Magnesium is the most reactive metallic element utilized for structural applications. This implies that even its alloys are highly susceptible to corrosion, which limits their application in industries such as automotive, aeronautics, etc. In this sense, sol-gel route has emerged as an important alternative technique of synthesizing protective coatings for magnesium alloys. Coating produced using this technique have been seen to offer great advantages such as high adhesion, use of low toxic material and a relatively simple surface preparation procedure. Sol-gel processing consists of successive reactions of hydrolysis and condensation of the corresponding precursors. The most common precursors used are the alkoxysilanes. Unfortunately, the coatings synthesized from these precursors present a relatively high vulnerability to cracking during the drying and curing processes. In order to solve this drawback, the organoalkoxysilanes have been recently incorporated into the sol-gel synthesis, resulting in the production of hybrid coatings with improved morphological and protective features. Nevertheless, the synthesis of these coatings is normally carried out in acidic medium where magnesium is not stable and reacts spontaneously, which can deteriorate their morphological properties. Therefore, in the design of hybrid coatings on magnesium alloys should be employed a catalyst of moderately low pH. Acetic acid is the most promising desired acid, however, little has been done on the effects of its concentration, immersion time and aging time on the developed alloy.
Researchers led by Dr. Fernando Viejo at Industrial University of Santander in Colombia, evaluated the effects of the catalyst (acetic acid) concentration, the immersion time and aging time during the synthesis and deposition of TEOS-GPTMS hybrid sol-gel coatings on the AZ31 magnesium alloy. They also purposed analyze how such synthesis parameters affected the morphology, composition and corrosion performance of the resultant coatings. Their work was published in the research journal, Surface & Coatings Technology.
The research team commenced by monitoring the hydrolysis and condensation reactions as they occurred within the sols. They then varied both the catalyst concentration and the immersion time during deposition in order to evaluate the corrosive power of the sols synthesized and their interaction with the magnesium surface. The team then analyzed the performance of the hybrid coatings by evaluating the effects of different aging time on the composition, morphology and corrosion resistance.
The study showed that by suitable control of catalyst concentration, the immersion time and the aging time, it is possible to mitigate surface corrosion processes on the AZ31 magnesium alloy during the sol deposition. By doing this, the researchers have been able to obtain continuous, homogeneous coatings with no evidence of cracking, which decreased the corrosion current density of the parent alloy by about one order of magnitude and provided, simultaneously, a protection range up to 150 mV. Finally, the relatively high stability of the sols synthesized is of considerable economic interest at industrial level because it opens the opportunity of synthesizing multiple coatings with the same sol before gelation, thus reducing the production costs.
TEOS – tetraethoxysilane (Si(OC2H5)4)
GPTMS – (3-glycidyloxypropyl)trimethoxysilane (C9H20O5Si)
C.A. Hernández-Barrios, C.A. Cuao, M.A. Jaimes, A.E. Coy, F. Viejo. Effect of the catalyst concentration, the immersion time and the aging time on the morphology, composition and corrosion performance of TEOS-GPTMS sol-gel coatings deposited on the AZ31 magnesium alloy. Surface & Coatings Technology volume 325 (2017) pages 257–269.
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