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The aim of this paper is to investigate the synergism effect between lanthanum salt (La(NO₃)₃) and benzotriazole (BTAH) on the corrosion inhibition of commercial brass and to further study the inhibition mechanism.

Potentiodynamic polarization curves were carried out on bare brass and brass treated with additions of optimum concentration of BTAH, La salt and La salt+BTAH to the basal deposition solutions in 3.5 wt. percent sodium chloride solution. The inhibition mechanism of the composite conversion coatings on brass obtained in optimal deposition techniques were investigated by electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), X‐ray diffraction (XRD), and FT‐IR reflection spectra.

A “Critical La(NO₃)₃ content” and “Critical BTAH content” were both observed, at which the coatings prepared performs the highest protectiveness, and La(NO₃)₃ and BTAH had an excellent synergism effect on the corrosion inhibition of brass. The corrosion mechanisms for uncoated and coated brass are different. A remarkable enhancement of the brass's corrosion protection was obtained by the formation of composite conversion coatings consisted of Cu(I)BTA and La coordinate thing except for Cu₂O and La₂O₃, which acted as a barrier avoiding the release of metal ions and inhibited the diffusion of the oxygen.

The results from this paper showed that La(NO₃)₃ and BTAH could be used together to prepare the novel composited conversion coatings on commercial brass for the good corrosion inhibition.