Host-Guest Systems in the Development of Anticorrosive Polymeric Films
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L.R.M. ESTEVAO, RS.V. NASCIMENTO DQO - Instituto de Quimica, Universidade Federal do Rio de Janeiro, CT BI A, 6' andar, Cidade UniversitAria, Rio de Janeiro, R.J. Brazil
ABSTRACT Interactive dispersion-type LDPE/EVA films with anticorrosive properties were produced using host-guest systems of thermally stable volatile corrosion inhibitors (VCI) dicyclohexylammonium p-nitrobenzoate and phosphate, adsorbed on NaY zeolites and diatomaceous earths. Carbon steel specimens were either directly exposed to the inhibiting systems studied or wrapped with the additivated films. Accelerated corrosion tests were carried out where the inhibitive properties of the systems being studied were determined by mass loss of the test coupons. The influence of the mineral support on the volatility of the corrosion inhibitors was evaluated by TG/DTG techniques. The obtained results may be regarded as a confirmation of the hypothesis that VCI volatility may be controlled by their deposition on determined supports. Also, both VCIs had their efficiency enhanced by zeolites, and the phosphate greatly benefited from the use of diatomaceous earth. INTRODUCTION It is common practice to enclose metallic objects in polymeric films for protection during transportation and storage. However, one of the major drawbacks of this procedure is the condensation of water vapor inside the contained space, giving rise to a highly corrosive environment of moisture and oxygen. In order to reduce or prevent the attack on these metals, techniques have been developed in which volatile corrosion inhibitors (VCI) are incorporated into these films, drastically reducing corrosion rates. VCIs are salts of moderately strong bases and weak volatile acids'. They have the ability to vaporize and condense on a metallic surface, fostering breakdown of contact with the aggressive medium, and thus making the metal less susceptible to corrosion. The first application of these products was probably during World War II, when it became necessary to develop new ways of protecting military equipment and replacement parts during shipment and storage. The usual procedures of coating objects with oil were found to be inappropriate, since they involved a difficult and time-consuming removal of the oil prior to usage2. This cleaning procedure was exceedingly troublesome, especially in subzero temperatures, when the oils become increasingly viscous. For the past five decades VCIs have been vastly studied and a number of them have gained commercial importance. Dicyclohexylammonium nitrite (DICHAN) and benzoate3 , diisopropylam-monium nitrite (DIPAN), cyclohexylamine carbamate (CHC)4 , among others, have been found to effectively inhibit the rusting of steel. VCIs incorporated into films are transferred to the metal surface by diffusion through the gas phase and are then either adsorbed directly onto the metal or dissolved in surface moisture films'-6. Hence, one of the major parameters responsible for the efficiency of such products is their vapor pressures. Their molecules in the vapor ph
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