31 August 2010
A conducting polymer film acts as a self healing coating to protecting metals from corrosion, say researchers in Japan.
Steel is used to construct many different structures but is susceptible to corrosion, which can limit its practical uses and lifetime. Structures such as bridges or boats are often exposed to salt solutions that rapidly corrode them. This is a large problem and costs related to corrosion in developed countries amounts to approximately four per cent of their gross national product.
Damian Kowalski and coworkers at Hokkaido University have developed a new type of coating using an intrinsically conducting polymer (ICP), polypyrrole, which could be used as an alternative to expensive and toxic chromates currently used.
Ions are released when the coating is damaged and react with the metal to repair itself
ICPs are, in effect 'synthetic metals', capable of conducting electrical currents or ions. Kowalski doped polypyrrole with heteropolyanions (PMo12O403- and HPO42-). When the polymer coating is damaged, healing ions are released to the affected site, react with the steel forming an insoluble iron molybdate salt in the defect zone. This is different to other systems where usually a monomer is released to recreate the coating in the damaged region.
The key to the system is the control of the healing, explains Kowalski, 'in our work we have demonstrated how to control the release of these healing ions using an ion-permselectivity approach'. This stops the healing ions reacting with the metal before the coating is damaged, significantly increasing the lifetime of the coating.
Paul Braun, an expert in self-healing coatings at the University of Illinois, US, is impressed by the novel approach. Braun sees one possible advantage of Kowalski's system is its size, as it is 'much thinner than other coatings, which will be a distinct advantage for some applications'.
Kowalski is now developing the system to improve the healing response of the coating, attempting to reduce the size even further.