| 简介： |
|
Yangyong Wang, Xinli Jing, Junfeng Qiang
School of Environmental and Chemical Engineering, Xi’an Jiaotong University, Xi’an 710049, P.R. China
Since the first report by DeBerry that PANI electrodeposited on stainless steels can enhance their anticorrosion performance in acidic solutions, tremendous papers have focused on polyaniline’s corrosion property for various metals such as aluminum, copper, silver, iron and steel, to mention just a few. The polyaniline used ranges from the emeraldine base form (EB) to the emeraldine salt form (ES) and some of their derivatives. Many researchers believe that EB exhibits better anti-corrosion performance in acidic corrosion medium than ES, while in neutral environment ES better and also the corrosion protection effectiveness depends on the doping acids used. However, several researchers pointed out that only the conducting protonated PANI was effective for corrosion protection while EB was useless. The corrosion protection mechanisms of PANI have been studied by many researchers. For ES, there mainly exist Wessling’s redox mechanism[1] and Schauer’s MCP mechanism[2] . For EB, a few considered it was some kind of anodic protection, while others assumed a compact barrier and permeable-selective property and the high diffusion resistance toward corrosive ions was responsible for the action of corrosion protection. However, some others assumed PANI was not very effective for corrosion protection for the main reason that the PANI film from solution cast was porous and poor adhesion onto steels. The adhesion could be improved by electrochemical co-polymerization or blending with other polymers and therefore corrosion protection performance could be enhanced. While still others believe that the above-mentioned blends cannot offer effective corrosion protection effect. In this paper, we investigate the corrosion protection performances of particulate EB for cold rolled steel (CRS) in 3.0wt% NaCl aqueous solution.
EB was chemically synthesized according to reference[3]. Dispersion of EB in water or 3.0wt% aqueous NaCl and the epoxy coating containing 10.0wt% EB (denoted as EBd/EP) were prepared according to reference[4] and[5] respectively. Dimensions of cold rolled steel panels for the testing of corrosion potential, XPS analysis and cyclic voltammetry (CV) testing were 100 20 1mm3, 10 10 1mm3 and 4×5 1 mm3 respectively. All panels were pre-treated according to reference. Measurements of cyclic voltammograms, corrosion potentials and Tafel plots were carried out using a conventional three-electrode electrochemical cell with a platinum counter electrode and a saturated Ag/AgCl reference electrode. The working electrodes were the various coated and uncoated CRS coupons or coated platinum electrodes. The corrosion medium was 3.0wt% aqueous NaCl at room temperature. The pH values were detected by exactitude pH test paper.
When the water or 3.0wt% NaCl solution dispersed EB contacts with CRS, the pH value of the medium around the EB particle changed from 7.0 to 9.0 in 10 to 20 minutes and hold the line thereafter. After drying of CRS coupled with EB particles under dynamic vacuum(50℃) for 5 hours, the color of EB changed from the original coffee to blue and the CRS takes on off-white other than the newly polished silvery white. XPS analysis indicates that the off-white film is made of Fe2O3 and be the same both in water and 3.0wt% aqueous NaCl dispersed cases. SEM confirmed that this off-white oxide is continuous and compact. Electrochemical tests (Tafel plots) indicate that the corrosion potential of the treated coupons shifted toward anodic direction from -649 mV (vs. Ag/AgCl) to -444mV(vs. Ag/AgCl), and the corrosion current density diminished from 200µA/cm2 to 26.8µA/cm2, which was resulted from passivation of CRS surface. FTIR analyses demonstrate that EB was reduced after reaction with CRS in both cases.
CVs show that EB film cast from NMP solution on platinum possess good oxidation-reduction reversibility in the initial several scans, but degraded and peeled off soon (about 10 scans). For the EBd/EP film, no redox current was detected at the beginning because of the barrier effect of epoxy coating while it was observed after 2760 scans (ca. 13h) and remained nearly the same intensity for 3000 scans. It is therefore obvious that EB particles dispersed in epoxy coating exhibit steady redox capability and it is possible to prepare PANI anti-corrosion coating in this manner. It was found that the color of the testing medium changed gradually to brown coupled with the peel off of EB film after replacing the platinum with CRS (4×5×1mm3). Obviously, EB alone do not exhibit good corrosion protection performance due to its poor adhesion and much micro-cracks, which permit the contact of corrosive medium with CRS. EBd/EP coating showed much better barrier effect (than pure EB coating) together with steady and favorable redox characteristic. More detailed reactions between EB and CRS are under investigation for the now.
In order to distinguish the barrier effect from passivation one, we prepared four groups of coupons with EP and EBd/EP according to the protocol of
Table 1.Corrosion potential-immersing time plots of all coupons show only coupons C and D exhibited higher equilibrium corrosion potentials, which we attributed to the passivation effect of the EBd/EP primer in both cases. Coupon D expressed lower corrosion potential than that of coupon C in the beginning, but they gradually reached the same for long enough time. These results indicate that EBd/EP is more penetrable than EP as a topcoat. We thereby concluded that coatings containing EB particles exhibited favorable passivation effect and it was necessary to cooperate with a good barrier topcoat to produce excellent corrosion protect performance.
Table 1 Protocol of primers and topcoats on CRS
Film
A
B
C
D
Protocol
Topcoat
EP
EBd/EP
EP
EBd/EP
Primer
EP
EP
EBd/EP
EBd/EP
In conclusion, a compact oxide film was formed on the CRS surface with the help of EB particles dispersed in deionized water or aqueous NaCl solution and resulted in the increasing of corrosion potential and decreasing of corrosion current density. EB particles dispersed in epoxy coating exhibited well its redox characteristic and were more effective and practical than EB film alone in corrosion protection. While the existence of EB particles decreased epoxy coating’s barrier performance and therefore a good barrier topcoat was necessitate for EBd/EP film to improve its corrosion protection performance.
Acknowledgement
The authors would like to thank the financial support of this research by the National Natural Science Foundation of China (Grant no.59903005)
References
[1] Wessling B, Materials and corrosion, 1996, 47: 439
[2] Schauer T, Joos A, Dulog L, Eisenbach CD, Progress in organic coatings, 1998, 33: 20
[3] Jing XL, Yang QH, Zheng MS, China synthetic resin and plastics, 2001, 18(6): 10
[4] Jing XL, PhD dissertation, Xi’an Jiaotong University, 2001
[5] Wang YY, Qiang JF, Jing XL, Chinese science abstracts, 2001, 17(12): 1593
论文来源：Preprints of Third East Asian Polymer Conference,June,2004 |
|
