In 1985 David DeBerry [1] found that polyaniline, electro-deposited on passivated steel in strong acid environment, was enhancing the corrosion protection of this metal. No further research was done to find out what mechanism was reducing the corrosion rate and - even more important - if polyaniline deposited by other than electrochemical means on non-prepassivated metal surfaces would lead to any kind of corrosion protection, until our laboratory had developed first polyaniline containing paints and published first corrosion inhibition by such paints in 1988 [2]. It took another 5 years to come to reproducible results first by using pure polyaniline coatings from organic dispersions [3], which were deposited on untreated metals like normal steel, stainless steel and copper. We found that the corrosion potential was shifted to the more noble region and that the corrosion current was significally reduced or even completely eliminated at comparable corrosion potentials (Fig. 1).The SEM revealed that an oxide
layer was formed between the PAni coating and the metal surface (Fig. 2). Our research was continued together with Ron Elsenbaumer and Wei-Kang Lu [4], where we found the corrosion rate being reduced by several orders of magnitude and the oxide layer being composed mainly by Fe2O3 above a very thin Fe3O4 layer (Fig. 3).
Fig. 1: Corrosion current density-potential curves of various metals, both original and passivated
Fig. 2: SEM image showing that the second phase being to be the d2K-EPion of an oxide, initially extremely thin
Fig. 3: XPS analysis of passive iron oxide layers form in the presense of doped polyaniline
We suspected earlier [3] that the passivation reaction leading to the oxide could occur via a catalytic redox reaction of polyaniline (PAni). Our group later succeeded in developing dispersed PAni containing paints [5] which are capable of inducing the same passivation effect, but moreover industrially applicable and effective as corrosion prevention primer [7]. The work [6] was intended to find out what kind of reaction sequence is responsible for the passivation (oxide layer formation) and the corrosion protection induced by the PAni layer.
Our experiments are showing that the contact of polyaniline with iron in the presence of water changes polyaniline from the green to the yellow form, which can be understood as the reduction of polyaniline from the emeraldine salt to the leuco-emeraldine. This requires an equivalent oxidation of iron (to Fe2+). The back reaction is leading back to the normal polyaniline, although with some blueish component (emeraldine base or nigraniline) in the neutral environment. This back reaction is performed with the help of oxygen. In the degassed water solutions we also observe the reoxidation of the leuco-emeraldine into the emeraldine salt, but very interestingly we can see a gas formation at the surface of the film which we suppose is hydrogen.
Further observations using polyaniline containing paints have shown that the occurence of hydroxyl ions is very probable. We therefore come to the following reaction scheme describing the whole passivation reaction sequence of polyaniline on the iron surface, showing that polyaniline is acting as a redox catalyst and as a noble metal with respect to iron (Fig. 4).
Fig. 4: reaction scheme:
Fig. 5.1 and 5.2. are providing additional evidence for this mechanism, plus a first hint towards an evaluation of the more detail progress of the reaction: it seems, that an iron - ( Fe2+ ?) - PAni complex is formed, which is the reaction species for the next steps [8]. We assume that such a reaction occurs also with other metals like copper, aluminium or zinc which are all ennobled by PAni.
Fig. 5: MS spectra
Fig. 5.1: polyaniline with its various dimer components, see [8]
Fig. 5.2: the iron peak at m/e=54 [8]
Fig. 6.1 and 6.2 are showing additional XPS evidence for only Fe2O3 being produced during passivation [10].
Fig. 6.1-6.2: XPS spectra of iron [10]
Fig 6.1: untreated with Fe and FeOOH signal
Fig. 6.2: with pure Fe2O3 signal [8]
The main question for practical applications is, if this new, proven and convincing passivation procedure does also provide significant improvements in internationally accepted corrosion tests and later in practical continuous use. For the tests reported below, PAni containing primers [9] have been applied with a thickness of 20 and coated as described.
Especially the behaviour of the coating system with the passivation primer in scratches and other inquiries is of considerable interest. We had observed very early, that crossed scratches do not corrode under various corrosion conditions including laboratory tests and atmospheric corrosion, if the scratches are up to 2 mm wide.
Actual research [10] shows, that the passive oxide layer formation is observable for at least 400 nm towards the scratch center from the primer edge (Fig. 7). This might explain the extraordinary positive results in the corrosion tests described below.
Fig. 7: schematic view of passivation influence of pani in a scratch
3.1 Tests at DECHEMA Institute
The polyaniline primer CORRPASSIV (with Wilckens 2K-EP epoxy resin top coat) on sandblasted
St 37 steel was compared with specimens coated with only an epoxy top coat or with a zinc-rich primer (Wilckens Epoxid Zinkstaubfarbe EPB 7601) and the same epoxy resin top coat.
The test investigated crevice and pitting corrosion in accordance with ASTM G-48 in FeCl3 solution and contact corrosion with copper (with short-circuit – in accordance with DIN 50 919 – and without short-circuit). For crevice and pitting corrosion testing was performed on specimens with intact coatings and specimens with deliberately damaged coatings.
The results were as follows:
Of the steel specimens with damaged coatings, only those treated with the polyaniline primer CORRPASSIV passed the test. With intact coatings the specimens with zinc-rich primer and with CORRPASSIV primer passed. Corrosion was nevertheless found to be considerably less marked with the PAni primer, Fig. 8. Fig. 9: crevice corrosion of specimen with zinc-rich epoxy primer).
Fig. 8: corrosion was found to be considerably less marked with PAni primer
Fig. 9: crevice corrosion of specimen with zinc rich epoxy primer
The contact corrosion tests were performed in each case with a defined cruciform injury to the coating and an insulating bolted joint to copper.
In the test without short-circuit only the steel specimen treated with polyaniline primer displayed no corrosion attack at the damage site and no undermining of the coating. The comparison specimens displayed more or less marked rust formation at the damage site.
Fig. 10
In the test in accordance with DIN 50 919 with short-circuit by an external electrical connection to the contact corrosion element the cell current is measured directly.
All specimens exhibited formation of corrosion products in the crevice, but in the case of CORRPASSIV this was only slight (Fig. 10).
The specimen coated with polyaniline primer recorded a current of only approx. 1 mA, whereas the comparison specimens with and without zinc-rich primer showed much higher readings (around 2.5 - 3 mA).
3.2 Salt spray test
The (physically wetting) polyaniline primer CORRPASSIV was compared with a fast-drying two-component epoxy resin primer (Messrs. Finalin, Type 144) and a metal-reactive two-component primer with adhesion-promoting properties (Type 918). All three primers were coated with a two-component polyurethane paint (B412). CORRPASSIV, both with and without top coat, exhibited no rust formation beneath the primer.
The slight undermining receded after the test, and even within the cross and scratch injuries it was virtually impossible to detect any corrosion.
The salt spray test was performed in accordance with DIN 50021 with an exposure time of 288 hours.
The primer was tested on one plate without top coat (with a cruciform injury site), and on another with top coat (with scratch damage).
Both coating systems were also assessed after the test for adhesion, using the cross-hatch test (Fig. 11).
The cross-hatch test made it clear that the polyaniline primer displayed the best adhesion, and also that here the corrosion was confined solely to the damaged cross-hatch area and the undermining effect was by far the smallest.
The epoxy resin primers used for corrosion protection proved distinctly brittle and exhibited considerable rust formation beneath the layers peeling off (Fig. 12).
On the comparison specimens with clearly poorer adhesion properties, corrosion products were even found beneath the intact areas of primer, and the coatings displayed either considerable blistering or marked undermining in the region of the cross (Fig. 13). These results were confirmed in the specimens with polyurethane top coats. Here too CORRPASSIV displayed outstanding results thanks to its excellent adhesion to substrate and top coat.
With all three coatings corrosion was visible at the scratch. Here CORRPASSIV displayed considerably less "bleeding" of corrosion products than the comparison specimens.
The combination of polyaniline primer / polyurethane top coat showed only a very small measure of undermining (0 - 0.5 mm) (Fig. 14).
Fig. 11: CORRPASSIV without top coat: no rust under cross hatch cut
Fig. 12: standard system: corrosion attack even under two-component primer
Fig. 13: metal-reactive two-component primer with top coat: cracking and considerable undermining by rust
Fig. 14: CORRPASSIV with top coat: hardly any undermining, no flaking of paint system at cross-hatch
The comparison specimens exhibited varying degrees of marked undermining of the coating (up to 4 mm), sometimes with blistering as well.
3.3 Tests at FPL
* Institute3.3.1 Polyanilin primer against filiform corrosion [11]
The measurements performed are based on the dry and wet adhesion tests (stamp method) and the filiform HCl test (DIN 65 472). The tests were made on sandblasted AlMg1 alloy panels, coated with CORRPASSIV primer .
Following top coats were used:
1. Two component epoxy top coat, amine hardened - Topcoat 1 (2K-EP)
2. Two component epoxy top coat, amid hardened - Topcoat 2
3. Two component polyurethane top coat - Topcoat 3
The drying and curing state of the coatings were assessed by measuring the pendulum hardness using the König method (DIN 53 157, 1/87). The results of the Pendulum hardness measurements are shown in the Fig.15. It is the immediately evident that the pendulum hardness of the 2K-EP coating is much greater than that of the other coatings.
The results of the dry and wet adhesion measurement are summarised in Fig.16. The dry adhesion values at time 0 indicate good adhesion of all three coating systems investigated, the highest figure being achieved by 2-comp.epoxy topcoat amine hardened/CORRPASSIV. On exposure to water the adhesion of all coating systems falls of. The 2K-EP/CORRPASSIV coating was generally found to have the most stable wet adhesion.
Fig. 15: König pendulum hardness for the coatings investigated
Fig. 16: Dry and wet adhesion of coatings investigated
The filiform corrosion test was investigated using the HCl method after DIN 65 472. The test panels were scratched with a Siggens scratching tool and inoculated with hydrochloric acid fumes for one hour. After removal from the chamber the panels were left to stand in room air for a further hour. The panels were then stored for 6 weeks in a climate chamber at 40°C and 82% relative humidity.
The filiform corrosion was described in five ratings, which indicate a specific ratio of the the surface affected by corrosion to the length of the scratch. Rating I represents the most efficient corrosion control (surface value 0,0 - 0,5 mm²/cm) and rating V the poorest corrosion control (surface value > 25 mm²/cm).
Table 1: Results of the filiform test using the HCl method
|
Coating system |
Rating |
|
CORRPASSIV / Topcoat 1 |
I |
|
CORRPASSIV / Topcoat 2 |
III |
|
CORRPASSIV / Topcoat 3 |
IV |
It is clear that the CORRPASSIV/2K-EP coating possesses very good protective properties against filiform corrosion (Fig.17). This system was the first system ever found to meet the filiform corrosion test. Of the other coatings, 2-comp.epoxy topcoat amine hardened displays markedly better protection efficiency.
Fig.17: graphic reproduction of panel surfaces after the HCl test
3.3.2. Investigate the compatibility of selected top coats with CORRPASSIV [12]
The aim of the study was to investigate the compatibility between CORRPASSIV primer and top coats based on various binders. Properties being measured were pendulum hardness, dry and wet adhesion. Selected top coats have also been subjected to the salt spray test and outdoor weathering conditions.
The tests were made on sandblasted body panels from Mercedes Benz. Following laquer systems were used on CORRPASSIV primer:
1. 2-comp.epoxy top coat - amine hardened
2. 2-comp.epoxy top coat - amine hardened
3. 2-comp.polyurethane top coat - A
4. 2-comp.polyurethane top coat - B
5. 2-comp.acrylic top coat - 2K-AY
6. 1-comp.acryl top coat - 1K-AY
7. Intermediate layer - 2K-EP amine hardened
Top coat - 2-comp.polyurethane A
8. Intermediate layer - 2K-EP amine hardened
Top coat - 2-comp.polyurethane B
The drying and curing state of the coatings were assessed by measuring the pedulum hardness using the König method (DIN 53 157, 1/87). The results are summarised for the two layer systems in Fig.18.1 and for the three layer systems in 18.2. It is possible to rank the top coats in order of declining pendulum hardness:
2K-EP > EG5 > 1K-AY > 2K-AY > 2-comp.polyurethane A > 2-comp.epoxy top coat - amine hardened
Looking at the individual binders, it is not possible to establish a general correlation yet between the type of binder and the pendulum hardness.
The results of the dry and wet adhesion measurements are summarised in Fig. 19.1 to Fig. 19.4. As far as dry adhesion is concerned (at the time 0), the coating systems investigated can be ranked as follows:
2-comp.polyurethane topcoat A > 2K-AY > EG5 > 2K-EP/EG5 > 2-comp.epoxy top coat - amine hardened > 2K-EP/ 2-comp.polyurethane topcoat A > 2K-EP > 1K-AY
Exposing the coating systems to stress by immersion in water and tracing the changes in adhesion yields additional information about the resistance of the systems investigated, and it is also possible to say something about the compatibility of the polyaniline primer with the individual top coats.
Fig. 19.1 shows the pull-off figures for 2-comp.polyurethane topcoat A and 2K-EP/2-comp.polyurethane topcoat A coating systems on CORRPASSIV. The results indicate generally good compatibility of the 2-comp.polyurethane topcoat A/CORRPASSIV top coat/primer system. The same system, but with the addition of an intermediate layer 2K-EP, also displayed no evidence for adhesive failure; in the course of the measurements, however, there were signs of a weakening of the inter-layer adhesion between the 2K-EP intermediate layer and the 2-comp.polyurethane topcoat A top coat.
Fig.19.2 summaries the results for EG5 and 2K-EP/EG5 coating system. These systems were found to exhibit even better compatibility between CORRPASSIV primer and EG5 top coat: no adhesive failures were detected, and the detachement occurred primarily in the top coat zone. The adhesion between 2K-EP and EG5 was also found to be good.
Fig. 18.1: König pendulum hardness for the two-layer systems investigated
Fig. 18.2: König pendulum hardness for the three-layer systems investigated
Fig. 19.1: dry and wet adhesion of
2-comp.polyurethane topcoat A and 2K-EP/2-comp.polyurethane
topcoat A coating systems;
KD = cohesive failure in top coat, KG = cohesive failure in primer,
ZB = interlayer failure in top coat system
Fig. 19.2: dry and wet adhesion of EG5 and 2K-EP/EG5 coating systems;
KD = cohesion failure in top coat, KG = cohesion failure in primer
Fig. 19.3 shows the pull-off strength measurements for the 2K-EP and 2-comp.epoxy topcoat amine hardened coating systems. Both systems were found to display good compatibility: there were no adhesive failures, and the fracture site was mainly in the top coat zone.
Fig. 19.4 shows the results for the 2K-AY and 1K-AY systems. In the case of 2K-AY, despite the high dry adhesion value, there was found to be marked reduction in the wet adhesion of the Corrpassiv primer. In the final phase of the study it was almost entirely adhesive failure that was recorded, though the absolut figures for pull-off strength were still above the limits of full delamination. The low pull-off strength values show that the CORRPASSIV primer and the 1K-AY are not compatible and that the weak point in this system is in the interfacial zone between the two layers. Water diffuses into this region, resulting in premature cohesive failure. Accelerated blistering can be expected in this system.
The salt spray test was performed on scratched panels for three coating systems - 2K-EP, 2K-AY and 1K-AY. Under-film corrosion, blistering and degree of rusting were assessed. The results of this test are summarised in Table 2.
Table 2:
Results of salt spray test in accordance with DIN 50 021-SS|
Duration |
CORRPASSIV/ 2K-EP |
CORRPASSIV/ 2K-AY |
CORRPASSIV/ 1K-AY |
||||||||
|
[h] |
U* |
B** |
R*** |
U |
B |
R |
U |
B |
R |
||
|
24 |
k**** |
k |
k |
k |
k |
k |
k |
k |
k |
||
|
72 |
k |
k |
k |
k |
k |
k |
k |
m2/g4 |
k |
||
|
96 |
k |
k |
k |
k |
k |
k |
k |
m2/g4 |
k |
||
|
168 |
k |
k |
k |
k |
k |
k |
k |
m2/g5 |
k |
||
|
241 |
k |
k |
k |
k |
k |
k |
k |
m2/g5 |
k |
||
|
337 |
k |
k |
k |
k |
m2/g5 |
k |
k |
m2/g5 |
k |
||
|
505 |
k |
k |
k |
k |
m2/g5 |
k |
k |
m2/g5 |
k |
||
|
607 |
k |
k |
k |
k |
m2/g5 |
k |
k |
m2/g5 |
k |
||
|
770 |
k |
k |
k |
k |
m2/g5 |
k |
k |
m2/g5 |
k |
||
|
1035 |
k |
k |
k |
k |
m2/g5 |
k |
k |
m2/g5 |
k |
||
*U - under-film corrosion (DIN 53 167), **B - blistering (DIN 53 209),
***R - degree of rusting (DIN 53 210), k**** - none
The test showed that all three coating systems provide highly efficient corrosion protection and complete suppression of under-film corrosion over a period of more than 1000 h; especialy with top coat "2K-EP" no change whatever was found even after 1000 h. This must be regarded as an indication of the good corrosion control properties of the CORRPASSIV primer. In the case of 1K-AY, however, there was a marked tendency to form blisters, and this made itself felt after only 72 h. The same applied to 2K-AY top coat, though here the blistering was not observed until after 337h. The results of the salt spray test indicate a general need for individual investigation of the compatibility of the top coat with the CORRPASSIV primer.
Fig. 19.3: dry and wet adhesion of 2K-EP and 2-comp.epoxy topcoat amine hardened coating systems; KD = cohesive failure in top coat, KG = cohesive failure in primer
Fig. 19.4: dry and wet adhesion of 2K-AY and 1K-AY systems
KD = cohesive failure in top coat, KG = cohesive failure in primer
AB = adhesive failure
Two panels each of the same top coat systems as for the salt spray test were exposed to local climate and other environmental influences at Hook of Holland on 03.05.96. A first inspection of the panels will take place in October 1996.
4. Conclusions
The new organic metal polyaniline, almost a noble metal, ennobles steel and other metal surfaces while shifting their surface potential. It also passivates the conventionel metal by forming a metal oxide layer of up to 1 thickness.
The passivation is "self healing" as can be seen from the weak corrosion in scratches and other inquiries of the coating.
References:
[1] D. W. DeBerry, J. Electrochem. Soc., 132, 1022, 1985.
[2] German Patent P 37 29 566.7, Zipperling Kessler & Co., 1987.
[3] B. Wessling, Adv. Mater., 6, No 3, 1994.
F. Baron, graduation work (Zipperling).
[4] B. Wessling, W.-K. Lu, R. L. Elsenbaumer, ISCM 1994, Seoul (Proc. Synth. Met., 71, 2163-2166, 1995).
[5] B. Wessling, "Polyaniline an Organic Metal", information brochure of Zipperling Kessler & Co., 1995.
[6] B. Wessling, together with S. Schröder, S. Gleeson, H. Merkle, F. Baron, Materials and Corrosion, 47, 439, 1996.
[7] U. Ruhrberg, DECHEMA, test report: "Tests on ‘passivated’ steel specimens", for Zipperling Kessler & Co., 1994.
[8] V. Sauerland, R. Schindler, Synth. Met., in press.
[9] see technical informations "CORRPASSIV", Zipperling Kessler & Co., cf. http://www.Ormecon.de/.
[10] together with H.Ladebusch, Th. Strunskus, Technische Fakultät Universität Kiel, ongoing research, unpublished results.
[11] T. Schauer, A. Joos, E. Praschak, Forschungsinstitut für Pigmente und Lacke e.V., Stuttgart, "Supplementary investigation of protection afforded by selected coating systems based on a polyaniline primer against filiform corrosion of aluminium alloys", test report for Zipperling Kessler & Co., 1996.
[12] T. Schauer, A. Joos, E. Praschak, Forschungsinstitut für Pigmente und Lacke e.V., Stuttgart, "Investigation of the compatibility of selected top coats with primer 900226/32", test report for Zipperling Kessler & Co., 1996.
