Painting rusted steel: The role of aluminum phosphosilicate

• Aluminum phosphosilicate is an acid pigment which could act as mild phosphating agent.
• Aluminum phosphosilicate can phosphatize iron oxides on rusted surfaces.
• Aluminum phosphosilicate is compatible with acid binders.
• Aluminum phosphosilicate could replace chromate in complete painting schemes.
• Aluminum phosphosilicate primers improve paints adhesion on rusted surfaces.

Surface preparation is a key factor for the adequate performance of a paint system. The aim of this investigation is to employ a wash-primer to accomplish the chemical conversion of rusted surface when current cleaning operations are difficult to carry out. The active component of the wash-primer was aluminum phosphosilicate whose electrochemical behavior and the composition of the generated protective layer, both, were studied by electrochemical techniques and scanning electron microscopy (SEM), respectively. Primed rusted steel panels were coated with an alkyd system to perform accelerated tests in the salt spray chamber and electrochemical impedance measurements (EIS). These tests were conducted in parallel with a chromate wash primer and the same alkyd system. Results showed that the wash-primer containing aluminum phosphosilicate could be used satisfactorily to paint rusted steel exhibiting a similar performance to the chromate primer.