CORROSION ENGINEERING SECTION
Corrosionof Phosphoric Irons in Cement Grout G. Sahoo, *R. Balasubramaniam,t'* and S. Misra**
ABSTRACT Corrosion behavior of three cement-grouted phosplwric irons (Fe-0.llP-0.028C. Fe-0.32P-0.026C, and Fe-0.49P-0.02C, wt%) was studied using the potentiostatic polarization technique in 5% sodium chloride (NaC!)and compared with that of two commercial concrete reinforcement steels, a low-carbon steel (Fe-0.148 C-0.542 Mn-0.128 Si) and a microalloyed steel (Fe-0.151C-0.088P-0.197Si-0.149Cr-0.417Cu). The passive current density was highest in the low-carbon steel, whUe that of Fe-0.llP-0.028C, Fe-0.32P-0.026C, and the microalloyed steel was simUar(5 ?LA/cm2to 7 ?LA/cm2).Phosplwric iron Fe-0.49P-0.02C revealed the lowest passive current density. The breakdown of passivity by chloride ions occurred much below 500 mV vs. saturated calomel electrode (SCE) in the case of the low-carbon steeL The passive film was resistant to chloride in the case of phosphoric irons and the microalloyed steel. The improved corrosion resistance of plwsplwric irons was attributed to the inhibiting effect ofplwsphate that forms on steel in contact with cemenL KEYWORDS: passivejUm. pitting. plwsplwric irons, polarization, steel-reinforced concrete
INTRODUCTION Chloride-induced corrosion of reinforcement bars, which results in the deterioration of reinforced concrete (RC) structures, has been a major challenge to Submitted for publication September 2006: In revised form. July 2007. * Corresponding author. E-mail:
[email protected]. Department of Materials and Metallurgical Engineering. Indian
. Institute
of Technology. Kanpur-2080I6. India. .. Department of Civil Engineering. Indian Institute of Technology. Kanpur-2080I6. India.
civil engineers around the world. Reinforcement bars in normal RC construction are protected against corrosion by the passive film formed on these bars due to the high alkalinity of concrete pore solution (pH ~ 12). However, the presence of chloride ions in excess of a threshold value disrupts this passivating film,
rendering the bars susceptible to corrosion. I As far as the source of chloride ions is concerned, they may be either present in the initial constituents of concrete (e.g., in cases when marine sand or aggregate is used in construction) or diffuse into hardened concrete as in the case of marine structures2 or structures subjected to deicing salts.3 The reported annual direct cost of chloride-induced bridge infrastructure corrosion to the U.S. economy was estimated at $8.3 billion per year, and indirect cost was reported many times higher than this.4 Several methods such as the use of galvanized. epoxy-coated. or stainless steel reinforcement have been suggested and applications have been found. especially in the United States, Europe. and Japan. but cost effectiveness and other technical issues still prevent widespread use of these methods. In this regard. taking a clue from the extremely high corrosion resistance of the Iron Pillar in Delhi. India. made during the Gupta period (more than 1,600 years old). the use of iron of high phosphorus content (Le.. phosphoric irons) as reinforcement has been explored in this study. It has been established that the presence of phosphorus (0.25 wt%) in this pillar facilitates the formation of a protective passive film on the surface, which provides the pillar its exceptional corrosionresistant properties. 5-6
0010-9312/07/0001177/$5.00+$0.50/0
CORROSION-Vol.
63, No.1 0
@ 2007, NACE International
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CORROSION ENGINEERING SECTION
TABLE 1 Outline of the Experimental Program Series
W/C Ratio
A
0.45 with no chlorides
B
0.45 with 0.6% NaCI
Steels Used
Curing
Exposure Prior to Corrosion Tests
Duration
Seven days at room temperature inside the PVC mold used for casting
30-min immersion in 5% NaCI solution 20-day immersion in 5% NaCI solution
(by weight of cement added at the time of preparation of cement paste
A detailed study of the literature revealed that there have been no studies on corrosion behavior of phosphoric irons in concrete environments. It is known, however, that phosphorus-containing steels form a passive film, where the outer layer is enriched with POr, mainly iron(II) phosphate (Fe3[P04b),in sulfuric acid (H2S04)and hydrochloric acid (HClJenvironments.7-9 The incorporation of por in the outer layer leads to the formation of a bipolar passive film, with an inner anion selective Fe-O layer and an outer cation selective POr layer.7-9This outer cation-selective phosphate-enriched layer resists ingress of cr ions into the film. In case phosphate ions are added intentionally to the environment, they apparently enhance the chemisorption of dissolved oxygen that promotes gamma ferric oxide (y-Fe203)passive film formation. 10-11 Phosphates and hydrogen phosphates, like calcium hydrogen phosphates (CaHP04), sodium phosphate (Na3P04),and sodium hydrogen phosphate (Na2HP04), are known to be anodic corrosion inhibitors in concrete environments. Therefore, this was the motivation to undertake the present study on corrosion of phosphoric irons in cement grout. In modem steel making processes, the phosphorus content is controlled to about 0.05 wt% because phosphorus segregation to grain boundaries reduces ductility of steel. 12 This embrittlement due to phosphorus inclusion can be avoided by proper alloy design and heat treatments. 13-14 The suggested alloy design and heat treatments are not expected to affect the chemistry and morphology of P-rich inclusions. Further, the activity of phosphorus will not be lowered within steel and this has been confirmed by metallographic studies in the phosphoric irons. 15-16 Mechanical properties, comparable to that of normal reinforcement bars, have been obtained in the case of phosphoric irons,I5 and, hence, this study concentrates on the corrosion resistance of such irons in the highly alkaline environment of concrete. The corrosion behavior of three phosphoric irons of different phosphorus contents has been compared with two commercially available reinforcement bars (a conventional plain carbon steel bar and a microalloyed steel bar), using specimens cast in cement grout. The 976
potentiodynamic polarization technique was utilized to compare the behavior of phosphoric irons with traditional concrete reinforcement materials. EXPERIMENTAL PROCEDURES Experimental Program The outline of the experimental program followed in the study is given in Table 1. It can be seen from the table that two series of experiments were carried out-one using specimens cast with normal cement paste and the other using specimens cast with cement paste containing chlorides mixed in the mixing water (0.6% NaCI by weight of cement). Although the specimens for both series were cured in an identical regime, the corrosion tests were carried out after 30 min of immersion in 5% sodium chloride (NaCI)solution and 20 days of immersion in the solution in series A and B, respectively. The water-cement ratio of the cement paste, which has a decisive effect on the permeability of the matrix, was kept at 0.45 throughout. Materials Steel-Three different phosphoric irons, namely, PI' P2' and P3' a low-carbon commercial steel (T), and a low-carbon low-alloy commercial steel (C) were used. The chemical compositions of these samples were obtained using a direct reading optical emission spectrometer and are given in Table 2. The phosphorus contents in PI' P2, and P3were determined by wet-chemical analysis. The phosphoric irons were prepared by ingot casting route after melting in a high-frequency induction-melting furnace (175 kW, 1.000 Hz) in air. The alloys were prepared from calculated amounts of soft iron and Fe-P mother alloys (Fe-22P) depending on the required phosphorus content. The ingots were forged to 26-mm- and 16-mmdiameter circular bars after soaking at l,150°C. The specimens P2and P3were soaked at 1,150°C to obtain a dual-phase microstructure for improving the ductility of these high phosphorus-containing phosphoric irons P2and P3.15-16 The 26-mm-diameter forged bars were further hot-rolled into a 12-mm-diameter circular bar. CORROSION-OCTOBER
2007
CORROSION ENGINEERING SECTION
TABLE 2 Average Chemical Compositions
of Phosphoric Irons, T, and C (wt%)
Samples
C
P
Si
Mn
S
Ni
Cr
Mo
V
Cu
P1 P2 P3 T C
0.028 0.026 0.022 0.148 0.151
0.11 0.32 0.49 0.024 0.088
0.029 0.026 0.027 0.128 0.197
0.046 0.052 0.067 0.542 0.713
0.017 0.018 0.023 0.02 0.013
0.026 0.026 0.026
