Abstract
In order to evaluate the adaptability of hydroxyethane diphosphonic acid (HEDPA) as an environmentally benign alternative rust removal agent, a systematic investigation is being carried out. The effectiveness of HEDPA was thoroughly investigated as a function of acid concentration in the range 2-100 vol.% and at different temperatures in the temperature range 27 - 60°C.
The results suggest that the acid HEDPA is very effective in the rust removal process. The rate of rust removal by HEDPA is strongly dependent on the acid concentration and the solution temperature. Once an optimum threshold is reached an increase in either the acid concentration and/or temperature has a negative effect on the reaction kinetics. While the rust was completely removed by the 2 vol.% HEDPA within 3 hours at 27°C, the rust removal was completed within 30 minutes of treatment at 60°C. Similarly, while, the complete rust removal by 2 vol.% HEDPA was noticed after 3 hours, 10 vol.% HEDPA required chemical treatment for 0.75 hours at 27°C. However, chemical treatment with as received 100 vol.% HEDPA did not dissolve even 20% of the rust after 6 hours and the rust removal at 60°C was <2 %.
Chemical treatment of rusted steel samples with concentrated HEDPA solution (concentration range 5-100 vol.% HEDPA) has produced rust free steel samples with very rough surface topography. In addition, the chemical processing at higher temperatures (40 - 60°C) and higher HEDPA concentrations (5-100 vol.%) produced strong pungent smell and unpleasant cleaning environment. The most effective HEDPA concentration and temperature for the rust removal appears to be 2 - 4 vol.% and < 40°C respectively.
The activation energy for the rust dissolution process also appears to increase with an increase in the acid concentration. However, it appears that the increase is not very significant for the concentrations in the range 2-4 vol% HEDPA (2 vol.% - activation energy 11 ± 1 kcal/mole; 3 vol.% - activation energy 12± 3 kcal/mole and 4 vol% - activation energy 14 ± 2 kcal/mole). The activation energies for rust dissolution by 5, 10 and 20 vol.% HEDPA were found to be 20, 28 and 32 kcal/mole respectively. Above, 20 vol% HEDPA concentrations, a semi-quantitative determined value for the activation energy is 2 ± 2 kcal/mole. Prolonged treatment of samples with HEDPA allows the deposition of the reaction products onto the cleaned sample surface. The reaction product contains a mixture of various higher order iron phosphates.
