ABSTRACT
In this work we report the synthesis of different materials for immobilization of well-known pH indicators, to be used as additives in coatings for corrosion sensing: inorganic materials such as layered double hydroxides, silica nanocapsules and polymeric microcapsules (chitosan). The materials obtained were characterized by X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopies. Furthermore, the release profile of the obtained materials was performed using UV- Visible spectrophotometry under conditions relevant for the onset of corrosion processes, namely in the presence of NaCl and different pH conditions. The release and color changes were correlated with pH maps obtained by the scanning ion-selective technique for different galvanic couples (Al-Cu, Fe-Zn) and magnesium alloy AZ31. The results show that the materials obtained are able to change color, associated with the pKa of selected indicators and ascribed to pH changes in the alkaline direction as result of cathodic activity in different metals. These findings open prospects for the use of the prepared additives in functional coatings for corrosion sensing.
INTRODUCTION
Detection and mitigating actions are inter-related parts of the tackling of corrosion in a cost-effective way. Previous works available in the literature have shown the potential for using pH indicators as a simple and user-friendly approach for detection of corrosion activity. In fact, the association between the electrochemical processes occurring at the metal/solution interface, transduction into a measurable signal and correlation with level of degradation constitute an ever actual problem and challenge in the field of corrosion science and engineering.
In this work, we revisit the use of pH indicating molecules as active components of novel hosting structures for detection of corrosion. The designed systems encompass materials at micro and nanoscale with capacity for controlled release of substances, to be used as functional additives into polymeric coatings and thus impart self-sensing properties. The advantages associated with immobilization of pH indicators relatively to the direct addition of organic molecules into coating formulations can be the limitation of unwanted reactions between coating components and the pH indicators, and the localized nature of the response since only materials close to defects or sites where corrosion initiates will be signaled.