Galvanic Corrosion 4

Copper and its alloys are widely used as piping and heat exchanger tubes to handle waters with a wide range of chloride contents, from drinking water to seawater.  Sometimes there is a leak due to some form of localised corrosion and a small piece of new pipe, or a few new tubes are installed to enable use to continue.

Sometimes the new piece of pipe or tube fails very rapidly, much to everyone's surprise.  The reason for this is that, in some waters, there can be a significant potential difference (30 to 70mV) between new and filmed metal.  The potential  difference depends on the composition and temperature of the water.  If the area of the old (filmed) metal is very much greater than that of the new metal, accelerated corrosion will occur until the new metal forms a protective film.  Sometimes the corrosion is so rapid that perforation occurs before a protective film forms.

This is usually seen with copper and its alloys because they efficient cathodes, even when filmed, and can stimulate anodic dissolution readily.  This does not occur with stainless steels and other metals that form passive films.

Posted on: 7th April 2017

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Image (top left) by Agnieszka