*3.3. Corrosion Products*

It is well known that copper can accumulate at the surface of steel after a certain period of corrosion for copper-bearing weathering steels. Figure 4 show optical microscope pictures of the inclined cross section of test steels after an accelerated corrosion test at 25 ◦C for 288 h.

**Figure 4.** Optical microscope picture of inclined cross section of steel 1 (**a**), 2 (**b**), 3 (**c**), and 4 (**d**) after corrosion.

All samples were heat mounted to keep the rust layer from falling off. There is no evidence of copper segregation on the surface of steel 1 with less copper content (Figure 4a). With the increase of copper content in steel, sporadic copper segregation can be observed in the rust layer on the surface of steel 2 (Figure 4b), and the copper segregated on the surface of steel 3 begin to connect with each other locally (Figure 4d). The copper layer between the matrix and rust was obvious as shown in Figure 4d. The similar parts, which are much brighter in the back-scattered electron image are mainly pure copper of greater than 95% according to the EDS results. The copper layer is integrated in steel 4. However, this copper layer is easily ground away because pure copper is much softer than steel. It can be calculated that the thickness of the copper layer varies from a few microns to a dozen microns according to the tilt angle of the sample.

The oxides appear in black in Figure 4. These should primarily consist of iron oxide because copper is more stable than iron. After forming the copper layer between the rust and the matrix, the oxidation speed greatly slowed down before halting, and then the copper on the surface would oxidize slowly if the corrosion tests continued.

The bright gray part shown in Figure 4 is the matrix, which is an iron solid solution in which copper is the main constituent.

#### *3.4. Rust Removal from Corroded Steel*

By choosing an appropriate descaling reagen<sup>t</sup> and descaling condition, the rust could be removed while keeping the copper layers, which could be determined by the naked eye because of copper's distinctive color. Macrographs of the test steels before and after descaling are shown in Figure 5.

**Figure 5.** Macro-photograph of corrosion samples before (**a**) and after descaling (**b**) for steel 0, (**c**) for steel 1, (**d**) for steel 2, (**e**) for steel 3, and (**f**) for steel 4.

Before descaling, there is a thick rust layer on the steel surface, and a typical macrophotograph is shown in Figure 5a. After descaling, the rusts had obviously been removed, as shown in Figure 5b–f. With the increasing of copper content in steel, the copper on the steel surface becomes more and more obvious after rust removal. In order to show the copper layer on the surface of steel 4, the corners of rust removal surface were partially ground with sandpaper as shown in Figure 5f, which shows that the copper layer on the steel surface is dense and closely combined with the matrix.

In order to confirm the presence of a pure metallic Cu coating, the XRD analysis results for steel 2 at different stages are shown in Figure 6. Before corrosion, the original material is iron (Figure 6a), where copper should act as solid solution atoms. After corrosion, the surface of steel 2 is covered with a thick rust layer consisting of Fe2O3, Fe3O4, Fe, and FeO(OH), which may include α-FeOOH, γ-FeOOH, and amorphous ferric oxyhydroxide (FeOx(OH)3-2x, x = 0–1) [17,18]. There is little copper or copper oxide present, because these cannot be detected by XRD, as shown in Figure 6b. After descaling, there is approximately 80% Cu and 20% Fe with a small amount of Fe2O3 left on the surface of steel 2, as shown in Figure 6c. These results agree well with that shown in Figure 4.

**Figure 6.** XRD patterns on the surface of steel 2 under different conditions (before corrosion (**a**), after corrosion (**b**), after descaling (**c**) and after removing Cu coating (**d**)).

According to XRD results of all the descaled steel samples, the main contents of the descaled surfaces are pure copper and pure iron, with a small amount of Fe2O3 left. For steel 2, the copper content in the bulk steel is much smaller than that in steel 4, while the composition of the surface after descaling is similar. The higher iron content in the XRD pattern of steel 2 shown in Figure 6c may correspond to a relatively thinner copper coating.

The microstructures of the steel surfaces after descaling were observed using SEM, as shown in Figure 7. Except for steel 1, almost all other steel surfaces were covered with more than 95% copper according to EDS results of steels 2, 3, and 4, which is nearly pure copper, as shown in the XRD results, and the greater amount of iron recognized by the XRD should come from the matrix, as previously mentioned. However, there were some oxides left on the descaled surface according to the XRD results shown in Figure 6. Nevertheless, they were only found in small quantities. The carbon detected by EDS may have come from the microscope chamber, as did some of the oxygen in the spectra, because the EDS tends to over-measure the levels of light elements.

It can also be seen that the copper coating is not very dense for steel 2, as shown in Figure 7b. Thus, further oxidation of the matrix might occur when the corrosion test continues. For steel 4, the copper coating is denser than that of other steels, as shown in Figure 7. The "grain boundary" morphology shown in Figure 7d should come from the copper-rich phase precipitated in the matrix, as shown in Figure 1d. Therefore, the copper content of the "grain boundary" may be a little higher than that of the "grain".

**Figure 7.** SEM (**a**) for steel 1, (**b**) for steel 2, (**c**) for steel 3, and (**d**) for steel 4 micrographs on steel surface after descaling.

#### *3.5. Copper Enrichment Mechanisms*

#### 3.5.1. Movement of Copper Atoms

It is well known that the oxidation and corrosion rate of copper is relatively low compared to that of iron. Kondo [19] examined the distribution of copper in the scale of oxidized steel containing copper. The schematic diagram of corrosion and oxidation process of copper-bearing steel is shown in Figure 8. In the corrosion process, oxygen and other atoms tend to combine with iron atoms to form Fe2O3, Fe3O4 and other non-metallic corrosion products, while the combination of oxygen and copper requires higher energy and longer time. The combination of oxygen and iron atoms results in the increase of sample weight [20]. Meanwhile, the volume of the oxide layer expands with the increase of material weight, which indicates that the distance between the compound molecules in the oxide layer and the metal atoms in the matrix increases as shown in Figure 8b. As a result, the binding forces between iron atoms in the oxide molecule and those in the matrix are weakened, sometimes accompanied by some defects [21]. The appearance of defects is more conducive to the oxidation and movement of metal atoms.

Because of different bonds, the bonding force between metallic atoms is stronger than that between metal and oxide. With the formation of oxides on the surface of steel, the binding force between the copper atoms and the surrounding iron oxide molecules is weakened, whereas the bonding force between the copper atoms in the oxide layer and the iron (and/or copper) atoms in the matrix becomes stronger. The metallic atoms would move closer to each other. In Figure 8b, FexOy is used to represent corrosion products. As a result, the copper atoms move continuously from the inner oxide layer to the interface between the oxide and the metal matrix under the action of the resultant force, as shown in Figure 8c. After a period of corrosion, the continuous oxidation of iron atoms and the continuous movement of copper atoms lead to the formation of a certain number of copper atoms on the steel surface, which is macroscopically shown as a copper rich layer (Figure 8d).

**Figure 8.** Schematic diagram of copper enrichment mechanism. (**a**) Before corrosion, (**b**) initial stage of corrosion, (**c**) after a short period of corrosion, and (**d**) after a long period of corrosion.

In the oxidation process, the iron atoms in the lattice positions of the matrix crystal deviate from their original positions because they combine with oxygen to form ferric oxide. The migration of surrounding iron atoms and the increase of the microcosmic distance due to the weakening of the bonding force between the metal atoms and non-metal molecules is favorable for the directional movement of copper atoms. The oxidation of the matrix surface and diffusion of un-oxidized copper atoms occur at the same time, which form the quasi-layered microstructures of the matrix, copper enrichment layer, and upper rust layer after a period. Therefore, it can be deduced that the copper enrichment layer in the structure originates from the movement of copper atoms in the oxidized surface layer.

When steel is corroded, both iron atoms and solid solution copper atoms in the oxide layer deviate from their atomic lattice position. Under the experimental conditions, most copper atoms have time to move towards the matrix and combine to form a copper-rich phase. From the macroscopic point of view, a layer of "copper coating" has been formed, and the coating is generated by the matrix itself.

The force between the copper atoms moving from the oxide layer to the interface and the iron atoms in the matrix is a metal bond. Thus, there is no bonding problem.

It should be noticed that in the process of corrosion, some defects on the surface of the substrate, such as vacancies, will affect the concentration of copper, and some copper atoms will be immersed in the scale.
