131 research outputs found
Evaluation of the effects of cyclododecane on oil paintings
No full textThe solubility of oil paint components during the application of cyclododecane in solvent mixtures was evaluated in order to predict if the application of cyclododecane during restoration may significantly alter the chemical state of the paint layer in oil paintings. The chemical affinity between some of the oil binder components and non-polar cyclododecane could potentially lead to interactions or leaching during the application. In order to investigate these effects a set of samples taken from oil paintings from the early 1900s and 2008, were treated with cyclododecane in a solution, melted, and sprayed as aerosol. The samples were also submitted to a comparative extractive treatment with cyclododecane and organic solvents of different polarities. After the treatments, the extracted components were analysed by gas chromatography-mass spectrometry (GC/MS), which provided detailed molecular information on the composition of the extracts, together with a quantitative profile of fatty acids in extracted triglycerides, after saponification and derivatisation. The results show that applications of cyclododecane both as a spray and in a saturated solution in a hydrocarbon solvent determine the extraction of a low amount of lipids from the paint. On the other hand, when cyclododecane is applied in the melted form, there is an extraction of lipid components of the paint into the treatment solution
The Corrosion Behavior in Different Environments of Austenitic Stainless Steels Subjected to Thermochemical Surface Treatments at Low Temperatures: An Overview
No full textLow-temperature thermochemical treatments are particularly suitable for use in the surface hardening of austenitic stainless steels without impairing their corrosion resistance. In fact, when using treatment media rich in nitrogen and/or carbon at relatively low temperatures (<450 °C for nitriding, <550 °C for carburizing), it is possible to inhibit the formation of chromium compounds and obtain modified surface layers that consist mainly of a supersaturated solid solution, known as expanded austenite or S-phase. It has been observed that this hard phase allows the enhancement of corrosion resistance in chloride-ion-containing solutions, while the results were contradictory for chloride-free acidic solutions. This overview aims to discuss the corrosion behavior of low-temperature-treated austenitic stainless steels, taking into account the different microstructures and phase compositions of the modified layers, as well as the different test environments and conditions. In particular, the corrosion behavior in both chloride-ion-containing solutions and chloride-free solutions (sulfuric acid, sulfate and borate solutions) is discussed. The analysis of the international literature presents evidence that the microstructure and phase composition of the modified layers have key roles in corrosion resistance, especially in sulfuric acid solutions
From Austenitic Stainless Steel to Expanded Austenite-S Phase: Formation, Characteristics and Properties of an Elusive Metastable Phase
No full textAustenitic stainless steels are employed in many industrial fields, due to their excellent corrosion resistance, easy formability and weldability. However, their low hardness, poor tribological properties and the possibility of localized corrosion in specific environments may limit their use. Conventional thermochemical surface treatments, such as nitriding or carburizing, are able to enhance surface hardness, but at the expense of corrosion resistance, owing to the formation of chromium-containing precipitates. An effective alternative is the so called low temperature treatments, which are performed with nitrogen- and/or carbon-containing media at temperatures, at which chromium mobility is low and the formation of precipitates is hindered. As a consequence, interstitial atoms are retained in solid solution in austenite, and a metastable supersaturated phase forms, named expanded austenite or S phase. Since the first studies, dating 1980s, the S phase has demonstrated to have high hardness and good corrosion resistance, but also other interesting properties and an elusive structure. In this review the main studies on the formation and characteristics of S phase are summarized and the results of the more recent research are also discussed. Together with mechanical, fatigue, tribological and corrosion resistance properties of this phase, electric and magnetic properties, wettability and biocompatibility are overviewed
The “Expanded” Phases in the Low-Temperature Treated Stainless Steels: A Review
No full textLow-temperature treatments have become a valuable method for improving the surface hardness of stainless steels, and thus their tribological properties, without impairing their corrosion resistance. By using treatment temperatures lower than those usually employed for nitriding or carburizing of low alloy steels or tool steels, it is possible to obtain a fairly fast (interstitial) diffusion of nitrogen and/or carbon atoms; on the contrary, the diffusion of substitutional atoms, as chromium atoms, has significantly slowed down, therefore the formation of chromium compounds is hindered, and corrosion resistance can be maintained. As a consequence, nitrogen and carbon atoms can be retained in solid solutions in an iron lattice well beyond their maximum solubility, and supersaturated solid solutions are produced. Depending on the iron lattice structure present in the stainless steel, the so-called “expanded austenite” or “S-phase”, “expanded ferrite”, and “expanded martensite” have been reported to be formed. This review summarizes the main studies on the characteristics and properties of these “expanded” phases and of the modified surface layers in which these phases form by using low-temperature treatments. A particular focus is on expanded martensite and expanded ferrite. Expanded austenite–S-phase is also discussed, with particular reference to the most recent studies
Low‐Temperature Thermochemical Treatments of Face‐Centered Cubic Alloys: New Perspectives for Expanded Austenite From Austenitic Stainless Steels to High‐Entropy Alloys
No full textABSTRACT Thermochemical treatments performed at low temperatures (< ∼500°C) have received increasing interest for the surface modification of austenitic stainless steels. In fact, when treatment media rich in nitrogen and/or carbon are used at these temperatures, the formation of chromium compounds is inhibited and the interstitial atoms are retained in the face‐centered cubic lattice of austenite beyond the solubility limit. The obtained supersaturated solid solution, known as expanded austenite or S‐phase, has high hardness and can maintain or even increase the corrosion resistance in many environments. In the international literature, many studies are present that highlight the effects of the formation of this phase on tribological properties, fatigue resistance, corrosion behavior, wettability, biocompatibility, and magnetic properties of austenitic stainless steels. However, using analogous treatment conditions, expanded austenite can be obtained in many other alloys having a matrix with a face‐centered cubic lattice, such as austenitic steels, nickel and cobalt alloys, and the more recent medium‐ and high‐entropy alloys, but the studies on this topic are mostly at their very beginning. In this review, the characteristics and properties of expanded austenite and of the modified surface layers in which it is present are analyzed and discussed, considering all the different alloys in which this supersaturated phase can be produced. The role of alloy elements in promoting or hindering the formation of expanded austenite and the competing compound precipitates is taken into account. The opportunities and challenges of the low‐temperature treatments are highlighted, and possible future directions for the investigation are suggested
The Expanded Phases Formed in Stainless Steels by Means of Low-Temperature Thermochemical Treatments: A Corrosion Perspective
No full textSurface engineering of stainless steels using thermochemical treatments at low temperatures has been the subject of intensive research for enhancing the surface hardness of these alloys without impairing their corrosion resistance. By using treatment media rich in nitrogen and/or carbon, it is possible to inhibit chromium compound formation and obtain supersaturated solid solutions, known as expanded phases, such as expanded austenite or S-phase in austenitic stainless steels, expanded ferrite in ferritic grades, and expanded martensite in martensitic grades. These low-temperature treatments produce a significant increase in surface hardness, which improves wear and fatigue resistance. However, the corrosion behavior of the modified surface layers remains of paramount importance. In the international literature, many studies on this topic are reported, but the results are not always univocal, and there are still open questions. In this review, the corrosion behavior of the expanded phases and the modified layers in which they are present is critically analyzed and discussed. The relationships between the phase composition and the microstructure of the modified layers and the corrosion resistance are highlighted while also considering the different test conditions. Furthermore, corrosion test methods are discussed, and suggestions are given for improving the measurements. Finally, perspectives on future directions for investigation are suggested for encouraging further research
Editorial Board Members’ Collection Series: Improving Structural Integrity of Metals: From Bulk to Surface
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