*3.1. Microstructure*

Figure 1 shows the optical microstructures of the as-cast Cu-bearing steels. According to Fe-Cu phase diagram, at room temperature, the solid solubility of copper in iron is about 0.6%, while that of iron in copper is almost zero. Therefore, the copper-bearing steel is mainly composed of solid solution ferrite phase and copper-rich phase close to pure copper. Therefore, the main phases in Figure 1 should be α ferrite and a small amount of copper-rich phase.

For steels 3 and 4, there are obvious bright particle precipitates distributed along the grain boundaries and in the grains, and the grains are remarkably refined as a result of the precipitation of these phases. No bright precipitate can be observed in steels 1 and 2, which confirm that the bright phase is Cu-rich phase. The EDS analysis of steel 4 shown in Figure 2 shows that the bright precipitates are copper-rich phases with a small amount of iron in solid solution. Meanwhile, about 2.8–7.4 mass% of copper is distributed in α ferrite matrix. According to these results, there should be little of the copper-rich phase precipitated in steels 1 and 2, which agrees well with the optical observations shown in Figure 1. All the relatively larger white, gray, and black phases are α ferrite, which are shown in different colors because of their different crystal orientations.

**Figure 1.** Optical micrographs of Cu-bearing steel 1 (**a**), steel 2 (**b**), steel 3 (**c**), and steel 4 (**d**). The etchant was 4% nital.

**Figure 2.** SEM (scanning electron microscope) image and EDS (energy dispersive X-ray spectroscopy) results of steel 4.

The actual solid solubility of metals may be slightly higher than that shown in the phase diagram. Therefore, copper beyond the solid solubility content exists as copper atoms or as near pure copper phase in steel. With an increase of the copper content, the grain sizes of the steel became fine and uniform, as shown in Figure 1c,d, indicating that copper precipitated in excess of the solid solubility is more likely to accumulate at grain boundaries and hinder their migration.

## *3.2. Corrosion Kinetics*

Copper is widely employed as an outdoor building material for fabricating statues, sculptures, and monuments because of its aesthetic qualities, and good mechanical and antibacterial properties [14,15]. It is noted for its excellent corrosion resistance. Therefore, the corrosion behavior of copper-bearing steel is an important performance factor. The corrosion properties of Cu-bearing steels are shown in Figure 3, where steel 0 is copper free as a benchmark.

**Figure 3.** Corrosion behavior (weight gain per unit area in early (I), middle (II), and late (III) corrosion stage) of Cu-bearing steels.

The entire corrosion process (up to 288 h) for all the test steels can be clearly divided into three stages, as shown in Figure 3. Each stage shows distinctive slope behavior for all the samples.

In stage I, both steels 3 and 4 show higher corrosion rates. The corrosion is much more aggressive as the copper and iron begin the process of galvanic coupling at the very beginning of the corrosion process. In contrast, steels 0, 1, and 2 all show lower corrosion rates in stage I, which correspond to no obvious Cu-rich phases in the steels, and these relatively lower corrosion rates may be attributed to the formation of oxides in the beginning [16]. Therefore, in the initial stage of corrosion, steels 0, 1, and 2 exhibit surface corrosion of ferrite samples with relatively low corrosion rate, while steels 3 and 4 exhibit two-phase galvanic corrosion with relatively high corrosion rate.

The subsequent gain in weight in stage II becomes gradual for steels 0, 1, and 2. The growth rate of the corrosion rate of steel 0 is much greater than those of steels 1 and 2, which are even greater than those of steels 3 and 4, as shown in Figure 3. This phenomenon indicates that the surface corrosion products in the first stage are not dense, and the oxygen

penetrate through the open holes of the oxide layer to enter the metal matrix from the surface in stage II, and the matrix is directly corroded rapidly. Therefore, in the corrosion stages up to about 192 h, although the corrosion mechanism of steel changes when the copper content of steel reaches or exceeds 6.8%, a certain amount of copper in steel has the effect of reducing and stabilizing the corrosion rate. This phenomenon should be related to the corrosion products of copper-bearing steels.

When the corrosion time exceeds 192 h, the corrosion process enters the third stage. Although the copper-free steel (steel 0) was still corroded at a higher rate, the corrosion rate of all the experimental steels was reduced compared with that of the second stage. The decrease of corrosion rate of steel 0 should be related to the increase of thickness of corrosion products on steel surface. The corrosion rate of copper-bearing steel is obviously lower than that of the previous stage, especially the steel with higher copper content, which indicates that a protective layer is formed on the corrosion surface, which is related to copper in the steel.
