Toxic chemicals, such as chrome, are commonly used in the application of conversion coatings to various metal surfaces. In situ phosphatizing coatings (ISPCs) are an innovative approach for eliminating the requirement of a conversion coating which ends the need of toxic materials used in a multi-step coating practice. An ISPC is formulated by predispersing an in situ phosphatizing reagent (ISPR) into a paint system. In this study, an ISPR, an arylphosphonic acid, is used in a polyester–melamine paint to react in situ with the metal surface and to provide the acidic catalyst needed, while thermally curing the paint. A second polyester–melamine paint system is used as the control that uses the standard catalyst para-toluenesulfonic acid ( p-TSA) to catalyze the cross-linking reaction in the paint. These two paint systems are applied to bare 2024 T3 Al panels and to chromated 2024 T3 Al panels. The coated panels are treated in a corroding media for 2,400 h and monitored periodically using electrochemical impedance spectroscopy (EIS). Results of EIS data and the corresponding electrical equivalent circuit (EEC) show that the ISPC applied to both the chromated and untreated Al panels provide superior corrosion protection. At low frequency (0.01 Hz), the panels coated with the ISPC show 10 000 times more resistance than both the chromated and bare Al panels coated with the control polyester–melamine paint. The corresponding EEC shows that the panels with the control paint applied have double the amount of electrical components (resistors and constant phase elements). A physical interpretation is suggested for the EEC. The paint adhesion to the Al surface and the corrosion behavior are tested by salt-water immersion and salt fog. After the panels are exposed to the salt solution, a pressure tape is applied to test the adhesion. The results show that the ISPC applied on the chromated Al adheres the best. The simultaneous reaction of the ISPR catalyzing the curing of the paint and forming the metal–phosphate layer is the reason for the superior paint performance.