What materials are considered suitable for underground natural gas pipelines?

The correct answer, which identifies suitable materials for underground natural gas pipelines, highlights the use of polyethylene (PE) and coated steel pipe. Polyethylene is widely accepted for natural gas distribution due to its high resistance to corrosion, lightweight properties, and flexibility, which make it exceptionally suitable for underground installations. It can effectively withstand the pressures associated with gas transport and does not corrode, which can lead to leaks over time.

Coated steel pipe is another appropriate material because it offers the strength and durability needed for transporting natural gas under the ground. The external coating protects the steel from oxidation and other environmental factors that could compromise its integrity.

The other materials listed in the options, such as PVC and copper, are not suitable for underground natural gas pipelines. PVC is generally not approved for gas applications due to its inability to withstand high temperatures and pressures, as well as concerns over its durability. Copper is also not typically used for natural gas due to similar issues related to pressure and the potential for joint deterioration over time.

Aluminum and uncoated steel do not provide the necessary protective features for underground pipeline applications. Uncoated steel, in particular, is highly susceptible to corrosion, leading to significant safety concerns. Cast iron can be used in specific instances but is also