Re: Why are bags of magnesium joined to underground iron pipe-lines

What you have described is a form of cathodic protection used on virtually all immersed or buried carbon steel structures (water heater tanks, underground pipelines, underground storage tanks, submersible pumps, bridge structures, ship hulls, etc.). Sacrificial anodes of more active metals such as aluminum, magnesium, or zinc are installed on metal structures to control corrosion. Steel pipelines are cathodically protected by connection to a sacrificial magnesium anode buried in the same soil. The moist soil acts as an electrolyte.

The introduction of the sacrificial magnesium anode produces a galvanic cell in which the active metal (aluminum, magnesium or zinc) works as an anode and provides a flux of electrons to the structure, which imparts a cathodic (negative) charge to the structure to suppress corrosion. The cathode is protected because the anode will be corroded more quickly than the relatively cathodic metal structure. The anode gets progressively consumed, and is hence called a sacrificial anode. The cathodic protection reaction will not work in air, because air is not an effective electrolyte.

Sometimes, a low voltage DC current is applied to the connection between the anode and the cathode. This impressed current requires an investment in a reliable outside power supply, and is usually reserved for applications where sacrificial anodes will not work well.

Cathodic protection was first developed by Sir Humphrey Davy in 1824 as a method of controlling corrosion on British naval ships.