Highlighting performances of lanthanum-intercalated stevensite and the commercially available triphosphate aluminium (TPA) as anticorrosive pigments for brass in 3 % NaCl solution: Electrochemical and morphological study

Abstract

This study aims to develop and characterize the active corrosion protection properties of lanthanum ions intercalated through cation exchange with the clay fraction of stevensite which has been previously saturated with sodium. The corrosive medium was 3% NaCl solution, while the immersion time was 24 h. To improve these properties, the pigment was incorporated into an alkyd resin coating and tested for 240 h. This additive guarantees on the one hand a progressive release of the inhibitor and on the other hand it does not cause holes or defects in the polymer film over time. The two stevensite clay samples: homoionic (Na+-ST) and intercalated (La3+-ST) were characterized by mean X-ray diffraction (XRD) technique. The basal spacing d001 increased from 14.2 Å (Na+-ST) to 16.2 Å (La3+-ST), confirming the exchange of La3+ with Na+. The kinetic release of La3+ from the exchanged clay was followed during its dispersion in the studied medium. The cation exchange capacity (CEC) was about 73 meq/100 g. Electrochemical measurements revealed effective protection against corrosion. This property was improved by incorporating the before-hand intercalated stevensite into a paint system and compared to that of the commercial corrosion inhibitor (TPA), pigmented in the same type of coating. Corrosion products and the protective layer were analyzed by using SEM-EDS and optical microscope techniques. The results showed that it was possible to replace a large volume of TPA with a very small amount of lanthanum ions.