� ! 2*OH*� þ *H*<sup>2</sup> (6)

#### **Figure 6.**

*Potentiodynamic polarization curves of Cu (a) aged/Cu-MPS (b), cured/Cu-MPS (c), aged/Cu-APS (d) and cured/Cu-APS (e), in NaCl 3 wt % solution.*

$$2\text{O}\_2 + 2\text{H}\_2\text{O} + 4\text{e}^- \rightarrow 4\text{OH}^- \tag{7}$$

It could be observed that the pH value of the solution after the electrochemical experiments is a little higher than that before the experiment.

The anodic part of the curve copper shows three main regions to [15, 16]:

In the first region, an apparent Tafel behavior is observed and the current density increases up to a critical passivation value (*Jcp*), due to the oxidation of Cu (0) to Cu(I) (Eq. (8))

$$\text{Cu} \to \text{Cu}^+ + e^- \tag{8}$$

In the second region, The current density decease from *Jcp* to full passivation current density (*Jpas*). Cu(I) is rapidly transformed to an insoluble CuCl film (Eq. (9)) [17].

$$\text{Cu}^+ + \text{Cl}^- \rightarrow \text{CuCl} \tag{9}$$

In aqueous solution, *CuCl* would be unstable. It is immediately converted to the soluble cuprous complex *CuCl*� <sup>2</sup> (Eq. (10)) [18]. The current density suddenly increases again from *Jpas* to elevated anodic potential. We detected a small full passivation field (7 mV) in the case of the untreated copper. Thus, the dissolution of substrate is happening step by step.

$$\text{CuCl} + \text{Cl}^- \rightarrow \text{CuCl}\_2^- \atop \text{(surface)} \tag{10}$$

In the third region, the potential increases again before to stabilize when it reaches a limiting current density. *CuCl*� <sup>2</sup> is oxidized to *Cu*<sup>2</sup><sup>þ</sup> ions according to the following reactions (Eqs. (11) and (12)):

$$\text{CuCl}\_2^- \text{}\_{(surface)} \rightarrow \text{CuCl}\_2^- \text{}\_{(solution)} \tag{11}$$

$$\text{CuCl}\_2^- \_{\text{(solution)}} \rightarrow \text{Cu}^{2+} + 2\text{Cl}^- + 2\text{e}^- \tag{12}$$

However, according to some authors, other corrosion products could be also formed, such as Cu2O, CuO, Cu(OH)2 [14, 19–22].

**Figure 6** also shows that the specimen coated with silane coupling agents have a different behavior for untreated copper. In addition, both cathodic and anodic current decreased and the corresponding slopes vary, which indicated that the γ-MPS or γ-APS coating is mixed-type corrosion coating, i.e. inhibitor and barrier coating.

The electrochemical parameters such as *Ecorr,* corrosion potential, *Jcorr,* corrosion current density, *βc,* cathodic and *βa,* anodic slopes, *Rp,* polarization resistance, *CR*, corrosion rate and *PEF,* protective efficiency are calculated according to polarization curves and summarized in **Table 2**. The values of *Jcorr* and *Ecorr* were obtained by the extrapolation of anodic and cathodic Tafel curves. The *Rp* values are calculated by the next formula (Eq. (13)) [18],

$$R\_p = \frac{B}{J\_{corr}}\tag{13}$$

here, *B* is a constant that is calculated by using Stern–Geary equation (Eq. (14)) [23],

$$B = \frac{\beta\_c \beta\_a}{2.303(\beta\_c + \beta\_a)}\tag{14}$$

*Infrared Characterization and Electrochemical Study of Silanes Grafted into Surface… DOI: http://dx.doi.org/10.5772/intechopen.99782*


**Table 2.**

*Electrochemical kinetics parameters and protective efficiency obtained from potentiodynamic polarization curves.*

The values of corrosion rate (*CR*, millimeters per year (mmy�<sup>1</sup> )) are calculated using the expression (Eq. (15)) [8],

$$\text{CR} = 3.268 \times 10^3 \frac{J\_{corr}}{\rho} \frac{MW}{Z} \tag{15}$$

where ρ is the density of Cu in g.cm�<sup>3</sup> (= 8.92), MW is molecular weight of copper in g and Z is the number of electrons transferred in the corrosion reaction; Z = 2 in the case of Cu reaction.

After silane-modified copper surface, the potentiodynamic polarization curves moved toward anodic direction and toward less current density. The values of corrosion potential shift in the positive direction, denoting the beneficial effect of the two coupling agents' treatments on copper substrate corrosion. These two treatments produce best results mainly on the copper anodic oxidation reaction when the coating is the γ-APS, whose currents are reduced by about two orders of magnitude, at 0 mV/SCE. Nevertheless, the cathodic currents are reduced only by about one order of magnitude for the tow used coupling agents.

**Table 2** also depicts that the values of *Jcorr* and *CR* decease, after treatments, while the protective efficiency increases sharply to reach 96.62% for the cured/Cu-MPS specimen). By leaving the silane coupling agent coating in contact with the substrate for 24 h, Si–O–Si linkages formation begins to take place. This generates a more robust and adherent coating. The notable hindrance to the copper anodic oxidation process, watched from the noticeable diminution in corrosion current density and corrosion rate values and the increase in the polarization resistance and the protective efficiency, can mostly be ascribed to the strong Si–O–Si linkages [11]. The corrosive attack can be demonstrated only in the coating holes and after a period of time it causes a noticeable attack of the metal. Additionally, heat treatment helps the interconnected networks of the coupling agent on the surface through the elimination of water molecules [24]. More condensation takes place mostly in the outermost part of the silane film, leading to a polymolecular, denser, less permeable and, consequently, more corrosion resistant coating.

**Figure 6** also reveals that the untreated copper immersed in saline solution reaches passivity in a typical active-passive transition. Coupling agent modified copper in NaCl 3 wt % aqueouse solution features much wider potential range of full passivation, which offers further protection at elevated values of positive potential. The electrochemical parameters such as *Jcp*, critical current density of passivation, *Ecp*, critical passivation potential, *Jpas*, full passivation current density, *Epas*, full passivation potential, *Etp*, trans-passivation potential and *ΔE*, passivation range are listed in **Table 3**. Compared with the untreated copper (i.e., the blank),

the *Epas* and *Ecp* values shifted toward the cathodic direction and the *Jpas* and *Jcp* values decrease abruptly. The move of *Epas* and *Ecp* values toward cathodic potentials demonstrates higher corrosion resistance of the coupling agent modified copper. Moreover, the lower value of full passivation current density verifies a better tightness on the silane film formed on the copper surface after treatments.

The passivation ability of the substrate corresponding to the significant criterion of passivation kinetics is verified through the drop of *Jcp* value. Indeed, the area delimited by the peak of activity, identified by *Jcp*, corresponds to the amount of electricity required for the passivation of the material. The weak dissolution of the material is, on its side, verified by the decrease of this surface which is easily passive. These phenomena become more pronounced when the used silane is γ-APS.

In the fully passive range, the current density is independent of the potential. When the potential of full passivation finishes, *Etp* is reached. After this, the passive film continuity is damaged and the metal gets trans-passivated.

The passivation of silane-modified copper becomes more rapid and the passivation range, characterized by the difference between *Etp* and *Epas* (*ΔE* = *Etp* - *Epas*), is more extensive than that of pure copper. This confirms the relatively high corrosion resistance of silane modified copper.

At the end of the passivation, an unexpected increase in the current density has been detected, along with the evolution of molecular oxygen as well as the return of the active area, where copper dissolution takes place.

#### *3.2.2 Gravimetric measurements*

To investigate the effect of the treatments on the corrosion inhibition of copper in aerated NaCl 3 wt % aqueous solution at ambient temperature, gravimetric measurements were carried out. **Figure 7** shows the plot of the weight losses versus time curves of specimen blank (a) aged/Cu-MPS (b), cured/Cu-MPS (c), aged/Cu-APS (d) and cured/Cu-APS (e). The weight loss (*Δm*, mg cm�<sup>2</sup> ) and the corrosion rate (*CR*, mg cm�<sup>1</sup> h�<sup>1</sup> ) were calculated as follows (Eqs. (16) and (17)) [25]:

$$
\Delta m = \frac{W\_1 - W\_2}{A} \tag{16}
$$

$$CR = \frac{\Delta m}{t} \tag{17}$$

Where, *W1* and *W2* are the weight before and after exposure to saline solution, respectively, A is the total surface area and *t* is the immersion period.

According to **Figure 7**, the weight losses of untreated copper in NaCl 3 wt % solution increases with the increase in the immersion period as a result of the


**Table 3.**

*Characteristic passivation parameters obtained from potentiodynamic polarization curves.*

*Infrared Characterization and Electrochemical Study of Silanes Grafted into Surface… DOI: http://dx.doi.org/10.5772/intechopen.99782*

#### **Figure 7.**

*Variations of the weight losses with time for Cu (a) aged/Cu-MPS (b), cured/Cu-MPS (c), aged/Cu-APS (d) and cured/Cu-APS (e), in NaCl 3 wt % solution.*

#### **Figure 8.**

*Variations of the corrosion rate with time the different treatments.*

continuous dissolution of copper ions, due to the severe aggressiveness of the chloride ions.

aged/Cu-MPS decreases the weight loss of specimen, especially after 3 and 6 immersion days. Above this period, the weight loss value increases significantly to reach the value obtained for blank specimen after 12 immersion days. A further decrease in the loss of weight took place after curing and/or when the coating is γ-APS. The advantageous effect endures until 9 days of immersion, which indicates an improvement in coupling agent film durability. In fact, curing treatment catalyzes the formation of both chemisorbed and polymerized product forming more robust coupling agent layer. Nevertheless, after 12 immersion days this layer cannot resist whatever the silane coating used. This is also illustrated by the **Figure 8**, from which it can be seen that, after this period, there is no notable difference of the corrosion rate obtained for the untreated or silane coated copper.

#### **4. Conclusions**

The main conclusions of this study are summarized below:

• Room temperature aging allows a certain condensation between the loosely adsorbed silane molecules, which relatively improves the performances of the silanic layer.

