KH2PO4-KMnO4-Ca(NO3)2-La(NO3)3 quaternary anticorrosive coating for magnesium alloys

Abstract A new environmentally friendly chemical conversion coating for magnesium alloys was developed using a quaternary mixture of KH 2 PO 4 , KMnO 4 , Ca(NO 3 ) 2 , and La(NO 3 ) 3 . The methodology was based on sequential immersion processes that avoid the use of hazardous chemicals such as chromates or hydrofluoric acid. Electrochemical tests, salt spray exposure, and SEM/EDS characterizations were conducted to evaluate corrosion resistance, coating morphology, and chemical composition. Among all the tested formulations, the KH 2 PO 4 + KMnO 4 / La(NO 3 ) 3 + Ca(NO 3 ) 2 coating exhibited the best corrosion protection, outperforming commercial treatments such as DOW7 and SurTec 650. This coating formed homogeneous, 14 μm-thick layers with excellent adhesion and endurance, even after 2000 h in a salt spray chamber following paint application. The protective effect was maintained using low chemical concentrations and was unaffected by the sequence of application. Therefore, this quaternary mixture is promising for replacing traditional coatings in industrial applications.