Hydrogen Absorption in Iron Exposed to Simulated Concrete Pore Solutions Available to Purchase

Safe cathodic protection (CP) limits are required for prestressed steel in concrete to avoid the risk of hydrogen embrittlement (HE). A CP protection potential more positive than the reversible hydrogen potential is one approach for avoidance of HE. However, it has been demonstrated in the literature that the steel tendon can drop locally well negative of the reversible hydrogen potential of even an alkaline pore solution when subjected to impressed current CP. As a part of a broader effort using steel tendons, this preliminary study addressed some effects of concrete pore solution chemistry and metal surface condition on hydrogen absorption in iron. To accomplish this, the Devanathan-Stachurski permeation technique was used to investigate hydrogen absorption in 99.5% iron foils exposed to sodium hydroxide (NaOH), saturated calcium hydroxide (Ca[OH]₂), and saturated Ca(OH)₂ + 0.6 M sodium chloride (NaCl), all at pH 12.5. The foils used in this investigation were tested after various surface preparations: (a) polished, (b) with a thermal oxide formed by a heat treatment designed to simulate the stress relief oxide, and (c) with corrosion films to simulate an in-service tendon that was exposed to a marine environment for some time prior to CP. Hydrogen uptake in iron was most efficient for foils covered with Portland cement-based mortar, at least 2.5 times greater than that in NaOH of the same pH and hydrogen production rate. Absorption in saturated Ca(OH)₂ was somewhat less than that from the mortar cover. While chloride (Cl^–) had no direct effect on the hydrogen absorption rate, the corrosion product and the thermal oxide were found to decrease hydrogen absorption compared to polished iron. The thermal oxide acted as a complete barrier at all charging current densities investigated. The effectiveness of this thermal oxide barrier to hydrogen, however, was compromised by corrosion resulting from alternate immersion exposure to a Cl^– environment.