Natural gas transmission pipelines are a known to leak and thus release methane which is a greenhouse gas (GHG). There are several mechanisms for these emissions, including venting of lines, mechanical damage, and corrosion failure. With the projected pipeline life being described as “virtually forever”, slow corrosive attack can be an issue. The corrosive failure can be by either the exterior or interior attack. While there are practices in place to protect the exterior of the pipelines, including organic coatings and impressed cathodic protection, these techniques are not practical for the pipe’s interior. While dry natural gas is considered non-corrosive, the maintenance/inspect process of pigging of pipelines typicality results in black corrosion deposits being removed. These corrosion deposits are most likely a result of inadvertent water reacting with residual carbon dioxide (CO₂) forming carbonic acid and attacking the pipeline, particularly wherever this water pools.

This research examined the use of sacrificial coatings on the interior of natural gas pipelines to protect the pipeline from corrosion. These tests simulated the case where trapped water is in the bottom of the pipeline. A typical natural gas pipeline can have 100 bar of pressure. The gas stream can have 3 to 4 vol % CO₂. This can result in a CO₂ partial pressure of 3 to 4 bars. NACE SP0106 says that CO₂ at a partial pressure of greater than 2.1 bar in the presence of water is usually corrosive. Compression of the gas, which occurs every 40 to 100 miles, heats the gas which will accelerate the corrosion rate. The work examined the corrosion of an API 5L grade X65 steel in a test, 3 ½ wt. pct. NaCl solution with a 3.1 bar pressure of CO₂ at 40 °C. Results of the corrosion rate of the bare steel were compared with that of several sacrificial metals, including Al and Zn. The corrosion rate of the steel coupled to sacrificial metals was measure for both parts of the couple.