Long-Term Effects of Cathodic Protection on Prestressed Concrete Structures: Hydrogen Embrittlement of Prestressing Steel Available to Purchase

The issue of safe cathodic protection (CP) limits for prestressing steel in concrete was addressed in regard to concerns over hydrogen embrittlement (HE). The local environment at the steel-concrete interface was found to vary as a function of vertical position within a laboratory-scale marine bridge piling. Embedded pH electrodes indicated the pH within a steel crevice embedded within a concrete piling decreased from 11.5 to 6.5 in the atmospheric zone 30.5 cm (12 in.) above the water line. Hydrogen permeation was detected using embedded sensors at applied potentials (E_app) more positive than the reversible potential for hydrogen production calculated for alkaline pore solutions (pH > 12.6). A safe limit based on the reversible electrode potential (REP) would require knowledge of pH and E_app as a function of vertical position, as well as an understanding of their influence on HE. Constant extension rate tensile testing (CERT) was performed on notched prestressing steel tensile specimens at various cathodic polarization levels in: (1) saturated calcium hydroxide (Ca[OH]₂), (2) ASTM artificial ocean water, (3) under a mortar cover in artificial ocean water, and (4) in pH 4 and pH 6 Ca²⁺-containing environments simulating ferrous ion hydrolysis on corroding prestressing steel. CERT results were combined with permeation measurements to determine the relationship between steel mobile hydrogen concentration (C_H) and fracture initiation stress (σ_i) in each environment over a series of cathodic potentials. A relationship of the form σ₁ = σ₀ - αlog(C_H/C₀), where σ₀ is the fracture initiation stress in the absence of mobile hydrogen and C₀ is the mobile hydrogen concentration at or below which no hydrogen embrittlement is observed, was found independent of environment and pH. The previously reported fixed cracking threshold of -900 mV_SCE in Ca(OH)₂ solutions pH adjusted with hydrochloric acid (HCl), irrespective of pH (from 7 to 12.5), was explained. Decreasing pH in these environments produced a roughly constant C_H at E_app = -900 mV_SCE due to the opposing influences on hydrogen uptake of increasing hydrogen overpotential but decreasing availability of a Ca(OH)₂ recombination poison.