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Scientific Engineering of Anti-Corrosion Coating Systems based on Organic Metals (Polyaniline)

Dr. Bernhard Wessling, Ormecon Chemie GmbH & Co. KG, Ammersbek (a subsidiary of Zipperling Kessler & Co.)

Abstract

The new corrosion protection technology with polyaniline, an Organic Metal (conductive polymer), is presented. It is based on an immense surface ennobling and the formation of a passivating metal oxide. The requirements for efficiently working coating systems, comprising the dispersed Organic Metal containing primer, eventually an intercoat, and a top coat, are characterized. An integrated 4-step-method ("scientific engineering") has been developed and is successfully used for the systematic development of such coating systems. The combination of the measurement of the open circuit potential, a new scratch test, EIS and SKP together are a powerful tool for predicting the results of accelerated corrosion tests and real-time corrosion prevention performance. Organic Metal coating systems are out-performing even the best conventional anti-corrosion coating systems.

Keywords Polyaniline, Organic Metal, Conductive Polymer, Corrosion Protection, Ennobling, Passivation, Metal Oxide, Coatings, Electrochemical Impedance Spectroscopy (EIS), Scanning Kelvin Probe (SKP), Volta Potential, Open Circuit Potential, Potential Shift, Scratch Test, Accelerated Corrosion Tests.

1. Early work

We began our research in this area 1986 and prepared the first dispersed polyaniline (PAni) containing coating in 1987 [1]. Our goal was to find out, if corrosion protection was possible on just normal steel, without any pretreatment, especially without previous passivation in acid or under electrochemical conditions. We wanted to know, if corrosion protection was feasible using a PAni dispersion in a coating, hence without any electrochemical deposition of PAni. Our research was motivated by the publication of deBerry in 1985 [2] according to which polyaniline, electrochemically deposited on pre-passivated stainless steel in strong acid environment, was enhancing the corrosion protection of this metal. Similar observations, however with a composition not identical with polyaniline, had previously been made by Mengoli et al [3]. Both authors favoured the conclusion, that PAni, if electrodeposited on pre-passivated steel, was capable of maintaining the passive state.

This was the only public knowledge available, when we first began our work with the goal to find out about anti-corrosion effects based on PAni. Our proprietary knowledge at that time was how to disperse polyaniline, although still at a relatively low performance level, as we had succeeded with first dispersions of polyaniline only some 2, 3 years before (mainly in thermoplastic polymers). It was completely unknown, if a PAni dispersion coating (with the PAni-free insulating matrix around) would have any effect, and if, what the effect would be based on, and if not, how to make it to be an anti-corrosion coating. Nothing at all was known about any anti-corrosion effect of PAni, if applied non-electrochemically and on a non pre-passivated steel (or iron or any other metal) surface, and nothing at all was to be expected in this direction at that time.

In 1987, we surprisingly found some corrosion protection [1], and we also found that an oxide was formed, but this could have been any oxide or hydroxide, or even a first sign of rust, as we had not yet been able to see any improvement over state-of-the-art coatings. We followed these first findings in the next years, partially together with other groups, and also published another patent application [4] with some improvements in corrosion protection, but still not at all convincing for corrosion experts, still not performing reproducibly, and not performing any better than conventional coatings, but worse.

At that time, we had no idea, what mechanism was active if there was any beneficial and remarkable anti-corrosion effect of polyaniline deposited by other than electrochemical means on other than pre-passivated metal surfaces (stainless steel) under other than passivating conditions. The conductive polymer research community, and moreover the corrosion experts outside it, did not believe in the possibility to realize an anti-corrosion technique using polyaniline, especially not with a PAni dispersion. Also Allied and Americhem, with which together we had made the study leading to the second patent [4], stopped their work on this question and left the cooperation, as a success of the research was not at hand.

2. Breakthrough

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 stoichiometric 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 significantly 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. 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 feasible 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 our Pani dispersion (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]

3. The need for a systematic, scientific development tool

When we started our primer and coating system development, we were aware of the fact, that we had no practical experience in corrosion phenomena, and even less in coatings. On the other hand, corrosion and coatings experts did not believe in our findings, and we were unable to convince them to combine their experience in coatings with ours in the Organic Metal polyaniline, its synthesis and dispersion, and in "ennobling and passivation" by it.

We realized very soon [5c, 15], that a polyaniline containing primer alone was not sufficient as a practically convincing coating system, but that it needed a top coat. This top coat (or eventually an intercoat between primer and final top coat) had to fulfill certain requirements in order to be compatible with the primer. The primer itself had to meet the following demands:

The top coat (or the intercoat, resp.) had to offer

Having no practical experience and not enough time for the development of such coating systems, which should not only to be competitive, but moreover out-performing generally used state-of-the-art products, we had no other chance than to follow a scientific systematic route, in contrast to a "trial-and-error" strategy.

We started with the open circuit potential (and corrosion current density) measurement as described in [5b] in combination with a wheel-driven alternating immersion test according to DIN 50905 T4 as a 2-step screening tool. Systems which passed our test [29] were subjected to other laboratory tests, salt spray and outside weathering corrosion tests (cf. [16]). We found an acceptable correlation in performance and decided to let one of our systems be tested by a neutral paint research institute, the "Forschungsinstitut f�r Pigmente und Lacke" in Stuttgart.

Two studies were performed by them on our behalf. The first study was a comparison between a system composed of our primer 900 226/32 plus our selected epoxy top coat ("2-C EP") and 2 comparative systems having the same primer, but different top coats ("2-C AY" and "1-C AY", resp.). The study comprised measurement of dry and wet adhesion, and salt spray test performance. The system /32 + "2-C EP" performed by far at best [30], and the performance was in accordance with our previous internal results and [16]. This system is a first model system for our first commercial product (CORRPASSIVTM 4900), introduced in late 1996 [15].

The second study involved a system we had developed for aluminum, which requires a different primer used with the same "2-C EP" top coat. Here, the filiform corrosion was tested, both in laboratory as in an outside weathering site at Netherlands. Also this system performed very well, as documented by FPL [31]. This system has been further developed by us to a commercial product.

In parallel, the FPL investigated some other properties [32] of some systems comparable to those evaluated in the first study. They confirmed the immense potential shift found by us earlier [5] and found comparable potential values. The formation of the iron oxide Fe2O3 was confirmed, too. They also confirmed a link between barrier properties, stability of potential shift and corrosion protection performance.

However, for further development of more and different systems for different corrosion environments, we needed a tool which was not only capable of screening between "good" and "bad", but was moreover capable of delivering quantitative data enabling us to predict corrosion test results, both of accelerated and real-time tests and practical behaviour.

4. "Scientific Engineering": a new research and development tool for anti-corrosion coatings

This was the reason why we continued to study and develop other techniques [33] for evaluating those properties which we consider being the key requirements for good corrosion protection performance with Organic Metal coatings,

  1. the ennobling (i.e., the potential shift)
  2. the passivation (i.e., the oxide layer formation)
  3. adhesion of primer (+ top coat) also under corrosive conditions, and resistance against underfilm corrosion (i.e., minimum underfilm corrosion propagation velocity)
  4. optimal barrier properties to be maintained as long as possible under corrosive conditions.

These 4 properties are now measured by us with the following methods:

  1. open circuit potential measurement
  2. a scratch test developed by us
  3. Scanning Kelvin potential measurement (SKP)
  4. electrochemical impedance spectroscopy (EIS), using a new routine FFT technique.

Test 1 and 2 are used as a first screening. Test 3 and 4 are used for advanced screening, before they are combined with an immersion cycling, cycling climate test or salt spray test for those systems which pass the advanced screening successfully.

Industrially useful coating systems have been developed by us following this "Scientific Engineering Method". The results reported below were found on test panels coated with PAni containing primer [34] applied with a thickness of 20 �m and coated with various top coats.

4.1 Open Circuit Potential Measurement

This routine measurement can be performed under various well-known conditions [5b, 6, 8, 32] and does not need to be described further. Schauer et al. [32] have confirmed the range of the high potential shift found by us earlier [5]. Our internal specification is a potential shift to at least +100mV and a long term stability of it.

4.2 Scratch Test

This test can be performed in two ways, (a) by coating half of a panel with Pani containing primer, the other half with a conventional epoxy primer, then applying a top coat (e.g. 2-C EP) on both primers, or (b) by coating a panel completely with the coating system to be tested.

The coating is then injured with scratches 1 mm wide and 50 mm long, down to the metal. The panels are immersed in 10% salt containing water. After 1 night and after 100 and 300 hours, the panels are checked with respect to rust formation in the scratch: only those systems will be accepted to become commercial products, where no rust will be formed within the open scratch during this time (see Fig. 4). Conventional systems, also the very good ones, will develop rust in the open scratch, because the area there is not protected. They also develop rust under the coating, propagating at various velocities (cf. 4.3).

The protection provided by the polyaniline dispersion coating, the ennobling, however is a far-reaching effect because of its metallic nature. We are assuming furthermore, that it is not only the ennobling which provides protection for open scratches of this size, but also the passivation by Fe2O3 formation: this oxide can be found up to about 400 �m away from the coating edge as seen by XPS [35].

This protection can be effective more than 1 mm away from a coating edge, which makes anti-corrosion systems based on our Organic Metal a powerful edge corrosion protection technology. We even have sometimes found much bigger areas being protected even after removal of the coatings [36].

4.3 Scanning Volta Potential Measurements (Kelvin Probe)

The Volta potential measured with a Kelvin sensor is suitable for non-contact measurements of surface potentials even under undamaged surface coatings [37], [38]. The function principle and experimental set-up is shown in Fig. 5. The measurement object, the working electrode, and the reference electrode of the Kelvin probe form, due to the small gap between them, a capacitor. Between them a potential is developed, the amplitude of which gives a measure of the chemical nature of the material on the surface of the metal. A periodic variation in separation by means of an actuator built into the sensor changes the capacitance of the set-up. The resulting signal is converted to a measurement signal by means of a lock-in amplifier [39].

The resulting data is a scan of the surface Volta potential of the metal under the coating, which can - under certain conditions - be related with the corrosion potential. But for the purpose we are looking for, this is not a necessary relation [40]. In this part of our method, we only need to know the differences of potentials (a) between various sites, i.e. between the open scratch and the not injured areas of the coating (b) with the change in immersion time.

With this technique, we monitor and predict adhesion (or delamination) of the coating under corrosive attack. The underfilm corrosion propagation velocity (more precise: the corrosion potential propagation velocity, due to the first delamination of the coating before even first corrosion occurs) can be quantitatively measured.

In contrast to the sample design by Stratmann [38], we are measuring on samples like those used in our scratch test (4.2.). This leads to the phenomenon, that in the best of our systems, no potential negative enough for even to start any corrosion develops, so that no delamination or even rust formation or underrusting can occur.

Fig. 6 is showing a development of the Volta potentials in the open scratch and in the neighbourhood under the coating with time (polyaniline dispersion primer plus top coat). We are interested to follow the potential change especially in the first 24 hours of immersion in salt water (after usual conditioning in humid atmosphere). Typically, in the best system, a "W" form of the potential distribution forms, with no potential approaching strong enough negative values (Fig. 7), to allow corrosion. Only, when the bottom of the "W" form reaches values of -200mV or lower, a delamination (the prerequisite for corrosion) can be observed and corrosion may start. The delamination velocity is estimated with widening of the negative corrosion potential "W" valley, if it occurs at all (Fig. 8).

In contrast, scratches in coatings without Organic Metal primer have a big potential difference, a broad and deep valley, resulting in a high underrusting propagation velocity (Table 1). Very quickly, in the middle of the scratch the potential is strongly increasing now as the sign of rust formation.

We differentiate various polyaniline dispersion coating systems under development by comparison with the actually best performing benchmark, where no corrosion potential is reached in the scratch, hence no delamination occurs.

Three systems, each built up on the same primer, but top coated with different paints (Table 1), have all passed the scratch test but were different in their OCP. The question to be evaluated was: Do we see a difference in the delamination velocity? Table 1 contains the answer "Yes": after 24 hours, significant differences can be noticed. Note, that the performance order is in parallel with all other results. Primer formulations (same basis) without polyaniline under even the best top coats have 5-10 times quicker delamination. The best top coat on polyaniline dispersion primer alone (on epoxy primer or even on Zn-rich epoxy primer) does not perform comparably well at all. It is the most important conclusion, that underrusting or persistent passivation, resp., is also strongly influenced by the top coat, a conclusion which is not at all self-explanatory, but can be understood in view of [17, 7] and Fig. 1.

We set our internal specification as an underfilm corrosion propagation velocity of between 3 and 5 �m/h or less. Epoxy coating systems (primer plus top coat) are generally showing a velocity of around 20 to 60 �m/h, a factor of 10 or more faster than polyaniline dispersion coating systems.

Zn-rich epoxy primers (with epoxy top coat) do even show much quicker underfilm propagation, at least in the first 1-2 days. Such systems are not scratch-tolerant, they perform very well only with intact coatings. In comparison to that, our systems based on Organic Metals are performing extremely well both with and without scratches.

Only the best cataphoretic coatings on Zn-Phospate electrochemically pretreated steel in car manufacturing are showing figures (3-5 �m/h) comparable with polyaniline dispersion coating (0 - 5 �m/h). We are convinced to be able to achieve these numbers also with future product developments (as we actually see with some developmental products).

4.4 Electrochemical Impedance Spectroscopy

The technique is used by us to predict the performance of the complete primer / top coat (or eventually including an intercoat) system. We developed this technique as a routine method together with G. Popkirov based on his new FFT-EIS technique [41]. Measurements are carried out with a three-electrode system using a cell as described in [42]. The experimental set-up for this Fast Fourier Transformation Electrochemical Impedance Spectrometer is given in Fig. 9. A frequency-rich perturbation signal with a small amplitude is applied to the electrochemical cell controlled by an EG&G Princeton Research potentiostat Model 263A. In the present investigation a computer programmed sum of 42 sine waves distributed over 4 decades was used to synthesize the perturbation signal of a home-built signal generator The peak-to-peak amplitude of the perturbation voltage was usually 150 mV. The perturbation and the response signal are amplified and filtered by a Stanford Research Systems Inc. Model SR 640 Dual Channel Low-Pass filter. A/D and D/A conversion, timing and controlling were carried out by a 16-bit, 100 kHz transient recorder PC-card from United Electronic Industries, Inc. Model Win- 30/3016. Impedance spectra were evaluated by fast Fourier transformation of the perturbation and the response signal (Fig. 10).

For the experiments describing our procedure and representative examples, the following lacquer systems were used on polyaniline dispersion primer:

  1. two-component epoxy top coat - amine hardened = 2-C EP
  2. two-component acrylic top coat = 2-C AY
  3. one-component acrylic top coat = 1-C AY

The tests were made on sand blasted panels from Mercedes Benz and are from the same serie with those evaluated in 4.5 (cf. [30]).

Fig. 11a-c show experimental EIS data for the three different top coated steel panels as a function of immersion time. The measurements were carried out in 10% NaCl. Changes in the impedance spectra were observed over a six day period except for the 1-C acrylic top coat with a one day period.

The recording of the spectra takes about 5 - 20 seconds, the Nyquist and Bode plots are available for interpretation after another 15 seconds. The bottleneck for measuring more samples is only the number of available immersion cells, as they are placed and immersed in the same cell as they are measured. But as we are not measuring all of the systems under development, but only those which have passed the first screening, this is not a practically important limitation.

The performance of the complete system under evaluation can be assessed after 1 or 2 weeks at most. If their impedance spectra do not change significantly during this time (as shown by sample 1, which is a commercial polyaniline dispersion coating), we can expect a very good long-term corrosion protection performance.

Systems, which will fail, are loosing their capacitance during 1 week or even quicker. It can be seen, that the stability of the EIS spectra or loss of the pure capacitance behaviour is parallel to the results found in 4.5, a good or bad performance in VDA cycling test.

It should again be noted, that systems (samples #5 and #6) composed by a pure epoxy primer or a Zn-rich epoxy primer top coated with the same EP top coat as in the polyaniline dispersion coating /2-C EP (sample #1) again perfom worse (i.e., they loose impedance very quickly, Fig. 12).

Intermediate conclusion

Scratch test, OCP, RKS and EIS are probing each different aspects of the integral named "corrosion protection performance". Ennobling and passivation, primer adhesion (delamination/underrusting) or coating barrier properties, being tested somewhat in a separated manner, belong and work together. Both underrusting (delamination) and barrier property (impedance) are not only dependant on the primer or the topcoat, but the combination of both.

4.5 Climate cycling test (VDA) / Salt Spray Test

This test is used by us only after successful basic and advanced screening (4.1 to 4.4).

The systems described above (scratched panels) have been subjected to these accelerated corrosion tests for comparison of these test results with our 4-step method (cf. [30]). Underfilm corrosion, blistering and degree of rusting are summarised in Table 1. In this test, as in real life, both the performance of the intact coating on the metal itself, as also the injured (scratch) coating behaviour plays a role. These effects have been tested in the 4 tests shown above, and they are working together in accelerated corrosion tests and in outside weathering or other real corrosion environments.

The tests showed that all three coating systems provide highly efficient corrosion protection and complete suppression of underfilm corrosion over a period of more than 1000 h; especially with top coat 2-C EP no change at all was found even after 1000 h. The other top coats allow some blistering. None of the systems are really failing, but they are slightly different in their overall acceptable performance.

Our 4-step "Scientific Engineering" test method is able to differentiate much quicker and much more sensitively than do these generally accepted accelerated corrosion tests.

5. Conclusions and Outlook

The new Organic Metal polyaniline, almost a noble metal, ennobles steel and other metal surfaces while shifting their surface potential. It furthermore passivates the conventional metals by forming a metal oxide layer of up to 1 �m thickness. This exciting new technology is based on extremely fine dispersions of PAni in coating matrices of special composition.

Many internal and independant tests [4, 8, 20, 24] have shown that the presence of polyaniline at the metal surface (to be protected) alone in regardless what chemistry form or matrix is by far not sufficient for a reproducible and practically convincing high-performance corrosion protection coating.

As shown, the systematic combination of 4 tests, the measurement of the corrosion potential shift, a new simple scratch test, the Volta potential measurement with the Scanning Kelvin probe, and electrochemical impedance spectroscopy, eventually to be combined with salt spray or climate cycling test according to VDA (Association of German Automobilists) is a productive tool for testing and improving corrosion protection coating systems. In contrast to salt spray and climate cycling tests (3-6 months), useful results from measurements with EIS and the SKP can be obtained within 1-2 weeks (after complete drying of the coatings to be tested), from where accelerated corrosion test results and real-world corrosion prevention performance can be predicted.

Polyaniline dispersion coating coating systems for various industrial applications have been developed with this method [43]. They were specified and are released by several customers; practical industrial reference objects have been realized, e.g. pipelines, bridges, boats, container ships, steel constructions in chemical plants, hydraulic construction in waste water management, hydraulic service center in Airbus manufacturing etc. [44]. They are performing much better than any other conventional anti-corrosion coating. They can be applied at a 25 to 60% reduced coating thickness compared to conventional systems, without any loss of performance [45].

Recently, new systems have been developed with the help of these techniques, namely

Both systems will soon be presented for independent performance tests.

It might also be of interest, that our polyaniline dispersion coating systems have shown a well advanced performance on structural light metal alloys, like Mg alloys for automotive use. We are following this area, too, whereby we still need to develop our "Scientific Engineering" method for these metal substrates. Another system for structural Al applications (aerospace) is under development. Here again, the replacement of chromates is the environmentally important goal of the project.

Considering the short time we had for the development, our systematic 4-step method has helped us to develop coating systems of the highest performance at significantly lower coating thickness, because it led to a deep understanding of the phenomena during corrosion attack, ennobling and passivation. We were able to show, that

It would certainly be of interest to find out, if this method can also be applied on general, non-PAni-coating systems, to predict their performance.

Both, the Organic Metal anti-corrosion technology and our Scientific Engineering of anti-corrosion coating systems, have the potential to revolutionize corrosion protection. Not only new systems can be quickly developed with a high correlation between short term measurement and long-term performance, but also coatings with about 5 times longer lifetime for the metals to be protected, at 25 - 60% lower coating thickness, replacing chromates and zinc can be provided.

Considering the fact, that corrosion is responsible for the loss of about 4% of each countries Gross National Product per year, and considering, that the repair of the lost values not only requires financial, but even more environmental resources, our new technology could significantly contribute to reduce costs and to a future ecologically acceptable economy ("sustainable economy").

Figures

Fig. 1: Corrosion current density-potential curves of various metals, both original and coated with polyaniline [4b].

a) untreared with Fe and FeOOH signal

b) in the presence of polyaniline with pure Fe2O3 signal.

Fig. 2: XPS analysis of passive iron oxide layers [6]

Fig. 3: reaction scheme.

 

Left: heavy rust formation in the damaged site

Right: no corrosion attack - the ennobled and passivated metal surface is resistant, even without coating.

Fig. 4: St-14 panel after 300 hours in permanent immersion test (DIN 50905 T4) in 3% NaCl solution; (Click on the photograph for an enlarged view)

Fig. 5: Function principle of corrosion potential measurements using a scanning Kelvin-probe.

Fig. 6: 3D plot of the potential development in the scratch and under the coating; top t=0, bottom t=24 hours of immersion, for sample #2, cf. Fig. 7.

Fig. 7:          Development of a deep "W" corrosion potential valley in a coating system (#2) not meeting the specifications: at (top) t=0 (middle) t=12, (bottom) t=24 hours immersion time, reaching the corrosion potential around -200mV and showing underrusting (12-30 �m/h)

Fig. 8: Development of a shallow "w" potential distribution at elevated (passivation) level in the best performing coating system #1 at (top) t=0  (middle) t=12 (bottom) t=24 hours of immersion: no corrosion, no underrusting.

Fig. 9: Set-up for FFT electrochemical impedance spectroscopy.

Fig. 10: Time domain and power spectra for perturbation and response signal.

a) 2-C Epoxy top coat

b) 2-C Acrylic top coat

c) 1-C Acrylic top coat.

Fig. 11: Nyquist and Bode plots of selected top coats with PAni dispersion primer

Fig. 12: Bode plot of EIS spectra for sample #5 (EP/2-C EP) and #6 (Zn-EP/2-C EP) at a) t=0 b) t=70 c) t=140 hours of immersion

Table 1: Summary of RKS results for various coating systems.

 

 

Sample

 

mm/h

Potential E, (mV)

 

 

 

under coating

in scratch

1

Polyaniline/2-C EP

0 (-3)

0 to 150

100 to 150 (no rust)

2

Polyaniline/2-C AY

12-30

150 to 300

-100 to -300

3

Polyaniline/1-C AY

35-80

150 to 300

-50 to -300

4

Primer matrix without polyaniline 2-C EP

20-40

-200 to -100

-200 to 100 (rust)

5

EP primer / 2-C EP

50-60

-200 to -100

50 to 250 (rust)

6

Zn-EP primer/2-C EP

30-35

-150 to 0

-700 to -800

 

Table 2:    Results of salt spray test in accordance with DIN SS 50 021.

 Duration Polyaniline/2-C Epoxy top coat Polyaniline/2-C Acrylic top coat Polyaniline/1-C Acrylic top coat
[h] U1) B2) R3) U B R U B R
 24 n4) n n n n n n n n
 72 n n n n n n n m2/g4 n
 96 n n n n n n n m2/g4 n
 168 n n n n n n n m2/g5 n
 241 n n n n n n n m2/g5 n
 337 n n n n m2/g5 n n m2/g5 n
 505 n n n n m2/g5 n n m2/g5 n
 607 n n n n m2/g5 n n m2/g5 n
  n n n n m2/g5 n n m2/g5 n
1035 n n n n m2/g5 n n m2/g5 n
      1) U - underfilm corrosion (DIN 53 167)      2) B - blistering  (DIN 53 209)
      3) R - degree of rusting (DIN 53 210)           4) n - none

References

[1] B. Wessling, German Patent P 37 29 566.7, Zipperling Kessler & Co. (1987).

[2] D. W. DeBerry, J. Electrochem. Soc. 132, 1022 (1985).

[3] G. Mengoli, M. Munari, P. Bianco, M. Musiani, J. Appl. Polym. Sci. 26, 4247-4257, (1981).

[4] Joint Patent Application with Allied Signal, (Morristown/USA), PCT/US 93/00543

[5] a) B. Wessling, DE 43 34 628 A1, Zipperling Kessler & Co. (1993).

b) B. Wessling, Adv. Mater. 6, No 3, 226 (1994).

c) B. Wessling, PCT WO 95/00678, Zipperling Kessler & Co. (1993).

[6] Wei-Kang Lu, R. L. Elsenbaumer, B. Wessling, Syth. Met. 71, 2163-2166 (1995).

[7] B. Wessling, Synth. Met. 85, 1313-1318 (1997).

[8] Wei-Kang Lu, S. Basak, R.L. Elsenbaumer, "Corrosion Inhibition of Metals by Conductive Polymers", in: Handbook of Conducting polymers, T.A. Skotheim, R. Elsenbaumer, J.R. Reynolds (eds), 881-920 (M. Dekker 1998).

[9] T. Schauer, A. Joos, L. Dulog, C.D. Eisenbach, "Protection of iron with polyaniline primers against corrosion", to be published in Progress in Organic Coatings (in print).

[10] A.-M. Hugot-Le Goff, C. Palotta, J. Electrochem. Soc. 132 (11), 2805-2806 (1985); N. Boucherit, P. Delichere, S. Joiret, A.-M. Hugot-Le Goff, Mat. Sci. Forum 44&45, 51-62 (1989).

[11] in case of copper, the oxide is Cu2O (B. Wessling, J. Posdorfer, W. Strunskus, to be published)

[12] a) G. Nimtz, A. Enders, P. Marquardt, R. Pelster, B. Wessling, Synth. Met. 45, 197-201 (1991)

  b) G.Nimtz, R. Pelster, B. Wessling, Physical Review B, 49 (18), 12718-12723 (1994).

[13] a) C.K. Subramaniam, A.B. Kaiser, P.W. Gilberd, B. Wessling, Journal of Polym. Sci. Part B: Polym. Phys. 31, 1425-1430, (1993)

  b) C.K. Subramaniam, A.B. Kaiser, P.W. Gilberd, B. Wessling, Synth. Met. 69, 197-200 (1995)

  c) C.K. Subramaniam, A.B. Kaiser, P.W. Gilberd, C.-J. Liu, B. Wessling, Solid State Commun. 97 (3), 235-238 (1996).

[14] our own unpublished measurements

[15] Ormecon Chemie, Ammersbek (Germany) technical information (previously published by Zipperling Kessler & Co., Ormecon’s holding)

[16] Test Report of DECHEMA (D-Frankfurt), Title: Tests on "passivated" steel specimens; Scope of order: Corrosion test programme for crevice and pitting corrosion and galvanic corrosion of various pre-treated steel specimens (6/1994).

[17] B. Wessling, together with S. Schr�der, S. Gleeson, H. Merkle and F. Baron, Materials and Corrosion 47, 439 (1996).

[18] note: our Organic Metal is the first metal which can be reduced (2 e- and 2 H+ per dimer)

[19] the structures are comparable to those described in:

  B. Wessling, Polym Eng. & Sci., 31 (16), 1200-1206 (1991).

The thermodynamic reasons for the structure formation are explained by non-equilibrium thermodynamics, cf.:

a) B. Wessling, Synth. Met. 45, 119-149 (1991).

b) B. Wessling, Zeitschrift f. Physikalische Chemie 191, 119-135 (1995).

For more recent overviews about flocculation in dispersions, their thermodynamical basis and relationship with properties see:

a) B. Wessling in: Handbook of Conducting Polymers, Skotheim, Elsenbaumer, Reynolds (eds), 467-530 (M. Dekker 1998).

b) B. Wessling in: Handbook of Organic Conductive Molecules and Polymers, Hari Singh Nalwa (ed.), vol. 3, 497-632, (Wiley 1997).

[20] cf. P. Kinlen, D. Silverman, C. Jeffreys, Synth. Met. 85, 1327-1332 (1997).

[21] reference deleted (this has been done for a number of references that either introduce references to tradenames that do not benefit the reader, or reference results that are not available to the reader, and therefore provide no useful information - the reference numbers will be resequenced prior to publication)

[22] reference deleted

[23] cf B. Wessling: "Conductive Polymer / Solvent Systems: Solutions or Dispersions?"

a) Proceedings of SEAM (Search for Electroactive Materials), workshop at Brooklyn Polytechnic Institute, N.Y., Dec. 1996,

b) Proceedings of 3rd BPS (Bayreuth Polymer & Materials Research Symposion), Bayreuth (Germany) April 1997, manuscript available at: http://www.ormecon.de/Research/soludisp.

 see especially ch. 2.5 and 3 and Fig. 13.

[24] D.A. Wroblewski, B.C. Bencewicz, K.G. Thompson, C.J. Bryan, Polym. Prepr. 35 (1), 265 (1994).

[25] a) R. Racicot, T. Brown, S.C. Yang, Synth. Met. 85, 1263 (1997).

  b) M. Fahlmann, S. Jasty, A. Epstein, Synth. Met. 85, 1323-1326 (1997).

[26] reference deleted

[27] there are several incompatibility phenomena which may occur: (a) insufficient wetting or interlayer adhesion (b) insufficient curing of the top coat due to the chemical properties of the primer, as the primer contains an acid salt and provides an acid environment, which cannot be tolerated by all top coat systems

[28] later developments might be offered as single coat systems, comprising all 3 necessary effects (ennobling, passivation, sealing) together

[29] internal test requirements: (1) potential shift comparable as described in [5b] (2) similar or better corrosion performance - in blistering, corrosion in scratch, underfilm corrosion and its progression - compared with a zinc-rich epoxy primer + epoxy top coat (as used in [11])

[30] T. Schauer, A. Joos and E. Praschak, in Investigation of the compatibility of selected top coats with primer 900226/32, test report on behalf of Zipperling Kessler & Co. (Ormecon Chemie), Forschungsinstitut f�r Pigmente und Lacke e.V., Stuttgart, (1996).

[31] Test performed by Research Institute for Pigments and Paints, Stuttgart (Germany), Title: Filiform corrosion on aluminum: Polyaniline primer offers superior protection.

[32] Cf. [9], S.1 and Fig. 5.

[33] B. Wessling, J. Posdorfer, "Corrosion prevention with an Organic Metal (Polyaniline): surface ennobling, passivation, corrosion test results", Procedings Electrochem. Soc. (Dourdan), Sept. 1997

[34] reference deleted

[35] reference deleted

[36] cf. "Industrial reference objects" / Reports Ormecon Chemie, here: "Protection of cattle barn steel plates"

[37] M. Stratmann, H. Streckel and R. Feser, Corros. Sci. 32, 467 (1991).

[38] M. Stratmann, R. Feser and A. Leng, farbe und lacke 100, 2, 93 (1994).

[39] Technical Information UBM

[40] we prefer to measure the corrosion potential and its shift during ennobling with the open circuit potential technique as described in 4.1, which is more realistic in relation to the corrosion process

[41] G. S. Popkirov, R.N. Schindler, Electrochim. Acta 39, 2025 (1994); we are now using a principally comparable, but more modern and commercially useful apparatus

[42] U. Rammelt and G. Reinhard, farbe und lacke 98, 4, 261 (1992).

[43] Technical Information Ormecon Chemie, printed versions or Internet http://www.ormecon.de/Products

[44] Ormecon Chemie, Industrial Reference object reports; printed information or http://www.ormecon.de/corrprax

[45] reference deleted


Comments on this paper

1.    Dr Frank Lux