Mechanical and anticorrosion properties of nanosilica-filled epoxy-resin composite coatings

Applied Surface Science 292 (2014) 432–437

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Mechanical and anticorrosion properties of nanosilica-filled epoxy-resin composite coatings M. Conradi a,∗ , A. Kocijan a , D. Kek-Merl b , M. Zorko c , I. Verpoest d a

Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia Joˇzef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia d Department of Metallurgy and Materials, K.U. Leuven, Kasteelpark Arenberg 44, 3001 Heverlee, Belgium b c

a r t i c l e

i n f o

Article history: Received 1 July 2013 Received in revised form 26 November 2013 Accepted 29 November 2013 Available online 6 December 2013 Keywords: Nanosilica Epoxy Coating Hardness Corrosion resistance

a b s t r a c t Homogeneous, 50-␮m-thick, epoxy coatings and composite epoxy coatings containing 2 wt% of 130-nm silica particles were successfully synthetized on austenitic stainless steel of the type AISI 316L. The surface morphology and mechanical properties of these coatings were compared and characterized using a profilometer, defining the average surface roughness and the Vickers hardness, respectively. The effects of incorporating the silica particles on the surface characteristics and the corrosion resistance of the epoxy-coated steel were additionally investigated with contact-angle measurements as well as by potentiodynamic polarization and electrochemical impedance spectroscopy in a 3.5 wt% NaCl solution. The silica particles were found to significantly improve the microstructure of the coating matrix, which was reflected in an increased hardness, increased surface roughness and induced hydrophobicity. Finally, the silica/epoxy coating was proven to serve as a successful barrier in a chloride-ion-rich environment with an enhanced anticorrosive performance, which was confirmed by the reduced corrosion rate and the increased coating resistance due to zigzagging of the diffusion path available to the ionic species. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The total cost and environmental consequences of corrosion problems have become a major challenge to engineers [1]. Austenitic (AISI) stainless steel is known as an important engineering material, due to its generally high corrosion resistance combined with favourable mechanical properties, such as its high tensile strength [2,3]. The high corrosion resistance of this stainless steel is attributed to the presence of a passive film, which is stable, invisible, thin, durable and extremely adherent and self-repairing [4]. However, in many aggressive environments, such as a chlorideion-rich environment, AISI is still observed to suffer from pitting corrosion [5]. Therefore, in the past two decades, the modification of metallic surfaces by various coatings, organic or polymeric, has become part of an important procedure in enhancing particular surface properties, such as scratch resistance, oxidation and corrosion. Epoxy resin is one of the most common polymer matrixes that are widely used to protect steel reinforcements in concrete structures [6,7] because of its excellent mechanical properties, chemical

∗ Corresponding author. Tel.: +386 1 4701 972; fax: +386 1 4701 939. E-mail address: [email protected] (M. Conradi). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.155

resistance, good electrical insulating properties and strong adhesion to heterogeneous substrates. Epoxy coatings not only reduce the corrosion of a metallic substrate by providing an effective physical barrier between the metal and the environment containing an aggressive species, such as an enhanced chloride-ion concentration, O2 or H+ , they also serve as a reservoir for corrosion inhibitors that help the steel surface to resist attack from aggressive species. However, in spite of a successful application, pure epoxy coatings are often susceptible to damage by surface abrasion and wear [8] and show poor resistance to the initiation and propagation of cracks due to a highly cross-linked structure [9]. Therefore, many efforts have been made by researchers to improve the properties of epoxy by adding various nanofillers [10–18]. They studied the favourable effects of particle size, volume fraction and the quality of the dispersion on the mechanical response of the polymer composites [11,19–23]. In addition, a lot of attention has been paid to epoxy coatings containing nanoparticles that show a significantly improved barrier performance for corrosion protection by decreasing the porosity and zigzagging the diffusion path for the deleterious species [24]. This work investigates the influence of silica nanoparticles on the surface morphology, mechanical characteristics and anticorrosion behaviour of composite epoxy coatings. We focus on the direct incorporation of a low percentage of silica nanoparticles

M. Conradi et al. / Applied Surface Science 292 (2014) 432–437

into the epoxy matrix, expecting a dramatic improvement in the examined characteristics. 2. Experimental 2.1. Materials Epoxy resin (Epikote 828LVEL, Momentive Specialty Chemicals B.V.) was mixed with a hardener 1,2-Diaminocyclohexane (Dytek DCH-99, Invista Nederland B.V.) in the ratio 100:15.2 wt% and used as the matrix in the composite. Composite reinforcing silica (SiO2 ) nanoparticles with a mean diameter of 130 nm were synthesized following the Stöber–Fink–Bohn method [25]. Diglycidyl ether of bisphenol A (Sigma-Aldrich) was used as the silica surface modifier to prevent agglomeration. Imidazole (Sigma-Aldrich) served as a reaction catalyst. Austenitic stainless steel AISI 316L (17% Cr, 10% Ni, 2.1% Mo, 1.4% Mn, 0.38% Si, 0.041% P, 0.021% C, <0.005% S in mass fraction) was used as a substrate. 2.2. Surface modification of silica Silica and diglycidyl ether of bisphenol A (modifying agent) were mixed in the ratio 40:100 wt% and dispersed in 50 mL of toluene in the presence of imidazole (25 wt%). The mixture was then refluxed at 100 ◦ C for 2 h. To remove the by-product (imidazole) it was centrifuged three times using acetone as a solvent. The remaining silica was then dispersed in acetone and stirred at room temperature for 2–3 h. Finally, the acetone was removed and the silica was dried in an oven at 110 ◦ C for a few hours. 2.3. Steel substrate preparation The steel sheet with a thickness of 1.5 mm was cut into plates of 15 mm in width and 75 mm in length for the mechanical testing and into discs of 15 mm diameter for the electrochemical examination. Prior to the application of the coating the steel plates and discs were prepared by a mechanical procedure, initially ground with SiC emery paper down to 1200 grit, and then rinsed with distilled water. 2.4. Composite coating preparation Epoxy-based composites were prepared by blending with 2 wt% of 130-nm surface-modified SiO2 particles. To improve the dispersion of the silica particles in the coating, they were dispersed in epoxy resin using a solvent. Acetone was used as a solvent, since the Fourier-transform infrared spectroscopy (FTIR) and FTRaman analysis [26] indicated that the processing in acetone did not chemically alter the epoxy network. Prior to the addition of the silica particles, both the resin and the hardener were separately diluted in acetone with a 1:1 weight ratio. The nanoparticles were then dispersed in the epoxy resin/acetone solution using ultrasonification for 20–30 min at room temperature. After adding the hardener/acetone solution in the next step, the mixture was manually stirred for a few minutes. Finally, 10–20 ␮l of the silica/epoxy resin/hardener/acetone mixture was poured onto the steel substrate plates and a uniform film was then applied to the substrate using a wet film applicator. The prepared coatings were degassed under vacuum for 10–15 min to additionally remove the excess solvent. The composite coating was then cured in two steps. The composite coatings were first pre-cured at 70 ◦ C for 1 h and then post-cured at 150 ◦ C for another hour. The resulting coatings on the steel-substrate plates were 50 ␮m thick. For comparison, neat

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epoxy coatings without silica fillers were also prepared and cured in the same process as the composites. 2.5. Vickers hardness test The Vickers hardness measurements were performed using a FV-700 Future-Tech corp. device. The hardness values were measured immediately after the indentation with a 0.5 kg load for 15 s. The fracture-toughness values were calculated accordingly. Photomicrographs of the morphology of the indents were taken using a Nikon optical microscope. 2.6. Surface roughness A profilometer, model Form Talysurf Series 2 (Taylor-Hobson Ltd.), was employed for the surface analysis. The instrument has a lateral resolution of 1 ␮m and a vertical resolution of about 5 nm. It measures the surface profile in one direction. The topography of the surface was acquired by combining several measurements in parallel directions that were 2 ␮m apart. The samples were measured with a 1 mm2 spot size. TalyMap gold 4.1 software was used for both the image processing and the roughness analysis. The software offers the possibility to calculate the average surface roughness, Sa , for each sample, based on the general surface roughness equation (Eq. (1)):

1 1 Sa = Lx Ly

Lx Ly 0

  z(x, y) dxdy,

(1)

0

where Lx and Ly are the acquisition lengths of the surface in the x and y directions and z(x,y) is the height. To level the profile, corrections were made to exclude the general geometrical shape and possible measurement-induced misfits. 2.7. Contact-angle measurements The static contact-angle measurements of water (W) and glycerol (G) on a clean AISI 316L sample ground with SiC emery paper down to 1200 grit and on the pure epoxy and silica/epoxy composite coatings prepared on an AISI 316L steel substrate were performed using a CAM200 (KSV Nima) contact-angle goniometer. Liquid drops of 5 ␮l were deposited on different spots of the substrates to avoid the influence of roughness and gravity on the shape of the drop. The drop contour was analysed from the image of the deposited liquid drop on the surface and the contact angle was determined by using Young–Laplace fitting. To minimize the errors due to roughness and heterogeneity, the average values of the contact angles of the drop were calculated approximately 80 seconds after the deposition from at least five measurements on the studied coated steel. All the contact-angle measurements were carried out at 20 ◦ C and ambient humidity. As contact angles were only available for two polar liquids, an equation-of-state approach [27] was used to calculate the corresponding surface energies. 2.8. Potentiodynamic measurements Potentiodynamic measurements were performed on the AISI 316L stainless steel, ground with SiC emery paper down to 1200 grit and on the AISI 316L stainless-steel substrate coated with epoxy and silica/epoxy composite coatings in a 3.5 wt% NaCl solution (Merck, Darmstadt, Germany). The specimens were embedded in a Princeton Applied Research (PAR) Teflon holder and employed as the working electrode (WE). The reference electrode (RE) was a saturated calomel electrode (SCE, 0.242 V vs. SHE) and the counter

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Fig. 1. Optical micrographs revealing the typical morphology of a 0.5-kg Vickers indent for the 50-␮m-thick pure epoxy coating on AISI 316L (a) and for the 50-␮m-thick 130-nm silica/epoxy coating on AISI 316L (b).

electrode (CE) was a high-purity graphite rod. Potentiodynamic measurements were recorded using an EG&G PAR PC-controlled potentiostat/galvanostat Model 263 with a Softcorr computer program. The specimens were immersed in the solution 1 h prior to the measurement in order to stabilize the surface at the open-circuit potential (OCP). The potentiodynamic curves were recorded, starting at 250 mV vs. SCE more negative than the OCP. The potential was then increased, using a scan rate of 1 mV s−1 , until the transpassive region was reached. 2.9. Electrochemical Impedance Spectroscopy (EIS) A three-electrode, flat, KO2354 PAR corrosion cell (volume 0.25L) was used for the EIS measurements. The test specimen employed as a working electrode (WE) was exposed to the solution with an area of 1 cm2 . A platinum net and Ag/AgCl electrode were used as the counter (CE) and the reference electrodes (REs), respectively. The EIS investigations were carried out using a PARSTAT 2263 and the Power Suite program. The impedance spectra were obtained in the frequency range 100 kHz to 5 mHz, with the amplitude of the excitation signal being 10 mV. The impedance spectra at the corrosion potential were collected for increasing immersion times (from 1 h to 170 h). The solution used for the impedance spectroscopy was 3.5 wt% NaCl. For each sample the measurements were performed at least three times and a representative measurement was chosen to reflect the average measurement. 3. Results and discussion

measurements were performed on different spots all over each sample and used for the determination of the average values of HV and KIC. We observed a significant increase in the Vickers hardness upon adding silica to the epoxy coating (up to 40%); however, the difference in the fracture toughness of both coatings, epoxy and silica-epoxy, was not very pronounced. 3.2. Surface roughness To determine the effect of the epoxy and silica/epoxy coatings on the steel surface’s protection against the chloride-ion-rich environment, the surface roughness of the coated AISI 316L specimens was analysed in comparison to a clean AISI 316L surface that was initially ground with SiC emery paper down to 1200 grit. Fig. 2a–c shows the topography of the AISI 316L surface, the 50-␮m-thick pure epoxy coating and the 50-␮m-thick 130-nm silica/epoxy coating. Fig. 2d displays the average surface roughness Sa determined from the corresponding surface topography (see Eq. (1)). An examination of the silica/epoxy-coated steel surface with a profilometer revealed that the nanoparticles significantly changed the morphology of the steel surface. It is clear that the Sa for the AISI 316L surface and the pure epoxy is of the same order; however, a significant increase in the roughness is observed for the epoxy coating with embedded silica particles. Moreover, the incorporation of a small amount of silica fillers (i.e., 2 wt%) into the epoxy changes the surface morphology to such an extent that it plays an important role in influencing the coating’s anticorrosion properties, as we will see in the following sections.

3.1. Vickers hardness testing

3.3. Wetting properties

Typical Vickers indents in 50-␮m-thick pure epoxy and 130-nm silica/epoxy coating under a 0.5 kg load are presented in Fig. 1. It is clear that the indents in the epoxy coating (Fig. 1a) are consistently larger than those in the silica/epoxy coating (Fig. 1b) under the same loading conditions. In addition, a star-shaped Vickers impression was observed in the silica/epoxy coating, whereas the pure epoxy coating exhibits a pyramidal impression. The former also appears to exhibit a much greater degree of elastic recovery along the faces or sides of the impression. However, there was no apparent elastic recovery along the diagonals due to the presence of a permanent plastic deformation. The measured values of the Vickers hardness (HV ) and the corresponding fracture-toughness values (KIC ) of the pure epoxy and the 130-nm silica-epoxy coating are listed in Table 1. Here, ten

The improved water-repellent properties (i.e., the self-cleaning effect) of the surface are some of the crucial issues in corrosion protection. To analyse the surface wettability, five contact-angle measurements using two different polar liquids, water (W) and glycerol (G), were performed on different spots all over the sample and used to determine the average contact-angle values with Table 1 Measured Vickers hardness (HV ) and corresponding fracture toughness (KIC ) for 50␮m-thick pure epoxy and 130-nm silica-epoxy coating. Substrate

HV [GPa]

KIC [mN/m]

AISI + epoxy AISI + 130 nm silica/epoxy

0.11 ± 0.02 0.19 ± 0.02

0.66 ± 0.02 0.69 ± 0.02

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Fig. 2. Surface topography of AISI 316L surface (a) ground with SiC emery paper down to 1200 grit, (b) covered with 50-␮m epoxy coating, (c) covered with 50-␮m silica/epoxy coating and (d) average surface roughness, Sa , for all three surfaces under investigation. Table 2 Surface properties of AISI 316L substrate ground with SiC emery paper down to 1200 grit and AISI 316L substrate when blended with pure epoxy and 130-nm silica/epoxy coatings. Static contact angles were measured with water ( W ) and glycerol ( G ) and the corresponding surface energies were calculated using an equation-of-state approach. Contact angle [◦ ]

Substrate

 AISI AISI + epoxy AISI + 130 nm silica/epoxy

W

72.5 84.6 94.2



Surface energy [mN/m]

G



61.0 75.5 85.5

39.67 32.05 26.00

◦

an estimated error in the reading of  ± 1.0 and for the calculation of the surface energy. The wettability, however, was not possible to assess using non-polar liquids for the contact-angle measurements as they spread out on the coatings ( ∼ 0◦ ). The contact angles and the surface energies for the epoxy and the 130-nm silica/epoxy coatings on the AISI 316L substrate in comparison to the clean AISI 316L sample ground with SiC emery paper down to 1200 grit are reported in Table 2. It was observed that both investigated coatings, the pure epoxy and the 130-nm silica/epoxy, exhibited significantly higher values of static water-contact angles compared to the clean AISI 316L sample. In fact on the 130-nm silica/epoxy coating it was even higher than 90◦ . This suggests that the microstructure change of the epoxy coating upon adding the silica particles was reflected in an increased surface roughness that imparts a hydrophobic effect on the surface, which further leads to an enhanced anticorrosion effect. Changing from water to glycerol modified the wetting behaviour, and the contact angles typically exhibited lower values; however, they still followed the same trend as in the case of water as a probing liquid. As the contact angles were only available for two polar liquids, an equation-of-state approach [27,28] was used to calculate the surface energies with equation (Eq. (2)):



cos  = −1 + 2

s −ˇ( − )2 s 1 e 1

(2)

For a given value of the surface tension of a probe liquid  l (i.e., for water  l = 72.8 mN/m [29]) and  W measured on the same solid surface, the constant ˇ and the solid surface tension  s values were determined using the least-squares analysis technique. For the fitting with equation (1), a literature value of ˇ = 0.0001234 (mJ/m2 )−2 was used, as weighted for a variety of solid surfaces [27,28]. The calculated values of the solid surface energy (Table 2) dropped significantly when the clean AISI 316L substrate was covered with the pure epoxy coating and even more when covered with the 130-nm silica/epoxy coating, confirming the induced surface hydrophobicity. The low surface energy of the 130-nm silica/epoxy coating is most probably a consequence of the addition of lowsurface-energy additives, i.e., compounds with CH2 < CH3 bonds [30], which the silanated silica particles implant into the epoxy matrix. 3.4. Potentiodynamic measurements The potentiodynamic behaviours of the AISI 316L stainless steel and the AISI 316L substrate coated with the epoxy and silica/epoxy composite coatings in 3.5 wt% NaCl are shown in Fig. 3. The differences in the surface preparation of the pure alloy and the coated alloy significantly affected the polarisation and the passivation behaviour of the tested materials. After 1 h of stabilization at the OCP, the corrosion potential (Ecorr ) for the AISI 316L in NaCl was approximately –0.18 V vs. SCE. Following the Tafel region, the alloy exhibited a broad range of passivation. The breakdown potential (Eb ) for the AISI 316L in NaCl was approximately 0.19 V vs. SCE. In the case of the AISI 316L coated with the pure epoxy coating, the Ecorr in NaCl was approximately −0.17 V vs. SCE. The range of

Table 3 Corrosion parameters calculated from the potentiodynamic measurements. Material

Ecorr (mV)

Icorr (nA)

Rp (M)

Eb (mV)

AISI 136L epoxy on AISI 316L Silica/epoxy on AISI 316L

–180 –170 –220

550 4.7 1.3

0.04 4.5 17

190 130 700

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Fig. 3. Potentiodynamic curves recorded for (a) AISI 316L stainless steel, (b) AISI 316L stainless steel coated with epoxy coating and (c) AISI 316L stainless steel coated with silica/epoxy coating in 3.5 wt% NaCl solution. The scan rate was 1 mV s−1 .

passivation was moved to lower corrosion-current densities compared to the AISI 316L specimen and the Eb was 0.13 V vs. SCE. In the case of the silica/epoxy coating applied on the AISI 316L substrate, the Ecorr in NaCl was approximately −0.22 V vs. SCE. The range of passivation significantly increased up to 0.7 V vs. SCE and

Fig. 5. Bode plot for the silica-doped epoxy-coated AISI 316L in 3.5 wt% NaCl measured after 1, 10, 50, and 170 h of immersion: (a) absolute impedance (Z) as a function of frequency (f), (b) phase angle as a function of frequency (f). Bode plot of the bare substrate AISI 316L is shown for comparison.

was additionally shifted to the lower corrosion–current densities (Table 3). 3.5. Impedance

Fig. 4. Bode plot for the epoxy-coated AISI 316L in 3.5 wt% NaCl measured after 1, 10, 50, and 170 h of immersion: (a) absolute impedance (Z) as a function of frequency (f), (b) phase angle as a function of frequency (f). Bode plot of the bare substrate AISI 316L is shown for comparison.

The open-circuit impedances of the epoxy- and silica/epoxycoated AISI 316L substrates were traced over 170 h after the immersion of the electrode into the 3.5 wt% NaCl solution. Figs. 4 and 5 show the Bode–Bode diagrams for different immersion times of both the coated systems in the electrolyte. The impedance spectra of the bare substrate after 1 h of immersion are presented for comparison. In the Bode–Bode diagrams three distinctive segments are shown in the log |Z| vs. log f and the phase angle vs. log f spectra (f is the frequency in Hz, Z is the impedance). In the higher f region, log |Z| vs. log f shows a linear relationship for log |Z| vs. log f with a slope close to −1 (4a and 5a) and with the phase angle approaching −90◦ (4b, 5b). This behaviour is typical of a capacitor and is ascribed to the capacitive behaviour of the epoxy and silica/epoxy coatings. The second segment is in the middle f region (200 Hz to 0.2 Hz), where log |Z| vs. log f reaches a plateau and the phase angle tends towards 0◦ with increasing f for the epoxy-coated (Fig. 4b) substrates. This behaviour is typical of a resistor and corresponds to the resistance of the coatings and could be ascribed to the resistance of the ionic species through the coatings, due to the electrolyte uptake [4,5]. However, the phase angle in the middle frequencies for the silica/epoxy coatings does not approach 0◦ (Fig. 5b), indicating a diffusion process, especially after 100 and 170 h of immersion. A third segment lies in the low-frequency region (below 0.1 Hz), where an increase in Z with f is again observed. In this region, the

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polarization resistance (Rp ) can normally be determined, as Z is independent of f in the so-called DC limit (|Z| ≈ Rp ). However, the plateau in the low-frequency region was not reached here due to the very resistive behaviour of the coatings or due to the diffusion process. In the case of the silica/epoxy coating, the low-f resistive region is shifted by approximately one order of magnitude to the higher |Z| in the log |Z| vs. log f spectrum compared to the pure epoxy coating. This indicates the higher corrosion effectiveness of the silica/epoxy coating (5a vs. 4a). A prolonged immersion time leads to a linear decrease of the high- and middle-f resistive regions in the log |Z| vs. log f spectrum for the epoxy coating (Fig. 4a). This suggests that the capacitive and resistive properties of the coating differ as a result of the electrolyte penetration through the coating, as previously discussed by Perez et al. [4]. On the other hand, after a prolonged immersion time, mainly middle-f region changes of the spectra were observed for the silica/epoxy coating (Fig. 5). It is clear from the experiments that the addition of the silica into the epoxy coating (Fig. 5 vs. Fig. 4) changes the shape of the impedance spectra in the middle-f region, where the phase angle does not approach 0◦ (Fig. 5b). Such behaviour indicates a strikethrough diffusion process in the middle-f range, which was not observed in the pure epoxy coating. This indicates that the ionic species follow a zigzagged diffusion path, which results in a longer diffusion length when the silica particles are added to the epoxy coating. This was previously suggested by Shi et al. [31]. 4. Conclusion We successfully dispersed 130-nm silica particles into an epoxy matrix at a concentration of 2 wt%, after which the mixture was synthetized on austenitic stainless steel of the type AISI 316L in the form of a 50-␮m coating. The epoxy coating modified with silica nanoparticles showed a significantly enhanced hardness of ∼40% compared to the pure epoxy coating. However, a ∼5% increase in the fracture toughness upon adding silica into the epoxy coating was not very pronounced. The silica nanoparticles improved the microstructure of the epoxy coating, which was reflected in an increased surface roughness and the imparting of hydrophobicity to the coated surface, which leads to an enhanced anticorrosion effect. The low surface energy, on the other hand, was a consequence of the addition of low-surface-energy-additives into the epoxy matrix through a silica surface modification. The electrochemical monitoring of the coated steel in a 3.5% NaCl solution indicated the beneficial role of nanoparticles in significantly reducing the corrosion rate of the coated steel. The EIS measurements indicated that the incorporation of the silica particles increased the barrier performance of the coating through zigzagging the diffusion path that was available to the ionic species. Acknowledgement The authors gratefully acknowledge The Research Foundation–Flanders (FWO) for the financial support enabling research work at K. U. Leuven. This work was partly carried out within the framework of the Slovenian programme P2-0132, “Fizika in kemija povrˇsin kovinskih materialov” of the Slovenian Research Agency, whose support is gratefully acknowledged by M. Conradi and A. Kocijan. References [1] J.H. Potgieter, P.A. Olubambi, L. Cornish, C.N. Machio, E.S.M. Sherif, Influence of nickel additions on the corrosion behaviour of low nitrogen 22% Cr series duplex stainless steels, Corrosion Science 50 (2008) 2572–2579.

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