Substrate/Phosphate Coating Interaction via Potentiodynamic Measurements

The inherent metal quality, coupled with the metal surface condition prior to phosphating has been associated with the corrosion stability and adhesion characteristics of the subsequent phosphated and painted surface. The corrosion and adhesion properties are usually determined via long-term tests; however, our investigation of the potentiodynamic behavior of the metal substrates and phosphated coatings have shown that these electrochemical measurements can provide possible failure-mode explanations exhibited by subsequent accelerated testing. Fundamental properties of phosphate coatings on Fe, Zn, and Al substrates have also been investigated via potentiodynamic measurements. Indications of the reactivity of the different substrates with respect to ease of phosphateability may be derived via anodic and cathodic polarization measurements. Further, the effects of variations in the phosphate processing conditions have been correlated to polarization measurements. Finally, the successful treatment of different substrates requires variations in the chemical make-up of the phosphating bath, and a need for flexability in the chemical replenishment of the phosphating bath.