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Phosphate treatment of TMT rebar bundle to avoid early rusting: An option for single step process
Surface & Coatings Technology 201 (2006) 1583 – 1588 www.elsevier.com/locate/surfcoat
Phosphate treatment of TMT rebar bundle to avoid early rusting: An option for single step process M. Manna a,⁎, I. Chakrabarti b , N. Bandyopadhyay a a
Research and Development Department, Tata Steel, Jamshedpur 831001, India b Long Product Technology Group, Tata Steel, Jamshedpur 831001, India Received 28 November 2005; accepted in revised form 17 February 2006 Available online 4 April 2006
Abstract Thermo-mechanically treated (TMT) rebars are extensively used for reinforcement of cement concrete. In a large tropical country like India, where transit times are long and moisture levels are high, the rebars get easily rusted leading to a poor appearance. The present paper documents the work done by the authors to alleviate this problem. The work involved phosphate treatment on the surface of TMT rebars in bundle without additional process steps such as degreasing, rinsing and pickling. Rebars were treated in two alternate phosphate solutions one containing nitric acid and another nitric acid free. The phosphate layer obtained on the rebar surface was dense and black in appearances. When treatment was done in phosphate solution containing nitric acid, Zinc phosphate (hopeite) and zinc–iron phosphate (phosphophyllite) compounds formed. On the other hand, zinc phosphate (hopeite) in combination with iron phosphate (ludllamite) compounds formed when surface treatment was done in nitric acid free phosphate solution. The treated rebars were tested for extended exposure in normal atmosphere as well as under high humidity conditions. Also, Salt spray and Tafel study were conducted. In all cases the results were compared with those for untreated bars as control. Surface treatment carried out in nitric acid free phosphate solution showed 3–4 times improved resistance to red rust formation compared to bars treated in phosphate solution containing nitric acid. The bond strength of the rebar with concrete also increased marginally when treated in nitric acid free phosphate solution, whereas it reduced 10–15% for treatment in phosphate solution containing nitric acid. © 2006 Elsevier B.V. All rights reserved. Keywords: TMT; Phosphate treatment; Hoepite; Phosphophyllite; Ludllamite; Tafel study
1. Introduction Thermo-mechanical treatment (TMT) is a cost-effective way to produce high strength rebars for concrete reinforcement. The process relies on spraying high-pressure water on the rebar surface immediately after rolling to force formation of martensite [1] at the surface. The residual heat at the core ensures self-tempering of the martensite. This treatment results in an excellent combination of strength and toughness in the bars. Rebars of 8 and 12 mm diameter are often produced in coil form. These are straightened off-line under ambient condition by deploying a roll straightener. During this process, the protective high-temperature oxide scale gets dislodged and the consequent slightly cold worked bare rebar surface becomes prone to early red rust formation [2]. It is also possible that local ⁎ Corresponding author. Tel.: +91 657 2147445; fax: +91 657 2271510. E-mail address: [email protected] (M. Manna). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.02.041
galvanic cells may form on the bare steel surface resulting from compositional heterogeneity [3] to aggravate the problem. Hence, a suitable surface treatment is needed to tackle this problem of early red rust formation on the rebars. The product being bulk low-value steel commodity in nature, conventional passivating treatments involving elaborate preparation steps such as degreasing, rinsing and pickling become cost prohibitive. Often the bars are bundled on-line and the treatment
Table 1 Phosphating process conditions Passivation system
Phosphoric Zinc Nitric pH acid ml/l oxide acid g/l ml/l
Zn-phosphate 28 Zn-phosphate 18 with nitric acid
10 10
Temperature Treatment °C time (min)
- - - - 2.05 90 10 2.00 90
10 10
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M. Manna et al. / Surface & Coatings Technology 201 (2006) 1583–1588
Fig. 1. Photograph of untreated rebars.
Fig. 4. SEM micrograph of phosphate layer after treatment in phosphate solution containing nitric acid.
Fig. 2. Photograph of phosphate treated rebars.
should desirably be done without the need to unbundle and rebundle after the treatment. Commercial organic chemicals, such as oils [4–9], can prevent early rusting. But these are known to impair bond strength between cement and steel and are, therefore, unsuitable. Surface treatment with an inorganic chemical like phosphate appears worth examining. Phosphate treatment on steel surface is an industrially proven process and its disadvantages arising out of waste disposal and management can be managed in a commercially acceptable manner. Important advantages of phosphating process is that to protect steel surface from red rust formation and give good anchoring capability. The process depends upon formation of an integrated lattice of metal and crystalline phosphate [10–13]. The phosphate layer forms on the steel surface through different mechanisms [10].
The barrier performance of phosphate layer depends on the insulating ability and porosity of phosphate layer. Zinc phosphate has lower porosity and better insulating ability than manganese phosphate [14], and is, therefore, a preferred choice of coating for flat products. Phosphophyllite crystals on coldrolled steel and hopeite crystals on galvannealed steel has been found to form under various conditions of treatment time and composition of zinc phosphate bath [15]. Degreasing, cleaning, phosphate conversion coating, post cleaning, chromic acid sealing and drying are all considered important steps for phosphate treatment [16–18]. However, for the product under study, these steps are preferably avoided for consideration of cost. One step phosphating process is also studied [12,19]. By cathodic deposition process good quality phosphate coating of desired thickness is achievable even at ambient temperature without the need of an accelerator [20,21]. But hydrogenation of steel is the major limitation of this process [21]. While cathodic 100 O
90
(i) precipitation to the amorphous state (ii) crystallization and growth and (iii) reorganization of crystals
P Fe
80
Zn
Weight percent
70 60 50 40 30 20 10 0 1
3
5
7
9
Point analysis
Fig. 3. Photograph of the phosphate treated rebars unbundled after treatment.
Fig. 5. EDS depth analysis of phosphate layer after treatment in phosphate solution containing nitric acid.
64 100 144 196 256
1585
Zn 3 (PO 4)2 4H 2O Fe Zn 2 Fe (PO 4)2 4H 2O
0
4
16 36
Counts/S
M. Manna et al. / Surface & Coatings Technology 201 (2006) 1583–1588
10
20
30
40
50
60
70
80
90
2θ Fig. 6. XRD peaks of phosphate layer after treatment in phosphate solution containing nitric acid.
phosphating process may be good for flat steel surface, it is difficult to coat on uneven surfaces like that of the TMT rebars particularly when these are in the form of a tight bundle. The purposes of this paper is to study the effectiveness of the phosphate layer formation on the TMT rebar surface in phosphate solutions containing nitric acid and nitric acid free using one step dipping method of rebar in bundle and to study their effectiveness in protecting the product against early rusting without impairing the bond strength with concrete. 2. Experimental procedure The TMT rebars used for the experimental study were 12 mm in diameter. The basic composition of TMT material in weight percent is as follows:C: 0.18, Si: 0.35, Mn: 0.5, P: 0.03 the rest being Fe and it was measured using optical emission spectrometer according to standards ASTM E 415-99a [22]. The composition of the phosphate solution is shown in Table 1. The microstructure of the phosphate layer was examined by scanning electron microscopy (SEM. JEOL JXA 6400). An Energy Dispersive Spectroscopy (EDS. KEVEX Super dry
detector) was used for determination of chemical composition through depth of the phosphate layer. The structure of the phosphate layer was also determined using X-ray diffraction (XRD, Philips Analytical X-ray B.V. Machine). The fresh and surface treated TMT rebars were exposed to open atmosphere to monitor atmospheric corrosion resistance with time. The high humidity tests of two types of phosphate treated and fresh rebar (10 cm) were conducted in SC 450 humidity chamber (Weiss Technik). One humidity cycle comprised of exposure for 8 h at 50 °C under 95% humidity and 16 h at 20 °C under 75% humidity. Humidity controlled within the chamber was ±1/2%. The salt spray tests of phosphate treated and fresh rebar (10 cm) were conducted in WK111-340 salt spray cabinet (Weiss Technik). ASTM B117-03 was adopted for salt spray test [23]. This practice provides a controlled corrosive environment which has been utilized to generate relative corrosion resistance information for specimens of metals and coated metals in a given test chamber. Dissolution rate of the phosphate treated and fresh rebar surfaces were examined by Tafel test in 3.5% NaCl solution using Gamry DC105 system. The scan rate and immersion times for this test were 2 mV/S and 15 min respectively. Phosphate treated and untreated bars were cast in a square concrete block of size 10 cm. Quick setting cement (Convextra GP2) was used for casting purpose and curing time varied 48–60 h to achieve crushing strength of 200–300 kg/cm2
100
Weight percent
90 80
O
70
P
60
Fe Zn
50 40 30 20 10 0 1
3
5
7
9
Point analysis
Fig. 7. SEM micrograph of phosphate layer after treatment in nitric acid free phosphate solution.
Fig. 8. EDS depth analysis of phosphate layer for treatment in nitric acid free phosphate solution.
M. Manna et al. / Surface & Coatings Technology 201 (2006) 1583–1588 196
1586
64 36 0
4
16
Counts/S
100
144
Zn 3 (PO4 ) 2 4H 2O Fe Fe 3 (PO4 ) 2 4H 2O
10
20
30
40
50
60
70
80
2θ Fig. 9. XRD peaks of phosphate layer for treatment in nitric acid free phosphate solution.
of concrete structure according to the IS specification. The bond strength of rebar surface with concrete structure was evaluated as per IS: 1786 (1985) [24]. After curing of the block, load versus slip was observed with the help of a tensile testing machine (100 KN FUT make tensile testing machine), fitted with an appropriate precession slip measuring device as per IS: 1786 (1985). 3. Results and discussion 3.1. Appearance of fresh and phosphate treated rebar surface Surface appearances of both untreated and nitric acid free phosphate treated TMT rebar surfaces are shown in Figs. 1 and 2. It is evident that there was a change in surface appearance of the phosphate treated rebars compared to those of the untreated rebars. It is also evident from Fig. 3 that the coverage of phosphate layer was adequate even when the rebars were treated in a bundle. 3.2. Characterization of passive phosphate layer 3.2.1. Nitric acid containing phosphate treatment The surface treatment was carried out in phosphate solution containing nitric acid. The phosphate layer formed on the rebar surface was homogeneous and uniform as shown in Fig. 4. The depth–composition profile of the phosphate layer was carried
out by EDS. The distribution of P, O, Fe and Zn across the thickness of the coating is shown in Fig. 5. It can be seen that distribution of these elements throughout the thickness was almost constant. XRD study was carried out on the phosphate treated rebar surface to identify the phosphate compound formed. It was evident from the results of XRD analysis (Fig. 6) that the phosphate layer contained zinc phosphate (hopeite) as the principal phosphate component while, some zinc-iron phosphate (phosphophyllite) was also detected. 3.2.2. Nitric acid free phosphate treatment The surface treatment was also carried out in nitric acid free phosphate solution. The phosphate layer formed on the rebar surface in this case was inhomogeneous and non-uniform as shown in Fig. 7. The depth composition profile of the phosphate layer was carried out by EDS and the non-uniform distribution of P, O, Fe and Zn is shown in Fig. 8. It is apparent from EDS analysis at point 3 of the SEM micrograph that possibly some discontinuous metallic phosphide formed at isolated places along with phosphates. The XRD results are shown in Fig. 9. It can be seen that just as in the case of nitric acid containing solution reported earlier, hopeite remained the principal phosphate compound formed. Some Iron phosphate (ludllamite) compound was also detected. 3.3. Corrosion resistance behaviour of untreated and phosphate treated rebar surfaces 3.3.1. Atmospheric exposure test results Surface of untreated rebars was found to be extremely prone to red rust formation and the surface get rusted within 2–3 days
Fig. 10. Surface appearance of rebar after 45 cycles humidity test for treatment in nitric acid free phosphate solution.
Table 2 Percentage of surface covered with red rust in salt spray test Samples history
Fig. 11. Surface appearance of rebar after 15 cycles humidity test for treatment in phosphate solution containing nitric acid.
Untreated Nitric acid containing phosphate treated Nitric acid free phosphate treated
Red rust formation (%) 0h
5h
20 h
0% 0%
10–15% 95% 0% 1–2%
0%
0%
0%
50 h
100 h
100% 50%
100% 100%
0%
2%
M. Manna et al. / Surface & Coatings Technology 201 (2006) 1583–1588
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Table 3 Ecorr and corrosion rate of untreated and phosphate treated rebar surfaces
Fig. 12. Surface appearance of rebar after 100 h salt spray test for treatment in nitric acid free phosphate solution.
Fig. 13. Surface appearance of rebar after 100 h salt spray test for treatment in phosphate solution containing nitric acid.
exposure in open atmosphere. Rebar surface treated in phosphate solution containing nitric acid showed substantial improvement against atmospheric protection to prevent red rust formation and no red rust formed up to 60 days. Rebars surface treated with nitric acid free phosphate solution showed best atmospheric protection against red rust formation and no red rust formed up to 180 days. 3.3.2. High humidity and salt spray test results Phosphate treated rebar surface showed significant improvement with respect to delay in red rust formation under high humidity condition. Phosphate layer provides barrier as well as sacrificial protection to base steel to prevent red rust formation against highly humid and chloride rich environment. Surface treatment carried out in nitric acid free phosphate solution showed best protection against red rust formation. No red rust was observed for up to 45 cycles as shown in Fig. 10. Bars treated in phosphate solution containing nitric acid showed limited improvement and resisted red rust formation up to 15 cycles only as shown in Fig. 11. It is evident from Table 2 that treated rebar surfaces have better ability to prevent red rust formation in aggressive chloride environment compared to untreated bars. It is also evident that surface treatment carried out in nitric acid free phosphate solution performs better in preventing red rust formation when compared with treatment carried out in phosphate solution containing nitric acid as shown in Figs. 12 and 13.
Potential (V) vs Eref (Calomel electrode)
-0.200
Nitric acid containing Nitric acid free Untreated
-0.300 -0.400 -0.500 -0.600 -0.700 -0.800 -0.900 -7.0
Ecorr (mV)
Corrosion rate (mpy)
− 518.6 − 571.4 − 643.2
7.087 8.188 5.689
protection to base steel. Efficiency of phosphate layer to prevent red rust formation depends on its thickness and type of phosphate compounds consist in this layer. The type of phosphate compounds which are more active but dissolve in slower rate is more advantageous. From thermodynamic as well as kinetic points of view the most effective protection layer was formed on the rebar surface with nitric acid free phosphate solution (Table 3). 3.3.4. Pull-out test results The comparative bond strength of phosphate treated as well as untreated rebar surface with concrete is shown in Fig. 15. It is evident from this figure that surface treatment carried out in nitric acid free phosphate solution gives better bond strength with concrete compared to untreated surface bar. On the other hand bond strength reduced to the extent of 10–15% when rebar surface was treated in phosphate solution containing nitric acid. Such drop in bond strength can be attributed to a far smoother coating produced through treatment in nitric acid containing phosphate solution. 4. Conclusions 1. A black and dense phosphate layer formed on the surface of each of the rebar in the bundle after treating in phosphate solutions containing nitric acid as well as nitric acid free without additional process steps such as degreasing, rinsing and pickling. 2. Phosphate layer formed through treatment in phosphate solution containing nitric acid was homogenous and uniform. But phosphate layer formed through treatment in nitric acid free phosphate solution was inhomogeneous and non-uniform. 3. For treatment in both types of phosphate solution, Zinc phosphate (hopeite) was the principal compound formed.
Percentage increase in bond load compare to plain bar
3.3.3. Results of Tafel test Phosphate layers formed on the rebar surfaces through both types of treatment were more anodic compared to base steel as shown in Fig. 14 and, therefore, capable of providing cathodic
Material history Untreated Nitric acid containing phosphate treated Nitric acid free phosphate treated
200 180 160 140 120 100 80 60 40 20 0
Untreated Nitric acid containg Nitric acid free
0 -6.0
-5.0
-4.0
-3.0
0.05
0.1
0.15
0.2
0.25
Slip in mm
Log Current Density (A/cm2)
Fig. 14. Tafel plot of bare and phosphate treated surface in 3.5% NaCl solution.
Fig. 15. Percentage increase in bond strength in concrete of different treated and untreated bars compare to plain bars.
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M. Manna et al. / Surface & Coatings Technology 201 (2006) 1583–1588
When the solution contained nitric acid, some amount of zinc-iron phosphate (phosphophyllite) was also detected. On the other hand for treatment in solution free from nitric acid, iron phosphate (ludllamite) was found to form. 4. Surface treatment carried out in nitric acid free phosphate solution showed 3–4 times improved ability to prevent formation of red rust compared to treatment carried out in phosphate solution containing nitric acid. 5. The bond strength of the rebar surface with concrete increased marginally after treatment in nitric acid free phosphate solution while the same reduced 10–15% when treatment was carried out in phosphate solution containing nitric acid. This may be attributed to a far smoother coating produced through treatment in phosphate solution containing nitric acid. Acknowledgements The authors are grateful to Prof. R. C. Behera of NIT Rourkela, for conducting XRD analysis of phosphate layer. The assistance of Ms. Nitu Rani and Mr. B. Sharma in conducting experiments and characterization studies is also gratefully acknowledged. References [1] O.N. Mohanty, Product Awareness Seminar on TMT/CRS Rebars (Tatasteel technical report), 2001. [2] S.A. Bradford, 1st edition of Corrosion Control, Van Nostrand Reinhold, 1993, p. 74.
[3] K.R. Trethewey, J. Chamberlain, John Wiley and Sons, Inc., New York (1988) 273. [4] D. Chebabe, Z. Ait Chikh, N. Hajjaji, A. Srhiri, F. Zucchi, Corros. Sci. 45 (2003) 319. [5] E.I. Achouri, M.R. Infante, F. Izquierdo, S. Kertit, M. Gouttaya, B. Nciri, Corros. Sci. 43 (2001) 34. [6] A. Chetouani, A. Aouniti, B. Hammouti, N. Benchat, T. Benhadda, S. Kertit, Corros. Sci. 45 (2003) 1678. [7] G. Lendvay-Gyorik, G. Meszaros, B. Lengyel, G. lendvay, Corros. Sci. 45 (2003) 1692. [8] Sk. A. Ali, M.T. Saeed, S.U. Rahaman, Corros. Sci. 45 (2003) 260. [9] M. Hosseini, S.F.L. Mertens, M.R. Arshadi, Corros. Sci. 45 (2003) 1476. [10] E.L. Ghali, R.J.A. Potvin, Corros. Sci. 45 (1972) 583. [11] J. Donofrio, Met. Finish. 98 (6) (2000) 57. [12] B. Gruss, Met. Finish. 98 (6) (2000) 52. [13] G.N. Bhar, N.C. Debnath, S. Roy, Surf. Coat. Technol. 35 (1988) 171. [14] D. Wang, P. Jokiel, A. Uebleis, H. Boehni, Surf. Coat. Technol. 88 (1996) 147. [15] N. Satoh, T. Minami, Surf. Coat. Technol. 34 (1988) 331. [16] G. Gorecki, Met. Finish. 98 (6) (2000) 97. [17] M.R. Christian, C.B. Jeffrey, Met. Finish. 98 (9) (2000) 42. [18] G. Li, L. Niu, J. Lian, Z. Jiang, Surf. Coat. Technol. 176 (2004) 215. [19] S. Carpenter, M. Carpenter, Met. Finish. 100 (4) (2002) 73. [20] P.K. Sinha, R. Feser, Surf. Coat. Technol. 161 (2002) 158. [21] S. Jeganathan, T.S.N. Sankara Narayan, K. Ravichandran, S. Rajeswari, Surf. Coat. Technol. 200 (2006) 4117. [22] ASTM E 415-99a, ASTM International, 100 Barr Harbar drive, PO Box C700, West 19428-2959, United States. [23] ASTM B117-03, ASTM International, 100 Barr Harbar drive, PO Box C700, West 19428-2959, United States. [24] IS-1786 (1985), Indian Standards Specification, New Delhi, India, 1992.
Phosphate treatment of TMT rebar bundle to avoid early rusting: An option for single step process M. Manna a,⁎, I. Chakrabarti b , N. Bandyopadhyay a a
Research and Development Department, Tata Steel, Jamshedpur 831001, India b Long Product Technology Group, Tata Steel, Jamshedpur 831001, India Received 28 November 2005; accepted in revised form 17 February 2006 Available online 4 April 2006
Abstract Thermo-mechanically treated (TMT) rebars are extensively used for reinforcement of cement concrete. In a large tropical country like India, where transit times are long and moisture levels are high, the rebars get easily rusted leading to a poor appearance. The present paper documents the work done by the authors to alleviate this problem. The work involved phosphate treatment on the surface of TMT rebars in bundle without additional process steps such as degreasing, rinsing and pickling. Rebars were treated in two alternate phosphate solutions one containing nitric acid and another nitric acid free. The phosphate layer obtained on the rebar surface was dense and black in appearances. When treatment was done in phosphate solution containing nitric acid, Zinc phosphate (hopeite) and zinc–iron phosphate (phosphophyllite) compounds formed. On the other hand, zinc phosphate (hopeite) in combination with iron phosphate (ludllamite) compounds formed when surface treatment was done in nitric acid free phosphate solution. The treated rebars were tested for extended exposure in normal atmosphere as well as under high humidity conditions. Also, Salt spray and Tafel study were conducted. In all cases the results were compared with those for untreated bars as control. Surface treatment carried out in nitric acid free phosphate solution showed 3–4 times improved resistance to red rust formation compared to bars treated in phosphate solution containing nitric acid. The bond strength of the rebar with concrete also increased marginally when treated in nitric acid free phosphate solution, whereas it reduced 10–15% for treatment in phosphate solution containing nitric acid. © 2006 Elsevier B.V. All rights reserved. Keywords: TMT; Phosphate treatment; Hoepite; Phosphophyllite; Ludllamite; Tafel study
1. Introduction Thermo-mechanical treatment (TMT) is a cost-effective way to produce high strength rebars for concrete reinforcement. The process relies on spraying high-pressure water on the rebar surface immediately after rolling to force formation of martensite [1] at the surface. The residual heat at the core ensures self-tempering of the martensite. This treatment results in an excellent combination of strength and toughness in the bars. Rebars of 8 and 12 mm diameter are often produced in coil form. These are straightened off-line under ambient condition by deploying a roll straightener. During this process, the protective high-temperature oxide scale gets dislodged and the consequent slightly cold worked bare rebar surface becomes prone to early red rust formation [2]. It is also possible that local ⁎ Corresponding author. Tel.: +91 657 2147445; fax: +91 657 2271510. E-mail address: [email protected] (M. Manna). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.02.041
galvanic cells may form on the bare steel surface resulting from compositional heterogeneity [3] to aggravate the problem. Hence, a suitable surface treatment is needed to tackle this problem of early red rust formation on the rebars. The product being bulk low-value steel commodity in nature, conventional passivating treatments involving elaborate preparation steps such as degreasing, rinsing and pickling become cost prohibitive. Often the bars are bundled on-line and the treatment
Table 1 Phosphating process conditions Passivation system
Phosphoric Zinc Nitric pH acid ml/l oxide acid g/l ml/l
Zn-phosphate 28 Zn-phosphate 18 with nitric acid
10 10
Temperature Treatment °C time (min)
- - - - 2.05 90 10 2.00 90
10 10
1584
M. Manna et al. / Surface & Coatings Technology 201 (2006) 1583–1588
Fig. 1. Photograph of untreated rebars.
Fig. 4. SEM micrograph of phosphate layer after treatment in phosphate solution containing nitric acid.
Fig. 2. Photograph of phosphate treated rebars.
should desirably be done without the need to unbundle and rebundle after the treatment. Commercial organic chemicals, such as oils [4–9], can prevent early rusting. But these are known to impair bond strength between cement and steel and are, therefore, unsuitable. Surface treatment with an inorganic chemical like phosphate appears worth examining. Phosphate treatment on steel surface is an industrially proven process and its disadvantages arising out of waste disposal and management can be managed in a commercially acceptable manner. Important advantages of phosphating process is that to protect steel surface from red rust formation and give good anchoring capability. The process depends upon formation of an integrated lattice of metal and crystalline phosphate [10–13]. The phosphate layer forms on the steel surface through different mechanisms [10].
The barrier performance of phosphate layer depends on the insulating ability and porosity of phosphate layer. Zinc phosphate has lower porosity and better insulating ability than manganese phosphate [14], and is, therefore, a preferred choice of coating for flat products. Phosphophyllite crystals on coldrolled steel and hopeite crystals on galvannealed steel has been found to form under various conditions of treatment time and composition of zinc phosphate bath [15]. Degreasing, cleaning, phosphate conversion coating, post cleaning, chromic acid sealing and drying are all considered important steps for phosphate treatment [16–18]. However, for the product under study, these steps are preferably avoided for consideration of cost. One step phosphating process is also studied [12,19]. By cathodic deposition process good quality phosphate coating of desired thickness is achievable even at ambient temperature without the need of an accelerator [20,21]. But hydrogenation of steel is the major limitation of this process [21]. While cathodic 100 O
90
(i) precipitation to the amorphous state (ii) crystallization and growth and (iii) reorganization of crystals
P Fe
80
Zn
Weight percent
70 60 50 40 30 20 10 0 1
3
5
7
9
Point analysis
Fig. 3. Photograph of the phosphate treated rebars unbundled after treatment.
Fig. 5. EDS depth analysis of phosphate layer after treatment in phosphate solution containing nitric acid.
64 100 144 196 256
1585
Zn 3 (PO 4)2 4H 2O Fe Zn 2 Fe (PO 4)2 4H 2O
0
4
16 36
Counts/S
M. Manna et al. / Surface & Coatings Technology 201 (2006) 1583–1588
10
20
30
40
50
60
70
80
90
2θ Fig. 6. XRD peaks of phosphate layer after treatment in phosphate solution containing nitric acid.
phosphating process may be good for flat steel surface, it is difficult to coat on uneven surfaces like that of the TMT rebars particularly when these are in the form of a tight bundle. The purposes of this paper is to study the effectiveness of the phosphate layer formation on the TMT rebar surface in phosphate solutions containing nitric acid and nitric acid free using one step dipping method of rebar in bundle and to study their effectiveness in protecting the product against early rusting without impairing the bond strength with concrete. 2. Experimental procedure The TMT rebars used for the experimental study were 12 mm in diameter. The basic composition of TMT material in weight percent is as follows:C: 0.18, Si: 0.35, Mn: 0.5, P: 0.03 the rest being Fe and it was measured using optical emission spectrometer according to standards ASTM E 415-99a [22]. The composition of the phosphate solution is shown in Table 1. The microstructure of the phosphate layer was examined by scanning electron microscopy (SEM. JEOL JXA 6400). An Energy Dispersive Spectroscopy (EDS. KEVEX Super dry
detector) was used for determination of chemical composition through depth of the phosphate layer. The structure of the phosphate layer was also determined using X-ray diffraction (XRD, Philips Analytical X-ray B.V. Machine). The fresh and surface treated TMT rebars were exposed to open atmosphere to monitor atmospheric corrosion resistance with time. The high humidity tests of two types of phosphate treated and fresh rebar (10 cm) were conducted in SC 450 humidity chamber (Weiss Technik). One humidity cycle comprised of exposure for 8 h at 50 °C under 95% humidity and 16 h at 20 °C under 75% humidity. Humidity controlled within the chamber was ±1/2%. The salt spray tests of phosphate treated and fresh rebar (10 cm) were conducted in WK111-340 salt spray cabinet (Weiss Technik). ASTM B117-03 was adopted for salt spray test [23]. This practice provides a controlled corrosive environment which has been utilized to generate relative corrosion resistance information for specimens of metals and coated metals in a given test chamber. Dissolution rate of the phosphate treated and fresh rebar surfaces were examined by Tafel test in 3.5% NaCl solution using Gamry DC105 system. The scan rate and immersion times for this test were 2 mV/S and 15 min respectively. Phosphate treated and untreated bars were cast in a square concrete block of size 10 cm. Quick setting cement (Convextra GP2) was used for casting purpose and curing time varied 48–60 h to achieve crushing strength of 200–300 kg/cm2
100
Weight percent
90 80
O
70
P
60
Fe Zn
50 40 30 20 10 0 1
3
5
7
9
Point analysis
Fig. 7. SEM micrograph of phosphate layer after treatment in nitric acid free phosphate solution.
Fig. 8. EDS depth analysis of phosphate layer for treatment in nitric acid free phosphate solution.
M. Manna et al. / Surface & Coatings Technology 201 (2006) 1583–1588 196
1586
64 36 0
4
16
Counts/S
100
144
Zn 3 (PO4 ) 2 4H 2O Fe Fe 3 (PO4 ) 2 4H 2O
10
20
30
40
50
60
70
80
2θ Fig. 9. XRD peaks of phosphate layer for treatment in nitric acid free phosphate solution.
of concrete structure according to the IS specification. The bond strength of rebar surface with concrete structure was evaluated as per IS: 1786 (1985) [24]. After curing of the block, load versus slip was observed with the help of a tensile testing machine (100 KN FUT make tensile testing machine), fitted with an appropriate precession slip measuring device as per IS: 1786 (1985). 3. Results and discussion 3.1. Appearance of fresh and phosphate treated rebar surface Surface appearances of both untreated and nitric acid free phosphate treated TMT rebar surfaces are shown in Figs. 1 and 2. It is evident that there was a change in surface appearance of the phosphate treated rebars compared to those of the untreated rebars. It is also evident from Fig. 3 that the coverage of phosphate layer was adequate even when the rebars were treated in a bundle. 3.2. Characterization of passive phosphate layer 3.2.1. Nitric acid containing phosphate treatment The surface treatment was carried out in phosphate solution containing nitric acid. The phosphate layer formed on the rebar surface was homogeneous and uniform as shown in Fig. 4. The depth–composition profile of the phosphate layer was carried
out by EDS. The distribution of P, O, Fe and Zn across the thickness of the coating is shown in Fig. 5. It can be seen that distribution of these elements throughout the thickness was almost constant. XRD study was carried out on the phosphate treated rebar surface to identify the phosphate compound formed. It was evident from the results of XRD analysis (Fig. 6) that the phosphate layer contained zinc phosphate (hopeite) as the principal phosphate component while, some zinc-iron phosphate (phosphophyllite) was also detected. 3.2.2. Nitric acid free phosphate treatment The surface treatment was also carried out in nitric acid free phosphate solution. The phosphate layer formed on the rebar surface in this case was inhomogeneous and non-uniform as shown in Fig. 7. The depth composition profile of the phosphate layer was carried out by EDS and the non-uniform distribution of P, O, Fe and Zn is shown in Fig. 8. It is apparent from EDS analysis at point 3 of the SEM micrograph that possibly some discontinuous metallic phosphide formed at isolated places along with phosphates. The XRD results are shown in Fig. 9. It can be seen that just as in the case of nitric acid containing solution reported earlier, hopeite remained the principal phosphate compound formed. Some Iron phosphate (ludllamite) compound was also detected. 3.3. Corrosion resistance behaviour of untreated and phosphate treated rebar surfaces 3.3.1. Atmospheric exposure test results Surface of untreated rebars was found to be extremely prone to red rust formation and the surface get rusted within 2–3 days
Fig. 10. Surface appearance of rebar after 45 cycles humidity test for treatment in nitric acid free phosphate solution.
Table 2 Percentage of surface covered with red rust in salt spray test Samples history
Fig. 11. Surface appearance of rebar after 15 cycles humidity test for treatment in phosphate solution containing nitric acid.
Untreated Nitric acid containing phosphate treated Nitric acid free phosphate treated
Red rust formation (%) 0h
5h
20 h
0% 0%
10–15% 95% 0% 1–2%
0%
0%
0%
50 h
100 h
100% 50%
100% 100%
0%
2%
M. Manna et al. / Surface & Coatings Technology 201 (2006) 1583–1588
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Table 3 Ecorr and corrosion rate of untreated and phosphate treated rebar surfaces
Fig. 12. Surface appearance of rebar after 100 h salt spray test for treatment in nitric acid free phosphate solution.
Fig. 13. Surface appearance of rebar after 100 h salt spray test for treatment in phosphate solution containing nitric acid.
exposure in open atmosphere. Rebar surface treated in phosphate solution containing nitric acid showed substantial improvement against atmospheric protection to prevent red rust formation and no red rust formed up to 60 days. Rebars surface treated with nitric acid free phosphate solution showed best atmospheric protection against red rust formation and no red rust formed up to 180 days. 3.3.2. High humidity and salt spray test results Phosphate treated rebar surface showed significant improvement with respect to delay in red rust formation under high humidity condition. Phosphate layer provides barrier as well as sacrificial protection to base steel to prevent red rust formation against highly humid and chloride rich environment. Surface treatment carried out in nitric acid free phosphate solution showed best protection against red rust formation. No red rust was observed for up to 45 cycles as shown in Fig. 10. Bars treated in phosphate solution containing nitric acid showed limited improvement and resisted red rust formation up to 15 cycles only as shown in Fig. 11. It is evident from Table 2 that treated rebar surfaces have better ability to prevent red rust formation in aggressive chloride environment compared to untreated bars. It is also evident that surface treatment carried out in nitric acid free phosphate solution performs better in preventing red rust formation when compared with treatment carried out in phosphate solution containing nitric acid as shown in Figs. 12 and 13.
Potential (V) vs Eref (Calomel electrode)
-0.200
Nitric acid containing Nitric acid free Untreated
-0.300 -0.400 -0.500 -0.600 -0.700 -0.800 -0.900 -7.0
Ecorr (mV)
Corrosion rate (mpy)
− 518.6 − 571.4 − 643.2
7.087 8.188 5.689
protection to base steel. Efficiency of phosphate layer to prevent red rust formation depends on its thickness and type of phosphate compounds consist in this layer. The type of phosphate compounds which are more active but dissolve in slower rate is more advantageous. From thermodynamic as well as kinetic points of view the most effective protection layer was formed on the rebar surface with nitric acid free phosphate solution (Table 3). 3.3.4. Pull-out test results The comparative bond strength of phosphate treated as well as untreated rebar surface with concrete is shown in Fig. 15. It is evident from this figure that surface treatment carried out in nitric acid free phosphate solution gives better bond strength with concrete compared to untreated surface bar. On the other hand bond strength reduced to the extent of 10–15% when rebar surface was treated in phosphate solution containing nitric acid. Such drop in bond strength can be attributed to a far smoother coating produced through treatment in nitric acid containing phosphate solution. 4. Conclusions 1. A black and dense phosphate layer formed on the surface of each of the rebar in the bundle after treating in phosphate solutions containing nitric acid as well as nitric acid free without additional process steps such as degreasing, rinsing and pickling. 2. Phosphate layer formed through treatment in phosphate solution containing nitric acid was homogenous and uniform. But phosphate layer formed through treatment in nitric acid free phosphate solution was inhomogeneous and non-uniform. 3. For treatment in both types of phosphate solution, Zinc phosphate (hopeite) was the principal compound formed.
Percentage increase in bond load compare to plain bar
3.3.3. Results of Tafel test Phosphate layers formed on the rebar surfaces through both types of treatment were more anodic compared to base steel as shown in Fig. 14 and, therefore, capable of providing cathodic
Material history Untreated Nitric acid containing phosphate treated Nitric acid free phosphate treated
200 180 160 140 120 100 80 60 40 20 0
Untreated Nitric acid containg Nitric acid free
0 -6.0
-5.0
-4.0
-3.0
0.05
0.1
0.15
0.2
0.25
Slip in mm
Log Current Density (A/cm2)
Fig. 14. Tafel plot of bare and phosphate treated surface in 3.5% NaCl solution.
Fig. 15. Percentage increase in bond strength in concrete of different treated and untreated bars compare to plain bars.
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When the solution contained nitric acid, some amount of zinc-iron phosphate (phosphophyllite) was also detected. On the other hand for treatment in solution free from nitric acid, iron phosphate (ludllamite) was found to form. 4. Surface treatment carried out in nitric acid free phosphate solution showed 3–4 times improved ability to prevent formation of red rust compared to treatment carried out in phosphate solution containing nitric acid. 5. The bond strength of the rebar surface with concrete increased marginally after treatment in nitric acid free phosphate solution while the same reduced 10–15% when treatment was carried out in phosphate solution containing nitric acid. This may be attributed to a far smoother coating produced through treatment in phosphate solution containing nitric acid. Acknowledgements The authors are grateful to Prof. R. C. Behera of NIT Rourkela, for conducting XRD analysis of phosphate layer. The assistance of Ms. Nitu Rani and Mr. B. Sharma in conducting experiments and characterization studies is also gratefully acknowledged. References [1] O.N. Mohanty, Product Awareness Seminar on TMT/CRS Rebars (Tatasteel technical report), 2001. [2] S.A. Bradford, 1st edition of Corrosion Control, Van Nostrand Reinhold, 1993, p. 74.
[3] K.R. Trethewey, J. Chamberlain, John Wiley and Sons, Inc., New York (1988) 273. [4] D. Chebabe, Z. Ait Chikh, N. Hajjaji, A. Srhiri, F. Zucchi, Corros. Sci. 45 (2003) 319. [5] E.I. Achouri, M.R. Infante, F. Izquierdo, S. Kertit, M. Gouttaya, B. Nciri, Corros. Sci. 43 (2001) 34. [6] A. Chetouani, A. Aouniti, B. Hammouti, N. Benchat, T. Benhadda, S. Kertit, Corros. Sci. 45 (2003) 1678. [7] G. Lendvay-Gyorik, G. Meszaros, B. Lengyel, G. lendvay, Corros. Sci. 45 (2003) 1692. [8] Sk. A. Ali, M.T. Saeed, S.U. Rahaman, Corros. Sci. 45 (2003) 260. [9] M. Hosseini, S.F.L. Mertens, M.R. Arshadi, Corros. Sci. 45 (2003) 1476. [10] E.L. Ghali, R.J.A. Potvin, Corros. Sci. 45 (1972) 583. [11] J. Donofrio, Met. Finish. 98 (6) (2000) 57. [12] B. Gruss, Met. Finish. 98 (6) (2000) 52. [13] G.N. Bhar, N.C. Debnath, S. Roy, Surf. Coat. Technol. 35 (1988) 171. [14] D. Wang, P. Jokiel, A. Uebleis, H. Boehni, Surf. Coat. Technol. 88 (1996) 147. [15] N. Satoh, T. Minami, Surf. Coat. Technol. 34 (1988) 331. [16] G. Gorecki, Met. Finish. 98 (6) (2000) 97. [17] M.R. Christian, C.B. Jeffrey, Met. Finish. 98 (9) (2000) 42. [18] G. Li, L. Niu, J. Lian, Z. Jiang, Surf. Coat. Technol. 176 (2004) 215. [19] S. Carpenter, M. Carpenter, Met. Finish. 100 (4) (2002) 73. [20] P.K. Sinha, R. Feser, Surf. Coat. Technol. 161 (2002) 158. [21] S. Jeganathan, T.S.N. Sankara Narayan, K. Ravichandran, S. Rajeswari, Surf. Coat. Technol. 200 (2006) 4117. [22] ASTM E 415-99a, ASTM International, 100 Barr Harbar drive, PO Box C700, West 19428-2959, United States. [23] ASTM B117-03, ASTM International, 100 Barr Harbar drive, PO Box C700, West 19428-2959, United States. [24] IS-1786 (1985), Indian Standards Specification, New Delhi, India, 1992.