Corrosion of iron and steel substrates can be inhibited by coating them with any of the wide variety of compounds denoted generally as polyanilines. A polyaniline suitable for this type of application can be in either an electrically conductive salt (doped) form or an electrically nonconductive base form. Typically, polyaniline is dissolved in an organic solvent and the resulting solution is applied to a substrate by spraying, dipping, or brushing. The solvent is then allowed to evaporate leaving the substrate coated with a solid film of polyaniline, typically 1 to 200µm thick.
The figure illustrates the general molecular structures of aniline monomers and polymers. In general, the synthesis of a polyaniline involves mixing an aniline monomer with a protonic acid (e.g., hydrochloric acid), and a polymerization agent in an aqueous medium. The mixture is stirred and maintained at a temperature that depends upon the specific formulation and is typically a few degrees above or below 0 °C. The polymer forms as a blue-green precipitate, which is collected.
The polyaniline precipitate is washed with a protonic acid and/or organic solvent to obtain a polyaniline salt with a molecular structure like that shown in the second part of the figure. If desired, the polyaniline salt can be treated with an aqueous base (e.g., ammonium hydroxide) to convert the salt to a base with a molecular structure like that shown in the third part of the figure. The resulting polyaniline base is preferably in an oxidation state associated with a form of polyaniline called "emeraldine." The polyaniline base can then be recovered by such established techniques as washing with an aqueous base and organic solvents, followed by vacuum drying.
Preferably, a coating solution is made from a polyaniline in the base form because of the relatively high solubility of the base form. Solvents that can be used to make polyaniline base solutions include dimethyl sulfoxide, N-methylpyrrolidinone, and tetramethylurea. The concentrations of polyanilines in useful coating solutions lie approximately in the range between 0.5 and 5 weight percent.
Upon exposure to a corrosive, acidic environment, a polyaniline coating film in base form undergoes a conversion to a doped, electrically conductive salt form. The polyaniline film coating can also be applied to the substrate in a partially doped form; however, the partially doped polyaniline is generally more difficult to dissolve in a given solvent. Useful dopant anionic species that can be used to make partially doped polyaniline coatings include sulfonic acids and camphor sulfonic acid; solvents that can dissolve the resulting partially doped polyanilines include toluene and substituted phenols.
The tenacity of a polyaniline coating can be increased by pretreating the surface of the substrate with a suitable compound. Compounds useful for this purpose are some that simultaneously bind chemically, via covalent or ionic bonds, to the surface of the substrate and to the polyaniline. Among these compounds are phosphoric acid, polyphosphoric acid, and organic chelating agents in which atoms form multiple coordinate bonds with the substrate.
This work was done by N. Ahmad and A. MacDiarmid of the University of Pennsylvania for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
University of Pennsylvania
Center for Technology Transfer
3700 Market Street
Philadelphia, PA 19104-3147
Refer to MSC-22647, volume and number of this NASA Tech Briefs issue, and the page number.