Chrome-Free Single-Step In-Situ Phosphatizing Coatings on a Ti-6Al-4V Titanium Alloy

By Heather Neuder, Chhiu-Tsu Lin

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. How-ever, 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.