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Characterisation of phosphate coatings obtained using nitric acid free phosphate solution on three steel substrates: An option to simulate TMT rebars surfaces
Surface & Coatings Technology 203 (2009) 1913–1918
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Characterisation of phosphate coatings obtained using nitric acid free phosphate solution on three steel substrates: An option to simulate TMT rebars surfaces M. Manna ⁎ Research & Development Department, Tata Steel, Jamshedpur 831001, India
a r t i c l e
i n f o
Article history: Received 5 November 2008 Accepted in revised form 14 January 2009 Available online 24 January 2009 Keywords: TMT rebar GDOES SEM XRD EIS Corrosion
a b s t r a c t Phosphate coatings have been obtained on three steel substrates, (a) ferritic-pearlitic (F-P), (b) tempered martensitic (T-M) and (c) tempered martensitic containing oxide scale (T-M-O) at the top to simulate TMT (thermo mechanical treatment) rebar surfaces which are extensively used for composite concrete structure. Nitric acid free phosphate solution was used for the coating purpose. Scanning electron microscopy (SEM), glow discharge optical emission spectrometry (GDOES) and X-ray diffraction (XRD) techniques were used to characterise phosphate coatings. Acicular phosphate microstructure was obtained on T-M-O and T-M steel substrates, whereas coarser phosphate microstructure was obtained on F-P steel substrate. Thinner to thicker coatings were obtained on F-P, T-M and T-M-O steel substrates respectively. Oxide scale, on the T-M-O steel substrate promoted for deposition of phosphate compounds and thereby obtained thickest coating on T-M-O steel substrate. Zinc phosphate (hopeite) on T-M-O steel substrate and zinc phosphate (spencerite) on F-P steel substrate were detected as main phosphate respectively, whereas both zinc phosphates (hopeite and speccerite) on T-M steel substrate were detected in the same proportion. In addition to zinc phosphate, zinc iron phosphate (phosphophyllite) was detected on F-P and T-M steel substrates, whereas iron phosphate (beraunite) was detected on T-M-O steel substrate. A comparative performance against corrosion of all the phosphate coated steel substrates was evaluated by salt spray, Tafel and electrochemical impedance spectroscopy (EIS) studies. Test for extended exposure in open atmosphere as well as simulated highly humid condition were also conducted to identify coating performance. Phosphate coated T-M-O steel substrate showed 4–5 times improved resistance against corrosion than the phosphate coated F-P steel substrate. The bond strength of coated steel substrate with concrete was increased 2–26% for phosphate coating on T-M-O and T-M steel substrates while the same was decreased 4–12% for phosphate coating on F-P steel substrate. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Carbon steel has good mechanical properties and it is readily used as preferred material rather than corrosion resistance alloys in a variety of forms for large structures. Rebar is one of such carbon steel used extensively for composite construction of concrete structure. Presently, thermo-mechanical treatment (TMT) is a cost-effective way to produce high strength steel rebars. The process relies on spraying high-pressure water on the rebar surface immediately after rolling to force formation of martensite [1] with ferritic-pearlitic structure in the core. 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 rebars. 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. Red rust is formed on rebar surface due to formation of local galvanic cells resulting from
⁎ Tel.: +91 657 2147445; fax: +91 657 2271510. E-mail address: [email protected]. 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.01.024
compositional heterogeneity [2]. Hence, a suitable surface treatment is needed to tackle this problem. Commercial organic chemicals, such as oils [3–8], can prevent early rusting. But these are known to impair bond strength between cement and steel, therefore, unsuitable. Metal (Zn, Al, Ni) coating [9,10] on rebar surface improves the corrosion resistance property of the reinforcing steel but the product being bulk low-value steel commodity in nature, so metal coating may not be a good choice. Surface treatment with an inorganic chemical like phosphate and silicate appear worth examining. The phosphate treatment on metal surface is well known, whereby the surface of the metal is converted to an integral, mildly protective layer of insoluble crystal [11–14]. Different phosphate treatments are widely used for corrosion protection of screws, nuts, bolts, plain washer, brake components, clutch components, engine parts and several others [15]. Depending on different specific properties (corrosion resistance, paint adhesion, coefficient of friction) requirement on different metals (steel, cast iron, Mg alloys and Al alloys) surfaces, phosphate treatment has significant role [11,14,16–21]. The manganese phosphate coating obtained on AISI 4140 steel substrate after various heat treatments yield an important improvement in the corrosion resistance of the coatings [22]. Zinc phosphate has better
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M. Manna / Surface & Coatings Technology 203 (2009) 1913–1918
Table 1 Basic composition of steel in wt.% C
Si
Mn
P
Fe
0.18
0.35
0.5
0.03
~ 98.94
insulating ability than manganese phosphate [23] and therefore expecting for better protection capability. Phosphophyllite and hopeite crystals were obtained on cold rolled and galvanised steel substrate respectively [24]. Degreasing, cleaning, phosphate conversion coating, post cleaning, chromic acid sealing and drying are all considered important steps for phosphate treatment [16,25,26]. By cathodic deposition process, good quality even phosphate coating of desired thickness is achievable at ambient temperature without the need of an accelerator [27,28]. But hydrogenation of steel is the major limitation of this process [28]. While cathodic phosphating process may be good for flat steel surface, it is difficult to coat on uneven surfaces like rebars. Phosphate coating has been attempted in nitric acid containing phosphate solution on three steel substrates to simulate TMT rebar surfaces [29].One step phosphating process is also studied [23,30]. Surface treatment carried out in nitric acid free phosphate solution showed higher improved corrosion resistance than the surface treatment carried out in phosphate solution containing nitric acid [30]. The purposes of this paper are to study (i) the effect of phosphate coating in nitric acid free phosphate solution, on three steel substrates (to simulate TMT rebar surface) and (ii) the coatings characterisation. 2. Experimental procedure Three steel substrates, (a) ferritic-pearlitic (F-P) (b) tempered martensite (T-M) and (c) tempered martensite containing oxide scale at the top surface (T-M-O) were used for the experimental purpose. The basic composition of the steel in weight percent is shown in Table 1. Steel composition was analysed using optical emission spectrometer according to standard ASTM E 415-99a [31]. The process conditions for phosphate treatment on three steel substrates are shown in Table 2. The microstructure of the phosphate coating was examined by SEM (JEOL JXA 6400). Quantitative depth profiles of all the phosphate coatings were done using LECO(R) GDS-850A. The phosphate compounds present in the coating were identified by Grazing XRD (Philips Analytical X-ray B.V. Machine). The scan rate was 0.2°/s. ASTM D 4145 was adopted to find out coating adherence with the steel substrate [32]. All the phosphate coated steel substrates were exposed in open atmosphere to observe the time required for first appearance of red rust. The high humidity test was conducted to all coated steel substrates in a humidity chamber SC 450 (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 was used. Controlled humidity within the chamber was in the range of ±1/2%. The salt spray test was conducted to all coated steel substrates in a salt spray cabinet WK111340 (Weiss Technik). ASTM B117-03 was adopted for salt spray test [33]. 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 and charge transfer resistance of all phosphate coatings were examined by Tafel and electrochemical impedance spectroscopy (EIS) tests using Gamry DC105 system. All electrochemical experiments were conducted in 3.5% NaCl solution. The scan rate and immersion time
Table 2 Phosphating process conditions Phosphoric acid (ml/L)
Zinc oxide (g/L)
pH
Temperature (°C)
Treatment time (min)
28
10
2.05
90
10
Fig. 1. SEM images at the top surface of phosphate coating obtained on (a) F-P, (b) T-M and (c) T-M-O steel substrates.
for Tafel test were 2 mV/s and 15 min respectively. All phosphate coated and uncoated steel rods were cast in a square concrete block of 10 cm side. Quick setting cement (Convextra GP2) was used for casting purpose and curing time varied 48–60 h to achieve crushing strength of 19.61– 29.42 MPa of concrete structure according to the IS specification. The bond strength of steel surface with concrete structure was evaluated as per IS: 1786 (1985) [34]. After block curing, load versus slip was observed with the help of a tensile testing machine (100 kN FUT make tensile
M. Manna / Surface & Coatings Technology 203 (2009) 1913–1918
1915
testing machine), fitted with an appropriate precession slip measuring device as per IS: 1786 (1985) [34]. 3. Results and discussion 3.1. Characterization of phosphate coating Fig. 1(a–c) shows the SEM morphology of all three phosphate coated steel substrates. The phosphate coating, obtained on T-M and T-M-O steel substrate was acicular in nature where as the phosphate coating, obtained on F-P steel substrate was coarser and its look was like Luke warm.
Fig. 3. Grazing XRD peaks of phosphate coated (a) F-P, (b) T-M and (c) T-M-O steel substrates.
The GDOES elemental depth profile of all three phosphate coatings on three steel substrates is shown in Fig. 2(a–c). The thickest (16–22 μm) phosphate coating was obtained on T-M-O steel substrate whereas the thinnest (10–12 μm) phosphate coating was obtained on the F-P steel substrate. The thickness of the phosphate coating obtained on the T-M steel substrate was in-between and it was in the range of 12–16 μm. It is evident from GDOES depth profile that zinc content decreased and oxygen content increased at the top surface of all three phosphate coatings which can be attributed to the presence of high quantities of moisture. It is also evident from GDOES depth profile that the zinc content remained constant up to certain depth for the coating obtained on the F-P and T-M-O, whereas the zinc content gradually fall down for the coating obtained on T-M steel substrate. The comparative zinc content in the phosphate layer was highest for the coating obtained on F-P steel substrate, whereas the comparative zinc content in the phosphate layer was lowest for the coating obtained T-M-O steel substrate. Also, the iron content was high in the coating obtained on F-P and T-M steel substrates and less in the coating obtained on the T-M-O steel substrate. These results have good correlation with XRD results. It is evident from Fig. 3 that different phosphate compounds of zinc (hopeite and spencerite), zinc–iron (phosphophyllite) and iron (beraunite) present in coatings have been identified. Hopeite was the main phosphate compound in the coating obtained on the T-M-O steel substrate whereas spencerite was the main phosphate compound in the coating obtained on F-P steel substrate as evident from Table 3. The comparative percentage of hopeite and spencerite was close to same in the coating obtained on T-M steel substrate. The iron bearing phosphate, beraunite was detected in the coating obtained on T-M-O steel
Table 3 Percentage of different phosphate compounds in the coating Steel substrate
Hopeite
Spencerite
Beraunite
Phosphophyllite
T-M-O T-M F-P
78.6 50.4 28.2
20.6 43.6 67.8
0.8 0 0
0 6 4
Table 4 Corrosion rate and Ecorr of the coating in 3.5% chloride solution
Fig. 2. GDOES elemental depth profile of the phosphate coating obtained on (a) F-P, (b) T-M and (c) T-M-O steel substrates.
Steel substrate
Corrosion rate (mpy)
Ecorr (V)
T-M-O T-M F-P
0.36 0.62 1.618
−0.735 −0.638 −0.657
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M. Manna / Surface & Coatings Technology 203 (2009) 1913–1918
substrate whereas zinc–iron bearing phosphate, phosphophyllite was detected in the coatings obtained on T-M and F-P steel substrates. First, the iron oxide scale at the top of T-M-O steel substrate gets dissolved, thereby rising the local pH and promoting the iron and zinc phosphate formation, whereas the iron itself from T-M and F-P steel substrates, first gets dissolved being expected to less rise of pH at T-M and F-P steel substrates and less promoting of zinc phosphate and zinc–iron phosphate formation as mentioned in Eqs. (i)–(vi). It is expected more faster kinetic of reaction for dissolution of oxide scale than for dissolution of iron, thereby sharp rise of pH at the T-M-O steel substrate occurs. This in turn promotes for faster deposition of phosphate compounds on the T-M-O steel substrate. On the other hand, Fe get dissolve in slower rate, leads to least rise of pH at the F-P steel substrates. Least rise of pH at the F-P steel substrate results in slowest deposition of phosphate compounds on the F-P steel substrate as mentioned in Eqs. (iii)–(v). As a result, thickest to thinnest coating obtained on T-M-O, T-M and F-P steel substrates respectively. Fex Oy þ 2Hþ →Fe2þ þ H2 O
ðiÞ
Fe þ 2Hþ →Fe2þ þ H2
ðiiÞ
þ 3Zn2þ þ 2H2 PO−1 4 þ 4H2 O→Zn3 ðPO4 Þ2 4H2 O þ 4H
ðiiiÞ
þ 4Zn2þ þ 2H2 PO−1 4 þ 5H2 O→Zn4 ðOH−PO4 Þ2 3H2 O þ 6H
ðivÞ
þ Fe2þ þ 2Zn2þ þ 2H2 PO−1 4 þ 4H2 O→Zn2 FeðPO4 Þ2 4H2 O þ 4H þ 3Fe2þ þ 2H2 PO−1 4 þ 4H2 O→Fe3 ðPO4 Þ2 4H2 O þ 4H
ðvÞ ðviÞ
T-M-O steel substrate gets less time of contact with the phosphate solution under low pH condition. It results in less dissolution of iron in the solution, which in turn exhibits less iron in the coating. On the other hand, T-M and F-P steel substrates get more time of contact with the phosphate solution under low pH condition. It results in more dissolution of iron in the solution, which in turn exhibits more iron in the coatings. Initially, iron phosphate was obtained on T-M-O steel substrate due to faster rise of pH and less time available for Fe ions to diffuse in the bulk solution. After that, Fe ions were depleted at the T-M-O substrate thereby promoting hopeite and spencerite formation. The Fe ions dissolved from T-M and F-P steel substrates get longer time to diffuse in the bulk solution and deposit as zinc–iron phosphate. The comparatively high and low pH favoured for formation of hopeite and spencerite respectively, thereby expecting maximum to minimum content of hopeite in the coating obtained on T-M-O, T-M and F-P steel substrates respectively.
Fig. 4. Tafel plot of phosphate coated steel substrates in 3.5% NaCl solution.
Table 5 Time of appearance of first red rust on coated steel substrate, in different environments Steel substrate
Salt spray (h)
Humidity chamber (cycles)
Atmospheric (days)
T-M-O T-M F-P
100 40 25
45 18 10
180 70 50
3.2. Corrosion resistance behaviour of untreated and phosphate treated rebar surfaces 3.2.1. Atmospheric exposure test results All three phosphates coated steel substrates showed substantial improvement against atmospheric corrosion to prevent red rust formation and no red rust was observed up to 50 days for phosphate coated F-P steel substrate whereas no red rust was observed up to 180 days for phosphate coated T-M-O steel substrate as shown in Table 4. On the other hand no red rust was observed up to 70 days for phosphate coated T-M steel substrate. 3.2.2. High humidity and salt spray test results Phosphate coated steel substrate showed significant improvement with respect to delay in red rust formation under high humidity condition. Phosphate coating provides barrier as well as sacrificial protection to base steel to prevent red rust formation against highly humid and chloride rich environments. Phosphate coating obtained on T-M-O steel substrate showed the best resistance to prevent first red rust appearance whereas phosphate coating obtained on F-P steel surface showed the least resistance to prevent first red rust appearance. No red rust was observed for up to 10 cycles on phosphate coated F-P steel substrate whereas no red rust was observed for up to 45 cycles on phosphate coated T-M-O steel substrate. On the other hand, no red rust was observed for up to 18 cycles on phosphate coated T-M steel substrate. It is evident from Table 4 that phosphate treatment carried out on T-M-O showed better ability to prevent red rust formation in aggressive chloride environment comparatively to phosphate treatment carried out on the two other steel substrates. 3.2.3. Results of Tafel Test and EIS It is evident from Fig. 4 and Table 5 that free corrosion potential and corrosion rate were lowest for the phosphate coated T-M-O steel substrate whereas free corrosion potential and corrosion rate were highest for the phosphate coated F-P steel substrate. On the other hand, free corrosion potential and corrosion rate were inbetween for the phosphate coated T-M steel substrate. The phosphate compounds which have maximum resistance against dissolution in different corrosive environments are beneficial. From thermodynamic as well as kinetic points of view, the most effective protective phosphate coating was obtained on the T-M-O steel surface (Table 4). The electrochemical impedance test was carried out for all phosphate coated steel substrates in 3.5% NaCl solution to find out their comparative resistance against charge transfer. Fig. 5 represents the equivalent circuit diagram for electrochemical impedance test of all three phosphate coatings. The comparative resistance against charge
Fig. 5. Equivalent circuit diagram for EIS study of three phosphate coatings.
M. Manna / Surface & Coatings Technology 203 (2009) 1913–1918
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Fig. 6. Bode plot of phosphate coated steel substrates in 3.5% NaCl solution (a) impedance/0 h (b) impedance/120 h (c) phase shift/0 h (d) phase shift/120 h.
transfer of all the phosphate coatings in aggressive chloride environment for immediate exposure as well as after 5 days is shown in Figs. 6 (a–d) and 7(a,b). It is evident from Bode and Nyquist plots, that the coating obtained on T-M-O steel substrate showed maximum resistance against charge transfer whereas the coating obtained on F-P steel substrate showed minimum resistance against charge transfer. On the other hand, coating obtained on T-M steel substrate showed in-between resistance against charge transfer.
2. 3.
3.3. Pull-out test results The comparative bond strength of all the phosphate coated steel substrates to concrete structure is shown in Fig. 8. It is evident from this figure that phosphate coated T-M-O and T-M steel substrate gave 2–26% increment in bond strength whereas phosphate coated F-P steel substrate gave 4–12% drop in bond strength comparatively to bond strength of uncoated steel substrate to concrete structure. No crack or pill off occurs of phosphate coatings from all three steel substrates after bend test according to ASTM D 4145. Phosphate coatings with all three steel substrates have reasonably good adherence. Such drop in bond strength can be attributed to a far smoother coating (see Table 6) and lesser alkaline stability of spencerite in the environment of the concrete.
4.
5.
steel substrates. Thinner to thicker coatings were obtained on F-P, T-M and T-M-O steel substrates respectively. Oxide scale on T-M-O steel substrate promoted for faster deposition of phosphate compounds. Zinc phosphate (hopeite) on T-M-O steel substrate and zinc phosphate (spencerite) on F-P steel substrate were detected as main phosphate respectively, whereas both zinc phosphates (hopeite and speccerite) on T-M steel substrate were detected in same proportion. In addition to zinc phosphate, zinc iron phosphate (phosphophyllite) was detected on F-P and T-M steel substrates whereas iron phosphate (beraunite) was detected on T-M-O steel substrate. Phosphate coated T-M-O steel substrate showed 4–5 times improved corrosion resistance in different corrosive environments comparatively to phosphate coated F-P steel substrate. On the other hand, phosphate coated T-M steel substrate showed intermediate corrosion resistance. The bond strength of phosphate coated steel substrate to concrete was increased 2–26% after phosphate treatment on T-M-O and T-M steel substrates while the same was decreased 4–12% for phosphate coating on F-P steel substrate.
Acknowledgements 4. Conclusions 1. Coarser phosphate microstructure has been formed on F-P steel substrate whereas acicular phosphate was formed on T-M-O and T-M
The authors are grateful to Tata Steel for given approval to publish this research work in technical journal. The assistance of Mr. A. Chakraborty and Mr. V. Sharma for characterisation of the coating is also gratefully acknowledged.
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M. Manna / Surface & Coatings Technology 203 (2009) 1913–1918 Table 6 Roughness of the uncoated and phosphate coated three steel substrates
Fig. 7. Nyquist plot of phosphate coated steel substrates in 3.5% NaCl solution (a) 0 h (b) 120 h.
References [1] G. Jha, N. Bandyopadhyay, A.K. Singh, O.N. Mohanty, Product Awareness Seminar on TMT/CRS Rebars, 9th Nov. 2001, p. 47.
Fig. 8. Change in bond strength (%) of phosphate coated steel substrate to concrete structure.
Steel substrate
Uncoated
T-M-O
T-M
F-P
Roughness (μm)
1.65
2.35
1.98
1.15
[2] M.G. Fontana, Corrosion Engineering, 3rd Editions, McGraw-Hill International, 1986, p. 31. [3] K.R. Trethewey, J. Chamberlain, John Wiley & Sons, Inc., New York, 1988, 273. [4] D. Chebabe, Z. Ait Chikh, N. Hajjaji, A. Srhiri, F. Zucchi, Corros. Sci. 45 (2003) 319. [5] M. EI Achouri, M.R. Infante, F. Izquierdo, S. Kertit, H.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. Manna, G. Naidu, N. Rani, N. Bandyopadhyay, Surf. Coat. Technol. 202 (8) (2007) 1510. [10] M. Manna, N. Bandyopadhyay, D. Bhattacharjee, Surf. Coat. Technol. 202 (2008) 3227. [11] E.L. Ghali, R.J.A. Potvin, Corros. Sci. 45 (1972) 583. [12] J. Donofrio, Met. Finish. 98 (6) (2000) 57. [13] B. Gruss, Met. Finish. 98 (6) (2000) 52. [14] G.N. Bhar, N.C. Debnath, S. Roy, Surf. Coat. Technol. 35 (1988) 171. [15] US patent 5372658 Dec 13, 1994. [16] G. Gorecki, Met. Finish. 98 (6) (2000) 97. [17] US patent 5604040 Feb 1, 1997. [18] US patent 5653790, Aug 5, 1997. [19] L. Kousini, M. Azzi, M. Zertoubi, F. Dalard, S. Manimovitch, Surf. Coat. Technol. 185 (2004) 214. [20] D. Hawke, D.L. Albright, Met. Finish. (1995) 34. [21] A.S. Akhtar, D. Suage, P. Glaze, K.C. Worg, P.C. Worg, K.A.R. Mitchall, Surf. Coat. Technol. 187 (2004) 208. [22] Y. Totik, Surf. Coat. Technol. 200 (2006) 2711. [23] D. Wang, P. Jokiel, A. Uebleis, H. Boehni, Surf. Coat. Technol. 88 (1996) 147. [24] N. Satoh, T. Minami, Surf. Coat. Technol. 34 (1988) 331. [25] M.R. Christian, C.B. Jeffrey, Met. Finish. 98 (9) (2000) 42. [26] G. Li, L. Niu, J. Lian, Z. Jiang, Surf. Coat. Technol. 176 (2004) 215. [27] P.K. Sinha, R. Feser, Surf. Coat. Technol. 161 (2002) 158. [28] S. Jeganathan, T.S.N. Sankara Narayan, K. Ravichandran, S. Rajeswari, Surf. Coat. Technol. 200 (2006) 4117. [29] M. Manna, Corros. Sci. (2008), doi:10.1016/J.cosci.2008.11.021. [30] M. Manna, I. Chakrabarti, N. Bandyopadhyay, Surf. Coat. Technol. 201 (2006) 1583. [31] ASTM D 4145, ASTM International, 100 Barr Harbar Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States. [32] ASTM E 415-99a, ASTM International, 100 Barr Harbar Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States. [33] ASTM B117-03, ASTM International, 100 Barr Harbar Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States. [34] IS-1786 (1985), Indian Standards Specification, New Delhi, India, 1992.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Characterisation of phosphate coatings obtained using nitric acid free phosphate solution on three steel substrates: An option to simulate TMT rebars surfaces M. Manna ⁎ Research & Development Department, Tata Steel, Jamshedpur 831001, India
a r t i c l e
i n f o
Article history: Received 5 November 2008 Accepted in revised form 14 January 2009 Available online 24 January 2009 Keywords: TMT rebar GDOES SEM XRD EIS Corrosion
a b s t r a c t Phosphate coatings have been obtained on three steel substrates, (a) ferritic-pearlitic (F-P), (b) tempered martensitic (T-M) and (c) tempered martensitic containing oxide scale (T-M-O) at the top to simulate TMT (thermo mechanical treatment) rebar surfaces which are extensively used for composite concrete structure. Nitric acid free phosphate solution was used for the coating purpose. Scanning electron microscopy (SEM), glow discharge optical emission spectrometry (GDOES) and X-ray diffraction (XRD) techniques were used to characterise phosphate coatings. Acicular phosphate microstructure was obtained on T-M-O and T-M steel substrates, whereas coarser phosphate microstructure was obtained on F-P steel substrate. Thinner to thicker coatings were obtained on F-P, T-M and T-M-O steel substrates respectively. Oxide scale, on the T-M-O steel substrate promoted for deposition of phosphate compounds and thereby obtained thickest coating on T-M-O steel substrate. Zinc phosphate (hopeite) on T-M-O steel substrate and zinc phosphate (spencerite) on F-P steel substrate were detected as main phosphate respectively, whereas both zinc phosphates (hopeite and speccerite) on T-M steel substrate were detected in the same proportion. In addition to zinc phosphate, zinc iron phosphate (phosphophyllite) was detected on F-P and T-M steel substrates, whereas iron phosphate (beraunite) was detected on T-M-O steel substrate. A comparative performance against corrosion of all the phosphate coated steel substrates was evaluated by salt spray, Tafel and electrochemical impedance spectroscopy (EIS) studies. Test for extended exposure in open atmosphere as well as simulated highly humid condition were also conducted to identify coating performance. Phosphate coated T-M-O steel substrate showed 4–5 times improved resistance against corrosion than the phosphate coated F-P steel substrate. The bond strength of coated steel substrate with concrete was increased 2–26% for phosphate coating on T-M-O and T-M steel substrates while the same was decreased 4–12% for phosphate coating on F-P steel substrate. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Carbon steel has good mechanical properties and it is readily used as preferred material rather than corrosion resistance alloys in a variety of forms for large structures. Rebar is one of such carbon steel used extensively for composite construction of concrete structure. Presently, thermo-mechanical treatment (TMT) is a cost-effective way to produce high strength steel rebars. The process relies on spraying high-pressure water on the rebar surface immediately after rolling to force formation of martensite [1] with ferritic-pearlitic structure in the core. 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 rebars. 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. Red rust is formed on rebar surface due to formation of local galvanic cells resulting from
⁎ Tel.: +91 657 2147445; fax: +91 657 2271510. E-mail address: [email protected]. 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.01.024
compositional heterogeneity [2]. Hence, a suitable surface treatment is needed to tackle this problem. Commercial organic chemicals, such as oils [3–8], can prevent early rusting. But these are known to impair bond strength between cement and steel, therefore, unsuitable. Metal (Zn, Al, Ni) coating [9,10] on rebar surface improves the corrosion resistance property of the reinforcing steel but the product being bulk low-value steel commodity in nature, so metal coating may not be a good choice. Surface treatment with an inorganic chemical like phosphate and silicate appear worth examining. The phosphate treatment on metal surface is well known, whereby the surface of the metal is converted to an integral, mildly protective layer of insoluble crystal [11–14]. Different phosphate treatments are widely used for corrosion protection of screws, nuts, bolts, plain washer, brake components, clutch components, engine parts and several others [15]. Depending on different specific properties (corrosion resistance, paint adhesion, coefficient of friction) requirement on different metals (steel, cast iron, Mg alloys and Al alloys) surfaces, phosphate treatment has significant role [11,14,16–21]. The manganese phosphate coating obtained on AISI 4140 steel substrate after various heat treatments yield an important improvement in the corrosion resistance of the coatings [22]. Zinc phosphate has better
1914
M. Manna / Surface & Coatings Technology 203 (2009) 1913–1918
Table 1 Basic composition of steel in wt.% C
Si
Mn
P
Fe
0.18
0.35
0.5
0.03
~ 98.94
insulating ability than manganese phosphate [23] and therefore expecting for better protection capability. Phosphophyllite and hopeite crystals were obtained on cold rolled and galvanised steel substrate respectively [24]. Degreasing, cleaning, phosphate conversion coating, post cleaning, chromic acid sealing and drying are all considered important steps for phosphate treatment [16,25,26]. By cathodic deposition process, good quality even phosphate coating of desired thickness is achievable at ambient temperature without the need of an accelerator [27,28]. But hydrogenation of steel is the major limitation of this process [28]. While cathodic phosphating process may be good for flat steel surface, it is difficult to coat on uneven surfaces like rebars. Phosphate coating has been attempted in nitric acid containing phosphate solution on three steel substrates to simulate TMT rebar surfaces [29].One step phosphating process is also studied [23,30]. Surface treatment carried out in nitric acid free phosphate solution showed higher improved corrosion resistance than the surface treatment carried out in phosphate solution containing nitric acid [30]. The purposes of this paper are to study (i) the effect of phosphate coating in nitric acid free phosphate solution, on three steel substrates (to simulate TMT rebar surface) and (ii) the coatings characterisation. 2. Experimental procedure Three steel substrates, (a) ferritic-pearlitic (F-P) (b) tempered martensite (T-M) and (c) tempered martensite containing oxide scale at the top surface (T-M-O) were used for the experimental purpose. The basic composition of the steel in weight percent is shown in Table 1. Steel composition was analysed using optical emission spectrometer according to standard ASTM E 415-99a [31]. The process conditions for phosphate treatment on three steel substrates are shown in Table 2. The microstructure of the phosphate coating was examined by SEM (JEOL JXA 6400). Quantitative depth profiles of all the phosphate coatings were done using LECO(R) GDS-850A. The phosphate compounds present in the coating were identified by Grazing XRD (Philips Analytical X-ray B.V. Machine). The scan rate was 0.2°/s. ASTM D 4145 was adopted to find out coating adherence with the steel substrate [32]. All the phosphate coated steel substrates were exposed in open atmosphere to observe the time required for first appearance of red rust. The high humidity test was conducted to all coated steel substrates in a humidity chamber SC 450 (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 was used. Controlled humidity within the chamber was in the range of ±1/2%. The salt spray test was conducted to all coated steel substrates in a salt spray cabinet WK111340 (Weiss Technik). ASTM B117-03 was adopted for salt spray test [33]. 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 and charge transfer resistance of all phosphate coatings were examined by Tafel and electrochemical impedance spectroscopy (EIS) tests using Gamry DC105 system. All electrochemical experiments were conducted in 3.5% NaCl solution. The scan rate and immersion time
Table 2 Phosphating process conditions Phosphoric acid (ml/L)
Zinc oxide (g/L)
pH
Temperature (°C)
Treatment time (min)
28
10
2.05
90
10
Fig. 1. SEM images at the top surface of phosphate coating obtained on (a) F-P, (b) T-M and (c) T-M-O steel substrates.
for Tafel test were 2 mV/s and 15 min respectively. All phosphate coated and uncoated steel rods were cast in a square concrete block of 10 cm side. Quick setting cement (Convextra GP2) was used for casting purpose and curing time varied 48–60 h to achieve crushing strength of 19.61– 29.42 MPa of concrete structure according to the IS specification. The bond strength of steel surface with concrete structure was evaluated as per IS: 1786 (1985) [34]. After block curing, load versus slip was observed with the help of a tensile testing machine (100 kN FUT make tensile
M. Manna / Surface & Coatings Technology 203 (2009) 1913–1918
1915
testing machine), fitted with an appropriate precession slip measuring device as per IS: 1786 (1985) [34]. 3. Results and discussion 3.1. Characterization of phosphate coating Fig. 1(a–c) shows the SEM morphology of all three phosphate coated steel substrates. The phosphate coating, obtained on T-M and T-M-O steel substrate was acicular in nature where as the phosphate coating, obtained on F-P steel substrate was coarser and its look was like Luke warm.
Fig. 3. Grazing XRD peaks of phosphate coated (a) F-P, (b) T-M and (c) T-M-O steel substrates.
The GDOES elemental depth profile of all three phosphate coatings on three steel substrates is shown in Fig. 2(a–c). The thickest (16–22 μm) phosphate coating was obtained on T-M-O steel substrate whereas the thinnest (10–12 μm) phosphate coating was obtained on the F-P steel substrate. The thickness of the phosphate coating obtained on the T-M steel substrate was in-between and it was in the range of 12–16 μm. It is evident from GDOES depth profile that zinc content decreased and oxygen content increased at the top surface of all three phosphate coatings which can be attributed to the presence of high quantities of moisture. It is also evident from GDOES depth profile that the zinc content remained constant up to certain depth for the coating obtained on the F-P and T-M-O, whereas the zinc content gradually fall down for the coating obtained on T-M steel substrate. The comparative zinc content in the phosphate layer was highest for the coating obtained on F-P steel substrate, whereas the comparative zinc content in the phosphate layer was lowest for the coating obtained T-M-O steel substrate. Also, the iron content was high in the coating obtained on F-P and T-M steel substrates and less in the coating obtained on the T-M-O steel substrate. These results have good correlation with XRD results. It is evident from Fig. 3 that different phosphate compounds of zinc (hopeite and spencerite), zinc–iron (phosphophyllite) and iron (beraunite) present in coatings have been identified. Hopeite was the main phosphate compound in the coating obtained on the T-M-O steel substrate whereas spencerite was the main phosphate compound in the coating obtained on F-P steel substrate as evident from Table 3. The comparative percentage of hopeite and spencerite was close to same in the coating obtained on T-M steel substrate. The iron bearing phosphate, beraunite was detected in the coating obtained on T-M-O steel
Table 3 Percentage of different phosphate compounds in the coating Steel substrate
Hopeite
Spencerite
Beraunite
Phosphophyllite
T-M-O T-M F-P
78.6 50.4 28.2
20.6 43.6 67.8
0.8 0 0
0 6 4
Table 4 Corrosion rate and Ecorr of the coating in 3.5% chloride solution
Fig. 2. GDOES elemental depth profile of the phosphate coating obtained on (a) F-P, (b) T-M and (c) T-M-O steel substrates.
Steel substrate
Corrosion rate (mpy)
Ecorr (V)
T-M-O T-M F-P
0.36 0.62 1.618
−0.735 −0.638 −0.657
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M. Manna / Surface & Coatings Technology 203 (2009) 1913–1918
substrate whereas zinc–iron bearing phosphate, phosphophyllite was detected in the coatings obtained on T-M and F-P steel substrates. First, the iron oxide scale at the top of T-M-O steel substrate gets dissolved, thereby rising the local pH and promoting the iron and zinc phosphate formation, whereas the iron itself from T-M and F-P steel substrates, first gets dissolved being expected to less rise of pH at T-M and F-P steel substrates and less promoting of zinc phosphate and zinc–iron phosphate formation as mentioned in Eqs. (i)–(vi). It is expected more faster kinetic of reaction for dissolution of oxide scale than for dissolution of iron, thereby sharp rise of pH at the T-M-O steel substrate occurs. This in turn promotes for faster deposition of phosphate compounds on the T-M-O steel substrate. On the other hand, Fe get dissolve in slower rate, leads to least rise of pH at the F-P steel substrates. Least rise of pH at the F-P steel substrate results in slowest deposition of phosphate compounds on the F-P steel substrate as mentioned in Eqs. (iii)–(v). As a result, thickest to thinnest coating obtained on T-M-O, T-M and F-P steel substrates respectively. Fex Oy þ 2Hþ →Fe2þ þ H2 O
ðiÞ
Fe þ 2Hþ →Fe2þ þ H2
ðiiÞ
þ 3Zn2þ þ 2H2 PO−1 4 þ 4H2 O→Zn3 ðPO4 Þ2 4H2 O þ 4H
ðiiiÞ
þ 4Zn2þ þ 2H2 PO−1 4 þ 5H2 O→Zn4 ðOH−PO4 Þ2 3H2 O þ 6H
ðivÞ
þ Fe2þ þ 2Zn2þ þ 2H2 PO−1 4 þ 4H2 O→Zn2 FeðPO4 Þ2 4H2 O þ 4H þ 3Fe2þ þ 2H2 PO−1 4 þ 4H2 O→Fe3 ðPO4 Þ2 4H2 O þ 4H
ðvÞ ðviÞ
T-M-O steel substrate gets less time of contact with the phosphate solution under low pH condition. It results in less dissolution of iron in the solution, which in turn exhibits less iron in the coating. On the other hand, T-M and F-P steel substrates get more time of contact with the phosphate solution under low pH condition. It results in more dissolution of iron in the solution, which in turn exhibits more iron in the coatings. Initially, iron phosphate was obtained on T-M-O steel substrate due to faster rise of pH and less time available for Fe ions to diffuse in the bulk solution. After that, Fe ions were depleted at the T-M-O substrate thereby promoting hopeite and spencerite formation. The Fe ions dissolved from T-M and F-P steel substrates get longer time to diffuse in the bulk solution and deposit as zinc–iron phosphate. The comparatively high and low pH favoured for formation of hopeite and spencerite respectively, thereby expecting maximum to minimum content of hopeite in the coating obtained on T-M-O, T-M and F-P steel substrates respectively.
Fig. 4. Tafel plot of phosphate coated steel substrates in 3.5% NaCl solution.
Table 5 Time of appearance of first red rust on coated steel substrate, in different environments Steel substrate
Salt spray (h)
Humidity chamber (cycles)
Atmospheric (days)
T-M-O T-M F-P
100 40 25
45 18 10
180 70 50
3.2. Corrosion resistance behaviour of untreated and phosphate treated rebar surfaces 3.2.1. Atmospheric exposure test results All three phosphates coated steel substrates showed substantial improvement against atmospheric corrosion to prevent red rust formation and no red rust was observed up to 50 days for phosphate coated F-P steel substrate whereas no red rust was observed up to 180 days for phosphate coated T-M-O steel substrate as shown in Table 4. On the other hand no red rust was observed up to 70 days for phosphate coated T-M steel substrate. 3.2.2. High humidity and salt spray test results Phosphate coated steel substrate showed significant improvement with respect to delay in red rust formation under high humidity condition. Phosphate coating provides barrier as well as sacrificial protection to base steel to prevent red rust formation against highly humid and chloride rich environments. Phosphate coating obtained on T-M-O steel substrate showed the best resistance to prevent first red rust appearance whereas phosphate coating obtained on F-P steel surface showed the least resistance to prevent first red rust appearance. No red rust was observed for up to 10 cycles on phosphate coated F-P steel substrate whereas no red rust was observed for up to 45 cycles on phosphate coated T-M-O steel substrate. On the other hand, no red rust was observed for up to 18 cycles on phosphate coated T-M steel substrate. It is evident from Table 4 that phosphate treatment carried out on T-M-O showed better ability to prevent red rust formation in aggressive chloride environment comparatively to phosphate treatment carried out on the two other steel substrates. 3.2.3. Results of Tafel Test and EIS It is evident from Fig. 4 and Table 5 that free corrosion potential and corrosion rate were lowest for the phosphate coated T-M-O steel substrate whereas free corrosion potential and corrosion rate were highest for the phosphate coated F-P steel substrate. On the other hand, free corrosion potential and corrosion rate were inbetween for the phosphate coated T-M steel substrate. The phosphate compounds which have maximum resistance against dissolution in different corrosive environments are beneficial. From thermodynamic as well as kinetic points of view, the most effective protective phosphate coating was obtained on the T-M-O steel surface (Table 4). The electrochemical impedance test was carried out for all phosphate coated steel substrates in 3.5% NaCl solution to find out their comparative resistance against charge transfer. Fig. 5 represents the equivalent circuit diagram for electrochemical impedance test of all three phosphate coatings. The comparative resistance against charge
Fig. 5. Equivalent circuit diagram for EIS study of three phosphate coatings.
M. Manna / Surface & Coatings Technology 203 (2009) 1913–1918
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Fig. 6. Bode plot of phosphate coated steel substrates in 3.5% NaCl solution (a) impedance/0 h (b) impedance/120 h (c) phase shift/0 h (d) phase shift/120 h.
transfer of all the phosphate coatings in aggressive chloride environment for immediate exposure as well as after 5 days is shown in Figs. 6 (a–d) and 7(a,b). It is evident from Bode and Nyquist plots, that the coating obtained on T-M-O steel substrate showed maximum resistance against charge transfer whereas the coating obtained on F-P steel substrate showed minimum resistance against charge transfer. On the other hand, coating obtained on T-M steel substrate showed in-between resistance against charge transfer.
2. 3.
3.3. Pull-out test results The comparative bond strength of all the phosphate coated steel substrates to concrete structure is shown in Fig. 8. It is evident from this figure that phosphate coated T-M-O and T-M steel substrate gave 2–26% increment in bond strength whereas phosphate coated F-P steel substrate gave 4–12% drop in bond strength comparatively to bond strength of uncoated steel substrate to concrete structure. No crack or pill off occurs of phosphate coatings from all three steel substrates after bend test according to ASTM D 4145. Phosphate coatings with all three steel substrates have reasonably good adherence. Such drop in bond strength can be attributed to a far smoother coating (see Table 6) and lesser alkaline stability of spencerite in the environment of the concrete.
4.
5.
steel substrates. Thinner to thicker coatings were obtained on F-P, T-M and T-M-O steel substrates respectively. Oxide scale on T-M-O steel substrate promoted for faster deposition of phosphate compounds. Zinc phosphate (hopeite) on T-M-O steel substrate and zinc phosphate (spencerite) on F-P steel substrate were detected as main phosphate respectively, whereas both zinc phosphates (hopeite and speccerite) on T-M steel substrate were detected in same proportion. In addition to zinc phosphate, zinc iron phosphate (phosphophyllite) was detected on F-P and T-M steel substrates whereas iron phosphate (beraunite) was detected on T-M-O steel substrate. Phosphate coated T-M-O steel substrate showed 4–5 times improved corrosion resistance in different corrosive environments comparatively to phosphate coated F-P steel substrate. On the other hand, phosphate coated T-M steel substrate showed intermediate corrosion resistance. The bond strength of phosphate coated steel substrate to concrete was increased 2–26% after phosphate treatment on T-M-O and T-M steel substrates while the same was decreased 4–12% for phosphate coating on F-P steel substrate.
Acknowledgements 4. Conclusions 1. Coarser phosphate microstructure has been formed on F-P steel substrate whereas acicular phosphate was formed on T-M-O and T-M
The authors are grateful to Tata Steel for given approval to publish this research work in technical journal. The assistance of Mr. A. Chakraborty and Mr. V. Sharma for characterisation of the coating is also gratefully acknowledged.
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M. Manna / Surface & Coatings Technology 203 (2009) 1913–1918 Table 6 Roughness of the uncoated and phosphate coated three steel substrates
Fig. 7. Nyquist plot of phosphate coated steel substrates in 3.5% NaCl solution (a) 0 h (b) 120 h.
References [1] G. Jha, N. Bandyopadhyay, A.K. Singh, O.N. Mohanty, Product Awareness Seminar on TMT/CRS Rebars, 9th Nov. 2001, p. 47.
Fig. 8. Change in bond strength (%) of phosphate coated steel substrate to concrete structure.
Steel substrate
Uncoated
T-M-O
T-M
F-P
Roughness (μm)
1.65
2.35
1.98
1.15
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