Impressed-Current Cathodic Protection of Steel-Reinforced Concrete Pilings: Protection Criteria and the Threshold for Hydrogen Embrittlement

Safe cathodic protection [CP] limits for prestressing steel in concrete and the adequacy of CP using established criteria were evaluated in regard to hydrogen embrittlement (HE). Impressed-current CP was applied to laboratory scale pilings at current densities from 0.1 mu A/cm(2) to 3.0 mu A/cm(2) via a skirt anode located at the waterline. Adequate CP was achieved at positions 25 cm (9.8 in) above to 50 cm (19.7 in.) below the waterline, according to the 100-mV depolarization criterion, at an apparent applied current density of 0.33 mu A/cm(2). However the -780 mV(SCE) criterion was not met for currents as high as 1.33 mu A/cm(2) for these positions. Hydrogen production, absorption, and permeation in steel first was observed via embedded hydrogen sensors 50 cm and 25 cm above the water line at art applied current density of 0.33 mu A/cm(2). Observation of hydrogen production verified concerns that the local oxygen concentration might be depleted readily at modest CP levels and that local pH levels may be below 12.5. Experimentation demonstrated that steel crevice corrosion was initiated readily within chloride (Cl-)-contaminated concrete prior to CP application and that this corrosion was accompanied by acidification oft he local environment to pH less than or equal to 6 as a result of ferrous ion (Fe2+) hydrolysis. The mobile subsurface hydrogen concentration present within the steel reinforcement was determined for each applied cathodic current density. Although hydrogen production and uptake occurred at current densities as low as 0.33 mu A/cm(2), the critical hydrogen concentration for embrittlement (i.e., 2 x 10(-7) mol H/cm(3), as determined in prior research for bluntly notched prestressing steel) was not exceeded at area averaged current densities < 1.33 mu A/cm(2).