The Rectifier Transformer is simply an AC Transformer + Rectifiers to convert AC to DC. This is used to feed DC to the corrodable buried or water immersed structures.
The idea is to make the buried or immersed metallic structure a cathode. This will prevent corrosion of these metals.
The Actual DC current is quite huge , 100s of Amps in some cases like deep sea drill rigs, Industrial towers on the ground etc,.