Introduction

Ferritic stainless steel has gradually become an excellent alternative material in many application fields due to its excellent performance in toughness, ductility, weldability, stress corrosion cracking resistance and lower cost compared with austenitic stainless steel (Ref 1,2,3,4). To further expand the application ranges of ferritic stainless steel with improved mechanical properties and corrosion resistance, the addition of small amounts of alloying elements has become one of the hotspots for current research (Ref 2, 5, 6). Tin (Sn), as one of the cost-effective elements, has been used for a long time in the steel industry and possesses abundant reserves in China (Ref 7). It is generally believed that Sn is a harmful element for mechanical property of steel; however, 0.1 wt.% Sn addition to stainless steel or oil tanker cargo tank steel can increase their corrosion resistance in Cl−-containing acidic solutions (Ref 8, 9). It has been reported (Ref 10) that the uniform dispersion of Sn in steel can reduce the Fermi energy of steel, weaken the electrochemical activity and improve the corrosion resistance of steel in seawater environment. Kamimura et al. (Ref 11) have investigated the atmospheric corrosion behavior of Sn-containing steels in environment with high Cl−, and the results suggest that Sn slows down the corrosion rate of steel by inhibiting the formation of adsorptive intermediates (FeOHad and FeClad). Liu et al. (Ref 7) have reported that Sn, due to its high hydrogen evolution overpotential, enhances the corrosion resistance of low-alloy steel substrate in acid environment by suppressing H+ cathodic reduction. Nam et al. (Ref 12) have considered that the presence of SnO2 in the protective film on steel surface contributes to improved corrosion resistance.

Although Cu has a relatively lower hydrogen evolution overpotential than Sn, it is beneficial in passivating stainless steel in acid solution (Ref 13). Ujiro et al. (Ref 14) have reported that the deposited Cu uniformly covers the anodic surface on Cu-bearing ferritic stainless steel in chloride medium and thus favors the passivation. Moreover, some researchers have investigated the synergistic effect by Sn and Cu on the corrosion resistance of austenitic stainless steels. Pardo et al. (Ref 15) have reported that Cu favors the pit nucleation, but inhibited its growth; Sn exerts the opposite effect, however, favoring pit growth and inhibiting its nucleation. Nam et al. (Ref 12) have demonstrated that Sn and Cu favor the formation of a continuous and adherent passivation film on stainless steel surface. Zhang and Liu (Ref 16) have found that appropriate addition of Sn and Cu to ferritic stainless steel is beneficial in improving the pitting corrosion resistance.

In fact, the passivation behavior is critical for stainless steels so that a thin passivation film on the surface provides an ideal corrosion protection to steel substrate (Ref 17, 18). To date, the passivation behavior of ferritic stainless steel under the synergistic effect of Sn and Cu has rarely been reported. Therefore, this work aims to study the synergistic effect by Sn and Cu on the passivation performance of ferritic stainless steel in NaCl solution. Besides, the microstructure evolution by Cu addition has been discussed. Therefore, the obtained findings are of great significance in clarifying the synergistic effect by Sn and Cu to further improve the passivation performance of ferritic stainless steel, and give a potential guidance in developing new ferritic stainless steel with resource-saving, lower cost and high performance.

Experimental

Materials Preparation

The 430L ferritic stainless steels bearing Sn and Cu melted in a 30-kg vacuum induction furnace were used in this study (Ref 19). The melting process consisted of the following steps: (1) add pure Fe, Cr into crucible, and load Al, Si, Mn, Sn and Cu in hoppers sequentially; (2) start vacuuming and keep the pressure below 1 Pa in the furnace; (3) put argon into the furnace and melt the burden; (4) when the temperature of molten steel reaches 1600 °C, start the vacuum pump to reduce the pressure in the furnace until it maintains lower than 5 Pa for 600 s; (5) put argon into the furnace again; (6) add Al, Si, Mn, Sn and Cu in the order minutely; (7) by adjusting power, make the temperature reach around 1550 °C and then cast into ingot. The ingots with a diameter of 100 mm were hot-rolled to 5-mm plates at 1250 °C. The chemical composition of the obtained steels for investigation is presented in Table 1.

Table 1 Chemical composition of the obtained ferritic stainless steels for investigation (in wt.%)
Full size table

Microstructures Characterization

For microstructures observation, the three annealed samples (solution treated at 1000 °C for 300 s followed by water quenching) were treated according to standard metallographic procedures and then submitted to etching in 10 wt.% oxalic acid (H2C2O4) solution at 3.0 V for 90 s. The longitudinal morphologies of the samples were characterized by optical microscope (Olympus DSX510, Tokyo, Japan). The grain size statistics of the three experimental steels after heat treatment were obtained based on GB/T 6394-2002 “Metal average grain size determination method,” with the round cut point method being adopted for evaluating the grain size. The average grain size level number G is calculated by the following formula:

$$G = 6.644 \times \log \frac{MP}{L} - 3.288$$
(1)

where L is the length of the measurement line segment (or grid) used (mm), M is the magnification for observation and P is the number of intercept points on the measurement grid.

Potentiodynamic Polarization Curves Measurement

The samples for investigation with a thickness of 5 mm were cut from the middle position of the hot-rolled stainless steel plates. A working area of 1 cm2 of the sample surface was exposed for the electrochemical tests, and the non-working area was covered with distal epoxy glue. The prepared samples were sequentially ground via a series of SiC emery paper (#400, 600, 800, 1000, 1200, 1500 and 2000), and mirror-finished surface was obtained by three-step polishing using diamond pastes up to 0.25 μm.

Potentiodynamic polarization curves were measured by Gamry Reference 600 electrochemical workstation, glass electrolytic cell and super constant temperature water bath. The experimental system is a three-electrode system, the auxiliary electrode is a platinum electrode, and the reference electrode is a saturated calomel electrode (SCE). The steel samples were taken as working electrode. Before test, the working electrodes were initially immersed in the 3.5% NaCl solution for 1800 s to approach a steady state. The polarization curves were initiated at − 0.1 V versus SCE for 300 s to remove the passivation film naturally formed in the air. Then, scanning is started from the potential of the open-circuit potential of − 0.2 V with a scanning rate of 20 mV min−1. The scanning process proceeded to above the passivation potential, and finally, the polarization curve of the sample was obtained. All the potentiodynamic polarization measurements were taken at room temperature of 30 °C and repeated three times to verify the reproducibility. The surface morphology observation was also carried out by scanning electron microscopy (SEM) after the polarization curves measurements.

Composition Analysis of the Surface Passivation Film

X-ray photoelectron spectroscopy (XPS) was used to characterize the composition of surface passivation film on the steel samples exposed in 3.5 wt.% NaCl solution at 30 °C. The cathodic treatment was performed before the preparation of passivation film. Firstly, a constant potential of − 1000 mV versus SCE was applied for 180 s to remove the passivation film that naturally formed in the air. After that, a passivation potential of + 400 mV versus SCE was applied and maintained for 1 h to obtain a passivation film. Finally, the obtained sample was cleaned with absolute ethanol and distilled water.

The XPS characterizations were obtained using ESCALAB 250 (Thermo Fisher Scientific, USA). The photoelectron emission was excited by monochromatic Al Kα with a photoelectron energy of 12536 eV and an energy PE of 50 eV. The spectrum scan range was 0-1100 eV, the wide scan interval was 0.5 eV, and the narrow scan interval was 0.2 eV. The x-ray photoelectron spectra were acquired at 45° takeoff angle using an injection voltage of 3 kV, a current of 25 μA, a sputtering time of 1 min and a corresponding sputtering depth of approximately 5 nm. Gaussian–Lorentzian mix function and Shirley background subtraction were chosen to analyze the spectra via commercial Xpspeak (version 4.1) software. All the spectra peaks were calibrated by C 1s peak with a binding energy 284.6 eV.

Results and Discussion

Effect of Cu Addition on Microstructure of Ferritic Stainless Steel

Figure 1 gives the microstructure images of samples with different Cu addition recrystallized at 1000 °C for 300 s by optical microscope (OM). It is clear that all the three samples contain a ferrite phase owing to their middle content of Cr, free of Ni and ultra-low contents of C and N (Ref 19).

Fig. 1
figure 1

Optical microscopy images of ferritic stainless steel samples with different Cu addition annealed at 1000 °C for 300 s. (a) 0.20 wt.%, (b) 0.36 wt.% and (c)1.00 wt.% Cu

Full size image

To demonstrate the grain refinement by Cu addition, the grain size statistics have been carried out as indicated in Fig. 2. It shows that increasing Cu addition from 0.2 to 0.36 wt.% has little effect on increasing the average grain size. Further increasing the Cu addition to 1.0 wt.%, an average grain size of 5.44 μm has been observed. Clearly, the average grain size increases only slightly in a narrow range from 5.18 to 5.44 μm as the Cu addition increases in a wide range from 0.2 to 1.0 wt.% in the steel, indicating that Cu addition does not lead to great increase in the average grain size and may have a certain grain refinement effect on the ferritic stainless steel. It has been reported that the Cu enrichment at grain boundary can inhibit the grain size growing (Ref 20). The EDS detection at the grain boundary on Sample #3 with 1.0 wt.% Cu addition in Fig. 3 shows that Cu content reaches 2.22 wt.%, confirming the Cu enrichment at the grain boundary of ferritic stainless steel. Ye et al. (Ref 20) have shown that the precipitation phase of Cu at the grain boundary has pinning effect and thus inhibits the grains growing. Therefore, Cu addition from 0.2 to 1.0 wt.% only leads to the average grain size of ferritic stainless steel increasing slightly.

Fig. 2
figure 2

Statistics in average grain size of ferritic stainless steel samples with different Cu addition

Full size image
Fig. 3
figure 3

EDS detection of Cu content at grain boundary of ferritic stainless steel with 1.0 wt.% Cu addition

Full size image

Effect of Cu on Anodic Polarization Behavior

Potentiodynamic polarization has been employed to illustrate the effect of Cu addition on the corrosion behavior of Sn alloyed ferritic stainless steel in 3.5 wt.% NaCl solution at 30 °C, and the results are shown in Fig. 4. For the cathodic reduction process of each measured corrosion system, the cathodic current density (ic) keeps almost the same value and evolution behavior, indicating that Cu addition has no significant effect on the oxygen reduction at ferritic stainless steel surface in NaCl solution. For the anodic dissolution process of each measured corrosion system, the anodic current density (ia) maintains at a lower value and then shows a sharp increase at the critical pitting potential (Epit) about 0.2 V (versus SCE), indicating the passivation and then the pitting initiation, i.e., the passivated film breakdown evolution behavior for all the three ferritic stainless steel samples in NaCl solution as the applied potential positively evolves. As for the samples #1 and #2 with 0.20 and 0.36 wt.% Cu addition, respectively, they exhibit almost the same ia and Epit evolutions. However, sample #3 with 1.0 wt.% Cu addition has the lowest ia, indicating its best electrochemical stability among the three samples. Therefore, Cu addition to ferritic stainless steel mainly affects its anodic dissolution process and the stability of surface passivation film.

Fig. 4
figure 4

Effect of Cu addition on the potentiodynamic polarization behavior of Sn alloyed ferritic stainless steel in 3.5 wt.% NaCl solution at 30 °C

Full size image

Table 2 shows the obtained fitting parameters of Ecorr, icorr, Epit, and Epit − Ecorr for each curve in Fig. 4. The Ecorr varies little as the Cu addition increases in the ferritic stainless steel. Besides, icorr is significantly suppressed by Cu addition in increasing amount, indicating the best beneficial effect by 1.0 wt.% Cu addition to enhance the corrosion resistance of ferritic stainless steel substrate in 3.5 wt.% NaCl solution at 30 °C. Moreover, the evolution in Epit demonstrates the best performance in anti-localized corrosion of the steel with 1.0 wt.% Cu addition (Ref 21). Meanwhile, enhanced pitting resistance indexed by higher value of Epit − Ecorr is also observed by increasing the Cu addition. Therefore, an increasing amount of Cu addition has little influence on the cathodic process of ferritic stainless steel in 3.5 wt.% NaCl solution, but shows beneficial effect on enhancing both the corrosion resistance of steel substrate and its passivation behavior.

Table 2 Fitting parameters of corrosion potential (Ecorr), corrosion current density (icorr), critical pitting potential (Epit) and Epit − Ecorr for each potentiodynamic polarization curve in Fig. 4
Full size table

Morphology of Corrosion Products

The microstructure observation and potentiodynamic polarization characterization have demonstrated the beneficial effect by 1.0 wt.% Cu addition in refining the average grain size and improving the corrosion passivation of ferritic stainless steel. To explore the mechanism, Fig. 5 shows the corrosion morphology by SEM and EDS mapping of main elements on the ferritic stainless steel with 1.0 wt.% Cu addition after potentiodynamic polarization curve measurement. Observation in Fig. 5(a) finds that there is deposit of loose particles of different sizes at the bottom of corrosion pit, and close observation in Fig. 5(b) validates this. EDS mapping reveals the presence of Cu element in high amount as shown in Fig. 5(c), but almost no detection of Cl element as indicated in Fig. 5(d). The deposit of Cu particles at the bottom of corrosion pits can improve the passivation of the steel matrix and positively raise its corrosion potential with higher stability (Ref 22). Therefore, it is supposed that the deposition of Cu particles at the corrosion pit bottom is responsible for the better corrosion resistance of ferritic stainless steel with Cu addition in 3.5 wt.% NaCl solution at 30 °C.

Fig. 5
figure 5

Corrosion morphology of corrosion pit on the ferritic stainless steel with 1.0 wt.% Cu addition after potentiodynamic polarization curve measurement in 3.5 wt.% NaCl solution at 30 °C. (a) Corrosion pit; (b) particles deposition at the bottom of corrosion pit; (c) Cu element mapping; and (d) Cl element mapping

Full size image

Chemical Analysis of the Passive Film on the Surface of Steels

Usually, the corrosion behavior of stainless steel in a specific corrosive environment is strongly related to the chemical composition of the surface passivation film (Ref 5), and thus, XPS spectra measurements have been taken. As a representative, Fig. 6 shows the survey spectra of the passive film formed on the ferritic stainless steel sample 1# with 0.2 wt.% Cu addition, and constituent elements such as Fe, Cr, Sn, Cu and O are detected in the spectra. Signal for C 1s at 285.0 eV in the spectra is due to inevitable contamination during the sample transfer and/or testing. Aiming to analyze the chemical states of these elements in the passivation film, the high-resolution spectra of Fe 2p3/2, Cr 2p3/2, Sn 3d and O 1s were intentionally recorded. Based on the NIST XPS database (Ref 23), parameters in the standard spectra for specific elements and their oxides are listed in Table 3, including the chemical state binding energies (Eb) and the relative content, which were employed for spectra deconvolution to identify the chemical state of compositional elements in the passivation film on the three obtained stainless steel sample.

Fig. 6
figure 6

XPS survey spectra of the passivation film formed on ferritic stainless steel sample 1# with 0.2 wt.% Cu addition, recorded at the outmost surface after exposure for 1 h in 3.5 wt.% NaCl solution with 30 °C at potential of +400 mV vs. SCE

Full size image
Table 3 Parameters for deconvolution of obtained XPS spectra for the main species in the passivation film, and the atomic percent (%) of corresponding composition in the film on steel with different Cu addition
Full size table

In the present work, the Fe spectra are generally analyzed related to the valence state for ferritic stainless steel in NaCl solution (Ref 24), and Fig. 7 shows the high-resolution spectra of Fe element in the passivation film on ferritic stainless steel samples containing 0.20, 0.36 and 1.00 wt.% Cu. The relative intensities of Fe(II) and Fe(III) indicate that Fe(III) is the initial iron oxidized species in the passivation film. As the peak positions of FeO and Fe3O4 are too close to be identified (Ref 25, 26)], the determination of Fe3+ contribution from Fe2O3 or Fe3O4 is a difficult work (Ref 27). The Fe 2p3/2 spectra in Fig. 7 can be separated into three constituents including metallic Fe (707.5 ± 0.1 eV), Fe2+ (708 ± 0.1 eV) and Fe3+ (710.8 ± 0.2 eV) (Ref 28,29,30,31). Moreover, it can be found that the main composition of Fe species in the passivation film does not change significantly as the Cu addition varies in the ferritic stainless steel.

Fig. 7
figure 7

Deconvoluted high-resolution XPS spectra of Fe in the passivation film on the ferritic stainless steel samples with different Cu addition, recorded at the outmost passivation film surface after exposure in 3.5 wt.% NaCl solution for 1 h at 30 °C with potential of +400 mV vs. SCE. (a) 0.20 wt.%, (b) 0.36 wt.% and (c) 1.00 wt.% Cu

Full size image

The high-resolution spectra of Cr 2p3/2 are given in Fig. 8, and three constituents of metallic Cr (573.7 ± 0.2 eV), Cr2O3 (576.0 ± 0.1 eV) and Cr(OH)3 (577.2 ± 0.1 eV) are regarded as the primary composition of the passivation film (Ref 32,33,34). Moreover, the relative intensity ratio of Cr2O3 to Cr(OH)3 slightly increases as the Cu addition increases in the steel, indicating that Cu addition in higher amount may promote the formation of Cr2O3 at this sputtering depth. Considering that Cr2O3 is an important component in the inner layer of the ferritic stainless steel passivation film (Ref 35), it can therefore be concluded that the higher relative amount of Cr2O3 in passivation film is responsible for the improved corrosion passivation performance of sample 3# with 1.00 wt.% Cu addition.

Fig. 8
figure 8

Deconvoluted high-resolution XPS spectra of Cr 2p3/2 in the passivation film on the ferritic stainless steel samples with different Cu addition, recorded at the outmost passivation film surface after exposure in 3.5 wt.% NaCl solution for 1 h at 30 °C with potential of +400 mV vs. SCE. (a) 0.20 wt.%, (b) 0.36 wt.% and (c) 1.00 wt.% Cu

Full size image

Figure 9 shows the high-resolution spectra of Sn 3d in the passivation film on ferritic stainless steel samples containing 0.20, 0.36 and 1.00 wt.% Cu. The binding energies of Sn 3d3/2 and Sn 3d5/2 are approximately 494.60 and 486.18 eV, respectively (Ref 17). The positions of the peaks of stannous and stannic at Sn 3d5/2 and Sn 3d3/2 are approximately Sn2+ (3d5/2) 485.8 eV, Sn4+ (3d5/2) 486.3 eV and Sn2+ (3d3/2) 494.5 eV, Sn4+ (3d3/2) 495.4 eV (Ref 5). Therefore, the results in Fig. 9 confirm the existence of stannous oxide and hydroxide in the passivation film on ferritic stainless steel in NaCl solution (Ref 36). Moreover, it is interesting that for the steel containing 1.00 wt.% Cu, the intensity of Sn4+ peak in the passivation film is much higher than that in the passivation film on steel containing 0.20 wt.% and 0.36 wt.% Cu, indicating the presence of more Sn4+-containing oxides as the Cu addition increases in the steel substrate. As the Sn content in the original steel substrate is approximately the same (nearly 0.1 wt.%), it thus can be concluded that Cu element in higher addition can promote the formation of Sn4+-containing oxides in the passivation film. Based on this result, it may conclude that there is synergetic effect of Sn and Cu in the formation of Sn4+-containing oxides in the passivation film on ferritic stainless steel.

Fig. 9
figure 9

Deconvoluted high-resolution XPS spectra of Sn 3d in the passivation film on the ferritic stainless steel samples with different Cu addition, recorded at the outmost passivation film surface after exposure in 3.5 wt.% NaCl solution for 1 h at 30 °C with potential of +400 mV vs. SCE

Full size image

Figure 10 shows the spectra of O 1s in the passivation film on ferritic stainless steel samples containing 0.20, 0.36 and 1.00 wt.% Cu, and the asymmetry of O 1s peak indicates that the oxygen is present at several states, including the metallic oxides, hydroxide, adsorbed water and phosphate anions (Ref 37, 38). Analysis finds the presence of three components of O2– (530.0 ± 0.1 eV), OH– (531.6 ± 0.1 eV) and H2O (532.0 ± 0.1 eV) (Ref 37). The presence of OH–, primary constituent of the surface passivation film, relates to the presence of the hydroxide of Cr, Sn and Fe. Additionally, it is interesting that the content of bound water (H2O) in the passivation film increases predominantly with the Cu addition amount increasing in the steel substrate. According to the literature (Ref 38), the bound water in the passivation film has the ability to capture the dissolved metal ions and then favors the formation of a new passivation film against further attack by surroundings. Therefore, the passivation film on the ferritic stainless steel sample with 1.00 wt.% Cu addition has the best self-repairing ability compared with the other two samples with lower amount of Cu addition.

Fig. 10
figure 10

Deconvoluted high-resolution XPS spectra of O 1s in the passivation film on the ferritic stainless steel samples with different Cu addition, recorded at the outmost passivation film surface after exposure in 3.5 wt.% NaCl solution for 1 h at 30 °C with potential of +400 mV vs. SCE. (a) 0.20 wt.%, (b) 0.36 wt.% and (c) 1.00 wt.% Cu

Full size image

Mechanism of Sn and Cu Addition on Passivation of Ferritic Stainless Steel

A passivation film is prone to form on ferritic stainless steels surface, and based on the above analysis, it should consist of oxides/hydroxides of Fe, Cr, Sn and Cu, showing a good resistance in the neutral 3.5 wt.% NaCl solution at 30 °C. However, the chloride ion can adsorb on the surface and subsequently destroy the passivation film and induce pitting corrosion (Ref 39). The involved anodic and cathodic reactions are indicated by Eq 2 and 3, respectively (Ref 40):

$${\text{Fe}} \to {\text{Fe}}^{2 + } + 2{\text{e}}$$
(2)
$$2{\text{H}}_{2} {\text{O}} + {\text{O}}_{2} + 4{\text{e}} \to 4{\text{OH}}^{ - }$$
(3)

The reactions of Sn in the corrosion process are very complex (Ref 41), and the stepwise oxidation processes of Sn(0) → Sn(II) → Sn(IV) lead to the formation of various Sn(II)/Sn(IV) oxide/hydroxide species. Some possible reactions are listed as follows (Ref 42):

$${\text{Sn}} \to {\text{Sn}}^{2 + } + 2{\text{e}}$$
(4)
$${\text{Sn}} + 2{\text{OH}}^{ - } \to {\text{Sn}}\left( {\text{OH}} \right)_{2} + 2{\text{e}}$$
(5)

The anodically formed Sn2+ undergoes hydrolysis to form stannous oxide as indicated by Eq 6:

$${\text{Sn}}^{2 + } + {\text{H}}_{2} {\text{O}} \to {\text{Sn}}\left( {\text{OH}} \right)_{2} + 2{\text{H}}^{ + }$$
(6)

The Sn(OH)2 then chemically or electrochemically changes to a more stable configuration after steps of dehydration and oxidation as a function of both time and potential to form 2SnO·H2O, 5SnO·2H2O, 3SnO·H2O and/or SnO, which then can be oxidized to form SnO2·nH2O, where n are 0, 1 and 2 (Ref 17). Figure 11 shows the schematic diagrams in illustrating the effects of Sn and Cu on the passivation film on ferritic stainless steels surface in 3.5 wt.% NaCl solution. SnO2 is the stable solid corrosion product in the Sn-H2O system under all conditions, as shown in Fig. 11(a). The involved possible reaction for the formation of SnO2 is as follows:

$${\text{Sn}}\left( {\text{OH}} \right)_{2} + 2{\text{H}}_{2} {\text{O}} \to {\text{Sn}}\left( {\text{OH}} \right)_{4} + 2{\text{H}}^{ + } + 2{\text{e}}$$
(7)
Fig. 11
figure 11

Schematic diagram in illustrating the effects of formed Sn oxides (a) and Cu (b) on improving the passivation behavior of ferritic stainless steel in 3.5 wt.% NaCl solution at 30 °C

Full size image

However, Sn(OH)4 is unstable and further can be transformed to SnO2 by Eq 8:

$${\text{Sn}}\left( {\text{OH}} \right)_{4} \to {\text{SnO}}_{2} + 2{\text{H}}_{2} {\text{O}}$$
(8)

As both Sn and SnO2 have high hydrogen evolution overpotential (Ref 7), the presence of Sn and SnO2 at the bottom of corrosion pits can protect the steel from further attack by acidified NaCl solution.

For the corrosion pits initiation on passivation film, it consists of two steps (Ref 17). On the first step, the Cl− absorbs on the steel surface and induces the occurrence of metastable pits; on the second stage, the metastable pits are converted into stable pits that can continue to grow in the depth. After the passivation film breaking down, the solid-solution Cu in the steel matrix begins to dissolve and comes into action. Figure 11(b) summarizes the mechanism, whereby Cu increases pitting corrosion resistance in ferritic stainless steels in Cl−-containing medium. At the first step in this reaction, once the passive layer has been attacked by Cl− anions, Cu element from the steel matrix dissolves into NaCl solution to form Cu+, which can further react with Cl− to form CuCl and CuCl2−. Meanwhile, CuCl2− and CuCl are not stable, and they are easy to be reduced to form metallic Cu under the acidified environment and located at the bottom of corrosion pits as observed in Fig. 5. The Cu addition in higher amount of 1.00 wt.% can distinguish itself compared with Cu addition in lower amount. At the same time, the deposit of SnO2 in the passivation film under the synergistic of Cu increases. Moreover, it is reasonable that Cu and Sn have more noble equilibrium potentials than Fe and Cr and consequentially force Fe and Cr to be dissolved and form oxides layers to passivate the stainless steel surface (Ref 17). Therefore, the 0.1 wt.% Sn alloyed ferritic stainless steel with Cu addition in higher amount shows good corrosion resistance and better passivation behavior due to the synergistic effect of Sn and Cu.

Conclusion

The effect of Cu addition on the microstructure and passivation behavior of Sn alloyed ferritic stainless steel in 3.5 wt.% NaCl solution at 30 °C has been investigated, and the main conclusions can be summarized as follows:

  1. (1)

    The slightly increased average grain size of Sn alloyed ferritic stainless steel by increasing Cu addition to 1.00 wt.% indicates that Cu has certain grain refinement effect. The pinning effect by Cu enrichment at grain boundaries inhibits the grain size growing.

  2. (2)

    Addition of Cu has little influence on the cathodic corrosion process of ferritic stainless steel in 3.5 wt.% NaCl solution, but shows beneficial effect on improving both the passivation behavior and the corrosion resistance of steel substrate.

  3. (3)

    The deposition of Cu particles at the bottom of corrosion pit is supposed to be responsible for the better passivation behavior corrosion resistance and passivation behavior of ferritic stainless steel in 3.5 wt.% NaCl solution at 30 °C.

  4. (4)

    The content of Sn2+ and Sn4+-containing oxides in the passivation film on ferric stainless steel surface increases as the amount of Cu addition increases to 1.00 wt.% in steel matrix. There is synergistic effect between Sn and Cu in enhancing the corrosion resistance of ferric stainless steel matrix and improving its passivation behavior in NaCl solution.