Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.
Issue link: http://mp.epubxp.com/i/879896
50 OCTOBER 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 10 Corrosion rate = 0.0393 + 0.0092625* A – 0.0033625* B – 0.001525* C – 0.000325* A* B + 0.00105* A*C – 0.0005* B*C – 0.0148875* A 2 – 0.080375* B 2 – 0.0107125*C 2 (2) Analysis of Variance The analysis of variance (ANOVA) for regression parameters of the predicted response surface quadratic model for cor- rosion rate is shown in Table 2. A very high F-value (F model = 20.4956) and a ver y low probability value (P model = 0.0003) indicate that the model is significant. The lack of fit (P = 0.2004 > 0.05) is not significant, which is good for the model. 9-11 Furthermore, the fit of the model was verified by the coeffi- cient of determination R 2 . In this study, the FIGURE 1 3D graphs of interaction AB for corrosion rate. FIGURE 2 SEM micrographs of Cu surfaces formed in solutions at 40 and 80 °C. value of the determination coefficient (R 2 = 0.96) indicated that 96% of the variability in the response could be explained by the model. The adjusted determination coef- ficient (adj R 2 = 0.92) also indicated that the obtained model was significant. There- fore, temperature (A), pH (B), A 2 , B 2 , and C 2 are the significant model terms, whereas oth er mo d el t erms can b e c onsi d ered insignificant. Inf luence Effect and Optimization The three-dimensional (3D) response surface plots of the quadratic model were generated by DE software and utilized to assess the inf luence of the independent variables on the response. Figure 1 shows the 3D graph of interactive effect for corro- sion rate. The corrosion rate increases with the temperature and reaches the maximum in the range of 60 to 70 °C. When pH is above 9.0, the corrosion rate is almost con- stant. At temperatures below 60 °C, increas- ing temperature enhanced the number of activation molecules and diffusion coeffi- cients, which improved the dynamic of the reactions. Thus, more DO reacts with Cu in the cathode region of the metal surface. Meanwhile, the decrease of overpotential and solution viscosity also improved the intensity of reactions. Figueroa 12 suggested that the limit value of activation energy is relatively high, in the range from 25 to 55 °C. Th e transpor tation of cations was increased with rising temperature when the passive layer was dense and contained little water. Drogowska 13 obtained similar results and demonstrated that the elec- trodissolution process becomes energeti- cally close to a diffusion-controlled process between ionic lattices. Th e SE M micrographs of th e f ilms formed in pH 8.5 solutions at 40 and 80 °C are shown in Figure 2. The specimen sur- face is covered with a dense polyhedral oxide layer (Figure 2[a]) that consisted of cupric oxide (CuO). This layer has a large area of contact and better adhesion due to its geometry. Figure 2(b) illustrates that the oxides obtained had the same structure and were more compact at 80 °C. This film is believed to provide more protection against corrosion. Gassa 14 and Ribotta 15 studied the inf luence of temperature on di ss o lution an d p a ssivation of Cu by voltammetry. The sharpness and symmetry of the reduction current peaks at higher temperature revealed that the film is more compact and adhesive. Figure 3 shows that the corrosion rate increased from 0.005 to 0.036 mm/y with the increasing DO and temperature ini- tially, and then decreased to 0.024 mm/y under the experimental conditions. The maximum corrosion rate was observed in the range of 60 to 70 °C, while DO ranged from 0.5 to 1.5 mg/L. Increasing the content of D O re sult ed in th e transformation between Cu 2 O and CuO. The crystal lattice parameters of Cu 2 O is 4.2696 Å, while that of CuO are a = 4.6837 Å, b = 3.4226 Å, and c = 5.1288 Å. The molar volumes of Cu 2 O MATERIALS SELECTION & DESIGN