Layered Double Hydroxide Clusters as Precursors of Novel Multifunctional Layers: A Bottom-Up Approach

Cristina S Neves,João Tedim,Alexandre C Bastos,Angela Cunha,Daisy Rocha,Mikhail L Zheludkevich,Maksim Starykevich,Adelaide Almeida,Andrei N Salak,Mário G.S Ferreira

doi:10.3390/coatings9050328

Abstract

The specific microstructure of aluminum alloys is herein explored to grow spatially-resolved layered double hydroxide (SR-LDH) clusters on their surface. Upon chemical modification of LDHs via intercalation, adsorption and grafting with different functional molecules, novel surface-engineered surfaces were obtained. Crystal structure and phase composition were analyzed by X-ray diffraction (XRD) and surface morphology was observed by scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) and glow discharge optical emission spectrometry (GDOES) were used to correlate structural changes upon ion-exchange and interfacial modifications with chemical composition and surface profiles of the SR-LDH films, respectively. The protection conferred by these films against localized corrosion was investigated at microscale using the scanning vibrating electrode technique (SVET). LDH-NO3 phase was obtained by direct growth onto AA2024 surface, as evidenced by (003) and (006) XRD diffraction reflections. After anion exchange of nitrate with 2-mercaptobenzothiazole (MBT) there was a decrease in the SR-LDH thickness inferred from GDOES profiles. The subsequent surface functionalization with HTMS was confirmed by the presence of Si signal in XPS and GDOES analyses, leading to an increase in the water contact angle (c.a 144° ± 3°). SVET measurements of the SR-LDH films revealed exceptional corrosion resistance, whereas the bioluminescent bacteria assay proved the anti-microbial character of the obtained films. Overall the results obtained show an effective corrosion protection of the SR-LDHs when compared to the bare substrate and the potential of these films for biofouling applications as new Cr-free pre-treatments.

Highlights

Nowadays, there is a growing interest towards the development of coatings and smart interfaces enclosing multiple functionalities within the same matrix, aiming at improved performance, high durability, lower toxicity and cost-effectiveness [1].Aluminum is rarely found in nature in its metallic form as it is thermodynamically unstable, and must be processed from ores such as bauxite, using high energetic processes in order to be reduced back to the metallic form [2]
This d-value is lower than that reported for bulk Zn-Al-MBT layered double hydroxide films (LDHs) [19] and may indicate that the aforementioned anion exchange is incomplete
It should be noticed that when difference in the effective size of the parent and the substituting anions is rather high, the anion exchange is accompanied by fragmentation of the LDH crystallites

Summary

Introduction

There is a growing interest towards the development of coatings and smart interfaces enclosing multiple functionalities within the same matrix, aiming at improved performance, high durability, lower toxicity and cost-effectiveness [1].Aluminum is rarely found in nature in its metallic form as it is thermodynamically unstable, and must be processed from ores such as bauxite, using high energetic processes in order to be reduced back to the metallic form [2]. There is a growing interest towards the development of coatings and smart interfaces enclosing multiple functionalities within the same matrix, aiming at improved performance, high durability, lower toxicity and cost-effectiveness [1]. A recently proposed strategy relies on the formation of conversion films using the substrate as source of reactants to make a well adhered layer, capable of acting as the first protection level against ingress of H2 O, O2 and oxide disrupting species such as chlorides. A more recent trend relies on the use of LDHs as sealing of anodized layers [14,15,16] to impart additional active corrosion protection

Methods

Results

Conclusion