The arrangement for protecting a buried pipeline is illustrated in the figure. The buried pipe receives current from a DC power source via an auxiliary inert electrode buried in the ground. The pipe becomes the cathode and the auxiliary electrode the anode. The auxiliary electrode sometimes consists of scrap iron. In this case the iron will dissolve from the anode by reaction
Fe > Fe2+ + 2e and the electrode is described as a consumable anode. If the anode is a noble metal or an electrochemically inert material, the surrounding environment will be oxidized and in water reaction
2H2O > O2 + 4H+ + 4e will occur. In saline solutions, however, chlorine may be produced at the anode. This may present problems in confined spaces.
A range of materials have been used as non-consumable anodes for impressed-current systems. The sort of properties required by these anodes are
- good electrical conduction,
- low rate of corrosion,
- good mechanical properties, able to stand the stresses which they may be subjected to during installation and in service,
- readily fabricated into a variety of shapes,
- low cost,
- able to withstand high current densities at their surfaces without forming resistive barrier oxide layers, etc.
The following materials have been used as anodes: magnetite, carbonaceous materials (graphite), high silicon iron (14-18% Si), lead/lead oxide, lead alloys, platinised materials (such as tantalum, niobium, titanium). Platinum, with its high resistance to corrosion, would be an ideal anode material but has the major disadvantage of very high cost.
In practice, voltages up to 100 V and high current densities are possible on impressed-current anodes (see Table). Thus, large areas of a structure can be protected from a single anode and, because of the high driving voltage, the anode can be placed remote from the structure.
|Anode material||Max volts*||Typical anode current density
|Platinum / niobium||100||250 – 1500|
|Lead / silver / antimony||100||250 – 1000|
|High silicon iron||100||10 – 100|