Search Results

One measurement of the quality of a phosphated and painted metal substrate is through the use of accelerated test procedures involving corrosive environments. The subsequent test results have been found to have dependance upon the specific test procedure that was utilized. In particular, the effects of the acidulated rinse upon galvanized steel were strongly dependent upon the test procedure involving adhesion measurements after water immersion; whereas, a negligible effect was exhibited via a cyclic scab test procedure. Finally, it was determined that a chemical modification of the phosphate coating composition can alter the effects of the acidulated final rinse upon hot dipped galvanized steel with respect to adhesion performance.

Chemical conversion coatings represent those coatings formed by contacting a metallic substrate with a “chemical” solution that “converts” or “changes” the metallic surface from its original composition. The “conversion” process may arise via chemical or electrochemical interaction at the metallic surface, and produces a “coating” that is an integral part of the metal substrate. The presence of this “conversion coating” provides an increase in the corrosion resistance of the metal substrate, both with and without a subsequently applied organic film, and an increase in the adhesive capability of the metal substrate toward further organic film applications. Typical conversion coatings that are widely used in industry are phosphates (zinc, iron, manganese, calcium-zinc), chromates, chrome-free coatings, oxides (chemically or electrochemically formed); where each coating type provides the necessary properties that meet a particular requirement of industry

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.

The advent of cathodic electrodeposition in the automotive industry has brought about substantial changes in the pretreatment of steel and galvanized steel surfaces and the testing thereof. Cyclic scab blistering tests, water soak tests, coating solubility, coating porosity and ESCA/SEM/Auger surface analysis techniques have provided significant insights into the factors that optimize the performance of zinc phosphate coatings for cationic paint systems. Specifically, the crystal structure and iron content of the phosphate coating and a chromium based post-rinse have been shown to make a significant effect upon the subsequent corrosion resistance properties of the phosphate/metal/CED paint system. Further, ESCA/SEM surface analysis and accelerated testing have shown that the processing method substantially affects both the crystal structure and composition of the coating.