Chrome-free single-step in-situ phosphatizing coatings on a Ti-6Al-4V titanium alloy
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Surface pretreatment, such as chromate conversion, is generally required for the organic coating on Ti-6Al-4V titanium alloys. The surface modification process facilitates bond formation between the titanium surface and adhesives or paints. However, the hexavalent chromate used in the conversion coating is carcinogenic and must be eliminated.
Recently, the “green chemistry” technique of in-situ phosphatizing coatings (ISPC) has been developed at Northern Illinois University. In this study, a polyester-melamine coating system catalyzed by p-toluene sulfonic acid is used as a control paint and applied to bare (paint 1) and chromated (paint 3) Ti alloy panels. The arylphosphonic acid employed as an in-situ phosphatizing reagent (ISPR) is used to formulate ISPC polyester-melamine paint and to react in-situ with the metal surface when ISPC paint is applied on bare Ti panels (paint 2) and chromated Ti panels (paint 4). After soaking in 3% NaCl solution for 1500 hr, the electrochemical impedance, |Z|, was measured as 109, 1010, 1010, and 1010 ohm-cm2 for paints, 1, 2, 3, and 4, respectively. After 1000 hr salt spray (fog) testing, a disbondment from the “X” scribe is observed as 13 and <0.5, <1, <1 mm for paints 1, 2, 3, and 4, respectively. The simultaneous reaction of the ISPR in catalyzing the polymerization of paint film and forming the metal-phosphate layer is the reason for the superior ISPC paint performance.
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