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

Abstract

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 Ca 2+ -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 ( i ) in each environment over a series of cathodic potentials. A relation-ship of the form 1 = 0 -␣log(C H /C 0 ), where 0 is the fracture initiation stress in the absence of mobile hydrogen and C 0 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.

Evaluation of Delayed Fracture Property of High Strength Bolt Steels

“…Delayed fracture is not a problem only of high strength bolts, 1) but also of PC steel bars, 2) steel sheets, 3,4) reinforcing rods, [5][6][7] Al alloys 8,9) and stainless steels [10][11][12] for hydrogen pressure vessels and so forth.…”

Influence of Loading Rate on the Hydrogen-Assisted Micro-Damage in Bluntly Notched Samples of Pearlitic Steel

“…In the bluntly-notched geometry analyzed in this paper (see Figure 1) the distribution of hydrostatic stress σ reaches its maximum value at the center of the cross sectional area of the sample, so that hydrogen will be "pumped" to such a location according to Equation (1). The hydrogen concentration C profile in the transverse section of the notched specimen at the final instant of the test t = t HE is represented in Figure 9 in dimensionless terms (C r as the relative hydrogen concentration, i.e., the ratio C/C 0 where C 0 is the initial equilibrium concentration for the stress-free metal).…”

“…Prestressing steel wires are highly susceptible to hydrogen embrittlement (HE) or hydrogen assisted fracture (HAF) in cathodic environments [1][2][3][4][5]. Under such environmental conditions, hydrogen diffuses towards the internal regions of the material sample and can reach a critical concentration dependent on the stress-stain state at a given point and at a specific time [6].…”

Optimization of the simulation of stress-assisted hydrogen diffusion for studies of hydrogen embrittlement of notched bars

“…Cylindrical bars with circumferential V-and U-notches, where different triaxial stress-strain states can be generated, are frequently used in experiments of both inertenvironment fracture and HE of metals [9,14,15,[21][22][23]. Working their results out, e.g., to assess the critical local conditions for HAF events to occur [13][14][15], requires efficient means to find out transient hydrogen concentration distributions in such specimens.…”