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The effect of annealing on the corrosion behaviour of 444 stainless steel for drinking water applications
Corrosion Science xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Letter
The effect of annealing on the corrosion behaviour of 444 stainless steel for drinking water applications C. Bitondo a, A. Bossio a, T. Monetta a, M. Curioni b, F. Bellucci a,⇑ a b
University of Naples ‘‘Federico II’’, Dept. of Chemical Engineering, Materials and Industrial Production, 80125 Naples, Italy Corrosion and Protection Centre, The University of Manchester, Manchester M139PL, UK
a r t i c l e
i n f o
Article history: Received 21 May 2014 Accepted 7 June 2014 Available online xxxx Keywords: A. Stainless steel B. Polarization B. SEM C. Crevice corrosion
a b s t r a c t In this study, the corrosion behaviour of annealed and not annealed AISI 444 ferritic stainless steel in tap water with and without addition of selected concentrations of chloride ions was investigated. Cyclic potentiodynamic macro (large area) and micro (small area) polarization measurements (CPP), salt spray test, SEM and EDS analysis were employed to evaluate the pitting and crevice corrosion susceptibility of annealed and not annealed AISI 444. The results obtained indicate that annealing does not improve the resistance to pitting and crevice corrosion. Moreover, micro CPP indicates local susceptibility to pitting on both annealed and not annealed materials; such susceptibility was not evident from macropolarization tests. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The wide use of austenitic stainless steel in several industrial sectors is due to the combination of mechanical properties and corrosion resistance [1]. Recently, owing to the market competition, a substantial increase in the austenitic stainless steels price has been observed, with price per ton increasing from 2.500 to 13.200€ over the last five years [2]. This substantial price increase is attributed to the development of the Asian and South American economies, contributing to an increased demand for Ni and Mo, key alloying elements for the production of austenitic stainless steel. A possible route to minimize the impact of increased Ni and Mo cost on stainless steel price is the reduction of Ni and Mo content. Thus the replacement of austenitic stainless steels with ferritic steels, such as the AISI 444, is an option for some application. However, it is important that the ferritic stainless steel provides a solution of similar quality both in terms of mechanical properties and corrosion resistance. Areas in which the applications of ferritic stainless steels could increase in the near future range from industrial plants to the transport of drinking water. While the pitting corrosion resistance of the austenitic stainless steels 304 L and 316 L is widely studied [3,4], less attention has been paid to the corrosion behaviour of AISI 444 [5,6]. Tests performed with mixtures of hydrocarbons (immersion in heavy oil and cyclic reactivation EPR-DL), at temperatures typical ⇑ Corresponding author. Tel.: +39 081 7682402; fax: +39 0817682404. E-mail address: [email protected] (F. Bellucci).
of a distillation tower of crude oil (200, 300 and 400 °C) showed that AISI 444 is not substantially more susceptible to corrosion compared to the AISI 304 typically adopted for such applications [7]. On the other hand, tests performed at temperatures typical of distillation towers of the petrochemical industry (400, 475 °C) [6], indicate that at these temperatures the maximum precipitation of a phase occurs, resulting in an improvement of the mechanical properties but a decay of the corrosion resistance. Additional tests, performed to evaluate the effect of annealing at temperatures of 955, 980 and 1010 °C [8], revealed precipitation of titanium and niobium carbides and nitrides, which improve the mechanical properties and make the AISI 444 suitable for deep drawing. However, no tests were carried out to characterize the corrosion behaviour after annealing. Recently, due to new restrictions on the use of materials that might potentially release Ni ions, AISI 444 has been indicated as a potential substitute of 304 and 316 stainless steels, traditionally employed for drinking water applications [2]. Often, a press fitting system is used to join reliably and economically pipelines of diameter ranging from 15 to 108 mm for various type of industrial, residential and commercial applications. Press fittings have, at each end, a toroidal groove containing a synthetic rubber o-ring. During assembly, an appropriate tool induces plastic deformation of the toroid in order to provide the tightness to the coupling. Pipes and fittings can be made of both austenitic and ferritic stainless steel. If the plastic deformation of the fitting is not optimal, regions of stagnant fluid might develop under the o-ring or between the contacting metals. In these regions, a classical differential aeration
http://dx.doi.org/10.1016/j.corsci.2014.06.025 0010-938X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: C. Bitondo et al., The effect of annealing on the corrosion behaviour of 444 stainless steel for drinking water applications, Corros. Sci. (2014), http://dx.doi.org/10.1016/j.corsci.2014.06.025
2
C. Bitondo et al. / Corrosion Science xxx (2014) xxx–xxx
cell can develop and chloride concentration increases, leading to potential crevice corrosion problems. The pitting corrosion behaviour of steel AISI 444 (not annealed) was examined in [9] with the aim of investigating the possibility of replacing the drinking water tanks walls, currently made of AISI 304 and AISI 316L. From the tests conducted in synthetic drinking water, it was revealed that AISI 444 does not have a better resistance to pitting compared to the austenitic steels. However, corrosion resistance could be improved up to the levels offered by the austenitic steels by performing a surface treatment of passivation. On not annealed AISI 444, tests were conducted to evaluate the susceptibility to crevice corrosion [10]. It was shown that AISI 444 might suffer crevice corrosion, similarly to AISI 304 and AISI 316L [11,12] but, by using a suitable surface treatment with a mixture of HNO3 and HF, the corrosion resistance can be improved [9]. A recent study was conducted on not annealed steel AISI 444 [13], according to test described in the Japanese standard JIS G0592 (equivalent to the ASTM G192-08). This is an electrochemical method for the determination of the potential for crevice corrosion. The results showed that the presence of chloride ions has a detrimental effect on the crevice corrosion resistance of AISI 444. Nowadays, AISI 444 stainless steel is available on the market as annealed (more expensive) and not annealed. In this work, pitting and crevice susceptibility of annealed and not annealed AISI 444 stainless steel in tap water, contaminated with chloride ions, at room temperature was evaluated by using micro and macrocyclic potentiodynamic polarizations (CPP). The results from electrochemical testing were complemented by salt spray tests, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), in order to reveal the differences in corrosion behaviour between annealed and not annealed AISI 444 stainless steels. 2. Materials and methods The composition of the alloy used in this investigation is reported in Table 1. The annealing treatment was carried out industrially by heating the material at a temperature of about 1000 °C for 60 s as indicated by the supplier. In order to evaluate the corrosion behaviour of AISI 444, and the surface composition of the corroded areas, the following measurements were undertaken: (i) free corrosion potential (OCP), (ii) macroscopic cyclic potentiodynamic polarization (CPP), (iii) micropotentiodynamic polarization, (iv) metallographic test with EDS analysis, and (v) salt spray test. All the electrochemical tests were performed in tap water of the city of Naples whose hardness, pH and chemical composition is reported in Table 2. To simulate more aggressive environmental conditions, selected concentrations of NaCl were added to the tap water, to obtain a solution with low (0.02 wt% NaCl), medium (0.1% NaCl) and high (3.5 wt% NaCl) aggressiveness. 2.1. Electrochemical tests Aerated and deaerated CPP measurements were carried out on annealed and not annealed specimens of AISI 444 at room temperature in uncontaminated (pure tap water) and contaminated tap water with added NaCl. The measurements were performed in according to ASTM G3 and G5 using a 3 electrodes electrochemical
Table 2 Hardness, pH and chemical composition of Naples tap water. Total hardness (°F) Residual at 180 °C (mg/l) pH Nitrates (mg/l of NO3) Nitrites (mg/l of NO2) Ammonium (mg/l of NH4) Fluorides (lg/l of F ) Chlorides (mg/l of Cl )
39 569 7.3 12 <0.05 <0.03 400 53
cell, with a saturated calomel (SCE) as reference electrode (RE), a platinum foil as counter electrode (CE) and the stainless steel specimen as the working electrode (WE). Cell arrangement and procedure have been reported elsewhere [14]. The polarization curves were recorded using a Solartron SI1287 potentiostat connected with a Solartron SI1260, function generator, which was interfaced with a personal computer by a National Instruments GPIB-USB-HS data logger. Before the test, the corrosion potential (Ecorr) was monitored until it was steady. The ohmic drop between the working electrode and the reference electrode was not compensated. All the anodic potentiodynamic curves were initiated 30 mV below the corrosion potential and performed with a scan rate of 0.166 mV/s (10 mV/min). The direction of polarization was reversed at 0.1 mA/cm2. After reversal, the polarization was interrupted either when repassivation was observed, or when the original corrosion potential was reached. From the experimental polarization curves the corrosion current density, icorr, the pitting, Epit, and the protection, Eprot potentials were obtained. The icorr values were estimated by extrapolating the linear region of the anodic potentiodynamic response to the corrosion potential. In this paper the pitting potential is taken as the critical potential at which a dramatic increase in current is observed in the anodic polarization scan. All the specimens were observed by optical microscopy before and after polarization, in order to exclude the occurrence of crevice corrosion and to investigate the morphology of pitting. Selected specimens were also examined by SEM and EDS analysis (Hitachi TM3000). 2.2. Potentiodynamic micropolarization Potentiodynamic micropolarization measurements were carried out by using a microcapillary cell, described in detail elsewhere [15,16], and schematically represented in Fig. 1. On each specimen eight micro-areas were identified in two rows of four. This configuration allows to obtain a map of the electrochemical reactivity of both annealed and not annealed AISI 444 surface. With the micro-capillary cell setup, the test electrolyte is contained in a pipette tip, positioned on the area to be tested. The pipette is connected to a solution reservoir where both reference electrode (SCE) and counter electrode (platinum wire) are accommodated. For all the tests performed, a pipette tip size of 0.35 mm2 was used. Micro CPP tests were carried out according to the procedure outlined above for macro CPP, with the only difference that the scan rate used was 1 mV/s, due to the small volume of the solution available in the microcapillary [16]. All measurements were carried out on triplicate coupons and the average values are reported in this paper.
Table 1 Percentage chemical composition of AISI 444 used in this work.
AISI 444
Grade
C
Si
Mn
P
S
N
Cr
Mo
Ni
Ti
X2CrMoTi18-2
0.014
0.279
0.216
0.029
0.002
0.0148
17.859
2.074
–
0.4122
Please cite this article in press as: C. Bitondo et al., The effect of annealing on the corrosion behaviour of 444 stainless steel for drinking water applications, Corros. Sci. (2014), http://dx.doi.org/10.1016/j.corsci.2014.06.025
C. Bitondo et al. / Corrosion Science xxx (2014) xxx–xxx
3
Fig. 1. Schematic representation of the electrochemical set-up used to acquire micropotentiodynamic polarizations.
Fig. 3. Macrocyclic potentiodynamic polarization curves obtained from AISI 444 in artificially aerated tap water without the addition of chloride and with the addition of 0.1% and 3.5% NaCl: (a) not annealed, (b) annealed.
tates contained mainly titanium (in the range 45–90 wt%) and Niobium (in the range 15–20 wt%), a composition similar to that indicated by Silva et al. [7] and de Abreu et al. [17].
3.2. Cyclic potentiodynamic polarization
Fig. 2. SEM micrographs of ferritic steel AISI 444 acquired at 1500 magnification: (a) not annealed, (b) annealed.
2.3. Salt spray tests Salt spray tests were carried out in accordance with ASTM B117 standard on annealed and not annealed AISI 444 tubes, and on tubes to which an O-ring was applied to evaluate susceptibility to crevice corrosion. Specimens used in these tests where in the form of 35 mm diameter tubes. 3. Results and discussion 3.1. Microstructure In Fig. 2, the microstructure and grain size of not annealed (a) and annealed (b) AISI 444 are displayed. It is evident that grain size and the distribution of precipitates were not substantially affected by annealing. Unreported EDS analysis indicates that the precipi-
3.2.1. Influence of chloride ions on not annealed and annealed AISI 444 CPP is the electrochemical test currently used to evaluate the pitting potential, Epit, and the protection protential, Eprot. Susceptibility to pitting is based on the values of Epit and Eprot in a chloridecontaining environment [10]. Values of Eprot close to Epit indicate little susceptibility to pitting, while lower values of Eprot compared to Epit indicate susceptibility to pitting. CPP measurements carried out on annealed and not annealed AISI 444 in tap water with and without the addition of chloride are presented in Fig. 3a and b. It is evident that the values of Eprot are close to the values of Epit only for tests conducted in tap water without addition of chloride. Addition of 0.1% and 3.5% wt. NaCl resulted in a significant decrease of Epit and values of Eprot lower than Ecorr were observed for both annealed and not annealed AISI 444. In addition, it was found that Epit depends linearly on the logarithmic concentration of NaCl (Fig. 4) for both annealed and not annealed specimens in aerated and deaerated conditions. Similar logarithmic dependence was reported in the literature for the critical crevice potential, Ecrev, of not annealed AISI 444 [13]. Results from macroscopic CPP reported in this paper and in the literature [10,13] indicate susceptibility of AISI 444 to both pitting and crevice corrosion. High values of Eprot and Ecrev were observed
Please cite this article in press as: C. Bitondo et al., The effect of annealing on the corrosion behaviour of 444 stainless steel for drinking water applications, Corros. Sci. (2014), http://dx.doi.org/10.1016/j.corsci.2014.06.025
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C. Bitondo et al. / Corrosion Science xxx (2014) xxx–xxx
Table 3 Micropolarization results: mean and standard deviation of Ecorr and icorr. Average Ecorr, mV vs SCE icorr, lA/cm2
Annealed Not annealed Annealed Not annealed
13.0 35.5 1.1 0.7
St. Dev. 54 70 0.7 0.2
Fig. 4. Values of pitting potential as a function of chloride concentration for AISI 444 exposed to aerated and deaerated solutions. Dotted and full lines represent the best fitting lines for the annealed and not annealed AISI 444, respectively.
Fig. 6. Surface appearance of annealed AISI 444 after 250 h of exposure to salt spray test.
shown in Fig. 1. Micro CPP curves were obtained for both the not annealed and the annealed AISI 444 exposed to the tap water without addition of chloride. For each specimen 8 CPP curves were obtained at various locations. Fig. 5(a and b) shows the curves associated to the best and the worst behaviour exhibited by the annealed and not annealed steel. For the not annealed specimen, the curve obtained from the region labelled as 2 in Fig. 5a, displays behaviour similar to that observed macroscopically, while the curve obtained from the region labelled as 8 displays an Eprot of about 0.5 V below the Epit. A similar behaviour is observed in Fig. 5b, displaying curves obtained on the annealed material, but the reduction in Eprot compared to Epit is more substantial. It is evident that the micropolarization data do not directly agree with the macroscopic polarization data, suggesting a local susceptibility to pitting of AISI 444 even in presence of low Cl content. The data generated by micropolarization (Ecorr and estimated icorr) are reported in Table 3. The values of icorr are an estimate of the real corrosion rate due to the high scan rate adopted. As can be seen from data reported in Table 3, the annealed specimen exhibit a higher average and a wider variation of icorr and lower values of Ecorr suggesting that the surface might be locally more susceptible to corrosion in tap water environment.
3.4. Salt spray test Fig. 5. Best (full line) and worst (dotted line) micropolarization curves obtained in Naples tap water: (a) not annealed, (b) annealed.
only for values of Cl on the order of 50 ppm (this investigation) and 20 ppm [13]. On the basis of these results, therefore, AISI 444 can be recommended for all drinking water applications where only very low values of Cl contents are expected. 3.3. Micropolarization In addition to the macro CPP tests, further CPP measurements were carried out on the microscale, by using the technique of micropolarization [16]. The experimental set-up is schematically
Salt spray tests were carried out in addition to the electrochemical tests by exposing to the salt fog environment the annealed and not annealed specimens in presence and absence of crevice promoters for 10 days. No evidence of pitting corrosion was observed on both annealed and not annealed specimens without O-ring (absence of crevice promoters). However, both annealed and not annealed specimens suffered crevice corrosion attack extended for the 6% and for the 63% of the surface covered by the O-ring for the annealed (see Fig. 6) and not annealed specimen, respectively. Results reported above clearly indicate that the annealing treatment adopted in the industry does not improve the corrosion resis-
Please cite this article in press as: C. Bitondo et al., The effect of annealing on the corrosion behaviour of 444 stainless steel for drinking water applications, Corros. Sci. (2014), http://dx.doi.org/10.1016/j.corsci.2014.06.025
C. Bitondo et al. / Corrosion Science xxx (2014) xxx–xxx
tance of AISI 444. This result could be attributed to the fact that the heat treatment does not appear to induce substantial changes in the alloy microstructure. 4. Conclusions The results reported in this work indicate that 1. Metallographic observations and EDS analysis suggest that annealing treatment commercially available for the AISI 444, does not modify grain size and composition of intergranular precipitates, and therefore does not improve the corrosion resistance. 2. Macroscopic CPP indicate that the AISI 444 steel is susceptible to pitting corrosion, with a linear dependence of Epit on the logarithm of the chloride concentration. In addition, a protection potential was observed only for specimens exposed to tap water containing 50 ppm of Cl . Macroscopic CPP could not resolve the differences between the not annealed and the annealed material. 3. Microscopic CPP indicate that on both annealed and not annealed specimens local areas with values of Eprot lower than Ecorr are present, suggesting a susceptibility to pitting even in tap water. Additionally, compared to the not annealed steel, the annealed material displays lower values of Ecorr and higher values of icorr in tap water. This result also suggests that the annealed treatment does not improve the corrosion resistance of AISI 444. 4. Preliminary salt spray exposure test also indicates no beneficial effect of the heat treatment on the susceptibility to crevice corrosion of the AISI 444.
References [1] A.J. Sedriks, Corrosion of Stainless Steels, A Wiley-Interscience Publication, 1996. Volume 15 of Corrosion Monograph Series. [2] A. Burkert, J. Lehmann, A. Burkert, J. Mietz, P. Gumpel, Technical and economical stainless steel alternatives for civil engineering applications, Mater. Corr. 64 (9999) (2013) 1–17.
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[3] L. Peguet, B. Malki, B. Baroux, Influence of cold working on the pitting corrosion resistance of stainless steels, Corros. Sci. 49 (2007) 1933–1948. [4] A. Kocijan, C. Donik, M. Jenko, Electrochemical and XPS studies of the passive film formed on stainless steels in borate buffer and chloride solutions, Corros. Sci. 49 (2007) 2083–2098. [5] J.P.S.E. Machado, C.C. Silva, A.V.C. Sobral-Santiago, H.B. de Sant’Anna, J.P. Farias, Effect of temperature on the level of corrosion caused by heavy petroleum on AISI 304 and AISI 444 stainless steel, Mater. Res. 9 (2006) 137–142. [6] J.A. Souza, H.F.G. Abreu, A.M. Nascimento, J.A.C. de Paiva, P. de Lima-Neto, S.S.M. Tavares, Effects of low-temperature aging on AISI 444 steel, J. Mater. Eng. Perf. 14 (3) (2005) 367–372. [7] C.C. Silva, J.P.S.E. Machado, A.V.C. Sobral-Santiago, H.B. de Sant’Ana, J.P. Farias, High-temperature hydrogen sulfide corrosion on the heat-affected zone of the AISI 444 stainless steel caused by Venezuelan heavy petroleum, J. Petr. Sci. Eng. 59 (2007) 219–225. [8] H.F.G. de Abreu, A.D.S. Bruno, S.S. M Tavares, R.P. Santos, S.S. Carvalho, Effect of high temperature annealing on texture and microstructure on an AISI-444 ferritic stainless steel, Mater. Charact. 57 (2006) 342–347. [9] T. Bellezze, G. Roventi, A. Quaranta, R. Fratesi, Improvement of pitting corrosion resistance of AISI 444 stainless steel to make it a possible substitute for AISI 304L and 316L in hot natural waters, Mater. Corr. 59 (9) (2008) 727–731. [10] T. Bellezze, A. Quaranta, G. Roventi, R. Fratesi, Studio della corrosione interstiziale degli acciai inossidabili AISI 316L, AISI 304L e AISI 444, La metal. ita. 1 (2008) 7–11. [11] B. Cai, Y. Liu, X. Tian, F. Wang, H. Li, R. Ji, An experimental study of crevice corrosion behaviour of 316L stainless steel in artificial seawater, Corros. Sci. 52 (2010) 3235–3242. [12] Q. Hu, G. Zhang, Y. Qiu, X. Guo, The crevice corrosion behaviour of stainless steel in sodium chloride solution, Corros. Sci. 53 (2011) 4065– 4072. [13] Chan-Jin Park, Yong-Heon Lee, Initiation and repassivation of Crevice corrosion of type 444 stainless steel in chloride solution, Met. Mater. Int. 10 (5) (2004) 447–451. [14] T. Monetta, A. Acquesta, V. Maresca, R. Signore, F. Bellucci, P. Di Petta, M. Lo Masti, Characterization of aluminum alloys environmentally friendly surface treatments for aircraft and aerospace industry, Surf. Int. Anal. 45 (2013). 1522– 152. [15] G. Castagnola, A. Squillace, C. Bitondo, M. Montuori, F. Acerra, T. Monetta, F. Bellucci, Electrochemical and mechanical behaviour of similar fsw lap joints, ASST 2009 Conference, 12–14 May 2009, Leiden, Netherlands, Corr. Man. 90 (2009) 11–14. [16] M. Jariyaboon, A.J. Davenport, R. Ambat, B.J. Connolly, S.W. Williams, D.A. Price, The effect of welding parameters on the corrosion behaviour of friction stir welded AA2024-T351, Corros. Sci. 49 (2006) 877–909. [17] H.F.G. de Abreu, A.D.S. Bruno, S.S. M Tavares, R.P. Santos, S.S. Carvalho, Effect of high temperature annealing on texture and microstructure on an AISI-444 ferritic stainless steel, Mater. Charact. 57 (2006) 342–347.
Please cite this article in press as: C. Bitondo et al., The effect of annealing on the corrosion behaviour of 444 stainless steel for drinking water applications, Corros. Sci. (2014), http://dx.doi.org/10.1016/j.corsci.2014.06.025
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Letter
The effect of annealing on the corrosion behaviour of 444 stainless steel for drinking water applications C. Bitondo a, A. Bossio a, T. Monetta a, M. Curioni b, F. Bellucci a,⇑ a b
University of Naples ‘‘Federico II’’, Dept. of Chemical Engineering, Materials and Industrial Production, 80125 Naples, Italy Corrosion and Protection Centre, The University of Manchester, Manchester M139PL, UK
a r t i c l e
i n f o
Article history: Received 21 May 2014 Accepted 7 June 2014 Available online xxxx Keywords: A. Stainless steel B. Polarization B. SEM C. Crevice corrosion
a b s t r a c t In this study, the corrosion behaviour of annealed and not annealed AISI 444 ferritic stainless steel in tap water with and without addition of selected concentrations of chloride ions was investigated. Cyclic potentiodynamic macro (large area) and micro (small area) polarization measurements (CPP), salt spray test, SEM and EDS analysis were employed to evaluate the pitting and crevice corrosion susceptibility of annealed and not annealed AISI 444. The results obtained indicate that annealing does not improve the resistance to pitting and crevice corrosion. Moreover, micro CPP indicates local susceptibility to pitting on both annealed and not annealed materials; such susceptibility was not evident from macropolarization tests. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The wide use of austenitic stainless steel in several industrial sectors is due to the combination of mechanical properties and corrosion resistance [1]. Recently, owing to the market competition, a substantial increase in the austenitic stainless steels price has been observed, with price per ton increasing from 2.500 to 13.200€ over the last five years [2]. This substantial price increase is attributed to the development of the Asian and South American economies, contributing to an increased demand for Ni and Mo, key alloying elements for the production of austenitic stainless steel. A possible route to minimize the impact of increased Ni and Mo cost on stainless steel price is the reduction of Ni and Mo content. Thus the replacement of austenitic stainless steels with ferritic steels, such as the AISI 444, is an option for some application. However, it is important that the ferritic stainless steel provides a solution of similar quality both in terms of mechanical properties and corrosion resistance. Areas in which the applications of ferritic stainless steels could increase in the near future range from industrial plants to the transport of drinking water. While the pitting corrosion resistance of the austenitic stainless steels 304 L and 316 L is widely studied [3,4], less attention has been paid to the corrosion behaviour of AISI 444 [5,6]. Tests performed with mixtures of hydrocarbons (immersion in heavy oil and cyclic reactivation EPR-DL), at temperatures typical ⇑ Corresponding author. Tel.: +39 081 7682402; fax: +39 0817682404. E-mail address: [email protected] (F. Bellucci).
of a distillation tower of crude oil (200, 300 and 400 °C) showed that AISI 444 is not substantially more susceptible to corrosion compared to the AISI 304 typically adopted for such applications [7]. On the other hand, tests performed at temperatures typical of distillation towers of the petrochemical industry (400, 475 °C) [6], indicate that at these temperatures the maximum precipitation of a phase occurs, resulting in an improvement of the mechanical properties but a decay of the corrosion resistance. Additional tests, performed to evaluate the effect of annealing at temperatures of 955, 980 and 1010 °C [8], revealed precipitation of titanium and niobium carbides and nitrides, which improve the mechanical properties and make the AISI 444 suitable for deep drawing. However, no tests were carried out to characterize the corrosion behaviour after annealing. Recently, due to new restrictions on the use of materials that might potentially release Ni ions, AISI 444 has been indicated as a potential substitute of 304 and 316 stainless steels, traditionally employed for drinking water applications [2]. Often, a press fitting system is used to join reliably and economically pipelines of diameter ranging from 15 to 108 mm for various type of industrial, residential and commercial applications. Press fittings have, at each end, a toroidal groove containing a synthetic rubber o-ring. During assembly, an appropriate tool induces plastic deformation of the toroid in order to provide the tightness to the coupling. Pipes and fittings can be made of both austenitic and ferritic stainless steel. If the plastic deformation of the fitting is not optimal, regions of stagnant fluid might develop under the o-ring or between the contacting metals. In these regions, a classical differential aeration
http://dx.doi.org/10.1016/j.corsci.2014.06.025 0010-938X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: C. Bitondo et al., The effect of annealing on the corrosion behaviour of 444 stainless steel for drinking water applications, Corros. Sci. (2014), http://dx.doi.org/10.1016/j.corsci.2014.06.025
2
C. Bitondo et al. / Corrosion Science xxx (2014) xxx–xxx
cell can develop and chloride concentration increases, leading to potential crevice corrosion problems. The pitting corrosion behaviour of steel AISI 444 (not annealed) was examined in [9] with the aim of investigating the possibility of replacing the drinking water tanks walls, currently made of AISI 304 and AISI 316L. From the tests conducted in synthetic drinking water, it was revealed that AISI 444 does not have a better resistance to pitting compared to the austenitic steels. However, corrosion resistance could be improved up to the levels offered by the austenitic steels by performing a surface treatment of passivation. On not annealed AISI 444, tests were conducted to evaluate the susceptibility to crevice corrosion [10]. It was shown that AISI 444 might suffer crevice corrosion, similarly to AISI 304 and AISI 316L [11,12] but, by using a suitable surface treatment with a mixture of HNO3 and HF, the corrosion resistance can be improved [9]. A recent study was conducted on not annealed steel AISI 444 [13], according to test described in the Japanese standard JIS G0592 (equivalent to the ASTM G192-08). This is an electrochemical method for the determination of the potential for crevice corrosion. The results showed that the presence of chloride ions has a detrimental effect on the crevice corrosion resistance of AISI 444. Nowadays, AISI 444 stainless steel is available on the market as annealed (more expensive) and not annealed. In this work, pitting and crevice susceptibility of annealed and not annealed AISI 444 stainless steel in tap water, contaminated with chloride ions, at room temperature was evaluated by using micro and macrocyclic potentiodynamic polarizations (CPP). The results from electrochemical testing were complemented by salt spray tests, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), in order to reveal the differences in corrosion behaviour between annealed and not annealed AISI 444 stainless steels. 2. Materials and methods The composition of the alloy used in this investigation is reported in Table 1. The annealing treatment was carried out industrially by heating the material at a temperature of about 1000 °C for 60 s as indicated by the supplier. In order to evaluate the corrosion behaviour of AISI 444, and the surface composition of the corroded areas, the following measurements were undertaken: (i) free corrosion potential (OCP), (ii) macroscopic cyclic potentiodynamic polarization (CPP), (iii) micropotentiodynamic polarization, (iv) metallographic test with EDS analysis, and (v) salt spray test. All the electrochemical tests were performed in tap water of the city of Naples whose hardness, pH and chemical composition is reported in Table 2. To simulate more aggressive environmental conditions, selected concentrations of NaCl were added to the tap water, to obtain a solution with low (0.02 wt% NaCl), medium (0.1% NaCl) and high (3.5 wt% NaCl) aggressiveness. 2.1. Electrochemical tests Aerated and deaerated CPP measurements were carried out on annealed and not annealed specimens of AISI 444 at room temperature in uncontaminated (pure tap water) and contaminated tap water with added NaCl. The measurements were performed in according to ASTM G3 and G5 using a 3 electrodes electrochemical
Table 2 Hardness, pH and chemical composition of Naples tap water. Total hardness (°F) Residual at 180 °C (mg/l) pH Nitrates (mg/l of NO3) Nitrites (mg/l of NO2) Ammonium (mg/l of NH4) Fluorides (lg/l of F ) Chlorides (mg/l of Cl )
39 569 7.3 12 <0.05 <0.03 400 53
cell, with a saturated calomel (SCE) as reference electrode (RE), a platinum foil as counter electrode (CE) and the stainless steel specimen as the working electrode (WE). Cell arrangement and procedure have been reported elsewhere [14]. The polarization curves were recorded using a Solartron SI1287 potentiostat connected with a Solartron SI1260, function generator, which was interfaced with a personal computer by a National Instruments GPIB-USB-HS data logger. Before the test, the corrosion potential (Ecorr) was monitored until it was steady. The ohmic drop between the working electrode and the reference electrode was not compensated. All the anodic potentiodynamic curves were initiated 30 mV below the corrosion potential and performed with a scan rate of 0.166 mV/s (10 mV/min). The direction of polarization was reversed at 0.1 mA/cm2. After reversal, the polarization was interrupted either when repassivation was observed, or when the original corrosion potential was reached. From the experimental polarization curves the corrosion current density, icorr, the pitting, Epit, and the protection, Eprot potentials were obtained. The icorr values were estimated by extrapolating the linear region of the anodic potentiodynamic response to the corrosion potential. In this paper the pitting potential is taken as the critical potential at which a dramatic increase in current is observed in the anodic polarization scan. All the specimens were observed by optical microscopy before and after polarization, in order to exclude the occurrence of crevice corrosion and to investigate the morphology of pitting. Selected specimens were also examined by SEM and EDS analysis (Hitachi TM3000). 2.2. Potentiodynamic micropolarization Potentiodynamic micropolarization measurements were carried out by using a microcapillary cell, described in detail elsewhere [15,16], and schematically represented in Fig. 1. On each specimen eight micro-areas were identified in two rows of four. This configuration allows to obtain a map of the electrochemical reactivity of both annealed and not annealed AISI 444 surface. With the micro-capillary cell setup, the test electrolyte is contained in a pipette tip, positioned on the area to be tested. The pipette is connected to a solution reservoir where both reference electrode (SCE) and counter electrode (platinum wire) are accommodated. For all the tests performed, a pipette tip size of 0.35 mm2 was used. Micro CPP tests were carried out according to the procedure outlined above for macro CPP, with the only difference that the scan rate used was 1 mV/s, due to the small volume of the solution available in the microcapillary [16]. All measurements were carried out on triplicate coupons and the average values are reported in this paper.
Table 1 Percentage chemical composition of AISI 444 used in this work.
AISI 444
Grade
C
Si
Mn
P
S
N
Cr
Mo
Ni
Ti
X2CrMoTi18-2
0.014
0.279
0.216
0.029
0.002
0.0148
17.859
2.074
–
0.4122
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Fig. 1. Schematic representation of the electrochemical set-up used to acquire micropotentiodynamic polarizations.
Fig. 3. Macrocyclic potentiodynamic polarization curves obtained from AISI 444 in artificially aerated tap water without the addition of chloride and with the addition of 0.1% and 3.5% NaCl: (a) not annealed, (b) annealed.
tates contained mainly titanium (in the range 45–90 wt%) and Niobium (in the range 15–20 wt%), a composition similar to that indicated by Silva et al. [7] and de Abreu et al. [17].
3.2. Cyclic potentiodynamic polarization
Fig. 2. SEM micrographs of ferritic steel AISI 444 acquired at 1500 magnification: (a) not annealed, (b) annealed.
2.3. Salt spray tests Salt spray tests were carried out in accordance with ASTM B117 standard on annealed and not annealed AISI 444 tubes, and on tubes to which an O-ring was applied to evaluate susceptibility to crevice corrosion. Specimens used in these tests where in the form of 35 mm diameter tubes. 3. Results and discussion 3.1. Microstructure In Fig. 2, the microstructure and grain size of not annealed (a) and annealed (b) AISI 444 are displayed. It is evident that grain size and the distribution of precipitates were not substantially affected by annealing. Unreported EDS analysis indicates that the precipi-
3.2.1. Influence of chloride ions on not annealed and annealed AISI 444 CPP is the electrochemical test currently used to evaluate the pitting potential, Epit, and the protection protential, Eprot. Susceptibility to pitting is based on the values of Epit and Eprot in a chloridecontaining environment [10]. Values of Eprot close to Epit indicate little susceptibility to pitting, while lower values of Eprot compared to Epit indicate susceptibility to pitting. CPP measurements carried out on annealed and not annealed AISI 444 in tap water with and without the addition of chloride are presented in Fig. 3a and b. It is evident that the values of Eprot are close to the values of Epit only for tests conducted in tap water without addition of chloride. Addition of 0.1% and 3.5% wt. NaCl resulted in a significant decrease of Epit and values of Eprot lower than Ecorr were observed for both annealed and not annealed AISI 444. In addition, it was found that Epit depends linearly on the logarithmic concentration of NaCl (Fig. 4) for both annealed and not annealed specimens in aerated and deaerated conditions. Similar logarithmic dependence was reported in the literature for the critical crevice potential, Ecrev, of not annealed AISI 444 [13]. Results from macroscopic CPP reported in this paper and in the literature [10,13] indicate susceptibility of AISI 444 to both pitting and crevice corrosion. High values of Eprot and Ecrev were observed
Please cite this article in press as: C. Bitondo et al., The effect of annealing on the corrosion behaviour of 444 stainless steel for drinking water applications, Corros. Sci. (2014), http://dx.doi.org/10.1016/j.corsci.2014.06.025
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C. Bitondo et al. / Corrosion Science xxx (2014) xxx–xxx
Table 3 Micropolarization results: mean and standard deviation of Ecorr and icorr. Average Ecorr, mV vs SCE icorr, lA/cm2
Annealed Not annealed Annealed Not annealed
13.0 35.5 1.1 0.7
St. Dev. 54 70 0.7 0.2
Fig. 4. Values of pitting potential as a function of chloride concentration for AISI 444 exposed to aerated and deaerated solutions. Dotted and full lines represent the best fitting lines for the annealed and not annealed AISI 444, respectively.
Fig. 6. Surface appearance of annealed AISI 444 after 250 h of exposure to salt spray test.
shown in Fig. 1. Micro CPP curves were obtained for both the not annealed and the annealed AISI 444 exposed to the tap water without addition of chloride. For each specimen 8 CPP curves were obtained at various locations. Fig. 5(a and b) shows the curves associated to the best and the worst behaviour exhibited by the annealed and not annealed steel. For the not annealed specimen, the curve obtained from the region labelled as 2 in Fig. 5a, displays behaviour similar to that observed macroscopically, while the curve obtained from the region labelled as 8 displays an Eprot of about 0.5 V below the Epit. A similar behaviour is observed in Fig. 5b, displaying curves obtained on the annealed material, but the reduction in Eprot compared to Epit is more substantial. It is evident that the micropolarization data do not directly agree with the macroscopic polarization data, suggesting a local susceptibility to pitting of AISI 444 even in presence of low Cl content. The data generated by micropolarization (Ecorr and estimated icorr) are reported in Table 3. The values of icorr are an estimate of the real corrosion rate due to the high scan rate adopted. As can be seen from data reported in Table 3, the annealed specimen exhibit a higher average and a wider variation of icorr and lower values of Ecorr suggesting that the surface might be locally more susceptible to corrosion in tap water environment.
3.4. Salt spray test Fig. 5. Best (full line) and worst (dotted line) micropolarization curves obtained in Naples tap water: (a) not annealed, (b) annealed.
only for values of Cl on the order of 50 ppm (this investigation) and 20 ppm [13]. On the basis of these results, therefore, AISI 444 can be recommended for all drinking water applications where only very low values of Cl contents are expected. 3.3. Micropolarization In addition to the macro CPP tests, further CPP measurements were carried out on the microscale, by using the technique of micropolarization [16]. The experimental set-up is schematically
Salt spray tests were carried out in addition to the electrochemical tests by exposing to the salt fog environment the annealed and not annealed specimens in presence and absence of crevice promoters for 10 days. No evidence of pitting corrosion was observed on both annealed and not annealed specimens without O-ring (absence of crevice promoters). However, both annealed and not annealed specimens suffered crevice corrosion attack extended for the 6% and for the 63% of the surface covered by the O-ring for the annealed (see Fig. 6) and not annealed specimen, respectively. Results reported above clearly indicate that the annealing treatment adopted in the industry does not improve the corrosion resis-
Please cite this article in press as: C. Bitondo et al., The effect of annealing on the corrosion behaviour of 444 stainless steel for drinking water applications, Corros. Sci. (2014), http://dx.doi.org/10.1016/j.corsci.2014.06.025
C. Bitondo et al. / Corrosion Science xxx (2014) xxx–xxx
tance of AISI 444. This result could be attributed to the fact that the heat treatment does not appear to induce substantial changes in the alloy microstructure. 4. Conclusions The results reported in this work indicate that 1. Metallographic observations and EDS analysis suggest that annealing treatment commercially available for the AISI 444, does not modify grain size and composition of intergranular precipitates, and therefore does not improve the corrosion resistance. 2. Macroscopic CPP indicate that the AISI 444 steel is susceptible to pitting corrosion, with a linear dependence of Epit on the logarithm of the chloride concentration. In addition, a protection potential was observed only for specimens exposed to tap water containing 50 ppm of Cl . Macroscopic CPP could not resolve the differences between the not annealed and the annealed material. 3. Microscopic CPP indicate that on both annealed and not annealed specimens local areas with values of Eprot lower than Ecorr are present, suggesting a susceptibility to pitting even in tap water. Additionally, compared to the not annealed steel, the annealed material displays lower values of Ecorr and higher values of icorr in tap water. This result also suggests that the annealed treatment does not improve the corrosion resistance of AISI 444. 4. Preliminary salt spray exposure test also indicates no beneficial effect of the heat treatment on the susceptibility to crevice corrosion of the AISI 444.
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Please cite this article in press as: C. Bitondo et al., The effect of annealing on the corrosion behaviour of 444 stainless steel for drinking water applications, Corros. Sci. (2014), http://dx.doi.org/10.1016/j.corsci.2014.06.025