Final answer:
3) Galvanic cell, To protect buried steel gas pipes from corrosion, one may use cathodic protection through an impressed current system or by attaching a sacrificial anode, usually made of zinc or magnesium, thereby preventing the steel from corroding.
Step-by-step explanation:
When protecting buried steel gas pipes against corrosion, methods of cathodic protection may be used. These methods include using an impressed current system, where an external power source provides the current required for protection, or by attaching a sacrificial anode made of a more anodic (reactive) material than the steel pipe.
This sacrificial anode typically made of zinc or magnesium, corrodes in preference to the steel, effectively making the steel pipe the cathode and therefore protecting it from corrosion. Additionally, a galvanic cell is essentially created between the sacrificial anode and the steel pipe, with the soil acting as the electrolyte. Therefore, the correct answers are 1) Impressed current and 2) Sacrificial anode.
For protecting buried steel gas pipes against corrosion, **1) Impressed current** and **2) Sacrificial anode** methods are commonly used for cathodic protection. In impressed current systems, an external power source supplies a direct current to the pipeline, creating a protective environment.
Sacrificial anodes are made from more reactive metals like zinc or magnesium, which corrode sacrificially to protect the steel. The corrosion occurs on the sacrificial anode rather than the steel pipe. **3) Galvanic cell** is essentially the principle behind sacrificial anodes, making option 3 relevant. Thus, both impressed current and sacrificial anode methods contribute to effective cathodic protection for buried steel gas pipes.