Cathodic protection is an electrical way of stopping rust.

Rust is chemical and electrical. Metal dissolves in some solutions and gives off electricity. Metal can be ‘plated’ onto other metal electrically. All ‘batteries’ work on this principle and everyone knows that batteries drive loads of the things we use daily. Not many people know that our gas and oil comes to us through pipes that are inclined to rust, but are protected by ‘cathodic protection’. Some people know that metal boats are protected by cathodic protection, and have seen lumps of metal attached to hulls for this purpose. These lumps of metal dissolve in the water and give off electricity which prevents the hull from rusting. When you put two different metals in contact and submerge them in liquid (or wetness) one of the metals dissolves and discharges an electrical current into the liquid. The liquid (or damp material) is the ‘electrolyte’ and gets ‘charged up’ with electricity. It’s ‘electrical potential’ is increased. Electricity works by ‘pressure’ and anything with a higher ‘pressure’ gives off electricity to anything with a lower ‘pressure’. The electrolyte is then at a higher electrical ‘pressure’ than the metal that is not dissolving and so the electricity passes into it. The metal that is dissolving is the ‘anode’ from which the electrical current passes into the electrolyte and the other metal is the cathode into which the current passes because the electrical pressure must be balanced out. (everything tries to equalise). The dissolving metal is sacrificed to prevent the subject metal from corrosion, and this method is known as ‘sacrificial cathodic protection’. There are limits to which sacrificial cathodic protection can be used but the same principle can be used by causing a manufactured electrical pressure which is ‘impressed’ into the electrolyte. The electricity is then ‘drained’ out of the subject metal……. boat hull or pipeline…. and this interferes with the natural tendency of the metal to dissolve….or rust!

Impressed current cathodic protection

Electricity is generated by a sort of pumping action which causes it to flow backwards and forwards in ‘waves’, but this is no use for our purposes so we have to get it going in one direction through a circuit known as a ‘rectifier’. At the same time we can control the amount of current by transforming it, so the apparatus is known as a transformer-rectifier. A transformer-rectifier can be regarded as an electrical pump which is sucking the electricity out of the pipeline (etc) and pumping it into the ground (or sea … or swamp… or wherever else you want to pump it). The effect of this is amazing. It stops rust! And it’s cheap!

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