Ormecon Chemie, D-22949 Ammersbek (Germany)
Comment to
M.M. Attar, J.D. Scantlebury
Polyaniline as a possible inhibitor for the corrosion of mild steel
J. Corr. Sci. Eng., Vol 1, paper 8
The a.m. publication claims to introduce into the new field of corrosion protection by conductive polymers (Organic Metals), and to contribute experimental results for verification or disproval of previous publications.
The text does not meet either of the two goals.
As the one having opened this field (corrosion protection with Organic Metals like polyaniline, by providing a dispersion in a paint matrix, and applying it on the metal surface to be protected in a non-electrochemical way) about 10 years ago, I feel somewhat authorized to comment.
There was earlier work by Mengoli et al.[1] and DeBerry[2], who had coated passive stainless steel with polyaniline under passivating conditions using electrochemical techniques. They both concluded, that polyaniline was capable of maintaining an already achieved passive state of the steel.
In contrast to that, we tried to find out, if polyaniline as such - dispersed in paints or other dispersion media - was capable of providing corrosion protection, in the beginning while having no idea if there was any chance at all, and if yes, by which mechanism polyaniline could work. We found first motivating results in 1987[3], however with no really convincing performance.
Further work together with Allied Signal[4] supported our idea, but again with no results convincing any corrosion or paint engineer. Therefore, our partners did not continue their work.
We continued our research and found 1993, that under certain conditions, when having polyaniline in direct contact with the metal to be protected (iron, steel, copper, aluminum, magnesium and others), the corrosion potential was significantly shifted towards more positive values, and a relatively thick passivating iron oxide layer was formed.[5] We interpreted these phenomena as being an "ennobling" and a "passivation". Internal corrosion performance tests showed a drastic anti-corrosion effect, out-performing even zinc-rich epoxy coating systems. These results were confirmed by independent studies[6,7,8]. An introduction can be found under http://www.ormecon.de/Products/PAni/passivation.en in our web server.
Between our discovery and today, this idea has attracted some attention also by other groups; they are in principle confirming the potential anti-corrosion power of polyaniline, however, they have not been able yet to repeat our results with a comparable performance.[9] Actually, there is only one source for commercially useable coating systems providing excellent corrosion prevention.[10] These coating systems have been widely tested and have found some first practical acceptance.[11]
Since 1993, we have continued our research parallel to practical product development. We found out about the composition of the passivating oxide layer[12] - a -Fe2O3 in up to 1 µm thickness - and the reaction mechanism[13] (a catalytic mechanism involving reduction of the Organic Metal and re-oxidation of it to the metallic form by oxygen, cf. http://www.ormecon.de/Research/abstract/reactionnew.gif).
We also elaborated an integrated method, comprised by 4 experiments and measurements, for supporting our practical paint development work with quantitative and reproducible data which can be correlated with real-world corrosion processes.[14]
Considering this history and the status of our work, including confirmation from independent side, the text by Attar and Scantlebury is not referencing correctly the state of the science and art, and their summary of what was claimed in the various publications is not correct.
The experimental set-up is not appropriate for answering the title question. Even if a water "solution" of inhibitive pigments for paints is "traditionally been used as the starting point for the investigation", it is not correct to just adapt such techniques for any other new principle, especially when considering the available information on the corrosion mechanism of polyaniline[14] and on its physical and chemical properties (solubility of the dopant in water).
First, it is mentioned that a "solution" of polyaniline in water was made. This has not been verified in the experimental description. Polyaniline is not soluble at all, and especially not in water.[15] The authors did even not achieve a fine dispersion, as can be concluded from their experiment ("by shaking PAni powder in water"). The description given in the text leads us to conclude, that only a "suspension" of relatively big polyaniline particles has been achieved. However, the dopant (HCl) is soluble in water, so that a partial neutralisation takes place when adding protonated PAni to water.
A suspension of polyaniline in water is highly acidic (between pH 1 and 2, depending on concentration; the authors missed to report their pH). Polyaniline can only positively interact with the metal to be protected if a fine and stable dispersion is used. Dispersions in water, however, always seem to promote corrosion instead of preventing it (this is based on the reaction mechanism, as described in [13]). The test performed by the authors was nothing else than looking, if HCl (the dopant for polyaniline used in this study) or NaCl (in case of the neutral PAni) is able to corrode metal - they do, and both results do not surprise us.
It is unappropriate to write (as the authors do) "... this work does not substantiate the previous claims[16] of PAni being a corrosion inhibitor ...", as their experiments did not test the previous claims: we never claimed, that a polyaniline suspension in water would act as a corrosion inhibitor, and we did not claim, that it acts like an "inhibitor", but in a new way, via "ennobling" and "passivation", with very specific mechanisms based on its property as being both a metal and a catalyst.
It is somewhat funny to read "... it might be, that when dispersed in a paint binder having different solubility characteristics a different behaviour might be seen." And: "The use of an organic polymeric dispersion medium is an obvious possibility". Was it too hard for the authors to check literature (or to follow the discussion in the "corrosion list") and to find out that what "might be" was in fact realized 5 years ago, and their proposal of using an organic polymeric binder as "an obvious (??) possibility (??)" was worked on by us since 10 years? What was "obvious" 10 years ago? Nothing, as only electrochemically applied PAni was believed to maintain an already achieved passive state.
Why do Attan and Scantlebury only assign the use of dispersed PAni in a paint as a possibility? It is reality, although not yet in broad use.
We suggest to start a more serious evaluation using our coating systems. We do not recommend that corrosion labs make their own PAni dispersion paints, as the dispersion of polyaniline is by 2 orders of magnitude more complex than any other material being used in paints. Its measured surface tension as a raw material (as delivered) is about 70 mN/m, its effective surface tension is between 150 and 1500 mN/m.
Furthermore, the use of HCl as the protonating dopant is not recommended (instability). Our polyaniline, ORMECON®, is highly stable and dispersable. We provide samples coated with optimized primers and compatible top coats for evaluation.
