1. Introduction
Q235 steel has good plasticity, toughness and weldability and is widely used in construction and engineering structures, such as vehicles, boilers, containers, etc. In addition, before the end of the 1940s, ordinary carbon steel (Q235) was used for pipeline steel [
1]. However, the corrosion resistance of Q235 steel is poor, and Q235 steel will suffer corrosion in weakly alkaline, neutral and acidic environments (including the atmosphere, water and soil), which will cause significant losses of economy, resources and energy and threaten the safety of the corresponding grounding infrastructure and personnel. The rapid development of urbanization and industrialization has led to the deterioration of the environment. “Smog weather” usually occurs in winter, but pollutants are present in the air all year round. Fine particulate matter (PM 2.5) in the atmosphere enters the soil with rain, which causes the transformation of soil quality [
2,
3]. Rainfall, growth absorption, water loss from terrain, and human activities all affect changes in the water content in the soil environment and subsequently affect soil corrosion. In addition, soil corrosion is affected by soil resistivity, soluble salts, water content, pH and the interactions of the factors. The factors often vary with time and space, and they are very complex [
4]. Therefore, the rules and mechanism of soil corrosion still need to be further explored and studied.
Soil corrosion is one of the main causes of grounding infrastructure failure, and it is hidden and difficult to detect. Soil is a complex electrolyte system. Water provides soil with an electrolyte [
5]. There are many kinds of soil in China, and the corrosion resistances of the same material in different soils are very different. Currently, natural environment corrosion test nets have been built in our country. Particles in sand have no adsorption effect on ions, and the corrosion of metal materials has certain characteristics [
6]. Studying the electrochemical corrosion mechanism of Q235 steel in sand under a natural air-dried state (moisture from saturated to dry) is helpful for evaluating the corrosion status of Q235 steel through rainfall or human activities to guide engineering application.
In recent years, the corrosion of Q235 steel in soil environments with different properties [
6], such as magnesium chloride polluted sandy silt soil, silt soil containing sodium chloride, sand [
7], diatomite soil [
8], sodium bentonite [
9], bentonite clay [
10] and alternating wet and dry soil [
11] was studied by electrochemical testing techniques and morphological composition testing techniques. The simulated acid rain heavily increased the corrosion rate of Q235 steel in the acidic soil in Yingtan [
12]. The Q235 steel in acidic soil in Singapore was mainly corroded locally and with severe corrosion [
13]. The electrochemical characteristics of the sand were analyzed from the characteristics of the interfaces of the three phases, the basic model and the equivalent circuit fitting. In sandy soil (gas, liquid, solid multiphase corrosion system), the cathode distribution on the metal surface depends on the total length of the three phases boundary per unit area (L
tpb), which is an important factor affecting the corrosion behavior [
14]. To further explore the influence of pore fluid on the electrochemical corrosion of Q235 steel in sand containing simulated haze aqueous solution (HA solution), the electrochemical corrosion of Q235 steel in sand under a natural air-dried state was studied based on electrochemical theory.
4. Discussion
The E
ocp of Q235 steel in sand gradually positively skews, indicating that the corrosion kinetics of Q235 steel in sandy soil decrease with the increase in age [
17]. In the high-frequency region, solid, solution and gas in sand containing HA solution will all form conductive paths, and the impedance spectrum will fluctuate greatly when the water content is low [
15,
18]. In sandy soil, Cl
− has greater mobility and aggression, while SO
42− has more charge. The impedance spectrum radius of Q235 steel in sand containing HA solution is smaller than that in sand without HA solution [
15]. At 8 d, the water component of the sand containing the HA solution is 12.5%, which is near the limit volume content of the liquid bridge (6%–12%). The arc radius of the capacitive loop reached a minimum, and the sand had the strongest corrosion on the Q235 steel. The water content in the liquid bridge limit volume is respectively about 6% and 12% for the loose simple cube arrangement and the compact tetrahedron arrangement. When the water content is 6%–12%, the water content in the sand is near the liquid bridge limit volume. Water content continues to decrease, and there are lenticular or annular water films on the contact points of sand particles that are not connected with each other [
15].
Table 4 shows that after the water component reaches less than the limit liquid bridge volume (8 d), the fitting parameters of the impedance spectra change greatly, which may be caused by the complex distribution of the pore liquid. With decreasing water in sand, the solution resistance (R
e), sand layer resistance (R
s) and charge transfer resistance (R
ct) increase, but the order of magnitude of R
s changes little. In addition, the order of magnitude the diffusion impedance (W) representing tortuosity fluctuates greatly, which is probably because the complex pore structure of sand containing HA solution [
19]. Q
dl is a constant phase element, the values of n are all less than 0.8 and the interface capacitance deviates from the ideal capacitance.
The pitting characteristics of anode branch for polarization curve also indicate the faster corrosion kinetics of Q235 steel in the early age (1–5 d) [
17]. The I
o of Q235 steel is above 3 μA/cm
2 at 1 d–13 d, the corrosion degree of Q235 steel is above medium, the corrosion degree is below 3 μA/cm
2 at 14 d and the corrosion of Q235 steel is mild [
19]. The overall corrosion rate decreases gradually from 10
−1 to 10
−3 and the average corrosion rate is 0.1629 mm/a. HA solution accelerates the corrosion of Q235 steel in sand without HA solution (average corrosion rate, 1.51 × 10
−2 mm/a). The addition of HA solution increases the corrosion rate of Q235 steel by orders of magnitude, and the corrosion potential E
0 basically shifts to the negative direction, which increases the corrosion tendency [
15].
The surface of Q235 steel has highly variable brown-yellow corrosion products (iron oxides, about 70–200 μm), which is closely related to the porous structure of sand containing the HA solution. The part in contact with the pore solution (HA solution) can directly carry out electrochemical corrosion, and the corresponding product presents a darker color [
20]. The SEM images of the corrosion products (
Figure 10) showed that the corrosion products were in the shape of lamellar, flocculent clusters, rice grains, etc.
The EDS results (
Table 6) indicate that the corrosion products are mainly composed of Fe, O and C. And small amounts of Na and Cl are present. Therein, Fe and O are in compositions of brown and yellow iron oxides. In the O1s fine spectrum (
Figure 11b), the peak at 529.34 eV represents the formation of iron oxides, while the peak at 530.99 eV may be caused by the coexistence of various iron oxides and adhesive sand particles (SiO
2), with multiple overlapping components [
21]. In the Fe2p fine spectrum (
Figure 11c), the characteristic peak at 710.74 eV represents the formation of Fe2p
3/2(Fe
2O
3), and the peak at 724.14 eV represents the formation of Fe2p
1/2(Fe
2O
3) [
22]. In the Na1s fine spectrum (
Figure 11d), the peaks at 1071–1071.5 eV represent sodium compounds. In the C1s fine spectrum (
Figure 11e), the peak at 284.8 eV represents the chemical state of C-C, while the peak at 288.46 eV represents the chemical state of O-C=O. On the one hand, the metal matrix is an iron–carbon alloy, and on the other hand, adventitious carbon is adsorbed on the sample [
23].
Fe
2O
3 is the main component of the corrosion products. Anode dissolution will produce Fe
2+, which may react with H
2O in the neutral pore liquid to form Fe(OH)
2 (Formula (3)) and further oxidize to form Fe
2O
3 on the Q235 steel [
24,
25]. When the steel is in contact with the sand, a circuit is formed through the sand to form a corrosion battery, and an electrochemical reaction can occur.
The HA solution and porous structure of sand affect the electrochemical corrosion of Q235 steel, and
Figure 12 shows that HA solution strongly aggravates the corrosion of Q235 steel in sand [
15].