2.1. Film preparation and Characteristics Measurement
The film thickness can be controlled by precisely adjusting the drop amount, as shown in Figure 1
. HPMCP-1, HPMCP-2 and HPMCP-3 correspond to 600, 1200 and 1800 μL, respectively. The corresponding thicknesses are 200, 360, and 580 μm, respectively. HPMCAS-1, HPMCAS-2 and HPMCAS-3 correspond to 600, 1200 and 1800 μL, respectively. The corresponding thicknesses are 180, 360, and 560 μm, respectively. The results show that the thickness of the HPMCP and HPMCAS films could be accurately adjusted and controlled.
Raman spectroscopy was utilized to investigate the material characteristics of HPMC derivatives. The Raman spectra of HPMCP and HPMCAS are presented in Figure 2
. The characteristic peaks of HPMC are depicted, including those at 1360 cm−1
(COH bending) and 1450 cm−1
]. Moreover, a comparison of the two curves of HPMCP and HPMCAS shows that there is no obvious different absorption peak. It reflects that although phthalate, acetate, and succinate were added to HPMC to meet the requirements of acid and moisture resistance, HPMCP and HPMCAS preserved the structural properties of HPMC. Therefore, Raman spectroscopy could be used to assess the material, decomposability, and uniformity properties of HPMCP and HPMCAS [32
2.2. Anti-Corrosion Behavior
a shows the Nyquist plots of the four measured samples. The first sample is uncoated high speed steel (HSS). A series of samples are denoted HPMCP-1, HPMCP-2 and HPMCP-3, respectively. Figure 3
b shows the Nyquist plots of HPMCAS with varying thickness. It can be seen that the left part was a semi-circle, and the right part was an incomplete semi-circle. Hence, it can be determined that the plot is comprised of two time constants. The equivalent circuit diagram shown in Figure 4
was used to simulate the actual conditions [34
As shown in Figure 4
, Rs represents the resistance of saline solution; Rf represents the resistance of the HPMC derivatives film; CPE_film is the capacitance of the HPMC film; Rct is the resistance between the steel and solution, also called the charge-transfer resistance; CPE_dl is the capacitance of the double electrode layer. CPE was used in this study as opposed to the capacitance in the traditional equivalent circuit model as there was an uneven current potential distribution; CPE would be more accurate according to previous studies [35
In the equivalent circuit diagram, Rf corresponds to the left semi-circle in Figure 3
; the larger the Rf, the larger the radius in the Nyquist plot, and the better the anti-corrosion ability. Rct corresponds to the right semi-circle. As the aim of the current study is to investigate the anti-corrosion ability of the film, we focus on Rf.
The original data was fitted with an equivalent circuit diagram, and the fitting data is shown in Table 2
. The film resistance (Rf) of HPMCP-1, HPMCP-2, and HPMCP-3 are 989, 1260, and 1368 Ω, respectively. The film resistance of HPMCAS-1, HPMCAS-2, and HPMCAS-3 are 987, 1339 and 1535 Ω, respectively. For both materials, the film resistance increased with increasing film thickness, as did the penetration depth [38
], indicating enhanced resistance ability with increasing film thickness.
By comparing the Rf for HPMCP and HPMCPAs, the results clearly demonstrate that HPMCP and HPMCAS were at the same scale. However, the increase in impedance for the HPMCAS can be attributed to it having a better hydrophilicity than HPMCP. The high capacitance, CPE_film, is related to the high extent at which water has penetrated the film [39
]. Comparison of CPE_film-T for HPMCAS and HPMCP shows that the former had a larger CPE_film-T, i.e., greater moisture content. This is consistent with the experiment on contact angles. Compared to HPMCP, HPMCAS showed a smaller contact angle, namely, a better hydrophilic property. Higher hydrophilicity of HPMCAS compared to HPMCP and bare HSS may result from high-wettability. This leads to an increased concentration of corrosive substance on the HSS surface.
Previous results show that HPMCP and HPMCAS films had demonstrated promising anti-corrosion behavior. The potentiodynamic polarization (PP) method was further used to record the variation in current and potential during the experiment. The polarization curves for HPMCP and HPMCAS with different thicknesses are shown in Figure 5
. The curves are divided into cathodic and anodic polarization. Cathodicpolarization is the section before the lowest point, representing hydrogen reduction in the experiment: 2H+
. Anodic polarization is the right section after the lowest point, representing metal oxidation in the experiment: M → Mn+
The bottom point of the curve represents the corrosion potential. The corrosion current (Icorr) was measured using Tafel extrapolation. Within 50 mV of the corrosion potential, a linear region, called the Tafel region, was obtained. The tangent lines of cathodic polarization (slope βa) and anodic polarization (slope βc) intersect in the horizontal axis at the point of corrosion current (Icorr), which represents the corrosion rate. In the present study, Icorr was used to evaluate the anti-corrosion ability of the film [40
], the data of which are shown in Table 3
The electrochemical corrosion measurements of HSS, HPMCP and HPMCAS are shown in Table 4
. The Tafel plots for the HPMCP yield corrosion potentials of Ecorr = −388.1, −294.9, and −230.5 mV for HPMCP-1, HPMCP-2, and HPMCP-3, respectively, which are more positive than that of the bare HSS, where Ecorr = −547.5 mV. Moreover, the corrosion current (Icorr) of HPMCP-1, HPMCP-2, and HPMCP-3 were 6.8, 5.2, and 0.8 μA/cm2
, respectively, which are significantly lower than that of the HSS sample (26.3 μA/cm2
The Tafel plots for the HPMCAS yield a corrosion potential of Ecorr = −294.4, −211.1, and −176.7 mV for HPMCAS-1, HPMCAS-2, and HPMCAS-3, respectively, which are more positive than that of the bare HSS. Moreover, the corrosion current (Icorr) for HPMCAS-1, HPMCAS-2, and HPMCAS-3 was 1.8, 1.4, and 1.7 μA/cm2
, respectively, which are significantly lower than that of the HSS. From Table 3
, it can be seen that for HPMCP and HPMCAS, the corrosion current decreased with increasing film thickness, indicating a reduced corrosion rate. Thus, there is a positive correlation between film thickness and the corrosion resistance performance.
Comparison of HPMCP-3 and HPMCAS-3 shows that the corresponding Icorr decreased considerably when we used the phthalate function group, suggesting the formation of hydrophobic properties. The electrochemical measurement results show that the HPMCP film provided better protection against corrosion of the HSS than the HPMCAS.
In terms of viscosity, the values for HPMCP and HPMCAS were 100 and 200 mPa·s, respectively. The high viscosity of HPMCAS resulted in poor film formation, causing defects and inferior smoothness of the film [42
], and, therefore, poor corrosion resistance. HPMCP had low material viscosity, hydrophobic surface and low moisture content resulted in promising corrosion resistance performance.