3.5.2. Influence Factors

The formation of the quasi-layered structure may be affected by the following factors: the corrosion rate, corrosion time, and copper content in the steel.

The corrosion rate should correspond to the movement speed of the copper atoms in the oxide layer, although the values of these two are difficult to measure at present. If the corrosion rate is very high, the copper atoms are not able to move to the interface, because all the iron atoms around them are quickly oxidized and the copper atoms lose the driving force. Thus, no copper enrichment layer will be formed. However, if the corrosion rate is rather low, the moving velocity of the copper atoms toward the metal matrix may also be very low. In this process, the copper atoms themselves may be oxidized before they form the copper enrichment layer, and the copper atoms inside the metal matrix have no reason to move to the surface at 25 ◦C. Thus, no copper enrichment layer will be formed in this case either.

The effect of the oxidation time is related to the copper content in the steel. As previously mentioned, the formation of a copper enrichment layer is a process in which copper atoms in the oxide layer keep moving and accumulate on the surface of the matrix. Therefore, the continuous copper enrichment layer on the matrix surface can be formed in a relatively short time if the copper content is high. In contrast, if the copper content is low, the corrosion time will be longer, and the thickness of the oxide layer will be greater, allowing sufficient copper atoms to form a continuous layer of copper, which can explain the early corrosion deceleration stage of steels 3 and 4 in the corrosion weight gain curve shown in Figure 3.

#### *3.6. Electrochemical Corrosion Test*

Figure 9 shows the polarization curves of all test steels (a) and steel 4 (b) before corrosion, without Cu coating, and after corrosion and descaling, with Cu coating (under the experimental conditions, only steel 4 shows a complete copper coating), in 3.5% NaCl solution.

**Figure 9.** Potentiodynamic polarization curves of all test steels (**a**) and steel No.4 with Cu coating (**b**) in 3.5% NaCl solution.

The electrochemical parameters Ecorr, Icorr, jcorr, and *v* were obtained by fitting the curve are listed in Table 2. With the increase of copper content in steel, Ecorr tends to move positively, which implies a better corrosion resistance property. At the same time, when the copper content is high enough, the corrosion current density and corrosion rate of the experimental steel are reduced, and the passivation interval has been extended, which also shows that the corrosion resistance of the steel is improved. Relevant research results also reported that high-copper-bearing steel has excellent corrosion resistance [22].


**Table 2.** Electrochemical parameters of test steels.

Ecorr of steel 4 with Cu coating is more positive than that of steel 4 without Cu coating. The OCP of steel 4 with Cu coating shifted significantly to a positive (anodic) direction, suggesting the formation of an efficient barrier for the aggressive medium [23,24].

The polarization potential of the steel treated by corrosion and descaling shifts positively and the corrosion current per unit area is reduced by nearly one order of magnitude, which is similar to the result of polyaniline coating on mild steel [25]. Therefore, the corrosion resistance of Cu-bearing steel can be significantly improved by the copper coating after corrosion and descaling treatment at 25 ◦C.
