Steel In Sulphuric Acid Solution Research Articles

Adsorption of thiophene derivatives on steel in sulphuric acid solutions

Measurements of the corrosion rate of mild steel in 1N H 2SO 4 solution at 25°C with and without addition of thiophene mono-derivatives (1.10 −5–2.10 −2 mole/1.) were performed. Adsorption of these compounds was elucidated. Results show that thiophene and its derivatives are adsorbed on the steel surface according to Parsons and Hill de Boer isotherms. This suggests that the adsorptive properties of the compounds under consideration approach to a physical type. From th eadsorption isotherms some thermodynamic data for the adsorption process (ΔG° d8, and f) are calculated and discussed.

Synergistic action of n-dodecylamine withn-capric acid on the corrosion of steel in sulphuric acid solution

Studies were performed on the effect of mixtures of n-capric acid (Ac) with n-dodecylamine(Am) on the corrosion rate of mild steel in IN H 2SO 4 at 25°C. The concentration of Ac was changed within the range from 3·10 −5 to 1·5 10 −4 mole/1, and that of Am from 1·10 −5 to 2·10 −4 mole/1. The maximum increase in the inhibitive efficiency of Ac by additions of Am was observed at concentrations close to that of the saturated solution of Ac in 1N H 2SO 4. It was found that Ac and Am mixtures were adsorbed at the metal surface following the adsorption isotherm of Frumkin. This may be written: θ s 1 − θ s exp ⁡ f m i x θ s = K c A m c A c where θ 5—coverage degree by both substances, ƒ mix—parameter connected with changes of the adsorption energy with coverage, K—equilibrium constant of the adsorption reaction, c Am, c Ac—concentrations of Am and Ac respectively. The standard free energy of adsorption of the mixture of Ac with Am is approximately equal tothe sum of the standard free energies of adsorption of these compounds taken separately, and fin,: is the arithmetical mean of the respective single values. This suggests that the ratio of the number of Ac to Am molecules in the adsorbed layer is 1 : 1. Most probably, micellae having such a composition (AcAm) already exist within the sulphuric acid solution.

Anodic protection of stainless steel by galvanic coupling with platinum

Stainless steel in sulphuric acid solutions at various concentrations and temperatures is passivated and does not undergo corrosion if coupled galvanically to a sheet of platinum when its surface area is in a definite ratio with the surface area of the stainless steel. The ratio is 1 with 38 per cent aqueous sulphuric acid at room temperature, but 100 with 52% H2SO4 solution at 75°C.The values of the ratio necessary for the passivation of stainless steel have been determined and related to the results of the corresponding polarization curves for the anodic dissolution of stainless steel and for the cathodic reduction of oxygen and of hydrogen ion in solution. The results obtained by galvanic coupling of stainless steel with platinum, palladium and gold show that platinum has a dual effect on toe passivation of stainless steel. At first stainless steel goes into the zone of unstable passivity, since the overvoltage of platinum for discharge of hydrogen ion is low; in the next stage stainless steel goes into the zone of stable passivity, since oxygen in solution is reduced more easily on platinum than on stainless steel.The efficiency of platinum is due to this dual activity, not shared by palladium or gold.

Potentiostatic response of passive metals to the rate of temperature change

The effect of temperature on the steady anodic current of passive stainless steel in sulphuric acid solution containing no oxidizing agent was investigated in the temperature region below 100°C by raising the temperature in three different ways. The temperature dependence of the anodic current appeared to alter with the rate of temperature rise. By applying a rapid temperature rise, a straight line extending over the wide range of temperature was obtained on the Arrhenius plot, giving an apparent activation energy of about 20 Kcal/mol. Arrhenius lines were obtained for iron, nickel, chromium, titanium, zirconium and molybdenum in the passive region by means of the steady temperature rise and compared with that of stainless steel. The deviation from Arrhenius line with the rate of temperature rise was more marked for chromium as well as stainless steel than for other metals. This result is briefly discussed in terms of alteration of surface condition.