Corrosion study of cola soft drink cans

Resumo

The study of corrosion in aluminium packaging is fundamental to ensuring the quality and safety of the product. The varnish layer present on metal packaging aims to minimize any interaction between the food or drink and the metal, as well as minimizing the corrosion process. The acidic nature of soft drinks, as well as the ions present in solution, such as phosphates, benzoates or chlorides, play a strong role in the oxidative process of aluminum. This study evaluated the corrosion of the body of aluminum cola cans. The beverage was first characterized and the results obtained were: pH =2.5; 7.5 ^oBrix; chloride concentration of 0.17g/L, acidity 0.17 %m/m; density 1.04 g/mL and concentration of reducing sugars was 11.3 %m/m.  The corrosion of aluminum in model solutions with different concentrations of citric acid and NaCl was studied via linear polarization for samples with and without varnish. The statistical analysis carried out, via factorial design 2³ with a triplicate at the center point, showed that the corrosion rate (CR) of the sample with varnish was significantly influenced by temperatures above 45 ^oC in the range of chlorides studied. On the other hand, the CR of the sample without varnish was strongly influenced by the combination of high temperatures and high chloride concentrations.  Removing the varnish resulted in a higher activation energy (AE = 67.48 kJ/mol) when removed with acetone and then polished with sandpaper compared to the sample with varnish (AE = 88.92 kJ/mol). From a kinetic point of view, this showed the protective effect of this layer and its effectiveness against corrosion. The ΔH^o and ΔG^o activation parameters of the process were positive, showing that corrosion is an endothermic process with increased disorder when the varnish is removed, respectively. It is therefore important to buy the product with the packaging in good condition to guarantee the integrity of the protective layer and its quality, since oxidation of the aluminum can lead to products that compromise the quality of the food or drink.