Passivation of Metals by Coating with Polyaniline: Corrosion Potential Shift and Morphological Changes

By Bernhard Weßling

In 1985 David DeBerry^[1] reported a change in the corrosion behavior of stainless steel which has an electroactive polyaniline coating. He deposited polyaniline electrochemically from a pH 1.0 perchloric acid solution onto stainless steel and concluded that the coating appeared to be deposited over the passive metal oxide film (present on the metal surface in an acid environment). This resulted in a form of anodic protection due to the polyaniline which has redox states capable of maintaining the native passive film on the metal.

Several groups have repeated such experiments and have qualitatively confirmed that "pitting" in particular is suppressed in these coated samples compared to uncoated passive stainless steel in acid environments.^[2]

We have been interested in coating metals like iron, steel, stainless steel or other metals non-electrochemically by using polyaniline dispersions or polyaniline containing lacquers. The difference between the earlier approaches mentioned above and our approach is that we applied the polyaniline coating to the pure metal surface, with no previous (electrochemical) passivation. Initial experiments have demonstrated some improvement in corrosion protection.^[3] Progress in lacquer formulation has been made on the basis of long-term salt spray studies,^[4] but the effects observed in this practical test were not totally convincing. Here, recent successes in reproducible corrosion protection through coating various metals with polyaniline are reported.

Fig. 1. Corrosion current density - potential curves of various metals, both original and passivated.

The metallic samples used were bare iron ST 37, stainless steel V2A and copper with a specimen size of 90 x 50 x 2 mm. All metallic samples were prepared before measurement as follows: a) burr removal and straightening of all edges of the sample plates with a file; b) dry grinding with emery cloth (no. 50 and 100); c) smoothing with emery cloth (no. 100 and 200); degreasing with ethyl acetate (3 times: no. 1 and 2 with a brush, no. 3 ultrasonic). In contrast to earlier^[3,4] procedures, polyaniline was deposited from pure polyaniline dispersions.^[5] Polyaniline used in these dispersions was Versicon.^[6] The coating process was repeated at least 5 times (after complete drying of the previous coating step) and a maximum of 20 times to increase thickness. The results described below are taken from experiments with 15 coating layers on the metal surface.

Corrosion potential and corrosion current measurements were made using computerized systems arranged according to DIN 50918. In general, salt water has been used (1 M NaCl) except for the measurement of the V2A samples (0.1 M H₂SO₄ solution).^[7] The applied potential (between Pt-reference electrode and metal sample electrode) was changed in 15 steps from 0 - 5 V, thereby (indirectly) causing the change of cell potential. After that three measurements of sample potential (cell potential) and current density every 30 s were made; the highest current value was taken. The measurement was completed at 20 mA/cm2. SEM studies were performed using a CamScan CS 24 apparatus with internal semiquantitative elemental analysis capability (EDAX).^[8] Iron plates have been studied, which have been polished before polyaniline coating.

The most important result is the significant and reproducible shift of the corrosion potential, mainly together with a decrease in specific corrosion current (or decreased gradient angle of the current curve). Figure 1 shows the shifted potential for polyaniline-coated metals along with the original corrosion potential/current curve. The corrosion potential shift is at least 100-200 mV, and the best reproducible values were up to 800 mV for iron, and 300 mV for stainless steel. Also, less current is measured at the shifted potential. Especially for copper, the current curve increases only very slowly, so that about 1/3 of the original current density is reached at a potential about 2 V higher.

Fig. 2. Corrosion current density/potential for iron passivated with polyaniline with different coat thicknesses.

It is interesting that the shift in the corrosion potential can even be seen with metal samples which have been coated by polyaniline from dispersion to allow proper reaction with the metal, and in which the polyaniline coating has been removed before electrochemical measurement. Figure 2 shows the corrosion potential/current changes of iron samples which have been coated with different numbers of coating layers and the polyaniline layer removed after complete interaction of the polyaniline with the metal and before electrochemical measurement. The necessity of a sufficiently thick polyaniline layer is clear (one polyaniline layer is equivalent to about 100 nm thickness).

There is a surprising change in the optical appearance of the metal surface after removal of the polyaniline layer: whereas the original metal surface appeared quite shiny (but was not perfectly polished), the metal surface appeared light to dark grey, matt and spotted (Fig. 3).

Fig. 3. An untreated iron plate (right) exhibits rust after a short time in salt water or after one anodic corrosion current measurement. The passivated plate (left, passivation was performed only on the lower half of the plate and the polyaniline layer was removed after passivation) exhibits no rust even after a corrosion current density measurement.

SEM studies have shown that the passivation is a complex multistep process. The SEM studies were done with perfectly polished iron plates because chemical and morphological changes are then easier to detect and because perfectly polished plates cannot be passivated with polyaniline as quickly as "normal" iron. It was hoped that polished, passivated plates would show us different stages of the passivation process, because optically grey passivated areas could be detected right beside original shiny areas.

Fig. 4. SEM image showing that the first phase of passivation seems to be an etching step - grain boundaries and crystallic orientation become visible.

The first step of interaction between polyaniline and iron after coating seems to be an etching step (Fig. 4). The first few microns of iron (and "dirt") are removed. After this, the fresh iron surface is coated with an iron oxide layer (Figs. 5 and 6). Elemental analysis with an EDAX system shows that the pure iron surface contains virtually no oxygen, the freshly etched areas have some oxygen (only a very thin oxide layer, ca. 10 to 20 nm) and the passivated areas exhibit a high oxygen content. We are now planning to study the chemistry of the oxide layer using the ESCA technique.

Fig. 5. SEM image showing that the second phase seems to be the deposition of an oxide, initially extremely thin.

The experiments described above lead to the following conclusions: a) Proper coating of metals with pure polyaniline from dispersion and subsequent interaction of polyaniline with the metal surface lead to a significant shift of the corrosion potential in the direction of noble metals. b) The whole process, which can be described as "the passivation of metals by polyaniline" only leads to practically useful improvements in corrosion protection if a corrosion potential shift has occurred. c) The formation of a passive metal oxide layer, which prevents the metal from being corroded, proceeds without an electrochemical step, if, as shown, polyaniline is deposited on the metal surfaces using dispersions. d) The chemical process leading to the passivated metal surface is not yet fully understood. We suggest that the metal is stoichiometrically oxidized (after an initial etching step) with the polyaniline layer functioning as catalyst. After formation of the metal oxide layer the metal sample is passivated, but only as long as the metal oxide layer is present. Therefore, the polyaniline coating should remain on the metal surface, which allows for a continuous repair of eventual chemical degradation of the metal oxide interface, using the regenerated polyaniline layer.

Fig. 6. SEM image showing passivation is achieved with a several micrometer thick oxide layer coating the whole surface.

Initial weathering tests have shown that the corrosion velocity of metal samples coated according to this new process is reduced by at least a factor of 2 to 5. Many practical improvements in the coating process, coating formulation and barrier coating on top of the polyaniline layer, which have to fulfill a lot of practical requirements, still remain to be done. In parallel to this we will continue exploring the mechanism of formation of the passive metal oxide interface and its precise chemical nature.

References

1. D. W DeBerry. J Electrochem. Soc. 1985, 132. 1022.
2. A. MacDiarmid, personal communication at the lot. Coof Synthetic MetaIs 1986. Kyoto, Japan.
3. German Patent P 37 29 566.7, 1987, Zipperling KessIer & Co.
4. US Patent appl. 823416, 823511 and 823512 (1992) Allied-Signal, Zipperling KessIer & Co. and Americhem Inc.
5. B. Wessling, Synth. Met. 1991, 907 and 1057.
6. Allied Signal Inc., Morristown/USA, production based on a licence given by Zipperling Kessler & Co, Ahrensburg, Germany and Americhem Inc., Cuyahoga Falls, Ohio/USA.
7. Construction of the experimental setup and performance of most of the electrochemical measurements by F. Baron, Zipperling Kessler & Co, Ahrensburg. 1993.
8. Institute for Scanning Electron Microscopy, B. BöhIken, Hamburg.

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