Aluminum alloy 2024-T3 protection by magnesium-rich primer with chromate-free metal salts

• Published in: Progress in organic coatings 2015-01, Vol.78, p.446-454
• Main Authors: Merten, Bobbi Jo E., Battocchi, Dante, Bierwagen, Gordon P.
• Format: Article
• Language: English
• Subjects: Aluminum alloy; Aluminum base alloys; Chromate-free; Coatings; Corrosion inhibitors; Corrosion prevention; Corrosion protection; Formulations; Lithium carbonate; Magnesium; Magnesium nitrate; Mg-rich primer; Pigments; Primers (coatings); Sprays
• ISSN: 0300-9440; 1873-331X
• DOI: 10.1016/j.porgcoat.2014.09.013

•Improved AA 2024-T3 protection shown by Li2CO3, Mg(NO3)2, and Mg containing primer.•Li2CO3, Mg(NO3)2, and Mg film reduced blistering and corrosion in 1600hrs ASTM B117.•Mg(NO3)2 reduces the negative effects of the Mg oxidation reactions.•Li2CO3 improves defect protection when formulated with Mg(NO3)2 and Mg at low PVC. This research examines novel Mg- and metal salt-rich primer formulations with low Mg metal pigment loading. The chromate-free metal salts were incorporated to assist in the passivation, corrosion inhibition, and pH buffering of the aluminum substrate. The total pigment volume concentration (PVC) in the examined coatings ranged from 6 to 20%, which is far below its critical pigment volume concentration (CPVC) as well as that of traditional Mg-rich primers (PVC≈45%). The formulations were applied to aluminum alloy (AA) 2024-T3 panels and coated with a polyurethane topcoat. Non-pigmented and Mg-rich primer controls were used for comparison. The coating systems were subjected to accelerated weathering by ASTM B117 salt spray chamber and removed periodically for analysis by electrochemical impedance spectroscopy (EIS) and electrochemical noise measurements (ENM). Corrosion potential was also measured. Conventional, macroscopic examination was used to rank coating performance. The present findings support the significance of CO3 compounds in the protection of aluminum and its alloys by Mg-rich primers. It is proposed that the addition of Li2CO3 and Mg(NO3)2 to the primer facilitate the production of Mg(OH)2 and MgCO3 precipitates to improve protection. Anti-corrosion and anti-blistering effects were observed for formulations containing Li2CO3, Mg(NO3)2, and a low percentage of Mg metal particulates. The Mg(NO3)2 appears to reduce blistering while the Li2CO3 enhances defect protection. These effects are demonstrated throughout 1600h of exposure to salt spray. Continued optimization presents the opportunity to design and implement unique cathodic protection plus corrosion inhibitor coating systems for use on aluminum substrates.