Layered double hydroxides for corrosion-related applications-Main developments from 20 years of research at CICECO.

This work describes the main advances carried out in the field of corrosion protection using layered double hydroxides (LDH), both as additive/pigment-based systems in organic coatings and as conversion films/pre-treatments. In the context of the research topic "Celebrating 20 years of CICECO", the main works reported herein are based on SECOP's group (CICECO) main advances over the years. More specifically, this review describes structure and properties of LDH, delving into the corrosion field with description of pioneering works, use of LDH as additives to organic coatings, conversion layers, application in reinforced concrete and corrosion detection, and environmental impact of these materials. Moreover, the use of computational tools for the design of LDH materials and understanding of ion-exchange reactions is also presented. The review ends with a critical analysis of the field and future perspectives on the use of LDH for corrosion protection. From the work carried out LDH seem very tenable, versatile, and advantageous for corrosion protection applications, although several obstacles will have to be overcome before their use become commonplace.

In situ kinetics studies of Zn-Al LDH intercalation with corrosion related species.

Kinetic parameters for three anion exchange reactions - Zn-LDH-NO3→ Zn-LDH-Cl, Zn-LDH-NO3→ Zn-LDH-SO4 and Zn-LDH-NO3→ Zn-LDH-VOx- were obtained by in situ synchrotron study. The first and the second ones are two-stage reactions; the first stage is characterized by the two-dimensional diffusion-controlled reaction following deceleratory nucleation and the second stage is a one-dimensional diffusion-controlled reaction also with a decelerator nucleation effect. In the case of exchange NO3-→ Cl- host anions are completely released, while in the case of NO3-→ SO42- the reaction ends without complete release of nitrate anions. The exchange of Zn-LDH-NO3→ Zn-LDH-VOx is a one-stage reaction and goes much slower than the previous two cases. The latter is characterized by a one stage two-dimensional reaction with an instantaneous nucleation. As a result, at the end of this process there are two crystalline phases with different polyvanadate species, presumably V4O124- and V2O74-, nitrate anions were not completely released. The rate of replacing NO3- anions by guest ones can be represented as Cl- > SO42- > VOxy-.

One-step synthesis and growth mechanism of nitrate intercalated ZnAl LDH conversion coatings on zinc.

An approach for the synthesis of ZnAl-NO3 LDH conversion coatings on zinc in an aqueous acidic Al(NO3)3/NaNO3 solution is demonstrated for the first time. The growth mechanism has been investigated using time resolved structural, microstructural and analytical methods. A LDH growth model involving both electrochemical and chemical processes is suggested.

Mechanism of corrosion inhibition of AA2024 by rare-earth compounds.

The mechanism of corrosion protection of the widely used 2024-T3 aluminum alloy by cerium and lanthanum inhibitors in chloride media is described in detail in the present work. The corrosion process was investigated by means of scanning Kelvin probe force microscopy (SKPFM), in situ atomic force microscopy, and scanning electron microscopy coupled with energy dispersive spectroscopy. Employment of the high-resolution and in situ techniques results in a deep understanding of the details of the physical chemistry and mechanisms of the corrosion processes. The applicability of the SKPFM for mechanistic analysis of the effect of different corrosion inhibitors is demonstrated for the first time. The inhibitors under study show sufficient hindering of the localized corrosion processes especially in the case of pitting formation located around the intermetallic S-phase particles. The main role of Ce(3+) and La(3+) in the corrosion protection is formation of hydroxide deposits on S-phase inclusions buffering the local increase of pH, which is responsible for the acceleration of the intermetallics dealloying. The formed hydroxide precipitates can also act as a diffusion barrier hindering the corrosion processes in active zones. Cerium nitrate exhibits higher inhibition efficiency in comparison with lanthanum nitrate. The higher effect in the case of cerium is obtained due to lower solubility of the respective hydroxide. A detailed mechanism of the corrosion process and its inhibition is proposed based on thermodynamic analysis.