**About the Editor**

**Marina Cabrini** has been a professor of Science and Technology of Materials at the Mechanical Engineering Faculty of the University of Bergamo since 2001. Her courses are "Metallic Materials" and "Polymer, Composites and Ceramics" in Mechanical Engineering, "Biomaterials" for Technology Engineering for Health and "Electrochemistry and Electrochemical Technologies" for the PhD student in Engineering and Applied Science of the University of Bergamo.

Her research activity is on electrochemistry and corrosion, primarily focused on the environmentally assisted cracking of traditional and innovative steels for the oil and gas industry. She has conducted some research on biomaterials, on the electrochemical characterization of the kinetic of passivity film formation on rebar in concrete, on corrosion inhibitors for chloridecontaminated concrete and on the corrosion evaluation of carbon steel in CCTS (carbon capture transport and storage).

Nowadays, she is working on the corrosion behavior of aluminum, titanium and nickel alloys obtained by means of direct metal laser sintering in collaboration with Polytechnic of Turin, on the corrosion and stress corrosion cracking of aluminum alloys (AA7075 and AA 2024) welded by means of friction stir welding, the effect of loading on the hydrogen diffusion and embrittlement of low alloyed high strength steels and the corrosion in oil and gas and geothermal systems.

#### **Preface to "Corrosion and Protection of Materials"**

The corrosion phenomenon is the deterioration of metallic structures caused by the reaction of the metal with the environment. Several recent case studies pointed the attention towards the importance of the security and monitoring of facilities and infrastructures. The first estimate of the economic damages caused by corrosion and the protection of materials was carried out in the 1970s by the British Government, with the conclusion that the amount of expenses for the restoration of the damaged structures was around 3% of the gross domestic product (GPD), and that 90% of this figure would have been easily saved if the technical notions of protection and choice of materials already acquired had been applied correctly. A similar study commissioned published on the web site of the NACE (National Association of Corrosion Engineering) indicates a surprisingly identical value: 3.4% of GDP (2013)! It is impressive how in forty years the economic importance of the corrosion of materials has not decreased at all, on the contrary—considering the growth of GDP, the data from an economic point of view are certainly alarming. On the basis of these considerations, it could be inferred that over more than forty years, not much has been done for the prevention of corrosion, but obviously this consideration is superficial. It is much more realistic to think that the conditions in which materials operate are in many plants more and more exasperated (for example, think of the increase in the depth of exploitation of oil fields), as there is a constant search for more performing materials, with high weight/performance ratios in order to reduce energy consumption, not to mention the increased sensitivity towards the environment that led to the abandonment of some anticorrosion processes and/or methods (an example for all the use of chromates as corrosion inhibitors or anticorrosive pigments) and the research on environmentally friendly systems. In addition, new alloys, with different microstructures, like high entropy alloys, ultra-fine grains metals, or new technologies of production and installation, like additive manufacturing, innovative coating or friction stir welding, need to improve the consolidated knowledge on the corrosion mechanisms and protection methods of metallic materials. At the base of each correct design must therefore be an increased sensitivity about the increase in its useful life, both to reduce the energy impact of production plants and disposal, and to reduce the possibility of contamination of the environment with the fluids of the process or with the same substances used for corrosion protection. The aim of this Special Issue was to draw a panorama of the actual research on the corrosion and protection of traditional and innovative materials in natural environments or in the industrial context. This Special Issue contains thirty papers by 140 authors of 16 countries, with the geographical distribution observed in Figure 1, testifying to the importance of studying corrosion problems all over the world. Several different items are presented. The first papers of the volume are on alloys of new technologies, and it is opened by an invited review on the corrosion of additively manufactured aluminum alloys; the other two papers are, respectively, on alloy 625 obtained by additive manufacturing and on CrFeCoNiNbx high-entropy alloys. The following seven papers are mainly on the mechanism of corrosion, including two interesting reviews, one on the alternate current corrosion on carbon steel under cathodic protection and the other on the electrochemical polishing of austenitic stainless steels. Studies on the mechanism of corrosion of pure iron, martensitic and austenitic stainless steels, nickel–aluminum–bronze, and Al–Zn alloy are presented in the following papers. Microbiological corrosion, biocide treatments and corrosion inhibitors are always topical in industrial practice. These topics are dealt with in the following five papers, with particular attention to the environmental impact of the chemicals studied, which must necessarily be eco-friendly. Many studies are underway regarding the development of anticorrosive coatings, the most current trend is aimed at ceramic and metal coatings, as demonstrated by six works published in this Special Issue. Stress corrosion cracking and hydrogen embrittlement are widely studied, but they remain of strong actuality. This item in the Special Issue is reported on in five papers regarding the traditional stress corrosion phenomena in sour environments or in pipeline steels, high-strength steels wires, as well as aluminum or magnesium alloys. Finally, the last four works present interesting aspects related to monitoring the corrosion of structures and the effects of corrosion on the residual mechanical resistance of civil structures, such as bridges or reinforced concrete. These arguments are of vital importance given the increase in the average life of infrastructures and their natural deterioration which makes the problem of their control and recovery of primary importance. Personally, I am honored to have had the opportunity to be the gues<sup>t</sup> editor of this volume, and I want to thank MPDI for giving me the opportunity; thanks you above all to Andy Zhang for his precious contribution in the realization of this Special Issue of *Materials*.

**Figure 1.** Geografical distribution of the authors of the papers present in this special issue.

**Marina Cabrini** *Editor*

## *Review* **Corrosion and Corrosion Protection of Additively Manufactured Aluminium Alloys—A Critical Review**

#### **Reynier I. Revilla \*, Donovan Verkens, Tim Rubben and Iris De Graeve**

Research group of Electrochemical and Surface Engineering, Department of Materials and Chemistry (MACH), Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium; Donovan.Verkens@vub.be (D.V.); tim.rubben@vub.be (T.R.); Iris.De.Graeve@vub.be (I.D.G.)

**\*** Correspondence: rrevilla@vub.be

Received: 28 September 2020; Accepted: 22 October 2020; Published: 28 October 2020

**Abstract:** Metal additive manufacturing (MAM), also known as metal 3D printing, is a rapidly growing industry based on the fabrication of complex metal parts with improved functionalities. During MAM, metal parts are produced in a layer by layer fashion using 3D computer-aided design models. The advantages of using this technology include the reduction of materials waste, high efficiency for small production runs, near net shape manufacturing, ease of change or revision of versions of a product, support of lattice structures, and rapid prototyping. Numerous metals and alloys can nowadays be processed by additive manufacturing techniques. Among them, Al-based alloys are of grea<sup>t</sup> interest in the automotive and aeronautic industry due to their relatively high strength and stiffness to weight ratio, good wear and corrosion resistance, and recycling potential. The special conditions associated with the MAM processes are known to produce in these materials a fine microstructure with unique directional growth features far from equilibrium. This distinctive microstructure, together with other special features and microstructural defects originating from the additive manufacturing process, is known to greatly influence the corrosion behaviour of these materials. Several works have already been conducted in this direction. However, several issues concerning the corrosion and corrosion protection of these materials are still not well understood. This work reviews the main studies to date investigating the corrosion aspects of additively manufactured aluminium alloys. It also provides a summary and outlook of relevant directions to be explored in future research.

**Keywords:** metal additive manufacturing; aluminium alloys; corrosion behaviour; microstructure; corrosion protection
