Evaluation of Environmentally Benign New Chemical Rust Removing Agent – Hydroxy Ethane Diphosphonic Acid (HEDPA)

--------------------------------------------------------------Abstract-----------------------------------------------------------In order to evaluate the adaptability of hydroxyethane diphosphonic acid (HEDPA) as an environmentally benign alternative rust removal agent, the present investigation was undertaken. The effectiveness of HEDPA was studied as a function of acid concentration in the range 2-20 vol. % and at different temperatures in the temperature range 23 o C -55°C. The results suggest that the acid HEDPA is very effective to remove the rust and the rate of rust removal depends upon the acid concentration and/or the solution temperature. Once an optimum value is reached, further increase in either the acid concentration and/or the solution temperature has a negative effect on the rust dissolution kinetics. In addition, it was also found that the chemical treatment with concentrated HEDPA solution (concentration range 5 -20 vol. % HEDPA) produces rust free steel samples with very rough surface topography. The chemical processing at higher temperatures (> 40 o C-55 o C), and higher HEDPA concentrations (> 5vol. %), produces strong pungent smell and an unpleasant cleaning environment. The most effective HEDPA concentration and temperature for the rust removal appears to be 2-4 vol. % and < 40 o C respectively. The activation energy for the rust dissolution process using 2-4 vol.% HEDPA was in the range 12 +2 kcal/mole. The activation energies for rust dissolution by 5, 10 and 20 vol . % HEDPA were found to be 20, 28 and 32 +2 kcal/mole respectively. The reaction product contains a mixture o f v arious higher order iron phosphates. The soils that were contaminated low concentration (< 0.5 wt. %) of with the react ion products improved the germination of seeds and the plants that were fed with solution containing the chemical reaction by products up to < 0.5 wt.% were healthy and increased in their size. Above 0.5 wt . %, the trends were reversed.