Polyanilines corrosion inhibition

There is a large potential for conducting polymers as corrosion-inhibiting coatings. For instance, the corrosion protection ability of polyaniline is pH-dependent. At lower pH polyaniline-coated steel corrodes about 100 times more slowly than noncoated steel. By comparison, at a pH of about 7 the corrosion protection time is only twice for polyaniline-coated steel. Another area of application involves creation of solid state rechargeable batteries and electrochromic cells. Polyheterocycles have been cycled thousands of times with retention of over 50% of the electrochromic activity for some materials after 10,000 cycles. IR polarizers based on polyaniline have been shown to be as good as metal wire polarizers. 

S. Fuhua, W. Xiutong, Y. Jianqiang, H. Baorong, Corrosion Inhibition by Polyaniline Copolymer of Mild Steel in Hydrochloric Acid Solution. Anti-Corros. Methods Mater. 2011,58,111-115. 

Jose, E., Pereira, S., Susana, 1., Cordoba, T., and Roberto, M. T. Polyaniline acrylic coatings for corrosion inhibition the role played by counter-ions. Corrosion Science, 47(3), 811-822 (2005). 

Ogurtsov, N.A., et al. 2004. Corrosion inhibition of aluminum alloy in chloride mediums by undoped and doped forms of polyaniline. Synth Met 143 (1) 43. 

Gasparac, R., and C.R. Martin. 2001. Investigations of the mechanism of corrosion inhibition by polyaniline. Polyaniline-coated stainless steel in sulfuric acid solution. / Electrochem Soc 148 (4) B138. 

Intrinsically conducting polymers (ICPs) are electroactive long-range conjugated polymers. They generally possess reversible redox performance, while metal corrosion is also a redox process therefore, it is possible that ICPs may find application for metal anticorrosion. It is true since the early report for corrosion inhibition performance of ICPs such as polyaniline (PANI) by DeBerry [5]. After more than 20 years of development, now ICPs have received much attention, since th may be a kind of alternative anticorrosion agents instead of the toxic heavy metal in anticorrosion coating, no matter they are used alone or as composite with substrate resin. 

Pereira da Sdva, J.E., Cordoba de Torresi, S.I., and Torresi, R.M. (2005) Polyaniline acrybc coatings for corrosion inhibition the role played by counter-ions. Corros. Sci., 47, 811-822. 

Polyanilines as Alternative Anticorrosion Materials A Molecular Level Model of the Corrosion Inhibition Process... 

The oligomers/trimers of polyaniline as shown in Fig. 2 have also proved to be effective in corrosion control. Their frontier orbital energies and measured oxidation potential indicate large positive oxidation potentials. Further analysis of frontier orbital effects suggests a similar mode of corrosion inhibition for the oligomers and full polymer. Parallel interactions occur for the oligomers relative to... 

There may be some concern that all conducting polymers behave as polyaniline, that is, that all conducting polymers exhibit the same pH dependence of corrosion protection that polyaniline does. However, since polyaniline is made conducting by treatment with acid whereas most other conducting polymers are not, it is extremely unlikely that all conducting polymers will exhibit similar pH dependence of corrosion inhibition. 

Significant corrosion inhibition of polyaniline-coated mild steel exposed to saline (3.59 NaCl) and acidic (0.1 N HCl) environments was reported by Thompson et al. [19]. These polyaniline coatings seemed to promote corrosion protection even where scratches existed in the protective coating. But no reliable quantitative characterization was provided in this report, and only a qualitative explanation was made from electrochemical alternating current impedance measurements or direct current linear polarization experiments. Several other conducting polymers including poly(3-hexylthiophene), poly(3-octylthiophene), poly(3-thienylmethylacetate)... 

More recently Ahmad and MacDiarmid [30] published a fundamental study on corrosion inhibition of steel with chemically deposited polyaniline. The study concludes that the corrosion protection for iron provided by this conducting polymer mostly arises from anodic protection, which requires much less current density (micro- to milliamperes per square centimeter) than a polymer can provide to the metal. Anodic protection of steel is achieved by the formation of a passive oxide layer, which can be induced by applying an anodic potential of about 0.1 V vs. SCE. The study showed that the minimum potential required for passivation of a sam-... 

IV. CORROSION INHIBITION OF STEELS USING POLYANILINE AND POLYPYRROLE... 

Visual observations and spectroscopic investigations on the various aluminum alloys coated with doped polyaniline and doped polypyrrole indicate that there is no long-range electrode polarization phenomenon like that observed on mild steel-coated samples. The corrosion protection of aluminum using conducting polymers is affected by the composition of the aluminum alloy. Corrosion inhibition is controlled solely by anodic protection. Doped polyaniline coatings provide better corro-... 

The plethora of data presented in this chapter and in the literature leaves no doubt that corrosion processes and mechanisms for inhibition are complex. Nonetheless, it is well established that conductive polymers can impart significant corrosion protection to selected metals such as steel, aluminum, and copper. Also, a picture is beginning to emerge of the corrosion inhibition mechanisms that rationalizes the observations made on corroding cold steel coated with polyaniline. For this system there are two cases to consider. Case I is where a steel surface exposed to a corrosive environment is completely covered by a coating of doped polyaniline. Case II is a case where a steel surface exposed to a corrosive environment is almost completely covered by a coating of doped polyaniline but also has small areas of bare steel exposed. 

The overall mechanism for corrosion inhibition for this case is simplistically illustrated in Fig. 31.40. Wes-sling [20] and Lu et al. [26,29] clearly demonstrated that when doped polyaniline is placed in contact with mild steel, the steel surface undergoes a rapid oxidation process to provide a layer of y-Fe203 at the polyaniline/ iron interface. This process is shown schematically in Fig. 31.40 by the transformation of (a) to (b) and occurs according to the equation... 

Fig. 31.40 Schematic illustration of the mechanism of corrosion inhibition provided by doped polyaniline on steel with no defects in the coating.

The overall mechanism for corrosion inhibition on exposed bare steel surfaces adjacent to the polyaniline coating (overcoated with epoxy) is outlined in Fig. 31.41. It is quite evident that the process of corrosion inhibition and passive oxide layer formation occurs by mechanisms that are more complex that those shown here. However, this figure highlights several important steps that are consistent with the electrochemical, visual, and spectroscopic data. 

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