Hyperbaric-Oxygen Accelerated Corrosion Test for Iron in Cement Paste and Mortar

Abstract

A novel accelerated corrosion test method which enhances oxygen supply has been proposed for reinforcing steel in concrete in this study. Oxygen reduction current density (ORCD) was measured by means of potentiodynamic polarization test for an iron specimen embedded in cement paste or mortar in a saturated Ca(OH) 2 solution in ambient air. The ORCD decreased with an increase in cover thickness and the current density was reciprocally proportional to the cover thickness from 1 mm to 10 mm, suggesting that diffusion limited oxygen reduction can be accelerated by reducing the cover thickness below 10 mm. The oxygen supply to iron surface in cement paste or mortar was enhanced by pressurized oxygen gas using a newly developed hyperbaric-oxygen accelerated corrosion test container. Iron specimens with 5 mm cement paste and mortar covers showed almost 25 times higher ORCD in 0.5 MPa oxygen gas than that in ambient air, respectively. The iron specimens covered with 5 mm of cement paste or mortar containing chloride ion were immersed in a saline solution and exposed to 0.5 MPa oxygen gas in the container for 30 days. The thickness of the rust layer formed for 30-days was in good agreement with that estimated from the ORCD obtained in 0.5 MPa oxygen gas, indicating that the corrosion was accelerated in proportion to the oxygen (partial) pressure. Furthermore, the rust formed in pressurized oxygen gas showed similar characteristics to that formed in a practical service environment. Thus, the hyperbaric-oxygen is beneficial and effective to validly accelerate the corrosion of reinforcing steel in concrete.

Effects of Oxygen Pressure and Chloride Ion Concentration on Corrosion of Iron in Mortar Exposed to Pressurized Humid Oxygen Gas

“…Thus, corrosion of Fe in mortar containing Cl − was successively enhanced in the pressurized humid oxygen gas at >95%RH utilized in the present study as well as the HOACT of Fe in mortal immersed in a NaCl solution in our previous study. 16 As was explained above, corrosion of Fe in mortar with high Cl − content was suppressed in 1.1 and 2.1 MPa of oxygen gas, despite of the sufficient addition of Cl − in mortar for depassivation. This result indicates that HOACT has an upper limit oxygen gas pressure to efficiently enhance the corrosion of Fe in mortar.…”

“…The entire mortar cover of 5 mm can be regarded as the thickness of the diffusion layer, δ, because the oxygen diffusion layer formed by the oxygen consumption on Fe surface grew over 10 mm in mortar cover thicker than 10 mm. 16 The diffusion coefficient, D, of molecules in aqueous solutions does not generally depend on the outside gas pressure, and micropores in mortar were filled with Ca(OH) 2 solution. The diffusion coefficient, D, of dissolved oxygen thus considered to be independent on the oxygen gas pressure.…”

“…3.7 μm in thickness in our previous study. 16 In the previous paper, the oxygen gas pressure of 0.5 MPa was the gauge pressure and the absolute pressure was 0.6 MPa. Note that the oxygen pressure given in this paper is absolute pressure.…”

“…have reported that the oxygen reduction reaction on a Fe sample in mortar was higher than that on a Fe sample embedded in cement paste. 16 This difference is attributed to the dissimilar characteristics of the micro-pores in cement paste and that in mortal containing fine aggregates and to the higher transportation coefficient in mortar. 28 Coarse aggregates in concrete generally form larger micro-pores than those in mortar and in cement paste, 29 implying that the diffusion of oxygen and resulted oxygen reduction reaction on iron in concrete is higher than that in mortar and in cement paste.…”

“…Recently, the authors have proposed a hyperbaric-oxygen accelerated corrosion test (HOACT) as a novel accelerated corrosion test which can reproduce rust almost identical to the practically formed rust in a simple process with less time consumption. 15,16 In our previous study, iron or steel samples embedded in concrete, mortar and cement paste were immersed in a NaCl solution and exposed to 0.5 MPa of pure oxygen gas in the developed HOACT container to accelerate oxygen reduction reaction. The obtained rust was as thick as that estimated based on the oxygen reduction current density measured under the same condition.…”