When pipe flanges are made of a dissimilar metal to the pipes they are connecting (i.e. they are higher or lower in the galvanic series) corrosion may occur. This is especially true of flanges that are in contact with electrolytic solutions, for example sea water or acidic chemicals. Galvanic corrosion is extremely hazardous; however, there are a number of ways you can reduce this risk.

You can order flanges made from alloys that are corrosion-resistant, i.e. high in the galvanic series. However, this in itself may actually increase the risk of corrosion unless the pipes are replaced with equally corrosion-resistant materials.

This comes as a surprise for people who don’t understand how galvanic corrosion works. Simply put, the further apart two metals are in the galvanic series, the faster corrosion will occur. This is because one (the lower resistance) acts as an anode, and the more highly resistant metal acts as the cathode. In the presence of an electrolyte such as seawater, ions flow from the anode to the cathode, resulting in corrosion. If the two metals are very near each other in the galvanic table, the flow will be slow. However, if they are far apart it will increase, leading to rapid corrosion.

In an ideal world, the entire pipeline would be composed of a super-alloy such as duplex. However, this isn’t usually feasible, so you must use highly resistant pipes and pipe fittings wherever you can, and in areas where they are dissimilar, work on reducing the likelihood of the two metals coming into contact with the electrolyte.