The reader does not disclose the composition of the gas except to say that carbon dioxide is present at a 6% maximum with no hydrogen sulfide. So lets talk about carbon dioxide and carbon steel in petroleum production operations. Carbon steel is a common material of construction for petroleum production operations and may be suitable for your application. Carbon dioxide is a weakly acidic gas that becomes corrosive when dissolved in water by forming carbonic acid. Velocity effects can be very important when dealing with carbon dioxide systems with corrosion rates approaching 500 to 1500 mils/yr. According to ASM Volume 13C - Corrosion: Environments and Industries, whether or not corrosion in these systems is controlled or uncontrolled depends on the factors governing the deposition and retention of a protective iron carbonate scale. Conditions that favor the formation of a protective iron carbonate scale include temperatures greater than 60 °C, increased pH above 4, and lack of turbulence. For more information I refer the reader to the chapter entitled "Corrosion in Petroleum Production Operations" in ASM Volume 13C. This chapter includes an equation for estimating corrosion rates for aqueous carbon dioxide/carbon steel systems based on the temperature and partial pressure of carbon dioxide as well as many literature references.