**Contents**


#### **Adrita Dass and Atieh Moridi**


#### **Preface to "Protective and Functional Coatings for Metallic and Ceramic Substrates"**

*Coatings* here provides a selection of 10 papers, published in 2019, from researchers and institutions based in various countries around the world (nine European, one American, and one Asian), allowing us to appreciate the variety and significance of ongoing research in the wide field of protective and functional coatings.

The most noteworthy investigations conducted in the area of surface protection are currently proceeding with an identically fast pace in the twofold direction of a deeper and increasingly reliable knowledge of degradation and protection mechanisms, and of the technological optimization of new materials selection and design, coatings deposition processes, and characterization methods.

A representative essay of the worldwide efforts toward more durable surfaces can be read in this short collection. Both organic and inorganic coatings are included among the protection strategies, from a large variety of deposition processes, with interesting examples of organic-inorganic composites being proposed (such as PVC-ZnO nanocoposites or MoS2-sunflower oil combinations). Major attention is devoted to protection from both electrochemical and chemical corrosion of different metallic alloys (stainless steel, bronze, and aluminum) and to advanced SiC-SiC composites exposed to the aggressive environments characteristic of gas turbines.

The delicate issues related to the modeling and experimental evaluation of mechanical properties of coatings are comprehensively investigated in papers concerning cavitation erosion and sliding wear resistance and stress-dependent elasticity of Ti–Al–N-based thin films.

A dedicated space was purposely devoted in the papers selection for examples of applicability of an advanced characterization technique (attenuated total reflectance Fourier transform infrared microscopy) for the evaluation of bituminous coatings from reclaimed asphalt, as representative of the important ongoing efforts to achieve reliable and affordable characterization and classification (and standardization) procedures of both newly conceived or combined materials.

Finally, as an example of the wide field of research on innovative functional coatings, a study is presented of highly active Mo-based alloys deposited in the form of films as electrocatalytic material for hydrogen evolution reaction.

A comprehensive review of directed energy deposition additive manufacturing of metallic components completes the collection, highlighting the complex physico-chemical interactions between matter and heat, which are at the basis of Additive Manufacturing (AM) production processes, and whose knowledge and modeling capacity owes much to the experience acquired in the study of high temperature coating deposition processes.

> **—Prof. Dr. Cecilia Bartuli University of Rome "La Sapienza"**

## *Article* **Effect of Synthesis Conditions on the Controlled Growth of MgAl–LDH Corrosion Resistance Film: Structure and Corrosion Resistance Properties**

#### **Muhammad Ahsan Iqbal \* and Michele Fedel \***

Department of Industrial Engineering, University of Trento, via Sommarive 9, 38123 Trento, Italy

**\*** Correspondence: muhammadahsan.iqbal@unitn.it (M.A.I.); michele.fedel@unitn.it (M.F.);

Tel.: +39-320-022-7519 (M.A.I.); +39-461-285-354 (M.F.)

Received: 10 December 2018; Accepted: 2 January 2019; Published: 7 January 2019

**Abstract:** In this study, a series of MgAl–layered double hydroxide (LDH) thin films were synthesized by a single step hydrothermal process at different synthetic conditions on AA6082, and the combined effect of reaction temperatures and crystallization times on in situ growth MgAl–LDH structural geometry, growth rate, and more importantly on the corresponding corrosive resistance properties are briefly discussed. The synthesis of LDH was performed at reaction temperatures of 40, 60, 80, and 100 ◦C, while the treatment time was varied at 12, 18, and 24 h. The as-prepared synthetic coatings were characterized by scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR), while the corresponding corrosion protection efficiency of the developed coating was studied through potentiodynamic polarization studies and electrochemical impedance spectra. The findings demonstrated that extended crystallization time and reaction temperature impart a significant effect on the oriented growth of layered double hydroxide, the surface morphology, and on the film thickness, which had a remarkable influence on the LDH corrosion resistance ability. The LDH coated specimen developed at 100 ◦C for 18 h reaction time showed a more compact and dense structure compared to the traditional platelet structure obtained at 80 ◦C for 24 h crystallization time, and interestingly that compact structure exhibited the lowest corrosion current density, up to five orders of magnitude lower than that of bare AA6082.

**Keywords:** MgAl–layered double hydroxide; EIS; XRD; FT-IR; AA6082; corrosion protection
