Corrosion and antifouling properties of copper-containing PEO coatings produced on steels

peer reviewedIn this work, antifouling copper-containing PEO coatings were produced on zinc-aluminized steels and their antifouling properties in circulating seawater were tested at the Hydrobiological Station Umberto D'Ancona located in Chioggia (Venice, Italy). The effect of the presence of the copper particles on the localized corrosion properties of the PEO coatings was also investigated in depth. In detail, the PEO-coated samples were produced and characterized in terms of microstructure and phase composition through SEM and XRD analysis. The antifouling properties of the samples were evaluated through stereo-microscope and SEM observations after up to 28 days of immersion and the corrosion properties were analyzed with EIS and SVET tests. The results, besides the successful incorporation of the copper particles into the coatings, evidenced the remarkable antifouling effect of the copper particles which also produced a clear selection in the type of algae that can colonize the samples. Considering the corrosion properties, the copper particles were found to be detrimental, due to the galvanic coupling with the substrate. Considering both results, the copper-containing PEO coating can be suggested only in combination with a topcoat which further increases the corrosion performance.9. Industry, innovation and infrastructur

Effect of salt bath nitrocarburizing and post-oxidation on static and fatigue behaviours of a construction steel

Several surface modification technologies are typically applied to improve the mechanical properties of the material surface of structural components subjected to severe conditions of load, wear and chemical erosion of the surface. The nitrocarburizing and post-oxidation heat treatment, also known as quench-polish-quench (QPQ), improves the fatigue, wear and corrosion resistance properties of the material, since it increases the surface hardness and generates surface compressive residual stresses. In the present contribution, the effects of the salt bath nitrocarburizing and post-oxidation heat treatment on the static and fatigue behaviours of 39NiCrMo3 construction steel have been investigated by experimentally testing plain as well as notched specimens. For comparison purposes, 39NiCrMo3 construction steel, both untreated and treated, and X5CrNiCuNb 16-4 stainless steel have been considered. First, the microstructure of the untreated and treated steel has been identified by metallographic analysis; micro-hardness measurements have been collected and residual stresses profiles have been obtained by using the X-ray diffraction technique. Then, experimental static and fatigue tests have been performed. Finally, the fracture surfaces have been analysed to locate fatigue crack nucleation sites

Study of the effect of functionalization with inhibitors on the corrosion properties of PEO-coated additive manufactured AlSi10Mg alloy

Plasma electrolytic oxidation (PEO) was carried out on SLM-produced AlSi10Mg alloy using various corrosion inhibitor additives. The study investigated the effects of these additives on microstructural evolution, phase composition, and corrosion behavior by comparing the functionalized coating to a standard base silicate PEO one. The results revealed that, in most cases, the additives significantly altered the coatings' thickness, density, and adherence, leading to improved corrosion resistance due to an enhanced barrier effect over electrolyte penetration. Long-term immersion electrochemical impedance spectroscopy (EIS) tests proved the additives’ corrosion inhibitor capability, as they all were able to enhance coating stability

On the microstructural evolution and phase transformation in two medium carbon high-silicon carbide-free bainitic steels

4th Mediterranean Conference on Heat Treatment and Surface Engineering (MCHTSE 2024) - Lecce, ItalyIn the last two decades, extensive research has been conducted on medium carbon steels with carbide-free bainitic microstructure, primarily due to their impressive combination of tensile properties. These steels demonstrate an ultimate strength higher than 1500 MPa and reaching elongation up to 20%. High silicon contents are included to prevent cementite precipitation from the untransformed austenite during the isothermal treatments for the bainitic microstructure formation. The exceptional mechanical performance exhibited by these steel grades are attributed to a multiphase composite microstructure consisting of ferritic bainite and carbon-enriched retained austenite. Bainitic ferrite forms during isothermal treatments via a displacive mechanism and thanks to the low isothermal temperatures (Tiso) guaranteed by the medium-high carbon content, it is characterized by submicro-nano scale thickness. This results in both, high yield and tensile strengths. On the other hand, the carbon-enriched austenite, which plays a crucial role in influencing the ductility through the TRIP effect, assumes a dual morphology: i) forming films sandwiched between ferrite subunits and ii) creating blocks positioned between the sheaves of ferritic bainite. In this work the effect of Tiso (300, 325 and 350 °C) on the microstructure and the bainitic transformation has been investigated in two medium carbon- Si rich Al bearing steels (0.38C-3.2Si-2.56Mn-0.1Al and 0.45C-2.8Si2.7Mn-0.56Al wt.%). The bainitic transformation and its kinetic was investigated by dilatometry, while a complete microstructural characterization was performed using optical microscopy, SEM, XRD and HV10 measurements. It emerged that the kinetic of the bainitic ferrite formation, the volume fraction and the morphology of the microstructural constituents is affected by both the chemical composition and the Tiso

Effect of Multi-Step Austempering Treatment on the Microstructure and Mechanical Properties of a High Silicon Carbide-Free Bainitic Steel with Bimodal Bainite Distribution

The effect of multi-step austempering treatments on the microstructure and mechanical properties of a novel medium carbon high silicon carbide-free bainitic steel was studied. Five different isothermal treatment processes were selected, including single-step isothermal treatments above martensite start temperature (at 350°C and 370°C, respectively), and three kinds of two-step routes (370°C + 300°C, 370°C + 250°C, and 350°C + 250°C). In comparison with single-step austempering treatment adopting a two-step process, a microstructure with a bimodal-size distribution of bainitic ferrite and without martensite was obtained. Bainitic transformation was studied using dilatometry both for single-step and two-step routes and the specimens were completely characterised by electron microscopy (SEM and TEM), X-ray diffraction (XRD) and standard tensile tests. The mechanical response of the samples subjected to two-step routes was superior to those treated at a single temperature

L-PBF fabrication of a novel high silicon carbide-free bainitic high strength steel

peer reviewedCarbide-free Bainitic microstructures are object of extensive research, due to the optimum combination of tensile mechanical properties, UTS exceeding 2000 MPa and elongation up to 20%. These are Si-based steels and incredible performances of bainite derive from the composite multiphase microstructure, consisting of submicro-nanoscaled bainitic ferrite, and carbon enriched retained austenite. Bainitic ferrite, formed via a displacive mechanism, provides a major contribute to strength from a bainitic ferrite with nano- or submicrometric thickness. Whereas carbon-enriched austenite is characterized by a two-fold morphology: i) films located between the ferrite sub-units and ii) blocks located between the bainitic ferrite sheaves. Moreover the addition of silicon contributes to the carbon enrichment of austenite since it inhibits cementite precipitation and improve the thermal stability of the untransformed austenite. Austenite plays a crucial role on the ductility of bainitic steels. Strain-induced martensitic transformation occurring under load and formation of twins in austenite enhances the total elongation and the strain hardening of the steels. In the latest years, additive manufacturing processes have gained significant importance due to the possibility to fabricate components with complex geometry and high material efficiency. In addition, it permits rapid prototyping, enables the production of highly customized and personalized parts for a large span of industrial sector and application. In this work new medium-carbon high silicon carbide-free bainitic steels have been fabricated through L-PBF process. The process parameters were optimized in order to obtain high density parts. Furthermore, the effect of the substrate preheating and a in situ isothermal heat treatment on the density, fabrication defects, and the microstructure have been investigated. OM, SEM, X-ray Diffraction, Image analysis were utilized to perform the material characterization. The results showed that substrate preheating and in-situ isothermal heat treatment leads to the formation of a carbide-free bainitic microstructure without requirement of post-fabrication heat treatment, that implies further energy consumption, sample oxidation and final machining to obtain the opportune quality. Furthermore, the substrate preheating lead to obtain components with lower amount of porosities and cracks deriving from the fast cooling rates and the developed residual stresses