Mechanical properties and corrosion behaviour of stainless steel reinforcing bars

Mechanical properties and corrosion behaviour of stainless steel reinforcing bars

Journal of Materials Processing Technology 143–144 (2003) 134–137 Mechanical properties and corrosion behaviour of stainless steel reinforcing bars H...

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Journal of Materials Processing Technology 143–144 (2003) 134–137

Mechanical properties and corrosion behaviour of stainless steel reinforcing bars H. Castro∗ , C. Rodriguez, F.J. Belzunce, A.F. Canteli University of Oviedo, Oviedo, Spain

Abstract Austenitic stainless steels have recently been used as reinforcing bars in special civil constructions in order to increase their expected life in corrosion environments. The complete mechanical characterization of two kinds of austenitic stainless steel rebars, 304LN and 316LN grades, obtained after hot and cold rolling schedules, was performed. Their mechanical properties and the influence on these of the rib region were obtained, as well as determining the fracture behaviour of these bars by means of their J–R curves. The experimental work was completed with the study of the corrosion behaviour of all the bars in a saline environment. All the results have been justified taking into account the fabrication method employed in each case. © 2003 Published by Elsevier B.V. Keywords: Stainless steels; Concrete-reinforcing bars; Fracture; Mechanical properties; Corrosion

1. Introduction It has long been recognized that carbon steel reinforcing bars have a low resistance to corrosion in chloride-bearing environments, resulting in many bridges, parking garages and marine structures having been severely damaged by corrosion of the reinforcement. Local factors such as use of road salt, acid rain, and, in coastal regions, salt water spray and chloride-laden air are potential dangerous environments. Although concrete provides protection for embedded steel, under the aforementioned conditions, the penetration of oxygen, water and chloride to the carbon steel allows rapid deterioration of the entire structure. An easy way to overcome the problem of corroding carbon steel, which leads to reinforced concrete failure, structural problems and costly repairs is to change the reinforcing material. The use of stainless steel, although initially expensive (the cost of stainless steel is nowadays 5–8 times more expensive than uncoated carbon steel), offers cost savings in the long term, eliminating rebar coatings, cement inhibitors, concrete sealers, thicker concrete overlay, maintenance costs, etc. [1,2]. Today, stainless steel reinforcing bars are considered an alternative construction when a design life of 75–100 years is demanded and life-cycle cost analysis has to be taken into account [3–5].

∗ Corresponding author. E-mail address: [email protected] (H. Castro).

0924-0136/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/S0924-0136(03)00393-5

Austenitic stainless steels are one of the best choices, as they combine very good corrosion behaviour with excellent mechanical properties (strength and toughness), especially when using LN grades, characterized by very low carbon levels (to prevent intergranular corrosion phenomena and improve weldability), and nitrogen alloying for increasing their mechanical strength. However, although the use of these materials is rapidly increasing, only a few countries have developed specific standards for stainless steel rebars, so their behaviour in these applications needs to be better known. A wide experimental program is being carried out at the University of Oviedo to analyse the mechanical, fracture and corrosion properties of different stainless steels grades intended to be use as concrete-reinforcing bars.

2. Materials and experimental procedure Two different austenitic stainless steels (304LN and 316LN grades) were furnished by ROLDAN S.A. (Ponferrada, Spain). The chemical composition of both grades is shown in Table 1. The external rib of the reinforcing bars was made in two different ways: hot and cold rolled. The hot rolled bars had a standard final diameter of 16 mm and were solution-treated and pickled after hot rolling. Cold rolled bars suffer a 15% reduction at room temperature, thus attaining the final geometry and a diameter of 12 mm.

H. Castro et al. / Journal of Materials Processing Technology 143–144 (2003) 134–137 Table 1 Chemical composition (wt.%) of the steels C

Si

Mn

Cr

Ni

Mo

135

3. Results N

Cu

S

P

304LN 0.023 0.38 1.49 18.55 8.77 0.21 0.18 0.18 0.022 0.027 316LN 0.030 0.36 1.31 17.62 11.70 2.87 0.20 0.10 0.004 0.023

Metallographic analyses performed on longitudinal and transversal sections always show regular austenitic grains, wholly recrystallized in the case of hot rolled bars and with frequent slip bands, especially near the surface ribs, for the cold rolled bars. Hardness profiles along different bar diameters were performed using a Vickers microhardness tester under a 500 g load. Tensile tests were performed directly on the reinforcing bars over a length of 200 mm and also on standard longitudinal specimens machined, respectively, with a diameter of 12 mm (hot rolled bars) and 8 mm (cold rolled bars). The J-integral crack growth resistance curves were determined using single-edge-notched bend specimens (SENB) under static conditions according to the ESIS procedure [6]. Specimens dimensions were 10 mm×10 mm×55 mm for the 16 mm diameter bars (hot rolled) and 8 mm×8 mm×55 mm for the 12 mm diameter bars (cold rolled). Fig. 1 shows the way these specimens were machined from the original reinforcing bars. All specimens were precracked in fatigue using a dynamic MTS testing machine with a maximum load of 100 kN and a load ratio Pmin /Pmax = 0.1. After the precracking process, all the specimens were side grooved to a thickness reduction of 20% as recommended in the standard in order to assure a strong triaxial state along the crack front during the test. Crack extensions were physically measured during the tests in such a manner that the data points were evenly spaced over the J–a plot. Finally, polarization resistance measurements, Tafel curves and anodic cyclic potentiodynamic polarization curves at room temperature were determined in all materials in contact with a 3 wt.% NaCl solution in distilled water according to ASTM G59 and G61 [7,8], using a potentiostat, a Ag/AgCl reference electrode and a scan rate of 0.6 V/h. All the results presented are the average values obtained after the performance of five different tests.

3.1. Mechanical properties Fig. 2 represents the microhardness measurements made along the bar diameters. Both steels, and especially after hot rolled rib shaping, show a clear surface hardening (rib region). The hardened region has a depth of only 1–2 mm. The effect of the rolling process on the bar hardness is also worth highlighting: cold rolled bars are approximately 100 Vickers units higher than hot rolled bars. Table 2 shows the tensile mechanical properties of all these steels (elastic modulus, E; yield strength, σ E ; tensile strength, σ R ; strain hardening coefficient, n; elongation, e; and area reduction, Z). The mechanical properties of both hot rolled steels are quite similar, while the non-ribbed specimens show lower strength and higher ductility than the specimens obtained directly from the reinforcing bars. This may be justified by the geometrical and hardening effect due to the rib. In contrast, the cold rolled 304LN grade has higher strength properties than the 316LN grade, due to its higher strain hardening behaviour. When the tensile behaviour of both hot rolled and cold rolled grades are compared, the hardening effect of cold rolling is notorious: yield strength is more than 70% higher for the cold rolled grades. It is also important to highlight the fact that the tensile properties obtained in all these tests are much higher than the minimum required (HV = 170 and σR = 620 MPa for the annealed 304LN grade). Fig. 3 shows the J–R curves obtained in the aforementioned toughness tests. The blunting line applied in each case is also represented in the same figure. Bar toughness is clearly lower after cold rolling, although the toughness obtained in these cases is also substantial. At the same time, as has already been seen with respect to tensile properties, the hot rolled J–R curves of both steels are similar, but after cold rolling, the 316LN grade shows a better fracture behaviour. Table 2 Tensile mechanical properties of ribbed and non-ribbed specimens σ E (MPa)

σ R (MPa)

n

e (%)

Z (%)

413 442

738.8 745.5

– –

47.4 47.7

62.8 64.2

Hot rolled without ribbed zone (Ø12 mm) 316LN 155335 398 720.2 304LN 155302 408 722.3

0.22 0.22

50.2 56.1

77.7 76.8

Cold rolled (Ø12 mm) 316LN 196816 304LN 178400

– –

24.4 22.9

44.9 44.7

0.088 0.074

23.3 22.6

71.3 66.9

Austenitic grades

E (MPa)

Hot rolled (Ø16 mm) 316LN 170867 304LN 172733

Fig. 1. Extraction of SENB specimens from the reinforcing bar.

711 764.95

884.4 931.4

Cold rolled without ribbed zone (Ø8 mm) 316LN 155384 678 814.9 304LN 182777 724 880.9

136

H. Castro et al. / Journal of Materials Processing Technology 143–144 (2003) 134–137

Fig. 2. Hardness bar profiles.

Fig. 3. J-integral versus crack extension (J–a).

The higher fracture resistance of the cold rolled 316LN grade may be justified by its greater ductility (Table 2) and especially due to the lower MnS inclusion content of this steel (see Table 1). Table 3 presents the critical J values, JIc (determined for a real crack extension of 0.2 mm) obtained in these tests using a blunting line calculated according to the ESIS procedure, which uses the strain hardening coefficient, n, of each grade. Once again, hot rolled bars and 316LN grade show the greater toughness. Finally, worthy of note are the very high JIc values obtained with all these products (also in the case of cold rolled bars) when comparing them with the usual values corresponding to carbon steel rebars (JIc lower than 100 kJ/m2 ). 3.2. Corrosion behaviour Linear resistance polarization methods have been used as an approximative technique for measuring corrosion rates in a chloride aqueous solution, as the polarization resistance obtained in these tests is inversely related to the corrosion rate. Also, uniform corrosion rates were obtained Table 3 Reinforcing bars JIc (kJ/m2 ) values

Hot rolled (Ø16) Cold rolled (Ø12)

316LN

304LN

438 348

312 203

under the same conditions using Tafel curves along with Faraday’s law. On the other hand, potentiodynamic testing was aimed to studying the pitting behaviour of these austenitic steels in the same chloride aqueous medium. Whether an alloy–environment combination will give rise to pitting depends on the separation between corrosion potential, Ecorr , and pitting or breakdown potential, Ep . The results obtained in the corrosion tests are given in Table 4. The beneficial effect of molybdenum on the localized corrosion behaviour (pitting) of austenitic steels in contact with chloride-containing solutions is evident when comparing Ep − Ecorr results. It is also seen that cold rolling decreases the pitting resistance of the bars, and so cold rolled 304LN steel is prone to pitting in this solution, as can be better seen when the whole potentiodynamic curve is represented (Fig. 4). The detrimental effect of cold rolling and the beneficial effect due to molybdenum are also clearly seen on the resistance polarization and Tafel results. Table 4 Corrosion parameters Austenitic grades

Rp (k)

Vcorr (mm per year)

Ecorr (mV)

Ep (mV)

Ep − Ecorr (mV)

316LN 304LN 316LN 304LN

607 265 72 23

0.9 0.6 1.5 1.7

−154 −139 −153 −132

+421 +247 +230 +10

+575 +386 +383 +142

(hot rolled) (hot rolled) (cold rolled) (cold rolled)

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Acknowledgements The authors would like to acknowledge the support given to this work through FEDER project 1FD97-1525. The contribution of ROLDAN S.A. (Ponferrada, Spain) supplying the steel used in the experiments is also gratefully acknowledged by the authors.

References

Fig. 4. Potentiodynamic polarization curves of 304LN steel, hot rolled and cold rolled.

4. Conclusions The mechanical properties, fracture and corrosion behaviour of austenitic reinforced bars were determined in relation to the rib shaping process (hot rolled and cold rolled), the following conclusions have been finally drawn. Cold rolled rib shaping allows a strong increase of the strength properties and hardness of the austenitic reinforcing bars while at the same time maintaining very high toughness levels, when compared with carbon steel rebars. This effect is especially important in the case of 304LN austenitic steel, due to its very high strain hardening potential. The detrimental effect of cold rolling and the beneficial effect due to molybdenum on the corrosion rate and localized corrosion behaviour (pitting) was clearly seen in contact with a chloride-containing aqueous solution. Under these conditions, cold rolled 304LN bars are prone to pitting while cold rolled 316LN bars, in contrast, maintain excellent corrosion behaviour along with very good mechanical properties.

[1] J.M. McGurn, Stainless steels reinforcing bars in concrete, in: Proceedings of the International Conference on Corrosion and Rehabilitation of Reinforced Concrete Structures, Orlando, December 1998. [2] A. Knudsen, Cost effective enhancement of durability of concrete structures by intelligent use of stainless steels reinforcement, in: Proceedings of the International Conference on Corrosion and Rehabilitation of Reinforced Concrete Structures, Orlando, December 1998. [3] L. Bertolini, P. Pedeferri, T. Pastore, Stainless steel in reinforced concrete structures, concrete under severe condition 2, in: Proceedings of the Second International Conference on Concrete under Severe Conditions, vol. 1, Tromso, Norway, June 21–24, 1998 (publicado por E&FN Spon). [4] The Concrete Society (UK): Concrete Society Technical Report 51, Guidance on the Use of Stainless Steel Reinforcement, Report of a Concrete Society Steering Committee, 1998. [5] A.K.C. IP, F. Pianca, B.B. Hope, Application of stainless steel reinforcement for highway bridges in Ontario, in: F.N. Smith, J.F. McGurn, G.Y. Lai, V.S. Sastri (Eds.), Nickel–Cobalt’97, Applications and Materials Performance, vol. IV, The Metallurgical Society, CIM, Montreal, 1997, pp. 227–284. [6] ESIS 92:P2, European Structural Integrity Society, Delft, Holland, 1992. [7] ASTM G59, Standard practice for conducting potentiodynamic polarization resistance measurements, Annual Book of ASTM Standards, vol. 03.02, PA, USA. [8] ASTM G61, Standard test method for conducting cyclic potentiodynamic polarization measurements for localized corrosion susceptibility of iron-, nickel-, or cobalt-based alloys, Annual Book of ASTM Standards, vol. 03.02, PA, USA.