Inhibitory Effects of Calcium Sulphate Concentrations on Cookware Aluminium Corrosion in Highly Acidic and Alkaline Media at Elevated Temperatures

Aluminium corrosion can deleteriously reduce the structural strength, integrity, safety, and serviceability of the metal. In addition, it is an inevitable issue of health concern in food environments because the substances resulting from the corrosion can have toxicological, mutagenic, and carcinogenic effects in the human body, potentially leading to many chronic ailments. This paper presents an inquiry into the inhibitory effects of calcium sulphate as a cheap, worldwide-available, and environmentally friendly food substance on corrosion of cookware aluminium in highly acidic and alkaline media. Coupons were produced from the aluminium 6061 alloy as a common cookware aluminium type and immersed for 1-48 hours in sequentially treated acidic media of 2.5, 2.7, and 3 pH and alkaline media of 10.5, 10.7, and 11 pH with anhydrite calcium sulphate concentrations of 0, 20, 25, 30, and 35% of the media at 30, 50, and 70°C temperature conditions. Corrosion penetration rates (CPRs), inhibition efficiencies, and micro-topographic changes of the coupons were evaluated relative to those of the coupons immersed in the untreated media. Results show a CPR range of 0.121-1.579 in the acidic and 0.101-1.105 mm/yr in the alkaline media. The CPR increased with increasing exposure temperature and media acidity and alkalinity levels but decreased with the immersion durations and the treatment concentrations to more or less constant values.  The highest corrosion inhibition efficiency of 74.34% occurred in the acidic media after a 3-hour immersion in the 2.7-pH medium at 30°C, with 35% treatment of the medium, while the highest value in the alkaline media was 72.95% after a 3-hour immersion in the 10.5-pH medium at 30°C, with 30% treatment of the medium. SEM micro-topographic analyses indicate that calcium sulphate facilitates corrosion inhibition of the cookware coupons in the media by forming coatings on them. Thermodynamic and kinetic analysis at the temperature conditions of the media shows that the inhibitory process of the sulphate is essentially by physisorption and is well fitted to the Langmuir adsorption isotherm model, as indicated by the Gibbs free energy, enthalpy, and entropy of the sulphate adsorption on the cookware coupons with average values of -23.345 kJ, -30.13 kJ, and 0.195 kJ/K, respectively. The study indicates that anhydrite calcium concentrations of greater than 35% can satisfactorily inhibit corrosion of cookware aluminium under the media conditions.