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The ubiquitous use of chromium and its derivatives as corrosion preventative compounds accelerated rapidly after the second industrial revolution, with such compounds now integral to modern society. However, the detrimental impact of chromium compounds on the environment and human health has prompted the need to revisit the majority of current industrial corrosion protection measures. This review retraces the origins of chromium replacement motivations, introducing the various legislative actions aimed at diminishing the use of chromium compounds, and critically reviews alternative corrosion preventative technologies developed in the recent decades to now. The review, herein, is intended for a broad audience in order to provide a concise update to an increasingly timely issue.

Electrochemical impedance spectroscopy (EIS) is a powerful tool to investigate properties of materials and electrode reactions. This Primer provides a guide to the use of EIS and a comparison with other electrochemical techniques. The analysis of impedance data for reduction of ferricyanide in a KCl supporting electrolyte is used to demonstrate the error structure for impedance measurements, the use of measurement and process models, and the sensitivity of impedance to the evolution of electrode properties. This Primer provides guidelines for experimental design, discusses the relevance of accuracy contour plots to wiring and instrumentation selection, and emphasizes the importance of the Kramers–Kronig relations to data validation and analysis. Applications of EIS to battery performance, metal and alloy corrosion, and electrochemical biosensors are highlighted. Electrochemical impedance measurements depend on both the mechanism under investigation and extrinsic parameters, such as the electrode geometry. Experimental complications are discussed, including the influence of non-stationary behaviour at low frequencies and the need for reference electrodes. Finally, emerging trends in experimental and interpretation approaches are also described.This Primer on electrochemical impedance spectroscopy (EIS) provides an experimental design guide to measure impedance and how these data are analysed. The range of applications that require EIS, from measuring battery performance to electrochemical biosensors, is highlighted. Limitations of the method along with emerging trends in experimental optimizations and data interpretation are also described.

A fundamental understanding of corrosion mechanisms is the key to developing suitable corrosion protection approaches, and for the prediction of service life of metallic structures. However, conventional corrosion testing methods such as mass loss and electrochemical testing do not guarantee estimation of \"true\" corrosion rate and often mask the underlying mechanisms, due to either low sensitivity or a lack of element‐resolved information. Relatively recent work in corrosion science has led to the development of a new class of corrosion testing approaches, namely atomic spectroelectrochemistry; whereby direct insight of dissolution and corrosion mechanisms can be obtained during electrochemical testing. Atomic spectroelectrochemistry provides real‐time and element resolved dissolution rate of material via coupling electrochemical flow cell with inductively coupled plasma – atomic spectroscopy. This concise review discusses the basic working principle of atomic spectroelectrochemistry and its recent applications in corrosion science to understand the true underlying corrosion mechanisms of a range of metallic materials.  

Additively manufactured high strength aluminium (Al) alloy AA7075 was prepared using selective laser melting (SLM). High strength Al-alloys prepared by SLM have not been widely studied to date. The evolution of microstructure and hardness, with the attendant corrosion, were investigated. Additively manufactured AA7075 was investigated both in the “as-produced” condition and as a function of artificial ageing. The microstructure of specimens prepared was studied using electron microscopy. Production of AA7075 by SLM generated a unique microstructure, which was altered by solutionising and further altered by artificial ageing—resulting in microstructures distinctive to that of wrought AA7075-T6. The electrochemical response of additively manufactured AA7075 was dependent on processing history, and unique to wrought AA7075-T6, whereby dissolution rates were generally lower for additively manufactured AA7075. Furthermore, immersion exposure testing followed by microscopy, indicated different corrosion morphology for additively manufactured AA7075, whereby resultant pit size was notably smaller, in contrast to wrought AA7075-T6.

The real-time dissolution of the single-phase compositionally complex alloy (CCA), Al 1.5 TiVCr, was studied using an inline inductively coupled plasma method. Compositionally complex alloys (CCAs), a term encompassing high entropy alloys (HEAs) or multi-principal element alloys (MPEAs), are—in general—noted for their inherently high corrosion resistance. In order to gain an insight into the dissolution of Al 1.5 TiVCr alloy, atomic emission spectroelectrochemistry was utilised in order to measure the ion dissolution of the alloy during anodic polarisation. It was revealed that incongruent dissolution occurred, with preferential dissolution of Al, and essentially no dissolution of Ti, until the point of alloy breakdown. Results were correlated with X-ray photoelectron spectroscopy, which revealed a complex surface oxide inclusive of unoxidised metal, and metal oxides in disproportion to the bulk alloying element ratio.

In this work, a flow-through electrochemical–ICP-AES platform for operando monitoring of pitting corrosion on the CoCrFeMnNi high-entropy alloy is introduced. This setup combines localized chloride injection, potentiostatic control, and online, element-resolved dissolution analysis, thereby addressing a long-standing gap in mechanistic studies of early pit initiation and repassivation. Experiments in 0.5 M H 2 SO 4 with Cl - injection enabled the continuous transfer of dissolved species from the electrode surface to the ICP-AES detector, achieving sub-ppb sensitivity and allowing quantification of Co, Cr, Fe, Mn, and Ni dissolution rates during the pitting process. The results reveal four characteristic stages, namely, incubation, initiation, propagation, and repassivation, with subtle but systematic differences between alloying elements. Co and Fe contribute slightly more during initiation, while Cr plays a dominant role during repassivation, reflecting its critical involvement in passive film regeneration. Charge analysis demonstrates that repassivation consumes a quantity of charge far greater than expected for compact passive films, pointing instead to a slow, iterative re-formation and partial dissolution of hydrated oxides. This methodology provides new mechanistic insight into the dynamic sequence of film breakdown, localized dissolution, and film repair in multicomponent alloys, and establishes a versatile framework for studying localized corrosion processes with element-specific resolution.

While multi-principal element alloys (MPEAs) remain a promising class of materials owing to several attractive mechanical properties, their corrosion performance is also unique. In this concise review, we present an emerging overview of some of the general features related to MPEA corrosion, following a decade of work in the field. This includes highlighting some of the key aspects related to the electrochemical phenomena in MPEA corrosion, and the relevant future works required for a holistic mechanistic understanding. In addition, a comprehensive database of the reported corrosion performance of MPEAs is presented—based on works reported to date. The database is assembled to also allow users to undertake machine learning or their own data analysis, with a parsed representation of alloy composition, test electrolyte, and corrosion-related parameters.

This review explores the aqueous corrosion of multi-principal element alloys (MPEAs) that have been prepared using additive manufacturing (AM), inclusive of laser powder bed fusion, directed energy deposition and electron beam melting. The impact of additive manufacturing techniques upon the microstructural features of MPEAs may influence corrosion resistance, through inducing an influence on grain size, phase distribution, and local nano-chemistry. Emphasis is placed on the comparative performance of MPEAs in various corrosive environments, highlighting key findings. The review aims to provide insights into the influence of AM as a processing method for MPEA production – in the context of impact upon corrosion.

The real-time dissolution of the single-phase compositionally complex alloy (CCA), Al1.5TiVCr, was studied using an inline inductively coupled plasma method. Compositionally complex alloys (CCAs), a term encompassing high entropy alloys (HEAs) or multi-principal element alloys (MPEAs), are - in general - noted for their inherently high corrosion resistance. In order to gain an insight into the dissolution of Al1.5TiVCr alloy, atomic emission spectroelectrochemistry was utilised in order to measure the ion dissolution of the alloy during anodic polarisation. It was revealed that incongruent dissolution occurred, with preferential dissolution of Al, and essentially no dissolution of Ti, until the point of alloy breakdown. Results were correlated with X-ray photoelectron spectroscopy, which revealed a complex surface oxide inclusive of unoxidised metal, and metal oxides in disproportion to the bulk alloying element ratio.

Aluminium alloy AA7075 (Al-Zn-Mg-Cu) specimens were prepared using selective laser melting, also known as powder bed fusion additive manufacturing. In the as-manufactured state, which represents a locally rapidly solidified condition, the prevalence of a previously unreported icosahedral quasicrystal with 5-fold symmetry was observed. The icosahedral quasicrystal, which has been termed nu-phase, was comprised of Zn, Cu and Mg.