Localized Corrosion on 2219 Aluminium Alloy Coated with a Titanium Based Conversion Coating

High strength aluminium alloys are widely employed in aerospace applications. However specific environmental conditions may trigger corrosion and therefore these alloys require additional protection. Most current conversion coatings used for this purpose are Cr(VI) based, and thus environmentally undesirable. A possible replacement is a Ti based coating, Nabutan STI/310. The aim of this work is to gain a better understanding of corrosion mechanisms of the Al alloy surfaces coated with Nabutan, focusing on the role of intermetallic precipitates in pitting corrosion. The second phase precipitates in Al 2219 alloy were characterized by means of Auger electron spectroscopy (AES), scanning Auger microscopy (SAM) and energy dispersive x-ray spectroscopy (EDX). The surface was then coated and the same precipitate was analysed to observe how the coating had deposited on the intermetallics and the matrix. Subsequently the behaviour of the precipitate in a 3.5% NaCl solution was investigated as a function of exposure time; 15 minutes, 45 minutes, 2, 4 and 8 hours. At the end of each exposure the sample was returned to microscope and the same precipitates re-analysed. The results showed how the coating deposited preferentially on the matrix. During the immersions, the coating itself suffered of a progressive degradation. Corrosion products rich in aluminium and oxygen were found to increasingly accumulate in a random manner over the matrix, as a function of time. Fe, and Mn from the alloy and Ti from the conversion coating did not show any major activity. Cu deposits were found after 8 hours of immersion.