steels are superduplex (Fe/25Cr/7Ni/3.5Mo/0.25N/0.7Cu/0.7W) and 6%Mo austenitic (Fe/20Cr/18-25Ni/6Mo/0.2N/0.7Cu). Commonly used nickel alloys include alloys 625 (Ni/20Cr/9Mo/3.5Nb) and alloy C-276 (Ni/15Cr/15Mo/3W). Titanium is usually used as commercially pure grades 1 or 2 and also grade 5 (Ti/6Al/4V), where higher strength is required.

All of these alloys have a high resistance to pitting and crevice corrosion in seawater. Some complex components require parts to be made of different alloys and the question then arises as to the risk of galvanic corrosion between these alloys. Research in a number of laboratories has shown that all of these alloys can be safely connected together in seawater, provided none of the alloys is working outside its limits of use.

In fact the work extended to cover all nickel-chromium alloys, where the molybdenum content exceeded 7wt%, and also all titanium alloys. Superduplex stainless steel piping has been safely connected to titanium plate heat exchangers on offshore platforms for many years. Piping in 6%Mo alloys has been safely connected to flanges overlaid with a range of nickel alloys, including alloys C-276 and alloy 686 (Ni/21Cr/15Mo/3.5W).

Posted on: 7th Oct 2016

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Image (top left) by Agnieszka