A comparative evaluation of corrosion protection of sol-gel based coatingssystems

Sol-gel technology offers a wide choice of chemistries and is currently per ceived as a potential replacement for current chromate treatments. The corr osion prevention behavior of a commercially available sol-gel surface pretr eatment with six different primers was evaluated using electrochemical impe dance spectroscopy. Eighteen different coatings systems were selected for e valuation. The test panels were prepared in three separate treatment groups of deoxidization only, Alodine 1200 chromate conversion treatment, and Lor d Aeroglaze sol-gel 110 non-chromate surface treatment, followed by six dif ferent primer coatings, and a polyurethane top coat. A variety of environme ntal conditions were generated based on Harrison's solution, salt fog and U V light exposure. Electrochemical impedance measurements were obtained from panels exposed to continuous immersion, alternate immersion in Harrison's solution and an alternate cycle of salt fog and UV chamber. The corrosion b ehavior of sol-gel based coatings was compared to that of chromate conversi on coatings and de-oxidized samples. Scribed panels were also tested to eva luate the corrosion resistance behavior of scratched or cracked coatings. O f all the 18 coatings systems investigated, the E-coat based system showed the best corrosion resistance performance. The sol-gel treated E-coat provi ded an acceptable corrosion protection level, a barrier resistance of over 10(8) Omega and performed similar to the conventional chromate system, as s een from the results of unscribed panels. However, the scribed panel data c learly demonstrates the superior performance of the Alodine 1200 treated sy stems over the sol-gel based coating systems. Cyclic salt fog/UV exposure d id not provide clear discrimination over the alternate immersion test. The scribed panel, on the other hand, did provide a higher degree of discrimina tion in a short time. (C) 2001 Elsevier Science B.V. All rights reserved.