  Next-generation gravitational wave detectors (GWDs) like the Cosmic Explorer
and Einstein Telescope require extensive vacuum tubing, necessitating
cost-effective materials. This study explores the viability of mild steel as an
alternative to austenitic stainless steel for UHV beampipes, focusing on
outgassing rates and surface chemistry after low-temperature bakeouts. Mild
steels exhibit significantly lower hydrogen outgassing rates, below 10$^{-14}$
mbar l s$^{-1}$ cm$^{-2}$ after bakeouts at 80{\deg}C for 48 hours. While water
vapor is the primary residual gas after such low-temperature bakeouts, repeated
treatments reduce its outgassing rate and modify surface conditions so that
such benefit is preserved after at least six months of exposure to laboratory
air. These findings position mild steel as an economical and efficient material
for future GWD beampipes.
