1. Introduction
A hot dip galvanizing (HDG) method using zinc (Zn) and a heavy-duty coating method that applies the spraying of zinc-rich paint on primer and that combines epoxy resin and fluoride resin have been largely favored [
1,
2,
3]. However, these methods showed several problems. In particular, the heavy-duty coating method requires re-coating within 10–15 years, and that significantly increases the maintenance costs in semi-permanent steel structures; besides, it may include certain toxic elements in the applied coating [
4]. Furthermore, the hot dip galvanizing method represents weak points, such as the size limitations of structural members, the thermal deformation of base materials and difficulties in welding after applying the coating in high-strength joint parts [
5]. The hot dip galvanizing method generated detached coating layers with time and showed deterioration in the external appearance in outdoor applications due to partially contaminated black areas and chloride carbonic zinc [
6].
In recent years, a thermal metal spraying method that produces an anti-corrosion layer on the surface of steel materials by melting Zn or Al, which has a relation to the sacrificial anode, using gas or electricity instead of the conventional corrosion resistance method, is attracting many researchers presently [
7,
8,
9]. Thus, it is of great importance to select an appropriate thermal metal spraying method depending on the environmental conditions, as the anti-corrosion performance of the arc thermal metal spraying method (ATMSM) is highly influenced by the types of metals used [
10,
11]. In addition, the thermal spraying layer has a porous structure, and the characteristics of the anti-corrosion criteria vary with the process of applying the sealing coating that fills the pores [
12]. Meanwhile, a quantitative study of this method with electrochemical experiments is needed, as conventional experiments, such as CASS (Copper ions accelerated salt spray) testing and salt spraying testing, have a qualitative limitation in the evaluation of the durability lifetime in ATMSM [
13,
14,
15,
16,
17].
Furthermore, sacrificial anode metals, such as tin (Sn), magnesium (Mg) and indium (In), which have a better quality of corrosion resistance than zinc (Zn) or aluminum (Al), have been under extensive research study [
18]. However, it seems that there are very few works that quantitatively evaluate the electrochemical performance of ATMSM using these metals [
19,
20,
21].
The objective of the study is to quantitatively evaluate by the electrochemical technique the influence of the metal wire type and the presence of the epoxy sealing coating treatment of the sprayed layer on the anti-corrosion performance of ATMSM.
2. Experiments
Table 1 shows the specimens that were used for the electrochemical experiments, which were conducted with the type of sprayed metal and the presence or absence of epoxy sealing coating. Pure zinc with a diameter of 1.6 mm, pure aluminum, Zn–Sn (65%:35% by mass (alloy)) and Al–Mg (95%:5% by mass (alloy)) were used as thermal spraying metals. Non-painted steel plates and hot dip galvanized steel plates produced at the factory were used for comparison. After blasting with grit, a thermal spray coating layer of a thickness up to 100 µm on the surface of steel plates is produced by using an arc thermal spraying gun, as shown in
Figure 1 [
7,
8,
9]. The Zn–Al thermal metal spraying method is a method that produces a corrosion resistance layer on the surface of steel materials using compressed air by melting metals, such as Zn and Al, with electric arcs.
Figure 1 demonstrates an arc sprayer used in a Zn–Al thermal metal spraying method. The metal spraying discharges the metal melted at an arc point through a circular slit, and that can be introduced into an air stream in which it can be diffused and cooled. Then, the diffused metal generates a layer on the surface of steel by avoiding collisions and forms a porous metal spraying layer according to the accumulation and solidification of such layers. Test specimens (Zn 73%:Al 27%) sealed with epoxy coating after the formation of the thermal spray coating layer were also prepared.
Table 1.
Specimens for the electrochemical experiments.
Table 1.
Specimens for the electrochemical experiments.
No. | Specimen Name | Specimens for Electrochemical Test | Epoxy Sealing Coating | Types of Spraying Metal | Anti-Corrosion Method |
---|
1 | NP | Non-painted specimen | No | - | - |
2 | HDG | Hot dip galvanizing (zinc) 400 g/m2 | No | - | Plating |
3 | Z100-NS | Zn (mass 100%) | No | Zn–Zn | Arc thermal metal spray |
4 | Z100-S | Zn (mass 100%) | Yes | Zn–Zn |
5 | A100-NS | Al (mass 100%) | No | Al–Al |
6 | A100-S | Al (mass 100%) | Yes | Al–Al |
7 | Z73-A27-NS | Zn–Al (mass 27%) | No | Zn–Al |
8 | Z73-A27-S | Zn–Al (mass 27%) | Yes | Zn–Al |
9 | Z65-S35-NS | Zn–Sn (mass 35%) | No | Zn·Sn–Zn·Sn |
10 | Z65-S35-S | Zn–Sn (mass 35%) | Yes | Zn·Sn–Zn·Sn |
11 | A95-M5-NS | Al–Mg (mass 5%) | No | Al·Mg–Al·Mg |
12 | A95-M5-S | Al–Mg (mass 5%) | Yes | Al·Mg–Al·Mg | - |
Common items | Steel plate: SS41, 15 mm × 15 mm × 1.6 mm thickness; experimental area: 0.78 cm2 Steel surface treatment: grit blast |
Figure 1.
Arc thermal metal spraying method.
Figure 1.
Arc thermal metal spraying method.
A 15 mm × 15 mm specimen was connected to electric wires in which all spaces, except for 0.78 cm2, were sealed to prevent electric contacts. Corrosion potential and polarization resistance were measured through the exposed section of 0.78 cm2.
An electrochemical system with three electrodes, where the specimen was configured as a working electrode (WE), graphite was configured as a counter electrode (CE), and a silver-silver electrode (Ag/AgCl) was configured as a reference electrode, was used in this study.
Figure 2 shows the setup for the electrochemical experiment. In addition, a VersaSTAT (Princeton Applied Research, Oak Ridge, TN, USA) was used to analyze the results of these experiments.
Changes in potential and currents were measured with a 1.0-mV/s projection speed in the range of −0.4–+0.8 mV based on the corrosion potential. Then, the polarization resistance was observed by calculating the obtained potential. The solution used in this experiment was a 3.5 wt% NaCl solution at 25 °C.
Figure 2.
Schematic diagram for test setup. WE: working electrode; CE: counter electrode.
Figure 2.
Schematic diagram for test setup. WE: working electrode; CE: counter electrode.
Table 2 shows sectional images produced using an SEM (scanning electron microscopy). In the case of the hot dip zinc galvanizing method, it shows a very dense, uniform and high adhesive layer that enables the specimen to have very good corrosion resistance in steel structures based on the sacrificial anode of Zn.
In the case of the specimens that were coated using pure Zn (No. 3) and Al (No. 4) by ATMSM, it appears that there is a rough surface accompanying many open holes. There are also some pores and micro-cracks in the image morphology, but these pores and cracks were not connected with each other nor traversing the coating from the coating surface to the steel substrate.
For the specimen with Zn–Al thermal metal spray (No. 5), the melted Zn (dark color) and Al (bright color) accumulated and formed a dense pseudo-alloy coating on the surface of the steel plates. A similar observation can be seen in specimen No. 6, too. As for Zn–Sn (65%:35%) in the mass specimen, there is rough surface accompanying many open holes. There are also many pores and micro-cracks in the image morphology. Specimen No. 8 shows a clear rough surface accompanying open holes, and also, there are some pores with a relatively large size.
From these SEM images, one can see that, based on the principal of a sacrificial anode, specimens coated with Zn, Al, Zn–Al, Zn–Sn and Al–Mg by ATMSM have a relatively high potential to resist corrosion.
Table 2.
SEM (scanning electron microscopy) images for each corrosion resistance method.