Hyperbaric-Oxygen Accelerated Corrosion Test for Iron
in Cement Paste and Mortar+1
, Sachiko Hiromoto2
and Eiji Akiyama3
1International Center for Young Scientists (ICYS), National Institute for Materials Science, Tsukuba 305-0047, Japan 2Research Center for Structural Materials, National Institute for Materials Science, Tsukuba 305-0047, Japan 3Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
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)2solution 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. [doi:10.2320/matertrans.M2018029]
(Received January 26, 2018; Accepted March 12, 2018; Published May 11, 2018)
Keywords: corrosion, reinforcing steel, concrete, rust, hyperbaric-oxygen, accelerated corrosion test
Concrete is indispensable to build large and load bearing constructions such as highways and railway viaducts. In Japan, many concrete constructions were built during the high economic growth period from the 1950s to 1970s, especially just before Tokyo Olympic year of 1964. More than 50 years after that, many of these constructions require repair or replacement as it is indicated by the frequent accident reports such as detachment and collapse of concrete.13) The detachment and collapse of concrete are ascribed to the corrosion of reinforcing steel which is embedded in concrete to burden tensile stress. The rust, corrosion product, of reinforcing steel expands, leading to cracking and detachment of the cover concrete. Therefore, understanding of the corrosion behavior of reinforcing steel is important to consider measures for the repair and maintenance of degraded concrete constructions or to build safe and long-life constructions.
Corrosion behavior of iron base materials is examined generally by the exposure to a practical service environment. However, the exposure test of iron in concrete takes several decades to proceed the corrosion until the cover concrete generates cracks and detachment because the iron surface has a corrosion protective oxide ﬁlm (passiveﬁlm) and the corrosion rate of iron is very low in alkaline concrete.4,5) It takes much longer period for corrosion-resistant and weather-resistant steels and steels in concrete containing repairing agents to be corroded to generate cracks. The exposure test under practical service condition hardly corrodes iron in
defect-free concrete in short period. Therefore, a novel accelerated corrosion test is necessary to shorten the test period to simulate the practical rust of reinforcing steel. The novel method is expected to provide accurate evaluation of the corrosion rate and the limit corrosion amount for the cracking of cover-concrete.
Various accelerated corrosion tests have been proposed for reinforcing steel: electrolytic corrosion test,6,7)wet-dry cycle corrosion test,810) autoclave test,11,12) etc. The electrolytic corrosion test accelerates reinforcing steel corrosion by applying relatively high anodic current to the steel. This test provides objective amount of corrosion in a relatively short period; however, the obtained rust sometimes contains chloride compounds. Takaya et al. used the electrolytic corrosion test to examine the relationship between the kind of rust compound, the crack width of cover-concrete, and the amount of corrosion. They reported that the relationship between the amount of corrosion and the crack width was not simulated by this test, because CaFeO2Cl was formed in addition to iron oxide and oxyhydroxide (Fe3O4(magnetite),
¡-FeOOH (goethite)) and £-FeOOH (lepidocrocite)), although CaFeO2Cl is not often formed in practical service environments.13)
The wet-dry cycle corrosion test accelerates reinforcing steel corrosion by the alternating repetition of dropping of salt solution and drying to simulate the real wet and dry environments. This test simulates the rust in practical service environment; however, it requires a long period and labor comparing with the other test methods.
The autoclave test accelerates reinforcing steel corrosion with high temperature and high vapor pressure which could alter the concrete properties. The accurate evaluation of concrete cracking behavior is then diﬃcult with this method. The above conventional accelerated corrosion tests eﬀectively accelerate reinforcing steel corrosion; however,
+1This Paper was Originally Published in Japanese in J. Japan Inst. Met.
Mater. 82 (2018) 17. The sample name described in Table 2 was changed.
+2Corresponding author, E-mail: DOI.Kotaro@nims.go.jp
they still need improvements. It is thus necessary to develop a novel accelerated corrosion test which provides the rust showing the same expansion ratio (i.e. rust composition and crystal structure) as the rust in practical service environment. The expansion ratio is the division of rust thickness by the thickness reduction of steel. The new test method should form the objective amount of rust in a short period with a simple procedure and without alternating the concrete properties.
The corrosion reaction of iron occurs with equivalent anode and cathode reactions: the iron oxidation reaction generates electrons which are consumed in cathodic reaction. The main cathodic reaction on iron surface in concrete is oxygen reduction reaction. It was reported that the iron corrosion in concrete is limited by oxygen diﬀusion.14)Consequently, the acceleration of cathode reaction by enhancing oxygen supply should be eﬀective to accelerate the whole corrosion reactions. Therefore we attempted to accelerate cathodic reaction, though most conventional accelerated corrosion tests aim to accelerate the anodic reaction.
The iron corrosion in concreate initiates by a breakdown of passive ﬁlm by chloride ions and/or neutralization.5,15,16) Thus, chloride ions are essential in the accelerated corrosion system. Conventionally, chloride ions are introduced in concrete by mixing of NaCl in concrete, by immersing concrete samples in NaCl solution, and by dropping or spraying NaCl solution on concrete surface. Though the NaCl mixing possibly alters the concrete properties and subsequently aﬀect the corrosion progress, an advantage of NaCl mixing is that the chloride ion concentration on iron surface is easily regulated. The immersion, dropping and spraying of NaCl solution requires a relatively long time for the diﬀusion of chloride ions to iron surface, and it is diﬃcult to regulate chloride ion concentration on iron surface. An advantage of these methods is that the chloride ion concentration gradient from concrete surface to iron surface of practical conditions can be simulated.
Electrophoresis method is originally to measure the eﬀective diﬀusion coeﬃcient in concrete (JSCE-G 571), that allows to form the chloride ion concentration gradient in
concreate in a relatively short period. The electrophoresis method can be thus used as a new chloride ion introduction method for accelerated corrosion tests. In this study, NaCl mixing was adopted to regulate chloride ion concentration on the embedded iron surface.
In this study, the oxygen ﬂux on iron surface in cement paste and mortar increased by exposing the specimens to high pressure oxygen gas. This method was suggested as a novel accelerated corrosion test for reinforcing steel in concrete, mortar and cement paste.17)NaCl was mixed in cement paste and mortar to regulate chloride ion concentration on the embedded iron surface.
2. Experimental Procedure
2.1 Iron specimens
Iron plate (99.5%, The Nilaco Corporation) with a thickness of 1 mm was used as a specimen. The iron plate was cut to coupons of 5©5 mm2, and the measurement surface was ground with SiC papers (Struers) up to#800 grid and rinsed ultrasonically in acetone for 5 min. A conducting wire was soldered to the iron coupons, and the soldered surface except the measurement area was coated with epoxy resin (SHO-BOND Corporation) for insulation.
2.2 Cement paste and mortar samples
The iron coupon specimens prepared in the previous section were embedded in cement paste and mortar with cover thicknesses of 110 mm and 2050 mm as shown in Fig. 1(a) and (b), respectively. The diameter and height of the samples with the cover thickness of 110 mm were 30 mm and 25 mm, respectively. The cover thickness was smaller than the distance from side and back surfaces to iron specimen (Fig. 1(a)). The samples with the cover thickness over 20 mm were made in polyvinyl chloride (PVC) pipe with a length of 100 mm and the sample was not removed from the pipe. The back surface was coated with epoxy resin to prevent the solution permeation (Fig. 1(b)).
Standard cement andﬁne aggregate for mechanical testing provided by Japan Cement Association were used to prepare
cement paste and mortar. The weight ratio of water-cement-ﬁne aggregate was 0.6:1:3. No chloride ion was mixed in the cement paste and mortar samples for cathodic polarization tests. Chloride ions were mixed in samples for hyperbaric-oxygen accelerated corrosion tests using 1.03 M NaCl solution. The used chloride ion concentration was higher than the limit chloride ion concentration for the corrosion-initiation.18)After 1 day of embedding, the cement paste and mortar samples without chloride ion were cured in distilled water, and those with chloride ions were cured in 1.03 M NaCl solution for 28 days.
2.3 Cathodic potentiodynamic polarization tests Cathodic potentiodynamic polarization tests were per-formed for iron specimens embedded in cement paste and mortar to examine the eﬀect of cover thickness and oxygen pressure on the oxygen reduction reaction on iron surface. The oxygen concentration on iron surface was expected to vary depending on the cover thickness and the oxygen pressure. Mercury-mercury oxide (Hg/HgO) electrode and Pt wire were used as reference and counter electrodes, respectively. Saturated Ca(OH)2 solution was used as an electrolyte at room temperature. After curing for 28 days, iron specimens embedded in the cement paste and mortar were immersed in the electrolyte for 10 min until the open-circuit potential (ocp) was stabilized, and then the potential of iron specimens was swept from ocp to¹0.6 V (vs. Hg/HgO) with a sweep rate of 20 mV min¹1.
The polarization tests to examine the eﬀect of cover thickness were performed in ambient air. The cover thickness of cement paste were 1, 2, 5, 10, 20 and 50 mm, and that of mortar were 3, 5, 10, 20 and 50 mm. The naked iron specimen (denoted by 0 mm-cover thickness) was used as a reference.
The polarization tests to examine the eﬀect of oxygen pressure were performed in a high-pressure container (shown in Fig. 2) ﬁlled with 100% oxygen gas at 0.5 MPa (about 5 atm), which allowed to supply 25 times oxygen to iron surface in comparison with that in ambient air. The cover thickness of cement paste and mortar was 5 mm.
2.4 Hyperbaric-oxygen accelerated corrosion tests Figure 2 shows a schematic illustration of high pressure container designed for the hyperbaric-oxygen accelerated corrosion test. The proof pressure of the container was designed to be higher than 2.0 MPa. The container has a cable port for electrochemical measurements.
The specimens with 5 mm cover of cement paste and mortar were immersed in 1.03 M NaCl solution in PTFE vessel which was placed in the high-pressure container. The internal pressure of the container was kept at 0.5 MPa (about 5 atm) with 100%oxygen gas for 30 days. An immersion test in ambient air was performed for 30 days as a reference.
After the immersion tests in 0.5 MPa oxygen gas and in ambient air, the iron specimens were extracted by crashing cement paste and mortar samples. The iron surfaces were observed using a one-shot 3D proﬁlometer (VR-3000, Keyence). The cross sections were characterized using a scanning electron microscope (Quanta FEG, FEI) equipped with an energy dispersion X-ray spectroscopy (Octane Elite, EDAX) by detecting back scattered electron. The crystal structure of rust formed on iron specimens was characterized by a laser Raman spectroscopy (RAMAN plus, Nanophoton) with a laser wave length of 532 nm. The iron specimens were stored in a vacuum desiccator with silica gel when the specimen was not analyzed immediately after the extraction. 3. Results and Discussion
3.1 Oxygen reduction reaction on iron specimen embedded in cement paste and mortar
Figure 3(a) and (b) show cathodic polarization curves of iron specimen in cement paste with cover thicknesses of 05 mm and 1050 mm, respectively. Cathodic current originating from oxygen reduction reaction was observed immediately from the ocp because oxygen reduction generally occurs in alkali solutions in the corresponding potential region. The current density increased with potential being less noble. The iron specimens in cement paste always showed lower current densities around ocp than the naked iron specimen (0 mm-cover thickness), indicating that the oxygen reduction reaction was limited by the oxygen diﬀusion in the cover. The current density around ocp decreased with an increase in cover thickness.
solution of cement paste and mortar to the direction of iron surface. The oxygen diﬀusion in concrete, mortar and cement paste is generally considered to be governed by Fick’s law. The oxygen concentration on iron surface thus depends on the oxygenﬂux; therefore, the oxygen concentration of iron surface decreased with an increase in cover thickness. The ORCD in cement paste and mortar decreased with a cover thickness to a certain value, while it became constant over that thickness as described later.
The ORCD in mortar was higher than that in cement paste. Apparent oxygen diﬀusion coeﬃcient in mortar was larger than that in cement paste,20)because pores of mortar, which are formed between cement paste and ﬁne aggregate, are larger than those in cement paste. The larger pores in mortar allowed faster oxygen diﬀusion than those in cement paste, leading to the higher ORCD in mortar.
Corrosion of iron in concrete proceeds with iron oxidation (anodic) and oxygen reduction (cathodic) reactions as described by eqs. (1) and (2), respectively. The oxygen diﬀusion-limited current density of iron in solutions is described by the equation containing diﬀusion coeﬃcient and thickness of diﬀusion layer (eq. (3)).21)
O2þ2H2Oþ4e !4OH ð2Þ
iL ¼zFDC¤ ð3Þ
where iL (A m¹2) is the oxygen diﬀusion-limited current density, z is the number of moles of electrons per mole of oxygen molecule in the oxygen reduction reaction (eq. (2)) and it is 4 in this study, F (C mol¹1) is the Faraday constant and it is 96485 in this study, D (m2s¹1) is the diﬀusion coeﬃcient of dissolve oxygen, C (mol m¹3) is the concentration of dissolved oxygen in bulk solution, and ¤ (m) is the thickness of diﬀusion layer. When the current density obtained by the cathodic polarization tests and the cover thickness are related in eq. (3), the whole cover can be deﬁned as a diﬀusion layer with a uniform oxygen concentration gradient.
It was assumed that the current density at the potential 50 mV lower than ocp corresponded to the oxygen diﬀ usion-limited current density. This was because the gradient of the polarization curve at around the potential 50 mV lower than ocp was smaller than that at further lower potential especially Fig. 3 Dynamic cathodic polarization curves of iron specimen embedded in cement paste with various cover thicknesses: (a) 0, 1, 2 and
5 mm, (b) 10, 20 and 50 mm.
with the thick covers and the current density at the potential lower than ocp was hardly inﬂuenced by the anodic reaction. Then, the current densities were obtained in Figs. 3 and 4, and plotted as a function of the reciprocal of cover thickness in Fig. 5(a). Figure 5(b) shows the magniﬁed plots in the cover thickness range from 10 to 50 mm.
In the cover thickness range from 1 to 10 mm for cement paste and mortar samples, the ORCD was reciprocally proportional to the cover thickness as shown in Fig. 5(a). The whole cover thinner than 10 mm thus behaved as a diﬀusion layer. In the thickness range from 20 to 50 mm of cement paste and mortar, the ORCD did not clearly depend on the cover thickness as shown in Fig. 5(b). This is presumably because the potential sweep rate of 20 mV/min was not slow enough to obtain the ORCD in the steady state, and then the oxygen concentration gradient formed by the oxygen consumption on iron surface appeared to be thinner than the cover thickness. It was also indicated that the thickness of the diﬀusion layer was thicker than 10 mm. Therefore, in the case with covers thinner than 10 mm, the oxygen concentration gradient in the cover material can be regulated depending on the cover thickness because the cover is clearly thinner than the diﬀusion layer. In the following hyperbaric-oxygen accelerated corrosion test, the cover thickness of 5 mm was used for cement paste and mortar samples to obtain the high oxygen concentration gradient in the cover material and to shorten the test period.
Figure 6 shows cathodic polarization curves of iron specimen embedded in cement paste and mortar and measured in pure oxygen gas at 0.5 MPa. The polarization curves in ambient air are shown for comparison. The ORCD in 0.5 MPa oxygen gas was higher than that in ambient air, both in cement paste and mortar samples. The ORCDs at the potential 50 mV lower than the ocp were listed in Table 1. The ORCD in 0.5 MPa oxygen gas in cement paste and mortar was 22.5 and 24.2 times higher than those in ambient air, respectively. These magniﬁcations coincided with the diﬀerence of oxygen (partial) pressure between in 0.5 MPa oxygen gas and in ambient air: the former was 25 times higher than the latter. This coincidence was interpreted by
Henry’s law: the concentration of dissolved oxygen is proportional to its partial pressure in the gas phase. The 25 times high concentration of dissolved oxygen in NaCl solution caused the 25 times high oxygen concentration gradient in cement paste and mortar. These results reveal that the oxygen reduction reaction on iron surface in cement paste and mortar can be accelerated in proportion to the oxygen (partial) pressure in the gas phase.
Fig. 5 Relationship between ORCD and reciprocal of cover thickness in the range of (a) 150 mm and (b) 1050 mm.
Fig. 6 Dynamic cathodic polarization curves of iron specimen embedded in cement paste and mortar with 5 mm of cover thickness in ambient air and 0.5 MPa oxygen gas.
3.2 Hyperbaric-oxygen accelerated corrosion tests of iron specimen in cement paste and mortar
Figure 7 shows the appearance of iron surfaces embedded in chloride-containing cement paste or mortar with a 5 mm-cover thickness and subsequently exposed to 0.5 MPa oxygen gas or ambient air for 30 days. Hereafter, the iron specimens embedded in chloride-containing cement paste and mortar and subsequently exposed to 0.5 MPa oxygen gas are denoted by CP-Fe-0.5 MPa and MR-Fe-0.5 MPa specimens, respectively (Table 2). The iron specimens embedded in cement paste and mortar and subsequently exposed to ambient air are denoted by CP-Fe-Air and MR-Fe-Air specimens, respectively (Table 2). CP-Fe-Air and MR-Fe-Air specimens did not show apparent corrosion as shown in
Fig. 7(a) and (b). Chloride ions in cement paste and mortar could damage the passive ﬁlm of iron; however, corrosion progress was prevented because oxygen supply was signiﬁcantly low. CP-Fe-0.5 MPa showed larger amount of rust than CP-Fe-Air as shown in Fig. 7(a) and (c). MR-Fe-0.5 MPa showed larger amount of rust than CP-Fe-MR-Fe-0.5 MPa as shown in Fig. 7(c) and (d).
Figure 8 shows cross section BSE images of iron specimens after the corrosion tests. CP-Fe-Air and MR-Fe-Air specimens showed a rust layer thinner than 0.2 µm as shown in Fig. 8(a) and (b) because of the small amount of oxygen gas in mortar and cement paste exposed to air. CP-Fe-0.5 MPa and MR-Fe-0.5 MPa specimens showed a rust layer with a thickness of several micrometers as shown in Fig. 8(c) and (d).
EDS mapping images of Fe and O corresponding to Fig. 8(d) are shown in Fig. 8(e) and (f ), respectively. Fe and O were uniformly distributed in the rust layer, indicating that the rust consisted of iron oxide and oxyhydroxide.
Mean thickness of rust layers of CP-Fe-0.5 MPa and MR-Fe-0.5 MPa specimens was calculated from each measurement value on 5 views randomly obtained on the same specimen, and summarized in Table 3. The rust thickness was estimated based on the ORCD at the potential 50 mV lower than ocp in Figs. 3, 4 and 6 and the expansion Fig. 7 Optical microscopic images of (a) CP-Fe-Air, (b) MR-Fe-air, (c)
CP-Fe-0.5 MPa and (d) MR-Fe-0.5 MPa surfaces after hyperbaric-oxygen accelerated corrosion test.
Table 2 Sample condition for hyperbaric-oxygen accelerated corrosion test.
ratio of rusts, and shown in Table 3 as a comparison. Here, the expansion ratio is obtained by the division of rust thickness by the thickness reduction of iron specimen, assuming that all the dissolve iron is oxidized to form the rust. The expansion ratios of¡-FeOOH,£-FeOOH and Fe3O4are 2.9, 3.1 and 2.1, respectively.22) The rust formed in this study consisted of
¡-FeOOH, £-FeOOH and Fe3O4, as described later. Thus, assuming that the expansion ratio of FeOOH is 3.0 and FeOOH and Fe3O4are present in same amount, the expansion ratio of the rust in this study was decided to be 2.55.
The empirical rust thickness of CP-Fe-Air and MR-Fe-Air specimens was less than the observation limit of 0.2 µm, which coincided to the estimated values of 0.06 µm and 0.13 µm, respectively. The empirical values of CP-Fe-0.5 MPa and MR-Fe-CP-Fe-0.5 MPa specimens were 1.7«1.1 µm and 3.7«1.2 µm, respectively, which apparently coincided to the estimated values of 1.4 µm and 3.2 µm, respectively. The coincidence between empirical and estimated values revealed that the iron corrosion was accelerated depending solely on the oxygen (partial) pressure. Consequently, the hyperbaric-oxygen is highly eﬀective to accelerate corrosion of iron embedded in cement paste and mortar.
3.3 Structure of rust formed in high pressure oxygen gas
Laser Raman spectrum of the rust of MR-Fe-0.5 MPa is shown in Fig. 9. The rust thickness of CP-Fe-0.5 MPa was under the detection limit of the Raman spectrometer. The rust formed in high pressure oxygen gas consisted of¡-FeOOH,
£-FeOOH and Fe3O4. Takayaet al.reported that ¡-FeOOH,
£-FeOOH and Fe3O4 were mainly formed in alkaline environment of concrete based on the X-ray diﬀraction measurement of the rust formed on reinforcing (RF) steel
which had been in service for 3040 years.21) The similar composition of the rust formed in this study to the real rust conﬁrmed that the developed corrosion test overcame the problems of the conventional accelerated corrosion tests, such as the diﬀerent composition and long test period. The hyperbaric-oxygen accelerated corrosion test is highly potential as a simple and appropriate accelerated corrosion test for iron specimen embedded in concrete.
The eﬀect of oxygen pressure on the cathodic reaction on iron specimen in cement paste and mortar was examined by dynamic cathodic polarization test. A novel accelerated corrosion test for RF steel in concrete was developed using high pressure oxygen. The ﬁndings are summarized as follows.
(1) The ORCD of iron specimen in cement paste and mortar was signiﬁcantly lower than that of naked iron specimen, and it decreased with an increase in cover thickness. The ORCDs in cement paste and mortar in 0.5 MPa oxygen gas were 25 times higher than those in ambient air, respectively.
(2) The ORCD of iron specimen in cement paste and mortar with a cover thinner than 10 mm was reciprocally proportional to the cover thickness. This result indicated that the cover thickness was in the range of the diﬀusion layer formed by the oxygen consumption on iron surface.
(3) The thickness of rust layers formed in cement paste and mortar in high pressure oxygen gas was equivalent to that estimated from the respective ORCDs under the assumption that the corrosion of iron in cement paste and mortar was limited by oxygen diﬀusion. This fact indicates that the corrosion of iron in cement paste and mortar can be accelerated depending on the oxygen pressure of the gas phase.
(4) The rust formed in high pressure oxygen gas showed similar composition and crystal structure to that formed under a practical service environment for several decades.
(5) The newly developed hyperbaric-oxygen accelerated corrosion test is highly potential for the acceleration of corrosion of RF steel in concrete, mortar and cement paste.
This work was supported by Cross-ministerial Strategic Innovation Promotion Program (SIP) “Infrastructure main-tenance, renovation and management” of Council for Science, Technology and Innovation (CSTI) (Funding agency: JST). This work was also partially supported by NIMS Molecule and Material Synthesis Platform in“ Nano-technology Platform Project”operated by MEXT, Japan.
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