Effect of UV Irradiation on the Alternating Wet and Dry Corrosion Behavior of Galvanized Steel in Sodium Chloride Solution

Abstract

In this paper, the corrosion performance of galvanized steel was investigated in a simulated marine environment, under UV irradiation coupling with an alternating wet and dry cycle in a NaCl solution. The surface morphology, composition, and corrosion performance of galvanized steel before and after different alternating wet and dry corrosion under UV irradiation were investigated. The results show that the corrosion current density gradually increases and the corrosion resistance decreases as a function of the alternating wet and dry corrosion cycles. Meanwhile, UV irradiation accelerates the increase in the corrosion current density and the decrease in the corrosion resistance. In addition, the corrosion product ZnO shows a semiconductor property, and the photo-induced electrons and holes produced under UV can participate in the corrosion reaction and promote the formation of loose corrosion products Zn(OH)₂, Zn₅(OH)₈Cl₂, and Al₂Cl₃(OH)₅·4H₂O, thus accelerating the corrosion of galvanized steel in the atmosphere environment.

1. Introduction

The marine environment is rich in resources, and the exploitation of the sea has become a hot topic. However, offshore equipment is extremely vulnerable to corrosion damage due to the complexity of the marine environment. Apart from the chloride corrosion, the equipment normally bears marine atmospheric corrosion under light radiation, splash, and wave-induced alternating wet and dry action, as well as wind, rain, waves, and currents of mechanical impact [1]. Galvanized steel is widely used in offshore vessels, oil rigs, island equipment, sewage pipes, and cross-sea bridges due to its high strength and low cost [2,3,4,5]. Therefore, the investigation of the effect of marine environmental factors on galvanized steel plays an important role in evaluating the service life of equipment in the marine environment.

Atmospheric corrosion is essentially an electrochemical corrosion process of materials in the dry/wet cycle under a thin electrolyte layer, variation of temperature, relative humidity (RH), rainfall, salt particles, and frequency of wet/dry cycling [6,7,8]. In the last few decades, many efforts have declared that temperature and RH have a significant effect on the corrosion performance of metal via variations in the physical and chemical state of the atmosphere/metal interface layer [9,10,11,12]. The salt particles in the air form droplets or electrolyte layers on the metal surface, formed when the temperature drops and RH rises. However, the droplets or the thickness of electrolyte layers decrease when the temperature rises and the RH drops [11]. According to Fick’s law, variations in the thickness of electrolyte layers affect the dissolution and diffusion of oxygen, resulting in different corrosion kinetics of metals [13]. The investigation demonstrated the corrosion rate under a thin electrolyte layer was much higher than that of the sample immersed in the solution, because of the accelerated dissolution and diffusion of oxygen in the thin electrolyte layer [14,15]. Meanwhile, the effect of temperature on the corrosion behavior of low-alloy steel in the marine atmosphere shows that the increase in temperature not only promotes the transport of aggressive ions Cl⁻ and the formation of local corrosion but also affects the solubility of oxygen gas in the thin electrolyte layer [12].

UV irradiation is also an indispensable factor affecting the corrosion of metal in the atmosphere corrosion when the metals are exposed to sunlight; the 2% ultraviolet (UV) in sunlight has a critical effect [16]. Many studies have confirmed that UV irradiation can affect the corrosion process of metals in the atmosphere. Song et al. [17] found that UV irradiation significantly increased the atmospheric corrosion rate and the positive optical voltage of carbon steel when they investigated the influence of UV irradiation on the atmospheric corrosion of Q235 carbon steel induced by NaCl. Liu et al. [18] also found the corrosion rate of carbon steel increased with the increase in temperature from 30 to 60 centigrade in the high-humidity tropical marine atmosphere. The effect of UV illumination on carbon steel was closely related to environmental temperature; the higher the environmental temperature, the greater the impact. A combination of electrochemical and spectroscopic techniques was built to study the corrosion behavior of Cu-Sn-pb ternary bronze under UV light; the results indicated that UV/visible illumination had a significant effect on the corrosion behavior of bronze covered with oxide films, and that visible light irradiation accelerated the dissolution rate of Cu and Pb and promoted the growth of corrosion layers on the bronze surface [16]. The study of the photoelectric cathodic protection performance of a coating system on stainless steel suggests that it has some corrosion protection after exposure to UV light [19]. TiO₂-coated steel revealed the corrosion rate increased due to the active oxygen generated by photoelectrochemical reaction under UV light [20]. The effect of UV irradiation on the corrosion of zinc was also investigated. Results show that UV irradiation apparently affected the atmospheric corrosion morphology and corrosion rate of zinc. Moreover, the corrosion products have a semiconductor property: the photo-induced electrons and holes produced under UV light which can participate in the corrosion reaction and affect the atmospheric corrosion rate of zinc [21,22,23].

Although the effect of UV on zinc and steel has been investigated, the investigation of the UV effect on galvanized steel is still very limited. The chemical composition and microstructures of galvanized steel are different from zinc and steel. The galvanization process with a molten 0.1–1 at% Al-Zn bath results in an outer layer (Zn-Al coating) and an intermetallic film in the form of Fe₂Al₅ Znx (0 < x < 1). Therefore, the degradation behavior of galvanized steel in wet/dry alternating corrosion environments with UV irradiation might be different. For this purpose, a wet/dry alternating corrosion test system coupled with UV irradiation was built to evaluate the degradation process of galvanized steel in a simulated marine atmosphere environment. The effect of UV irradiation on the variation of structure, composition, and corrosion electrochemical behavior of galvanized steel was systematically studied.

2. Materials and Methods

2.1. Materials

A commercial hot-dip Q345B galvanized steel was supplied by Shanghai Casting Enterprise Industrial Co., Ltd., Shanghai, China. The galvanized steel was cut into specimens measuring 45 mm × 35 mm × 5 mm using a wire cutter, and then a 3 mm hole was perforated in the middle of one side (as shown in Figure S1a). The specimens were degreased with acetone, rinsed with deionized water, and dried in cold air.

2.2. Alternating Corrosion Test System

Based on the temperature, humidity, and heat test chamber, a dry and wet alternating corrosion test system coupled with UV irradiation was built to simulate the dry and wet alternating corrosion state of galvanized steel in the marine environment, as shown in Figure 1. The corrosion medium was 3.5 wt.% NaCl solution, the test temperature was 45 ± 1 °C, the dry and wet ratio was 1:1 (1 cycle was 4 h, recorded as 1 T), the wet treatment was full immersion, the distance between the UV and the sample was 600 mm, and the dry treatment was placed under UV irradiation at a 250 mm distance. The UV irradiation was provided by a 15 W quartz UV lamp, a UV spectral range of 200–330 nm, and the UV irradiation intensity on the sample surface was 0.11 mV/cm⁻².

2.3. Characterization

Potentiodynamic polarization curves (PDP) and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion performance of the galvanized steel by an electrochemical workstation (CHI660D, Wuhan, China), with a conventional three-electrode cell, platinum plate as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and the galvanized steel as the working electrode. Before the testing, the corrosion test system was left to stand for 30 min, and the open circuit potential (OCP) was recorded for 600 s. The PDP test ranged from −0.3 V vs. OCP to 1.5 V vs. SCE, and the scanning rate was 1 mV/s. EIS test was performed at frequencies ranging from 100 kHz to 10 mHz by using a 10 mV amplitude sinusoidal voltage at OCP. The data were fitted using ZSimpWin3.5 software. The PDP and EIS measurements were carried out three times to ensure the repeatability of the measurements.

The morphology of the galvanized steels before and after different dry and wet alternating corrosion tests were characterized by scanning electron microscopy (SEM, S4800, Hitachi, Tokyo, Japan) with an energy dispersive spectroscopy (EDS, X-MaxN, Oxford, UK). The crystalline structure of the samples was investigated by X-ray diffraction (XRD, D8 ADVANCE, BRUKER, Ettlingen, Germany) with Cu-Kα radiation (λ = 0.15406 nm).

3. Results

3.1. Electrochemical Behaviors

Potentiodynamic polarization measurement has been widely used to evaluate the anti-corrosion performances of metal and coated metals [24,25]. Figure 2 shows the potentiodynamic polarization curves of galvanized steel after different cycles of alternating wet/dry corrosion with and without UV irradiation. After different cycles of corrosion, the corrosion potential of the specimens under UV irradiation was more negative and the corrosion current density was also greater, indicating that the galvanized steel was much easier to corrode under UV irradiation than that of the samples without UV irradiation. Meanwhile, it is clearly revealed that the corrosion of galvanized steel under UV is much more serious than that of galvanized steel without UV, as shown in the polarization curves of the sample with and without UV in the same cycles of corrosion (Figure S2).

In order to compare the corrosion rate of galvanized steel, the corrosion potential (E_corr) and corrosion current density (i_corr) were calculated from the polarization curves, as shown in Figure 3. From Figure 3a, the corrosion potential of the galvanized steel without UV decreased dramatically and reached a stable value with the increase in the cycles of corrosion. The corrosion potential of the specimen under UV shows a trend of first decreasing and then gradually increasing with the increase in the cycles, reaching a minimum value of −1.233 V at 5 T. The corrosion potential of the specimens under UV irradiation is more negative than that without UV irradiation. One interpretation is, in the atmosphere environment, zinc can form ZnO, which has a typical semiconductor property that can promote the formation of photoelectrons and migration to the surface of the galvanized steel resulting in a shift of corrosion potential to a negative direction under UV. Generally, the more negative the corrosion potential, the more prone the metal is to corrode [26]. Therefore, galvanized steel is easier to corrode under UV irradiation than the samples without UV. Figure 3b shows the corrosion current density of the specimen with and without UV irradiation. As for the sample without UV, the corrosion current density increased from 2.24 × 10⁻⁷ A·cm⁻² to the maximum of 6.23 × 10⁻⁶ A·cm⁻² after 10 T cycles of corrosion; however, the corrosion current density of the specimen with UV irradiation increased from 2.22 × 10⁻⁷ A·cm⁻² to the maximum of 5.76 × 10⁻⁵ A·cm⁻² after 10 T cycles of corrosion and kept at approximately 5.00 × 10⁻⁵ A·cm⁻² after 10 T cycles of corrosion. The corrosion current density of the specimen with UV irradiation was larger than that without UV irradiation. As described in a previous study, photoelectrons promote the generation of active oxygen to increase the corrosion of galvanized steel, resulting in a high corrosion current density under UV [20]. Figure S3 shows the Mott–Schottky (M-S) of the galvanized steels after different cycles of corrosion. After 1 T cycle of corrosion, galvanized steels with and without UV show a typical positive slope, indicating that both surface samples are n-type semiconductors. The sample without UV shows a similar semiconductor property from 1 T to 10 T, but the semiconductor property of the sample gradually decreased after 16 T, which might be attributed to an increase in corrosion resulting in zinc oxide conversion to zinc hydroxide. However, the semiconductor property of the galvanized steels with UV declines or even disappears after 3 T cycle corrosion. Under UV, ZnO on the surface of galvanized steel will prove that the formation of photo-induced electrons and holes participate in the corrosion reaction and then promote the formation of loose corrosion products, such as Zn(OH)₂, Zn₅(OH)₈Cl₂, Al₂Cl₃(OH)₅·4H₂O, which accelerate the corrosion of galvanized steel. From the above analysis, UV irradiation will accelerate the corrosion process of galvanized steel in a simulated marine atmosphere.

EIS was employed to evaluate the corrosion resistance and analyze the anti-corrosion mechanism of galvanized steel. Figure 4 and Figure 5 show the EIS curves of galvanized steel after different cycles of corrosion with and without UV. As shown in Figure 4a, the impedance modulus of galvanized steel without UV decreased gradually with the increase in cycles of immersion, and the capacitive arc gradually decreased, as shown in Figure 4b, indicating the corrosion resistance of galvanized steel decreased as a function of the corrosion cycles. Meanwhile, there are two-time constants appearing, which were attributed to the zinc coating and the electrochemical corrosion reaction at the solution/zinc coating interface. The EIS data can be demonstrated by a physical model shown in Figure 6. R_s is the solution resistance; R_c and Q_c are the coating resistance and capacitance, respectively; and R_ct and Q_dl are the charge transfer resistance and capacitance, respectively [27]. From Figure 4c, as for the sample with UV, the impedance modulus of galvanized steel decreased dramatically from 10⁴ Ω·cm² at 1 T to 1.81 × 10⁴ Ω·cm² at 3 T and decreased with the increase in the cycles of immersion. The capacitive arc shifted to low frequency, suggesting the composition of the corroded galvanized steel has a significant change due to some special reaction occurring under UV.

Figure 5 shows a comparison of the EIS curves with and without UV for the same corrosion cycle. The EIS curves with and without UV at cycle 1 T are very similar; however, with the extension of the corrosion cycle, the EIS curves change significantly and the impedance modulus at 0.01 Hz decreases sharply with UV, indicating that UV exacerbates the damage to the galvanized coating, leading to a sharp reduction in corrosion resistance. Meanwhile, the phase constant in the plots of the galvanized steel shift to a low frequency, suggesting the composition of the galvanized steel has a significant difference under UV. This result should be attributed to the formation of ZnO with the semiconductor property, which changes the reaction on galvanized steel under UV. The composition of the corrosion products on galvanized steel will be detected by XRD to further verify the reaction.

In order to analyze the anti-corrosion mechanism of galvanized steel, this study used the equivalent circuit model (Figure 6) to fit the impedance data. Figure 7 shows the R_ct and R_c of galvanized steel after different cycles of corrosion in 3.5 wt.% NaCl solution without and with UV. R_ct, which is inversely proportional to corrosion rate, is a parameter representing the resistance of the electron transfer across the metal surface. The higher R_ct is, the more difficult the corrosion reaction, and hence the lower the corrosion rate [25]. As shown in Figure 7a, the R_ct values of galvanized steel without UV are higher than the samples with UV. The results indicate that UV accelerates the electron transfer on the surface of galvanized steel, and then increases the corrosion rate of galvanized steel.

R_c can be used as the parameter to reflect the barrier property of zinc coating to corrosive agents [28]. It can be found in Figure 7b that the R_c value of galvanized steel without UV is much higher than the sample with UV before 5 T corrosion cycle, and the R_c value decreased dramatically, indicating that the zinc coating has a good barrier property to corrosive agents at the beginning, but this decreases due to the corrosion reaction that occurred on the coating. Polarization and EIS data suggest that UV will affect the corrosion reaction on galvanized steel and accelerate the corrosion rate. Using SEM and XRD, we further analyze the structure and composition of galvanized steel in the different corrosion cycles.

3.2. Structure and Composition

The corrosion morphology of the galvanized steel after different cycles was further analyzed using SEM and EDS; the results are shown in Figure 8 and

Table 1. Results of EDS analysis of galvanized steel after immersion in 3.5% wt.% NaCl solution with and without UV for 46 T.

Elements (W%)	Zn	O	Cl	Al	Other
Without UV	86.52	12.87	0.13	0.45	0.03
With UV	58.43	28.47	12.89	0.04	0.17

. From Figure 8, the corrosion products on the surface of the galvanized steel with and without UV increased with the increase in corrosion cycles. Meanwhile, the corrosion products on the surface of the galvanized steel with UV was much more than that of the samples without UV after 1 T, 10 T, and 25 T. For 46 T, the corrosion products on the surface of the galvanized steel with UV were looser than those of the sample without UV, but it is difficult to distinguish the number of corrosion products. Therefore, EDS was performed on the galvanized steel after the 46 T experiment. Most of the corrosion products on the sample without UV were Zn and 12.87% O, indicating the corrosion products mainly might be oxide and hydroxide compounds. However, 28.47% O was detected on the surface of the galvanized steel with UV, suggesting much more zinc oxide and zinc hydroxide were formed. Meanwhile, 12.89% Cl elements were also detected, indicating the corrosion of galvanized steel with UV is more serious than that of the sample without UV, and new corrosion products might form under UV. These results indicated that UV irradiation not only accelerated the corrosion of galvanized steel, but also changed the corrosion process of galvanized steel, and the composition of corrosion products changed significantly. XRD was used to further analyze the structure of corrosion products to illustrate the degradation mechanism of galvanized steel under UV.

The composition of the corrosion products on galvanized steel was characterized using XRD. As shown in Figure 9, as for the galvanized steel without UV, there was only a few ZnO formed on the galvanized steel after 1 T alternating wet/dry corrosion. The peak of ZnO increased slightly as a function of the immersion corrosion cycle. In contrast, the corrosion products of galvanized steel with UV showed significant diversity. Apart from the ZnO, Zn₅(OH)₈Cl₂H₂O (Simonkolleite) appeared at approximately 10° after 1 T of corrosion, indicating a new reaction occurred under UV [29]. Meanwhile, the peak of ZnO was higher than that of the sample without UV, declaring that UV accelerates the corrosion of the zinc coating. After 25 T corrosion cycles, FeOCl, AlOCl, and Al₂Cl₃(OH)₅·4H₂O appeared, indicating that the aggressive agents penetrated the zinc coating to the Fe₂Al₅ intermetallic layer, even to the matrix [29]. SEM and XRD results further verify that UV irradiation can change the corrosion reaction on galvanized steel.

3.3. Corrosion Mechanism

Based on the above results, the corrosion mechanism diagrams of galvanized steel in a simulated marine environment, under UV irradiation coupling with an alternating wet and dry in a NaCl solution, are proposed and presented in Figure 10. According to our previous study, the galvanization process with a molten 0.1–1 at% Al-Zn bath resulted in an outer layer (Zn-Al coating), and an intermetallic film (20–50 nm) formed between the zinc coating and the steel substrate, presumably in the form of Fe₂Al₅ Zn_x (0 < x < 1), which can inhibit the formation of Fe-Zn intermetallic phase or “outbursts” that are detrimental to the mechanical properties of the final coating [30].

As shown in Figure 10a, in a neutral NaCl solution, the Zn-rich phase is more active than the Al-rich phase; consequently, Zn²⁺ ions formed in an anodic reaction (in Equation (1)), whilst a cathodic reaction (O₂ reduction as shown in Equation (2)) occurred on the Al-rich phase. Zn²⁺ ions reacted with the OH⁻ from the cathodic reaction resulting in Zn(OH)₂. The Zn(OH)₂ will transfer to ZnO because of the unstable of Zn(OH)₂ (Equation (3)). Meanwhile, Al will dissolve to Al(OH)₄⁻ (Equation (4)) because of the increase in OH⁻ from O₂ reduction [29,31].

Zn → Zn²⁺ + 2e⁻

(1)

O₂ + 2H₂O + 4e⁻ → 4OH⁻

(2)

Zn²⁺ + 2OH⁻ → Zn(OH)₂ (or ZnO + H₂O)

(3)

Al + 4OH⁻ → Al(OH)₄⁻ + 3e⁻

(4)

However, under UV, the corrosion that occurs on the surface of galvanized steel is more complex and the corrosion products are numerous. According to the report, apart from ZnO, other compounds of zinc were produced on the surface of galvanized steel and the corrosion rates accelerated because of the photo-induced electrons and holes produced under UV [21]. This phenomenon was also detected in this study (Figure 9); Zn₅(OH)₈ Cl₂ was detected on the surface of galvanized steel under UV (Equation (5)):

5Zn²⁺ + 8OH⁻ + 2Cl⁻ → Zn₅(OH)₈ Cl₂

(5)

With the enhancement of the corrosion reaction on the sample, Cl⁻ aggressively reaches the Fe₂Al₅ intermetallic film, and the intermetallic layer reacts with Cl⁻ to form Fe²⁺ and Al³⁺, and then transfers to FeOCl and Al₂Cl₃(OH)₅·4H₂O or AlOCl under the UV catalytic. When the Cl⁻ reached the Fe matrix, the Zn-Al coating gradually lost its protective ability.

Fe → Fe²⁺ + 2e⁻

(6)

Al → Al³⁺ + 3e⁻

(7)

2Al³⁺ + 5OH⁻ + 3Cl⁻ + 4H₂O→Al₂Cl₃(OH)₅·4H₂O

(8)

2Fe²⁺ + O₂ + 2Cl⁻ → 2FeOCl

(9)

2Al²⁺ + O₂ + 2Cl⁻ → 2AlOCl

(10)

By the above analysis, under UV irradiation, the corrosion process of galvanized steel can be divided into three stages: (I) Zn-Al coating reacts with Cl⁻ aggressive agents, H₂O and O_2, to form ZnO or Zn(OH), Zn₅(OH)₈ Cl₂; (II) Fe₂Al₅ intermetallic layer reacts to form FeOCl and Al₂Cl₃(OH)₅·4H₂O or AlOCl after Cl⁻ aggressive agents penetrate the Zn-Al coating; (III) Cl⁻ aggressive agents reach the steel substrate and start to dissolve. UV has an important role in atmosphere corrosion, which can accelerate the destruction of the galvanized layer on the surface of Q345B, accelerating the corrosion medium into the substrate, resulting in the destruction of the galvanized layer and a certain degree of corrosion of Q345B.

4. Conclusions

In this work, a wet/dry alternating corrosion test system coupled with UV irradiation was built to evaluate the degradation process of galvanized steel in a simulated marine atmosphere environment.

(1)

The polarization and impedance results show the corrosion of galvanized steel increased as a function of the accelerating wet/dry corrosion cycles. The corrosion of galvanized steel under UV is more serious than that of galvanized steel without UV in the same corrosion cycles, which is explained by the formation of semiconductor ZnO to accelerate the corrosion process under UV.

(2)

In the NaCl solution, the Zn-Al coating reacts to form ZnO and Al(OH)₄⁻. The corrosion reaction gradually occurs, and corrosion products increase as a function of accelerating wet/dry corrosion cycles without UV irradiation. However, under UV irradiation, ZnO in galvanized corrosion products has semiconductor properties and the photovoltaic effect, promoting the formation of loose corrosion products Zn(OH)₂, Zn₅(OH)₈ Cl₂, and Al₂Cl₃(OH)₅·4H₂O, thus accelerating the corrosion of galvanized steel.