Corrosion Behaviour of Aluminium in HNO3 HNO3 – HCI and HNO3 – H2SO4 Mixtures

S. A. Khalil

Chemistry Department. Faculty Of Science, Al-Fatch Vniwrslty, Tripoli, (Libya)

The corrosion behaviour of Al in 1-6M HNO3 was studied at 15-40°C and also in mixtures of HNO3- HCI and HNO3 -H2SO4 at 20°C, by the weight loss method and measurement of the open-circuit potentials. The % inhibition of thiourea and N-phenylthiourea was determined in some selected acid solutions at 20° C. The following conclusions were derived : i) The corrosion rates follow the rate expression √ =kCn . ii) The activation energies determined for the corrosion reaction in 1-8M HNO3 vary from 54 to 33 kjmol indicating control mainly by a chemical effect in 1M HNO3 and diffusion in 6M HNO3. iii) At the lower temperatures Al is passivated with 6M HNO3 particularly, whilst at the higher temperatures no passivation occurs with 4 and 6M HNO3. iv) The corrosion rates obtained in HNO3-HCl mixtures are higher by 3-4 times these obtained in HNO3-H2SO4 mixtures of the same total molarity. These rates are dependent on the HCI or H2SO4 concentration in the mixture concerned and obey the rate equation v) Thiourea inhibits the corrosion in 6M HNO3, whilst accelarates it in 6M HNO3 4- 1M HCI mixture. vi) N-phenylthiourea acts as inhibitor in 6M HNO3, as well as, in 6M HNO3 + 1M HCI mixture; a 87.5 % inhibition was obtained in the former case, whilst 77.9 % inhibition obtained in the latter case. N-phenylthiourea is more efficient as inhibitor than thiourea. Its inhibitive effect is evidently due to surface adsorption and it is independent of the mechanism of metal corrosion.

Aluminium; HNO3; H2SO4