Introduction

Atmospheric corrosion of metals is an electrochemical process that occurs on an aqueous film formed over a metallic surface. The corrosion rate of metals in different atmospheres is influenced by various environmental factors such as temperature (Ref 1,2,3), relative humidity (RH) (Ref 4, 5), frequency of wet/dry cycles (Ref 6, 7), and air pollutants (Ref 7,8,9,10). Metals in the atmosphere are directly exposed to sunlight, which consists of approximately 2% ultraviolet radiation. Consequently, UV radiation is also an indispensable factor that affects the corrosion of different metals in the atmosphere. In recent years, several studies have investigated the influence of UV radiation on the corrosion behavior of metals exposed to the atmosphere. Song et al. reported the strong influence of UV radiation on the atmospheric corrosion process (induced by NaCl) of Q235 carbon steel, resulting in a significant increase in its corrosion rate (Ref 11).Thompson et al. found that UV radiation triggered the corrosion activity of Zn-based alloys after exposure for 4 months to the effluents (Ref 12). They observed that the photoinduced electrons migrated to the interface of the electrolyte and accelerated the cathodic reaction, thus accelerating the pitting corrosion of zinc alloys (Ref 12). Liu et al. studied the influence of UV radiation intensity on the corrosion behavior of Q235 in the south sea atmosphere and found that a considerable number of holds and fissures were distributed in the corrosion product layer. When the UV radiation intensity is increased to a certain degree, the corrosion rate of carbon steel is accelerated (Ref 13).

In general, stainless steels are widely used in various atmospheric environments owing to their excellent corrosion resistance. Nevertheless, atmospheric corrosion still occurs in these materials, particularly in an aggressive chloride ion environment. Most contemporary research on the effect of UV radiation on the corrosion performance of stainless steel focuses on the bulk electrolyte. Breslin et al. investigated the influence of UV radiation on the corrosion behavior of 316 SS in an NaCl solution. They found that the passive film of 316 SS was modified by UV radiation, and the pitting corrosion of 316 SS was improved (Ref 14). Moussa et al. proposed that UV radiation can improve the corrosion resistance of 304 SS in chloride-containing solutions and inhibit the pit growth of 304 SS (Ref 15). Stainless steels exposed to the atmosphere receive more UV radiation than those exposed to a solution. Thus, the corrosion behavior of stainless steel in an atmospheric environment is relatively different from that in a solution. However, there are few studies on the corrosion behavior of stainless steel in an atmospheric environment. Therefore, it is of great significance to study the corrosion behavior of stainless steel in the atmosphere.

The atmosphere of salt lakes, located in the northwest of China, is characterized by arid-rainless conditions and land salinization. Table 1 lists the climatic parameters of a typical salt-lake atmosphere. The high rate of sea salt deposition in this region exacerbates the atmospheric corrosion of metals in the region. Moreover, the salt-lake atmosphere is typically rich in MgCl2 salt particles with very low RH (i.e., <50%, or even approximately 30% RH), while the marine atmosphere is typically high in RH ( > 80% RH). Our previous research has demonstrated that the corrosion rate of stainless steel in a salt-lake environment is higher than that in a marine environment. This is mainly due to the differences in the DRH and ERH values between MgCl2 and NaCl (Ref 16). However, the influence of UV radiation on the environment was ignored in these studies. Furthermore, the metal in a salt-lake environment is more exposed to UV radiation than that in a marine environment. Thus, it is crucial to examine the influence of UV radiation on the corrosion behavior of 316 SS in a salt-lake environment.

Table 1 Climatic parameters of the salt-lake environment.
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Atmospheric corrosion is an electrochemical process; therefore, real-time atmospheric corrosion monitoring technologies such as atmospheric corrosion monitors (ACM) (Ref 17,18,19) and Kelvin probes (Ref 20) have been widely employed. However, the practical application of these methods is limited by many factors, some of which are not suitable for actual field monitoring, while others can only provide the severity of the degree of corrosion but not the direct information about existing electrochemical processes. However, in situ EIS has been used to obtain direct and real-time data on the corrosion behavior of metals using special micro-distance electrodes. Through this technique, corrosion rates along with the corrosion mechanisms of interface-related phenomena can be estimated (Ref 21,22,23,24).

Therefore, based on our previous work (Ref 16), we analyzed the effect of different UV radiation intensities on the corrosion behavior of 316 SS in detail, using in situ EIS, SEM, LSCM, WLI, and XPS technologies. For this purpose, the UV radiation intensities were set to 0.25, 0.5, 1, and 2 mW/cm2. Furthermore, the effect of UV radiation on the corrosion mechanism of 316 SS is discussed.

Experimental

Samples Preparation and Electrodes Preparation

For the accelerated wet/dry cyclic corrosion test and EIS analysis, specimens with dimensions 50× 25× 3 mm3 and 10× 10× 3 mm3 were cut from the as-received 316 SS plate. The composition of the 316 SS plates is listed in Table 2. To obtain the electrochemical signals, a comb-like micro-distance electrode was constructed on a 316 SS plate using the same method outlined in our previous work (Ref 21). The 316 SS specimens were separated by a thin insulating layer and sealed using epoxy resin. Figure 1(a) shows the arrangement of the in situ electrodes. Figure 1(b) illustrates the distance (approximately 60 μm) between the two specimens. The micro-distance electrodes were mechanically ground with SiC paper up to 2000 grit and then stored in a dryer for at least 24 h, followed by a wet/dry cyclic corrosion test.

Table 2 Chemical compositions of the 316 SS stainless steel (wt.%)
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Fig. 1
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Schematic diagram electrode design: (a) top view of the comb-like microelectrodes; (b) optical micrograph showing the distance between two plates

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Wet/Dry Cyclic Corrosion Experiment

To simulate the atmospheric environment of a salt lake, a wet/dry cyclic corrosion experiment was performed using a Weiss-Voetsch temperature and climatic test system (Fig. 2), and the effect of various UV radiation intensities on the corrosion behavior of 316 SS was investigated. The samples in the test chamber were divided into two layers separated by an anti-UV apron. The specimens in the top layer were irradiated with UV radiation, whereas those in the lower layer were not exposed to UV radiation. UV intensity was measured using a UVAB light meter (Tenmars-TM213). The test system was connected to an electrochemical station.

Fig. 2
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Schematic diagram elaborating in situ EIS test for performing wet–dry cyclic corrosion experiment under different UV radiation intensities

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The wet/dry cyclic corrosion experiment was carried out for 10 days to simulate real day and night conditions in the salt-lake environment. The wet/dry cycle included the following steps: (1) wetting the 316 SS surface with 20 μL/cm2 of 0.1 mol/L MgCl2 to simulate a salt-lake environment; (2) drying the surface in an oven maintained at 40 °C; (3) wetting the surface in the test chamber at 30 °C and RH (80%) for 60 min in the absence of UV light; (4) drying the surface in the test chamber fixed at 30 °C and RH (20%) for 120 min under UV light; and (5) repeating steps (3) to (4) 15 times.

Electrochemical Measurements

The in situ EIS test was carried out using a PARSTAT 2273 workstation. The EIS spectra of the micro-distance electrodes, under a thin liquid MgCl2 film, were measured in situ in the corrosion test chamber, as shown in Fig. 2. The amplitude of the AC voltage was maintained at 10 mV. The scanning frequency, which was set between 105 Hz and 10-2 Hz in the test process, was applied from high to low values during the test, and a total of 50 frequency points were obtained. The in situ EIS data were processed using the commercial software “ZSimpWin” (version 3.21).

Characterization

The two-dimensional (2D) surface morphologies of 316 SS were observed using SEM (XL30-FEG). Three-dimensional (3D) surface morphologies of 316 SS were observed using a non-contact laser scanning confocal microscope (LSCM, Zeiss LSM-700). The surface roughness of the specimens was recorded using LSCM. The composition of the corrosion products was assessed via XPS (ESCALAB 250) using monochromatic Al Kα radiation (hν = 1486.6 eV). For XPS, the measured current of the sample was 2 µA, and the size of the bombardment zone was 2 × 2 mm2. The binding energy (BE) was corrected using a carbon pollution standard (284.6 eV). Finally, XPS data were processed using “XPSPEAK 4.1.”

The corrosion products of 316 SS were carefully removed in a specific chemical solution (100 mL nitric acid + 900 mL distilled water) and vigorously stirred at 60 °C for approximately 20 min as per the International Organization for Standardization (ISO) 8407 standard. After removing the corrosion products formed on the surface of 316 SS, the pit morphologies and pit depth statistics were estimated using a white light interferometer (WLI, Micro XAM). The pit density was determined using image analysis software (Image J).

Results and Discussion

In Situ EIS

Figure 3(a) and (b) displays the Nyquist and Bode plots of 316 SS under different UV radiation intensities after 10 days of corrosion. Figure 3(c) shows the equivalent circuit obtained after fitting the in situ EIS results. As shown in Fig. 3(c), Rs represents the solution resistance, and Qr and Rr indicate the capacitance and resistance of the surface corrosion product, respectively. Qdl and Rct represent the capacitance and resistance of the double charge layer, respectively. The EIS fitting parameters of 316 SS are listed in Table 3. In this study, the reciprocal of the charge transfer resistance (1/Rct) was adopted to estimate the corrosion rate of 316 SS under different UV radiation intensities, as shown in Fig. 3(d). The 1/Rct values of the 316 SS are all lower than those without UV radiation. When UV radiation intensity was increased from 0.25 mW/cm2 to 2 mW/cm2, the UV radiation effectively restrained the corrosion rate. With further increase in the UV radiation intensity from 0.25 mW/cm2 to 0.5 mW/cm2, the inhibition influence of UV gradually increased. In contrast, the inhibitory influence of UV radiation gradually decreased when the UV radiation intensity was increased from 0.5 to 2 mW/cm2. That is, the inhibitory influence of UV radiation on the corrosion rate was the strongest when the UV radiation intensity was maintained at 0.5 mW/cm2.

Fig. 3
figure 3

In situ EIS results of 316 SS under various UV radiation intensities after 10 days of corrosion: (a) Nyquist plot, (b) Bode plot, (c) equivalent circuits, and (d) reciprocal of Rct

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Table 3 The fitting results of EIS parameters
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Morphologies of the Corrosion Products

Figure 4 displays the SEM morphologies of the surface corrosion products of 316 SS under different UV radiation intensities for an exposure time of 10 days. As evident in Fig. 4(a), some dispersed corrosion products are formed on the surface of 316 SS in the absence of UV radiation. When UV radiation intensity is increased from 0.25 to 0.5 mW/cm2, a small amount of corrosion products are formed, as shown in Fig. 4(b) and (c). However, the content of corrosion products gradually increases with increase in UV radiation intensity from 0.5 to 2 mW/cm2, as shown in Fig. 4(d) and (e).

Fig. 4
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SEM images of 316 SS after 10 days of corrosion under UV radiation intensity of (a) 0 mw/cm2, (b) 0.25 mw/cm2, (c) 0.5 mw/cm2, (d) 1 mw/cm2, and (e) 2 mw/cm2

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LSCM simultaneously provides information on the surface morphology and roughness of corrosion products. Figure 5 shows the 3D surface morphologies and surface roughness (Ra) values of 316 SS under different UV radiation intensities for an exposure time of 10 days. All results were obtained for the same parameters and area sizes. At a UV radiation intensity of 0.5 mW/cm2, the Ra value of 316 SS is approximately 0.360 μm (Fig. 5a). When the UV radiation intensity was increased from 0.25 mW/m2 to 0.5 mW/cm2, the Ra value of 316 SS gradually decreased and reached a minimum value of 0.116 μm, as shown in Fig. 5(b) and (c). Then, the Ra value gradually increases when the UV radiation intensity was increased from 0.5 mW/cm2 to 2 mW/cm2, as shown in Fig. 5(d) and (e).

Fig. 5
figure 5

LSCM images along with 3D surface roughness values of the surface of 316 SS after 10 days of corrosion under UV radiation intensity of (a) 0 mw/cm2, (b) 0.25 mw/cm2, (c) 0.5 mw/cm2, (d) 1 mw/cm2, and (e) 2 mw/cm2

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XPS of Corrosion Products

Figure 6 shows the XPS spectra of 316 SS under different UV radiation intensities for an exposure time of 10 days. In particular, the figure shows Fe 2p2/3, Cr 2p2/3, and O 1s core-level peaks. The XPS results confirm that Fe element is mainly presented as Femetal, Feox2+, Feox3+, Fe hyd, and Fex (LDH, layered double hydroxides) with binding energies of 707.2, 709.4, 710.7, 711.7, and 713.2 eV, respectively (Fig. 6a and b). Similarly, the Cr 2p2/3 peaks are mainly distributed at 574.6, 576.4, 577.4, and 578.8 eV, which are assigned to be Crmetal, Crox3+, Cr hyd, and Crox6+ (Fig. 6c and d). The O 1s peaks are mainly attributed to O2-, OH-, and H2O with binding energies of 530.3, 531.2, and 532.5 eV, respectively. (Fig. 6e and f). The fitting parameters for the XPS results are listed in Table 4 (Ref 25,26,27). The XPS results indicated that UV radiation intensities did not change the type of corrosion products; however, it varied their relative content on the surface of 316 SS.

Fig. 6
figure 6

XPS spectra observed for 316 SS under different UV radiation intensities after 10 days of corrosion in Fe 2p3/2, Cr 2p3/2 and O 1s

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Table 4 Parameters of XPS spectra
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Figure 7 shows the relative contents of hydroxide and oxide in the main Fe and Cr compounds of the corrosion products under different UV radiation intensities. Notably, the relative amount of [Feox] in corrosion products was remarkably higher under UV radiation than that in the absence of UV radiation, while a reverse trend was observed for [Fehyd]. These results indicate that UV radiation restrained the production of Fehyd and promoted the production of [Feox] to some extent. Similarly, UV radiation also influenced the relative content of Cr compounds in the corrosion products. The relative content of Crox3+ under different UV radiation intensities was higher than that in the absence of UV radiation. At a UV radiation intensity of 0.5 mW/cm2, the relative content of Crox3+ apparently increased, whereas the relative content of Crhyd appeared to be less than that without UV irradiation. Previous studies suggest that Cr2O3 thin layers are relatively compact in nature and display excellent protective effect, while the Cr(OH)3 thin layer is relatively porous and displays poor protective effect (Ref 28). Thus, when the UV radiation intensity is 0.5 mW/cm2, the high content of Cr2O3 in the corrosion products is beneficial for enhancing the corrosion resistance of 316 SS. Therefore, the strongest inhibitory influence of UV radiation observed on the corrosion rate at the UV radiation intensity of 0.5 mW/cm2 may be related to the enhanced protective effect of corrosion products. For a UV radiation intensity of 2 mW/cm2, the relative content of Crhyd apparently became higher than that in the absence of UV radiation, while in the case of Crox3+, the reverse trend was observed. Thus, the inhibitory influence of UV radiation gradually decreased when the UV radiation intensity was increased from 0.5 to 2 mW/cm2.

Fig. 7
figure 7

The relative contents of “hydroxide” and “oxide” in primary Fe and Cr compounds of surface corrosion products of 316 SS under different UV radiation intensities after 10 days of corrosion

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The above results indicate that UV radiation affected the relative content of corrosion products and inhibited the corrosion rate of 316 SS under different UV radiation intensities. Moreover, the inhibitory effect could be closely related to the intensity of UV radiation. The ratio of [Cr]/{[Cr]+[Fe]} in the corrosion products under different UV radiation intensities was further calculated, and the corresponding results are plotted in Fig. 8. The ratio of [Cr]/{[Cr]+[Fe]} under different UV radiation intensities was greater than that in the absence of UV radiation. A higher ratio of [Cr]/{[Cr]+[Fe]} in the passive film can enhance the corrosion resistance of stainless steel (Ref 29, 30). Fujimoto et al. found that Cr was selectively enriched in the passive film under UV radiation, thus inhibiting pitting corrosion on stainless steel surfaces (Ref 31). In this study, further enrichment of Cr content in the corrosion products by UV radiation is supposed to be the main factor affecting the corrosion rate of 316 stainless steel under UV radiation. The ratio of [Cr]/{[Cr]/[Fe]} initially increased and then decreased with a change in UV radiation intensity range of 0.25-2 mW/cm2. Thus, when the UV radiation intensity is 0.5 mW/cm2, the high ratio of [Cr]/{[Cr]/[Fe]} in the corrosion products is beneficial for enhancing the corrosion performance of 316 SS.

Fig. 8
figure 8

The ratio of [Cr]/{[Cr]+[Fe]} in corrosion products of 316 SS under different UV radiation intensities after 10 days of corrosion

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Analysis of Pits Morphology and Depth

Figures 9 and 10 shows the pit morphologies and pit sizes (depth and density) of 316 SS under different UV radiation intensities for an exposure time of 10 days. The maximum pit depth formed on the surface of stainless steel is closely related to the safety of stainless steel in service. Therefore, the maximum pit depth was analyzed using a statistical analysis method. Figure 10(a) presents the largest pit depth formed on 316 SS under different UV radiation intensities for an exposure time of 10 days. The maximum pit depth initially decreased gradually and then increased as UV radiation intensity changed in the range 0.25-2 mW/cm2. The maximum pit depth, formed on 316 SS under UV radiation intensities, was smaller than that without UV irradiation. The maximum pit depth was the smallest when the UV radiation intensity is 0.5 mW/cm2. In addition, Fig. 10(b) shows the pit density formed on 316 SS under different UV radiation intensities. The change in pit density followed a similar trend as that of the pit depth.

Fig. 9
figure 9

Representative morphologies of pits formed on 316 SS after 10 days of corrosion under UV radiation intensity of (a) 0 mw/cm2, (b) 0.25 mw/cm2, (c) 0.5 mw/cm2, (d) 1 mw/cm2, and (e) 2 mw/cm2

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Fig. 10
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Maximum pit depth (a) and pit density (b) of 316 SS under different UV radiation intensities after 10 days of corrosion

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Conclusions

In this study, the influence of different UV radiation intensities on the corrosion performance of 316 SS was studied in a simulated salt-lake environment. The main conclusions of this work are as follows:

  1. 1.

    The EIS results revealed that the corrosion rate of 316 SS was apparently inhibited by UV radiation, and the influence of this UV inhibition phenomenon initially increased and then decreased with increasing the UV radiation intensity from 0.25 to 2 mW/cm2. The inhibition effect of UV on the corrosion rate of 316 SS was the strongest when the UV radiation intensity was 0.5 mW/cm2.

  2. 2.

    UV radiation intensities did not change the type of surface corrosion products of 316 SS during the corrosion process; however, it altered the relative composition of the corrosion products. The increasing ratio of [Cr]/{[Cr] + [Fe]} in the corrosion products could improve the protection ability of the corrosion products formed on 316 SS.

  3. 3.

    The maximum pit depth and pit density formed on the surface of 316 SS initially decreased gradually and then increased with a change in UV radiation intensity range of 0.25-2 mW/cm2.