Troch-Nagels et al. [2], however, concluded, that PAni, unlike polypyrrole, after electrochemical deposition under comparable conditions, did not offer any corrosion protection.
Starting in 1986 we tried to achieve the goal of coating steel, which was not pre-passivated, under non-electrochemical conditions, but with a paint containing dispersed polyaniline. We were asking if some kind of corrosion protection - by whatever mechanism - could be realized by an interaction between a dispersion paint and a normal metal surface. This would be - in contrast to de Berry and others - a non-electrochemically applied PAni on a not pre-passivated metal surface. 1987, we achieved the first promising results [3]. Subsequent work (also published in various patents [4]) confirmed the previous findings, but did not show an exciting "quantum leap" in corrosion protection. Moreover, it was hardly reproducible and did not convince any paint manufacturer.
It was only in 1992/1993 that we finally found out after an in-depth evaluation of the interactions between various metal surfaces and coatings of polyaniline (applied as pure dispersion or as dispersion paints), that a remarkable corrosion potential shift ("ennobling") and an iron oxide layer formation ("passivation") together lead to a significant anti-corrosion effect [5]. In a study together with R. Elsenbaumer et al. [6], we found out, that the corrosion rate was reduced by a factor of up to 10.000! The iron oxide, which was formed between the metal surface and the polyaniline primer coating, was determined to be Fe2O3, later confirmed with even clearer XPS spectra [7].
In the meantime, we have evaluated the reaction mechanism, by which polyaniline as a redox catalyst is mediating the reaction between iron (or in analogous way other metals) and oxygen/ water to form the passivating oxide layer [8], see fig. 1.
Parallel product development towards commercially useful and competitive anti-corrosion coating systems have lead to various products, which have been successfully tested under practical and under various laboratory conditions [9], and are finding their place in the market in the meantime. It became also evident, that the conclusions, which were drawn from the basic research in dispersion and in the corrosion prevention mechanism of polyaniline, have lead to superior performance compared to other systems, which have been proposed as alternative techniques [10].
This is probably due to the fact, that the alternative methods do not fulfill all chemical, physical and technical requirements, which a corrosion prevention technology based on polyaniline in technical scale has to. The technique proposed by the NASA/Los Alamos group [36a] [11] not only is not practical for the general coatings industry, but also fails in terms of adhesion, reproducibility, and did not prove having a superior performance compared with high performing coating systems. The Monsanto variation [36b] [12] has yet to show being practically applicable and performing to an acceptable standard. The DSM concept [36c] [13] is actually only a form of polypyrrole potentially useful as a paint additive, but to our knowledge has not shown yet to offer any anti-corrosion performance in real paints.
We would conclude from our basic chemical, physical and theoretical evaluations of the polyaniline interactions with metal surfaces leading to corrosion prevention, and from our practical experience with the development, testing, and marketing of various PAni containing anti-corrosion paints (primers) in numerous applications, in widely differing corrosion environments, and summarizing the advantages and disadvantages known up to now, that the following requirements have to be fulfilled by a PAni containing coating for principally successful (and commercially attractive) applications:
figure caption:
fig 1: Reaction scheme for the catalytic function of polyaniline during
the formation of the passivative oxide layer between iron and a polyaniline
coating
[2] G. Troch-Nagels, R. Winand, A. Weymeersch, L. Renard, J. Appl. Electrochem. Soc. 139 (1992) 756
[3] patent application WO 88/00 798, Zipperling Kessler & Co, issued in the meantime in many countries in the world, e.g. US 5,567,355
[4] e.g. PCT/US 93/00543, in a cooperation between Zipperling Kessler and Allied Signal, Inc., priority January 1992
[5] B. Wessling (Zipperling Kessler) PCT/EP 94/02023, priority June 1993; in the meantime issued in several countries, e.g. Japan JP-P 2536817; cf. also: B. Wessling, Adv. Mater. 6 (1994) 226
[6] W.-K- Lu, R.L. Elsenbaumer, B. Wessling, Synth. Met. 71 (1995), 2163-2166
[7] B. Wessling, "Scientific and Commercial Breakthrough for Organic Metals", lecture at the ICSM '96 in Salt Lake City, to be published in Synth. Met. Conference Proceedings
[8] B. Wessling, Materials & Corrosion 47 (1996) 439-445
[9] Ormecon Chemie, technical informations "CORRPASSIV®"
[10] (a) K.G. Thompson, C.J. Bryan, B.C. Benicewicz, D.A. Wroblewski, Los Alamos National Laboratory Report LA-UR-92-360
D.A. Wroblewski, B.C. Benicewicz, K.G. Thompson, C.J. Bryan, Polymer Reprints 35 (1994) 265
(b) P. Kinlen, Proc. ICSM '96, Salt Lake City, to be published
(c) M. van Doorn, L. Bremer, Proc. ICSM '96, Salt Lake City, to be published
[11] a pure neutral PAni layer deposited from "solution" (dispersion), postdoped, and coated with a top coat
[12] a "soluble" polyaniline, presented as "PANDA", cf. Monsanto technical informations
[13] a water born latex, presented as "ConQuest", cf. DSM technical informations
[14] Ormecon Chemie developed and produces specially designed systems for steel, aluminum, galvanized steel, other metals, for general industrial or very aggressive (chemical) corrosion environments, for maritime corrosion, and for the various different application techniques, like brush or spray coating, roller coating, dip and spin coating, coil coating and others
