Dr. Bernhard Wessling
Ormecon Chemie GmbH & Co. KG, Ammersbek
(a subsidiary of Zipperling Kessler & Co.)
It was in 1993 that we found out how to reproducibly realize superior corrosion protection with Organic Metals. We discovered the phenomena responsible for the new corrosion protection principle: it was a surface ennobling (i.e., a shift of the corrosion potential of the metal surface by about 800 mV to the more noble range) and a new type of passivation (i.e., the formation of a stochiometric iron(II) oxide) [5]. For the basic studies, we used as well pure polyaniline dispersions as polyaniline dispersion coatings which were deposited on untreated metals like normal iron, stainless steel, copper and aluminum.
The corrosion current was significally reduced or even completely eliminated at comparable potentials (Fig. 1). Investigations with SEM revealed that an oxide layer was formed between the PAni coating and the metal surface. In cooperation with R. Elsenbaumer, we found that it is mainly composed by Fe2O3 (with an underlayer of Fe3O4) [6]. We repeated the study with a cleaner steel surface to begin with by XPS in cooperation with a group at Kiel [7] (Fig. 2) showing that there was no Fe3O4, but only a clean a-Fe2O3. (This does not exclude, that Fe3O4 might also occur in real situations, but it shows that a-Fe2O3 is the passivating layer composition formed by PAni, which has also independently been confirmed by R. Elsenbaumer [8] and T. Schauer et al. [9], although there is some dispute if it is the a- or the g-form which is formed).
It is of basic interest that an oxide like that has - to our knowledge - never been observed or analysed for the „passive“ state of stainless steel (or simple steel), which is not a stable property (after removing the steel from the passivating medium, its passivity is completely lost). In contrast, A.-M. LeGoff et al. have published, that the nature of the passive layer is FeOOH [10]. This means, that our new technology is also the first one which allows to produce Fe2O3 as a passivating oxide layer. [11]
In the meantime, in research together with G. Nimtz [12] et al. using microwave, and in work together with A. Kaiser et al. [13] using thermopower measurements, we had discovered that polyaniline not only is a conductive polymer, but a true, though „mesoscopic“ metal, and hence can be considered as an „Organic Metal“. Now we understood that the ennobling was possible due to its metallic property, as it is situated slightly less noble than silver in the galvanic serie [14].
Our group later succeeded in improving the dispersed PAni containing paints and coating systems (primer + top coat) [15] which are capable of inducing the same passivation effect, but are moreover industrially applicable and effective as corrosion prevention coating systems (CORRPASSIVTM). They perform significantly better than anti-corrosion coating systems composed by zinc rich epoxy primers and epoxy top coats [16].
We also discovered the reaction sequence which is responsible for the passivation (oxide layer formation) and the corrosion protection induced by the PAni layer. Fig. 3 shows an improved version of the reaction scheme elaborated by us [17]. It involves Fe-oxidation by PAni (the more noble metal, Emeraldine salt ES), which is thereby reduced to Leucoemeraldine base (LE) [18]; further oxidation of Fe (II) to Fe (III) and reoxidation of LE to PAni (ES) via the Emeraldine base EB occur both by oxygen; and Fe2O3 deposition by resulting OH-. This scheme shows furthermore, that our Organic Metal acts as a catalyst, and that the full catalytic cycle (ES ® LE ® EB and back to ES) will only take place, if the necessary H+ will not be removed by the surrounding medium, i.e. only, if the acidic pH will be maintained within the primer, e.g. due to the barrier property of the top coat.
It should be noted, that this new ennobling and passivation technology is only feasable with well advanced Organic Metal (PAni) dispersions in specially composed coatings. The particle size of PAni in our dispersions is about 70 nm. At such small particle size level, the dispersed Organic Metal phase will form the necessary network flocculation structures at very low concentrations [19] (we are using only about 2% PAni in our liquid coatings).
Other PAni formulations like „soluble polyaniline“ [20] do not offer any corrosion prevention effect. It is important to know, that the sample (3) mentioned in Section 4 and Table 4 in [20] is CORRPASSIVTM (in other publications P. Kinlen referred to it as „PAni/thermoplastic“), was transferred by us to Monsanto for their comparison tests. It performed comparably well as a zinc-rich primer with an epoxy topcoat [21]. The samples „PAni/Phenoxy“ and „PAni/Acrylic“ are PANDA products, which do not show any interesting anti-corrosion effect [22]. We hypothesize, that this is due to the special structure of PANDA, which is called a „solution“, but is in fact a fine dispersion with a (mainly insulating) dispersion stabilization layer adsorbed on the particle surface [23].
Other approaches like neutral PAni dispersions in NMP (wrongly been assigned as „solutions“) to be doped after application on the metal [24] are not only not effective, but also not practical [25].
N-PAni (the Emeraldine base, „undoped“) is also contributing some anti-corrosion effect, but by a factor of 100 to 1000 smaller [6]; this effect is probably due to its amine groups and could therefore be attributed to a pure inhibition mechanism.
It seems that the new effects of ennobling and passivation found by us are linked with well structured ultrafine dispersions (50 - 100 nm) of the Organic Metal in suitable matrices, forming a conductive and catalytically reactive continuous (and conductive!) network of flocculated PAni particles. [26]
