Chloride‐induced steel corrosion in alkali‐activated fly ash mortar: Increased propensity for corrosion initiation at defects

Abstract

Chloride contents at the steel-mortar interface that initiate steel corrosion were determined for carbon steel in alkali-activated fly ash mortar for three different exposure conditions: exposure to 1 M NaCl solution; leaching in deionized water and then exposure to 1 M NaCl solution; and leaching in deionized water, aging in air at 20°C and natural CO 2 concentration, and then exposure to 1 M NaCl solution. For comparison, a Portland cement mortar, exposed to 1 M NaCl solution, was studied. The median values of the corrosion-initiating chloride contents (average over the full length of the rebar) in the alkali-activated fly ash mortar varied between 0.35 and 1.05 wt% Cl with respect to binder, consistently lower than what was obtained for the Portland cement mortar, but with no clear trend regarding the exposure conditions. For most of the alkali-activated fly ash mortar specimens, preferential corrosion at the connection between the working electrode and the external measurement setup was observed, while preferential corrosion did not occur for the Portland cement mortar. Scanning electron microscopy and auxiliary experiments in synthetic solutions indicated that this behavior was caused by inhomogeneities at the steel-mortar interface in the alkali-activated mortar, likely due to its peculiar rheological properties in the fresh state. K E Y W O R D Salkali-activated materials, chloride, reinforcement corrosion, steel-concrete interface

A comparison of methods to assess the resistance of reinforcing steel against chloride‐induced corrosion in concrete—Particular consideration of 12% chromium steel

“…In the case of Series 1.4039, preinvestigations in 1 M NaOH solutions with sandblasted surface conditions were conducted. [ 24 ] The OCP of the 1.4571 wire is about 100 mV more negative than the OCP of the 1.0439 rebar under the same exposure. In both cases, no corrosion occurs in the area of the spot‐welded joint under accelerated polarization in solution.…”

Application of electrochemical methods for studying steel corrosion in alkali‐activated materials

“…A shift in the corrosion potential of the embedded steel reinforcement by approximately −200 mV has been established as a criterion indicating the transition from the passive to the active state for conventional concretes, [83,84] and subsequently adopted for AAMs. [63] Since both passivation and depassivation of the steel reinforcement are time-dependent phenomena, it is essential that sufficient time is allowed for the steel reinforcement to reach a steady passive or active state before the OCP is recorded.…”

“…[9] Despite the apparent inability of the LPR method in combination with the Stern-Geary equation to quantify the corrosion rate of the steel under localised attack, the polarisation resistance (R p ) derived from LPR measurements is a useful indicator of the state (passive or actively corroding) of reinforcement steel. For OPC mortars/ concretes, R p < 40 kΩ•cm 2 (combined with a decrease of the OCP) has been established as a criterion for active corrosion [83,84] and found to apply also for steel in low-Ca AAMs, [63] and a significant decrease of R p over a comparatively short time is usually an indication of the transition from the passive to the actively corroding state.…”

“…According to Angst and Büchler, [101] conventional GPM often yields more accurate values for R p than LPR measurements in the case of macro-cell corrosion (e.g., chloride-induced reinforcement corrosion), and the method can be even further improved by an alternative approach to evaluating the obtained data. However, to date, few studies [46,63,64] have applied GPM to study the corrosion of steel in AAMs (Table 6).…”

“…Leaching in deionised water and immersion in 1 M NaCl solution; accelerated carbonation: OCP, GPM [63,64] Four rebars embedded in cylindrical concrete samples (concrete cover of 15 mm) with an exposed area of 16 mm 2 (epoxy resin applied on the reinforcements)…”