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Alloy hardened steels offer excellent combination of mechanical properties, hardenability and corrosion resistance. 34CrMo4 is a medium carbon, low alloy steel widely used due to a good combination of high-strength, toughness and wear resistance. However, this steel experiences hydrogen embrittlement (HE), a complex phenomenon depending on the composition and microstructure. This work estimates de loss of the mechanical properties caused by hydrogen in electrochemically H-charged specimens in absence of mechanical stress but also, at low strain rate and constant load. H-charging for 2 and 6 h induce YS losses of about 40% and 71% and UTS losses of 39% and 59%, respectively. The synergistic effect of the stress and the H-charging process leads to a higher loss, 91%, and a faster brittle fracture even though hydrogen content is similar to those firstly H-charged and then tested in air.

Laser texturing can be used for surface modification of metallic alloys in order to improve their properties under service conditions. The generation of textures is determined by the relationship between the laser processing parameters and the physicochemical properties of the alloy to be modified. In the present work the basic mechanism of dimple generation is studied in two alloys of technological interest, titanium alloy Ti6Al4V and aluminium alloy AA2024-T3. Laser treatment was performed using a pulsed solid state Nd: Vanadate (Nd: YVO) laser with a pulse duration of 10 ps, operating at a wavelength of 1064 nm and 5 kHz repetition rate. Dimpled surface geometries were generated through ultrafast laser ablation while varying pulse energy between 1 µJ and 20 µJ/pulse and with pulse numbers from 10 to 200 pulses per spot. In addition, the generation of Laser Induced Periodic Surface Structures (LIPSS) nanostructures in both alloys, as well as the formation of random nanostructures in the impact zones are discussed., This work was supported by the Spanish Ministry of Science and Innovation [SMOTI MAT2009-13751] and the Spanish Ministry of Economy and Competitiveness [MAT2013-48224-C2-1-R-MUNSUTI]. Authors also acknowledge to Professor G. Dearden from the Laser Engineering Group, University of Liverpool, for his support for this study.

Ti-6Al-4V alloy was laser textured with cross groove patterns and then coated with graphene or MoS2, evaluating its tribological behaviour. The samples were textured by a solid state Nd:Vanadate (Nd:YVO4) laser, making crossed grooves with two different crossing angles, 45°and 60° shaping a rhombus-like pattern. The percentage of surface area textured was 18, 40 and 64%. Solid lubricants were deposited by the cloth burnishing technique. Bidirectional sliding wear tests were performed to evaluate the tribological behaviour of the alloy. Lubricant working lifetimes were longer on textured and coated surfaces than on polished surfaces. Graphene presented longer lifetime than the MoS2 for 18 and 40% of textured density but rather similar for the 64%., This work was supported by the Spanish Ministry of Economy and Competitiveness [MAT2013-48224-C2-1-R-MUNSUTI]. Authors also acknowledge Prof. José Vicente García Ramos from IEM-CSIC for his insightful comments on Raman spectroscopy and Prof. G. Dearden and Dr. W. Perrie from the Laser Engineering Group, University of Liverpool, for their support in this study.

© 2015 Elsevier B.V. Friction and wear rate behaviour of dual phase steel plate specimens (DP600) sliding against corundum balls, as a function of sliding speed (0.01-0.25. m/s) and contact frequency (1-16. Hz), has been performed under bidirectional ball-on-plate (reciprocating motion) dry sliding wear tests where the plate specimen is studied. The wear mechanism is mainly oxidative. The coefficient of friction and the wear rate show not only a high dependence on the sliding speed, but also on the contact frequency which appears to be the key factor in determining the wear behaviour of DP600 even at a constant sliding speed. A thorough assesment of the wear damage as a result of energy dissipation due to friction at the interface is also carried out., The authors gratefully acknowledge the funding by Ministerio de Economía y Competitividad (MINECO) under project Innpacto IPT-020000-2010-020 and project CONSOLIDER-INGENIO 2010 CSD 2008-0023 FUNCOAT.

Polypyrrole coating was successfully deposited on anodized 2024 unclad aluminum alloy showing that the presence of the anodic film is the key factor to ensure better adhesion to the polymer coating. Corrosion resistance of the conductive polymer layers grown for three and five cycles were evaluated in a 3 wt.% NaCl solution using polarization curves and impedance spectroscopy. It was found that the thermally treated polymer coating with higher thickness (five cycles) exhibited the best corrosion performance. This best coating shifts the corrosion potential towards nobler values, about 650 mV, and the exchange current density decreases two orders of magnitude regarding the anodic layer. The partial reduction of the structure of the polymer promoted by the thermal treatment plays a key role in the corrosion resistance of the coating allowing to the thermally treated polymer layer to act as a physical barrier against corrosion. © 2007 Elsevier B.V. All rights reserved.

The meat casing industry works with highly fibrous materials and low pH conditions. The pumps propelling the gel must therefore be highly corrosion-erosion resistant. In fact, the most common failure in this medium is corrosion, erosion or, most commonly, a combination of both. Coatings are thus required to protect the pump surface and maintain the operational conditions. TiN-PVD coatings seemed to be an attractive alternative amongst the range of options to extend pump lifetime. However, despite the greater chemical inertia expected of TiN, the combined effect of corrosion and erosion can induce a fast attack on the material. The present paper analyses failure of a TiN coating with a single-layer configuration in contact with an edible slurry of collagen used in the meat casing industry. The degradation mechanism comprises two different steps. Firstly, the acidic components reach the base steel through the pores, promoting severe dissolution of the steel substrate and leaving the TiN coating without support. Secondly, the coating detaches under working conditions and merges with the casing solution. Finally, the fibres together with the hard particles erode the pump walls, leading to fast deterioration. © 2006 Elsevier B.V. All rights reserved.

Corrosion and tribocorrosion studies of a TiBxCy/a-C coating deposited on AISI 316L steel have been performed in an aqueous solution of 0.26 vol.% acetic, 0.16 vol.% peracetic and 0.18 vol.% hydrogen peroxide (commercial product Oxonia 1 vol.%). The corrosion current density of the TiBxCy/a-C coating ranges on the same order as bare steel but with a significantly decreasing friction (0.1 vs. 0.6) and wear rate (~ 10 times lower). The compact microstructure of the coating hinders the access of the aggressive electrolyte to the substrate, preventing the onset of the corrosion attack, while maintaining an excellent tribological behavior in strong oxidant solutions.

CrN multilayer coatings are possible replacements for hard chromium because of their good mechanical and chemical properties. This paper focuses on X-ray photoelectron spectroscopy (XPS) to study the mechanism of electrochemical stripping of multilayered Cr-N based coatings. The samples were produced by electron beam (e-beam) physical vapour deposition (PVD), on mirror polished M2 steel. Multilayers constituted three separate layers of CrN with intermediate interlayers of Cr (0.1 μm). The total coating thickness was in the range 4.5-5.5 μm. Under galvanostatic conditions, anodic oxidation of CrN leads to replacement of nitrogen in the CrN coating by oxygen, forming oxy-nitrides on the samples surface. Contrary to findings described by some authors, we have not detected Cr6+ in this research. © 2006 Elsevier B.V. All rights reserved.

The present paper analyses the performance of CrN single-layers produced by electron beam PAPVD (EBPAPVD), finding that both corrosion and wear resistance are directly dependant on the structure and stoichiometry of the nitride. The nanolayer structure of the coatings is formed by periodically varying the nitrogen pressure during deposition resulting in layers with higher and lower N-content. This fact, which has not been described in the literature, causes different structure and morphology of the individual films providing excellent properties to the coating. For corrosion resistance, the CrN layer's greater compactness impedes penetration of the electrolyte and thus prevents the formation of a galvanic couple between the coating and the substrate. Moreover, a good wear resistance is obtained, retarding its delamination. © 2006 Elsevier B.V. All rights reserved.

In this work, the influence of the processing conditions on the microstructure and abrasive wear behaviour of a NiCrBSi hardfacing alloy is analysed. The hardfacing alloy was applied in the form of coatings onto a mild steel substrate (Fe-0.15%C) by different techniques: laser cladding (LC) and flame spraying (FS) combined with surface flame melting (SFM). In both cases, the appropriate selection of the process parameters enabled high-quality, defect-free NiCrBSi coatings to be obtained. The microstructure of the coatings was analysed by scanning electron microscopy (SEM), with attached energy dispersive spectroscopy (EDS) microprobe, and by X-ray diffraction (XRD). Their tribological properties were evaluated by micro-scale ball cratering abrasive wear tests using different abrasives: diamond, SiC and WC. Microstructural characterisation showed that both coatings exhibit similar phases in their microstructure, but the phases present differ in morphology, size distribution and relative proportions from one coating to another. Wear tests showed that in three-body abrasive conditions, despite these microstructural differences, the wear behaviour is comparable for both coatings. Conversely, in two-body wear conditions with diamond particles as the abrasive, it was observed that the specific wear rate of the material is sensitive to microstructural changes. This fact is particularly apparent in LC coatings, in which the zones of the layers with higher proportions of very small hard particles present a lower wear resistance. These results indicate that it is important to have good microstructural control of this material, in order to obtain coatings with an optimized and homogeneous tribological behaviour. © 2005 Elsevier B.V. All rights reserved.

Galvanized reinforcing steel with a cerium conversion coating have been studied in Ca(OH)2 saturated solution with and without chlorides. Electrochemical results reveal that cerium conversion coating provides an effective corrosion resistance compared to galvanized steel at short immersion times, 5 days without Cl- ions and 2 days with Cl- in the solution. The results suggest that cerium layer inhibits hydrogen evolution on the galvanized coating at early stages. At longer immersion times, galvanized steel with cerium conversion coating and galvanized steel describe similar corrosion behaviour in both electrolytes. There is not significant differences in the corrosion current density, about 5 μA/cm2, due to the presence of chlorides ions in the Ca(OH)2 saturated solution up to approximately 17 days of immersion. At longer immersion times, from 30 to 50 days, specimens in the chlorides containing solution exhibit higher corrosion activity than that recorded in the free Cl- solution revealing that cerium layer cannot inhibit the localized attack promoted by chloride ions. © 2006 Elsevier Ltd. All rights reserved.

The laser cladding of diesel engine exhaust valves is an emerging technology that provides excellent results in terms of the wear and corrosion resistance of the repaired materials. This work focuses on the deposition by laser cladding of Stellite 6 alloy coatings on two different base materials: an AISI 1045 carbon steel and an AISI 304 stainless steel. Following a process optimisation criterion, several of the fundamental laser treatment parameters were varied in order to obtain defect-free coatings with good adherence and minimal dilution. The metallographic structure of the clad material, irrespective of the substrate used, consists of a typical dendritic microstructure in a solid solution of Co. The mechanical strength of the layers was analysed by measuring HV0·1 microhardness and sliding wear behaviour. Laser coatings achieved hardness greater than 500 HV0·1, and presented excellent dry sliding wear behaviour, with a dimensional wear coefficient k one and a half orders of magnitude lower than that of the substrates. A combined oxidation and abrasion mechanism dominated the wear of the Stellite 6 layers, while in the case of AISI 1045 steel substrate, the wear was caused mainly by a surface oxidation mechanism. For the AISI 304 steel, the wear resulted from a combined adhesion, abrasion and plastic deformation mechanism. © 2006 Institute of Materials, Minerals and Mining.

[ES] El presente trabajo se centra en la obtención de capas de aleación base Cobalto (Tribaloy T-800) sobre un sustrato de acero inoxidable 18/8 (AISI 304) mediante la técnica de plaqueado láser. Tras los tratamientos, se caracterizaron las probetas mediante microscopía óptica y electrónica de barrido con microanálisis (EDS). La composición elemental del recubrimiento se determinó en un espectrómetro de emisión óptica con fuente de excitación (GDL). El estudio de la interfase reveló una perfecta adherencia entre el recubrimiento y el material base, sin defectos apreciables. La microestructura de las capas depositadas es dendrítica con un alto grado de refinamiento y una homogeneidad química a lo largo del cordón y con valores muy próximos a los del polvo de partida. En las zonas de solape entre cordones, se observó un crecimiento del grano debido al segundo tratamiento térmico recibido. La microdureza de las capas alcanza los 750 HV, valor considerablemente superior al del sustrato (200 HV). Asimismo, se obtuvo una mejora significativa de la resistencia a la corrosión del recubrimiento en medios salinos., [EN] The present work is based on obtaining Co based coatings (Tribaloy T-800) on plates of 18/8 stainless steel (AISI 304) by laser cladding technique. After the treatment, samples were characterized by optical and scanning electron microscopy with EDS analysis. The elemental composition of the coating was determined with a glow discharge lamp spectrometer (GDL). The study of the interface revealed a good adherence between the substrate and coating without substantial defects. For the laser cladded coatings, the microstructure was dendritic with a high degree of refinement and chemical homogeneity close to the original powder. The grain coarsening was observed in the overlapping zones due to the second heat treatment. Microhardness of the coatings reached 750 HV, a considerably higher value than the substrate hardness (200 HV). Also, the coating corrosion resistance in saline solutions had a great improvement., Al Proyecto del Plan Nacional de Investigación Científica, Desarrollo e Innovación Tecnológica 2000-2003, de referencia: MAT2001-3528-C03-02 de título: Optimización frente al desgaste y la corrosión de recubrimientos de NiCrBSi depositados por flame spray y láser.

To reduce the recovery or the replacement costs of diesel engine exhaust valves, they are manufactured with an economic base material, and a coating which is deposited on the seat valve in order to reach high hardness and good impact, corrosion and high temperature wear resistance (>550 °C) and without lubrication. In this work, appropriate laser cladding parameters have been determined to obtain Stellite 6 coatings over AISI 304 steel (as plane test specimens) and SAE EV8 steel (as valves) substrates. One and two superimposed tracks were deposited on the seat valves, and modifying the laser power as a function of the rotated angle at the beginning and the end of the circular tracks, pores and cracks have been minimized and the thickness of the track were made uniform. Hardness, dilution and final microstructure of the different coatings have been analysed. A 10 % dilution and 550 HV in the tracks over plane test specimens was observed, while valves with one track showed 25 % and 430 HV respectively. With two superimposed tracks the hardness was up to 470 HV in the upper track.Para abaratar el coste de recuperación o sustitución de válvulas de escape en motores diesel, estas se fabrican con un material base económico, recubriéndose el asiento de la válvula con otro material al que se exigirá elevada dureza y buena resistencia al impacto, a la corrosión y al desgaste erosivo en caliente (>550 °C) y sin lubricación. Partiendo de esta idea, en el presente trabajo se han determinado los parámetros adecuados para realizar, mediante la técnica de plaqueado láser, un recubrimiento con Stellite 6, sobre sustratos de acero AISI 304 (probetas planas) y SAE EV8 (válvulas reales). Sobre las válvulas, se depositaron uno y dos cordones superpuestos, se minimizó la presencia de poros y grietas, modificando el grado de solape inicial y final de los cordones circulares y la potencia en función del ángulo girado. Así, se consiguieron cordones de espesor uniforme. Tras los tratamientos, se estudiaron la dureza, dilución y microestructura final de los distintos recubrimientos, observándose, en los cordones sobre probetas planas, diluciones del 10 % y durezas de 550 HV, mientras que, en las válvulas con un cordón, eran del 25 % y 430 HV, respectivamente. Con dos cordones superpuestos, la dureza llegaba a 470 HV en el cordón superior.

Surface hardening thermo-chemical treatments provide metallic alloys with very hard corrosion and wear resistant coatings. As the layers obtained are so thin that dissuade from any machining operation after the treatment, it is useful to evaluate possible dimensional changes. This work studies the dimensional stability of alloy Ti6A14V after gas nitriding at 1100 °C and of superalloy MA956 after surface oxidation at the same temperature. For reference purposes, an analysis is also made of the deformations that appear on steel after conventional pack carburising treatment. In this study flat test specimens of 3 mm thickness were used, each of which had central drilled holes surrounded by six smaller outlying drilled holes. A calculation program allows the determination of changes in the drilled hole diameters and in the position of their centres as a consequence of the treatment. © 2002 Elsevier Science Ltd. All rights reserved.

Ti-Al intermetallic coatings on titanium have been produced by laser surface alloying. A 5-kW CO2 laser was employed to melt the surface of the titanium specimens and to alloy it with aluminium powder in a nitrogen atmosphere. The combination of titanium, aluminium and nitrogen and the specific characteristics of the high-power laser allow a new coating structure of titanium aluminides and nitrides to be produced. These coatings present a surface Al-TiAl3 composition that gradually evolves to Ti3Al close to the titanium base. A dendrite structure of titanium nitrides (TiN and Ti2N) and titanium aluminium nitrides (Ti2AlN) is formed throughout the laser track. The different structures obtained are described as a function of laser energy input during the process. © 2002 Elsevier Science B.V. All rights reserved.

Stellite layers produced for protective coatings in a direct process by laser remelting of the powder SF6 on plates of a high-temperature resistive alloy in Ar environment are investigated experimentally. Microphotographs and EDS analysis reveal a metallurgical bond between the substrate and coating as well as the structural homogeneity along the depth of the layer which is characterised by a chemical composition close to that of the powder. The laser-clad layers show a dendritic and fine-grained structure with a minor presence of impurities. For the coatings an improvement of mechanical properties in comparison to the base material such as the behaviour in wear resistance comparable to that of TiN and values of microhardness up to approximately 1000 HV are observed. On the other hand, corrosion properties depended on the defects and appeared on the surface as a result of the laser process parameters. © 2002 Elsevier Science Ltd. All rights reserved.

Electrochemical noise has been used to study the tribocorrosion behaviour of austenitic stainless steel AISI 316 in two different simulated body fluid solutions [phosphate buffer solution (PBS) and PBS plus bovine serum albumin). The analysis of potential and current transients permits not only the determination of the contribution of the aggressive medium to the total volume loss due to synergism between wear and corrosion but also the detection of changes in the mechanism produced by the mechanical/electrochemical attack. © 2014 Institute of Materials, Minerals and Mining.

TiN coatings were prepared by laser surface alloying of commercially pure (c.p.) Ti and Ti6Al4V alloy in a nitrogen atmosphere. Surface gas alloying was carried out using a 5 kW Spectra–Physics transverse flow CO2 laser operating in TEM 20 at a scan rate of 0.3 m\min and an input flow of 30 l\min. After laser gas alloying, surface tracks of a light gold colour could be seen by the naked eye. The microstructure of the Ti alloy revealed a surface layer of TiN dendrites in a nitrogen-enriched alpha matrix covering the melted surface. The corrosion properties of the material are discussed through the results of anodic polarization test and electrochemical impedance spectroscopy in HCl solution. An improvement in corrosion behaviour, associated with the presence of a good TiN coating, was observed.

The susceptibility of alloy 8090 T8171 to exfoliation corrosion has been analysed using the electrochemical impedance spectroscopy technique (EIS). Corrosion susceptibility was determined in a standard medium (EXCO tests). This acid solution creates a very severe attack in a short period of time and makes it possible to distinguish the different stages of the corrosion process. The EIS technique enables collection impedance spectra which show a variation with increasing exposure time and so is appropriate for recording these stages. The Nyquist diagram is initially described by a well defined capacitive semicircle, which suggests a corrosion process under activation control. This response is linked to the existence of small pits on the surface of the working electrode. However, when the immersion time increases, the attack causes the appearance of large pits on the electrode, leading to an impedance response comprised by two overlapping semicircles; the high-frequency semicircle related with the flat part of the working electrode and the low frequency arc associated with the part covered by pitting. © 1997 Elsevier Science Ltd. All rights reserved.

Electrochemical Impedance measurements in the range of 50 kHz to 50 mHz were applied to the study of an Al-Cu-Mg aluminium alloy (2024) in T4 condition and naturally aged for 6 y. Since the first hours of testing, the alloy was in an active state with high corrosion rates. At the beginning, the shape of the impedance plots suggested that the system is under charge transfer resistance control. However, when the experimental time increased, the system changed towards a mixed control. The impedance of this system is characterised by a high frequency capacitive arc which gives information on the performance of the unattacked surface. At lower frequencies, a diffusion control is apparent suggesting the specific contribution of the surface in which the localised attack takes place. A good agreement was found between the experimental results and a model circuit. © 1997 Elsevier Science Ltd. All rights reserved.

This paper describes a study of the influence of the butyl group as substituent in n-butylamine used as a mild steel corrosion inhibitor in hydrochloric acid solution. An inhibition mechanism is proposed. n-Butylamine, dibutylamine and tributylamine were studied in concentrations from 10−4 to 1 m and in the temperature range 278–308K using electrochemical, gravimetric and surface roughness techniques. The three inhibitors adsorbed on the mild steel according to a Frumkin isotherm.

Laser surface cladding has been carried out by means of a 5 kW CO2 continuous wave laser on both a mild and a stainless steel. A powder injection technique has been used to deliver a Ni-Cr-Al alloy onto the steel molten pool. After processing, samples were analyzed by optical and scanning electron microscopy in order to know the microstructure and composition of present phases. High temperature corrosion resistance tests were done in an oxidant environment at temperatures of 950 °C. Phase transformations and corrosion behaviour are discussed. A general conclusion is the suitability of these alloys to bear oxidation due to the formation of protective oxide layers on their surface.Se realiza un estudio sobre la obtención de recubrimientos de superficie mediante un láser continuo de CO2 de 5 kW de potencia de salida. Mediante la técnica de inyección de partículas, se realizaron plaqueados en superficie de Ni-Cr-Al sobre un acero suave y otro inoxidable de tipo 316. Tras el procesado, se estudió la microestructura de las probetas obtenidas. Posteriormente, se procedió a su ensayo en atmósfera oxidante a 950 °C, observándose la evolución de la microestructura, la formación de las capas de óxido y la cinética del proceso. Como conclusión general, destaca, independientemente de la base sobre la que se obtienen los recubrimientos, la gran resistencia del material a la oxidación, seguramente debida a la formación de capas de alúmina que actúan como barreras frente al ataque del oxidante.

High-power laser cladding has been used to prepare NiCrAl alloy plus zirconium oxide composite layers on an Incoloy 800H base. The metallographic structure before oxidation, the evolution of initial phases and the high-temperature oxidation (1300 K) behaviour of the coating were studied by means of optical and scanning electron microscopy. The results indicate that the new laser-produced alloy has a better oxidation resistance than the base metal as its weight gain is lower. This appears to be due to the formation of ceramic and metallic oxides on the surface of the specimen, which prevent the spread of alloy elements towards the exterior, and the entry of oxygen into the material.

Laser surface cladding of a NiCr powder mixture onto a 316 stainless steel was carried out. The coating microstructure, depth and composition were studied. The main variables of the process (scan speed and powder feed) were linked to the roughness, hardness and pitting corrosion properties in a saline solution of the layers obtained. A comparison with samples of a similar composition and the base stainless steel is given. Finally, resistance to oxidation at 900°C is tested. By optimizing the laser cladding parameters, layers with a high-quality surface can be produced. The new coating enhances mechanical and corrosion properties of the base materials.

Summary The properties of a stainless steel coating produced on a carbon steel using the high‐power laser surface cladding technique were studied. The cladding was obtained by injecting type 316 stainless steel powder. The composition and properties of the new coating were similar to those of a stainless steel of the same type obtained by conventional methods.

The processing and microstructural characteristics of nickel-based superalloys obtained by high-power laser surface cladding were studied. The processing variables were laser power, scan velocity and the composition of the feeder powder. The initial compositions consisted of a mixture of NiCrAl (71/18/11% by weight) and NiCrAl plus zirconium oxide composite (66/16/12/4% by weight). Following the process, the clad layers were characterized using analytical tools such as optical and scanning electron microscopy and thickness, substrate/ coating interface, cracks and their hardness.

The corrosion properties of hydrothermally sealed anodised 2024 aluminium alloy specimens in tartaric-sulphuric acid (TSA) and sulphuric acid (SA) are investigated using electrochemical techniques. A new mechanism is proposed to describe the improvements observed for the sealed porous layer grown in the mixed electrolyte. This mechanism involves the dissociation of the residual organic acid present inside the pores as a consequence of the pH increase during hydrothermal sealing (HT). Tartrate ions form chelate complexes with Cu(II) cations embedded in the oxide film. This results in a decrease in the heterogeneity of the film and consequently, better protective properties. © 2010 Elsevier Ltd All rights reserved.

Laser surface treatments are an attractive alternative for the production of new materials with high mechanical and corrosion-resistance properties. This technique permits the production of materials with a high added value on low-cost materials. The present research is aimed at obtaining NiCrAl alloys on iron-based materials using a continuous CO2 laser with 5–7 kW power. The microstructural characteristics and influence of iron on the type of phase in the new alloy is also analysed.

The steel used in high tension electricity pylons is usually protected against corrosion by hot dip galvanising. Because the majority of the material is exposed to rural conditions, its life span is satisfactory and it requires no maintenance. The life of the zinc coatings may initially be regarded as proportional to their mass regardless of the coating procedure used. This simplification hides the reality of differences in behaviour related to the structures of the coatings, and may result in different values for varying corrosive sources which are not always simple to identify. The present paper analyses the state of zinc coatings on steels which have been exposed to the elements for periods of 18–19 years in rural environments in Spain. The coatings with a richer iron structure resisted corrosion more effectively.

At present, one of the most important problems in automobile engines and transmission components is due to tribological processes (friction and wear) that in many cases come accompanied by corrosion processes due to the environmental conditions to which these materials are exposed during their lifetime. Both mechanisms can be minimized by means of the development and the application of adequate coatings that combine low friction with a high corrosion and wear resistance. The new tendencies in industrial PVD coatings to improve their properties are focused in the development of new multilayer and nanostructured coatings. These structures allow in a relatively simple way enhancing their tribological properties and the corrosion resistance that can not be reached by means of the traditional monolayer coatings. The background of this type of coatings consists of the stacking up of several layers with good individual tribological and mechanical properties, but every individual layer has a thickness that can be from hundreds of nanometres down to only 5-10 nm. The properties of these nanostructured coatings depend strongly on the thickness modulation of every individual layer. Concerning PVD coatings, the chrome nitride coatings have demonstrated to possess excellent wear resistance properties. In this work, multilayer Cr/CrN coatings with different individual layer thickness have been deposited on substrates of steel F1272 and silicon. The deposition has been carried out by means of the cathodic arc method alternating an atmosphere of pure Ar with a reactive mixture of N2/Ar. The multilayers obtained have been analyzed by means of Glow Discharge Optical Emission Spectroscopy (GD-OES) and in some cases by means of FE-SEM obtaining bilayer (Cr/CrN) periods of the order of 220 and 45 nm. The coating characterization has been complemented with hardness and composition measurements as well as by the performance of several wear and corrosion-wear tests. © 2008 Elsevier Ltd. All rights reserved.

In the recent years, solar energy has started to be considered as a useful energy resource for industry. Outside of its traditional applications, domestic heating and electricity productions, there are scarce references to topics like Surface Treatment of Metals. When solar energy is adequately concentrated, a sufficient energy density is reached to affect a metal thermically, from a mere surface heating to a total fusion depending on the power and duration of the treatment. The present paper sets out the initial results of work on treatment of carbon steel carried out at the Almeria Solar Platform. The results indicate that the maximum temperature (close to 1200 ° C) is reached only a few seconds or minutes after the beginning of the test as a function of thickness and flux density. This suppose an enormous advantage over conventional thermal treatment in terms of time. Another aspect of interest is that the energy is free of charge but, on the other hand, we cannot control it as we like because we depend on weather characteristics. Maximum hardness was found on the metal surface (950 HV) with a treatment depth of between 1.5 and 2 mm. The conclusions reached are therefore that solar energy treatment is ideal for production of structural modifications and even the formation of surface alloys. © 1991.

Conversion treatments based on immersion in Cr(VI) aqueous solutions are key technologies that combine low cost, easy application and high performance. However, they are environmentally problematic due to their carcinogenity and genotoxicity. Among the potential alternatives, treatments based on rare-earth compounds have drawn attention due to the stability of their oxides and their environmental acceptability. Despite the amount of work published, there is not yet an industrially suitable alternative treatment for aircraft aluminium alloys that is able to provide the required corrosion protection. A common feature of these alloys is the high level of copper in their chemical composition. Although the presence of copper in an alloy may enhance cerium deposition, high copper content alloys (AA2024, AA7075) have proven the most difficult to protect with Ce conversion coatings. In the present work, a commercial 8090-T8 aluminium alloy containing 1.15 weight percent (wt.%) Cu was coated with a Ce conversion coating at room temperature from a Ce bath without prior pretreatment of the specimens. Polarisation curves revealed that the presence of a cerium conversion coating (CeCC) reduces by two orders of magnitude the corrosion rate of the AA8090 alloy in a sodium chloride solution. Impedance measurements exhibited capacitive behaviour for the CeCC up to 216 h, showing that the cerium layer protects the bare alloy in the aggressive solution. Electrochemical tests have therefore revealed that these conversion layers afford long-lasting protection; withstanding up to 168 h in Salt Spray test. © 2008 Elsevier Ltd. All rights reserved.

The role of pre-treatment in the formation of a cerium conversion coating is investigated for the protection of AA2024-T3 and 7075-T6 alloys. The alloys were alkaline-etched and de-smutted in nitric acid, prior to cerium treatment in Ce(NO3)3 at 85 °C with H2O2 accelerator. Potentiodynamic polarization studies in 3.5% NaCl solution revealed a large shift of ∼ 300 mV of the corrosion potential below the pitting potential for the 7075-T6 alloy, which correlated with the development of a finely-textured, uniform coating. However, the formation of a uniform coating and protection was dependent upon the time of de-smutting, with non-uniform coatings resulting from extended times of de-smutting. In contrast, non-uniform coatings developed on the 2024-T3 alloy, with pitting potential at the corrosion potential, irrespective of the time of de-smutting. Findings for the 2024-T3 alloy indicate that extended de-smutting affects the enrichment of alloying elements. © 2008 Elsevier B.V. All rights reserved.

Selective attack in an AISI 304 stainless steel weld metal has been developed after three months in service in well water. Welding zones showed a severe corrosive attack that in some cases led to the steel perforation. Optical and scanning electron microscopy (SEM/EDX) revealed a selective attack. An in-depth analysis showed indications of microbiological activity which could be responsible of the severe attack. © 2006 Elsevier B.V. All rights reserved.

The improvement of the corrosion, wear and tribological behaviour by searching for new materials has entailed a greater expansion of PVD technology. Introduction of PVD systems in the industry is a real fact because this technique has been able to develop coatings whose response for many applications goes beyond used them until now. However the evolution of this technology requires minimize the economic costs in order to be a competitive process. A way to achieve this objective consists on recover the tool when the life time of the coating has finished or when deposition process has failed. Bringing this idea in the industry requires the stripping methods’ development due to surface’s tool must be clean —without coating’s remainders—, before a new deposition process. Current work shows galvanostatic stripping of chromium nitride coating deposited by two different PVD techniques: cathodic arc and magnetron sputtering. The electrochemical method has exhibited more advantages than chemical methods reported in literature, such a high sensitivity to detect final point of CrN coating stripping, capability to determinate structure of the coating —number of coatings, chemical composition, thickness…—, short times of the process and minimum damage on the substrate. These entire characteristics do possible think about a new deposition process on the stripped tool.

This work presents a novel approach of sliding ball-on-disk wear tests where the disc material is investigated. Each part of the wear track on the disc is in discontinuous contact with the counterbody. The contact frequency at each part of the wear track on the disc with the counterbody is defined by the rotation frequency of the disc. The sliding speed is however a function of both the rotation frequency and wear track diameter. In this work, the effect of the contact frequency on friction and wear was investigated on carbon steel in discontinuous sliding contact with corundum balls. Various sliding speeds were used while maintaining the contact frequency at a fixed value, and various contact frequencies were applied at constant sliding speeds. The wear rate of the disk material is shown to depend not only on the usual wear test parameters, namely sliding speed and contact load, but also on contact frequency. Moreover, contact frequency is shown to be a key factor determining the wear mode even at constant sliding speed and load. At contact frequencies above 9 Hz, the dominant wear mechanism is oxidational wear, while at frequencies below 4 Hz the dominant wear mechanism is adhesive wear. This transition from adhesive to oxidational wear takes place together with a change in the type of debris generated and in the value of the coefficient of friction. The validity of the Garcia-Ramil-Celis model proposed earlier for discontinous sliding contact conditions, is demonstrated for the case of carbon steel disks sliding against a chemically inert counterbody. © 2005 Elsevier B.V. All rights reserved.

The present paper focuses on the study of SCC behaviour of a new Al-Cu-Li alloy. For this purpose, two conventional media - NaCl and NaCl + H2O2 - were used for comparison with commercial alloys 7075 and 8090. This new alloy shows lower susceptibility to SCC than conventional alloys as it does not undergo environmentally-induced embrittlement in NaCl solutions and in 1 M NaCl + 0.3% H2O2 in which the 7075 and 8090 alloys, respectively, undergo environmentally-induced fracture. Solution composition was modified in order to determine the environmental conditions and strain rates under which this new alloy will crack due to a stress corrosion cracking phenomenon. The addition of 0.6 M sulphates to 1 M NaCl + 0.3% H2O2 solution allows the definition of a range of strain rate (between 10-7 and 10-6 s-1) in which this new alloy undergoes stress corrosion cracking. © 2006 Elsevier Ltd. All rights reserved.

After a biologically induced corrosion in a 304 stainless steel tube electrochemical tests were carried out on the affected areas to characterise the nature of attack of the steel covered by the excreted exopolymers. Once the corrosive attack has been developed, the samples were removed from the solution, dried out for 1 month and re-immersed in the same well water which caused the attack. Polarization curves and impedance measurements made on non attacked welds, reference stainless steel and the metal covered by the exopolymer confirm that a reactivation of the corrosion process in previously damaged welded areas is produced. However, the corrosive attack has not been so severe than the initially produced by bacteria and after 28 days of immersion the progress of the attack is probably due to the previous activation of the steel surface rather than the microbiological activity. © 2006 Elsevier Ltd. All rights reserved.

Galvanised and galvannealed steels are widely used due to their good corrosion resistance in aqueous solutions. However, when additional protection is required, organic coatings, corrosion inhibitors or conversion coatings are used to improve their corrosion protection. In this work, sol-gel coating was used to improve the corrosion behaviour of these two materials. This paper analyses the final protective properties of a sol-gel coating prepared by basic catalysis and its dependence on the sintering temperature and time of treatment. The influence of the sintering conditions on the galvanised and galvannealed substrates is a decisive factor for the coating quality and for the barrier affect against the aggressive media. While heat treatment time is the controlling factor for the galvannealed steels, the temperature is determining in the case of the galvanised. Corrosion mechanisms for sol-gel galvanised steels did not changed with respect to the uncoated steel. However for galvannealed steel, after coating the mechanism is not purely cathodic. © 2006 Springer Science + Business Media, Inc.

Aluminium-lithium alloys are suitable for aeronautical purposes because of their good mechanical properties and high damage tolerance. Although these alloys are less susceptible to stress corrosion cracking than conventional alloys, Al-Li-Cu-Mg alloy (8090-T8171) still experiences this problem in a NaCl + H2O2 solution. In this work it has been demonstrated that the addition of 10,000 ppm of CeCl3 to the medium inhibits the stress corrosion cracking of 8090 alloy by precipitation of cerium oxides/ hydroxides. The deposition of these compounds on the alloy surface decreases the pit density and slows the crack growth through the grain boundaries by hindering the anodic dissolution of T phases. © 2004 Elsevier Ltd. All rights reserved.

Chromium nitride-CrN-coatings have emerged as an alternative in machining applications thanks to thermal stability, low deposition temperature, excellent wear resistance and good corrosion resistance. However, under some circumstances it is often necessary to remove these kinds of coatings without attack of the base metal surface. In this work a stripping method based on electrochemical techniques has been developed to strip chromium nitride coatings in an oxidising alkaline solution process. Under galvanostatic conditions, it has been applied to strip CrN coatings with different configurations-monolayer and multilayer. The greater activity of the nitride in this medium leads to the formation of more soluble species. The complete removal of the chromium nitride layer is indicated by a sudden rise in the potential in time, which allows simple control of the stripping process. The surface appearance of the samples at the end of the coating removal process is good, since pitting is not detected and surface homogeneity is preserved, which are necessary conditions for redeposition of this type of coating. © 2005 Elsevier B.V. All rights reserved.

The work is based on the dry sliding wear of NiCrBSi coatings deposited on the plates of grey cast iron (DIN GG30) using the laser cladding technique, the parameters of which were such as to provide almost crack-free coatings with minimum dilution and very low porosity. The post-laser treatment microstructure showed up as a dendritic structure matrix which is a solid solution of Ni and chromium-rich precipitates with an average microhardness of over 850 HV 300 gf. Higher concentrations of precipitates are observed in the overlapped tracks, leaving the adjacent areas and some zones of the interface free of them. Wear behaviour was determined by using block-on-ring dry sliding linear contact at several loads (30-100 N) and sliding speeds (0.65-2.6 m/s). The density of the coating, friction coefficient and wear rate coefficient k (mm3/N m) were estimated using a method founded upon the PV factor theory. When load or speed increases, the friction coefficient diminishes. For a 30 N load, a superficial oxidation mechanism with loosening of the generated oxide layer predominated as the principle wear mechanism. Adhesion and oxidation were observed for higher loads. © 2005 Elsevier B.V. All rights reserved.

Laser surface treatments can be used as a powerful tool to repair moulds which have experienced severe damage during their service lifetime. The present paper studies the application of cladding techniques to repair moulds such as those used in the plastics industry. By means of a continuous wave diode laser operating at 600 W output power, a tool steel-annealed AISI A2-has been coated with two alloys with different protective properties: a high-speed steel with a high molybdenum content (AISI M2) and excellent mechanical properties; and a martensitic stainless steel (AISI 431) which combines good wear and corrosion resistance. After the treatment, the clad surfaces were characterised by means of optic and scanning electron microscopy. The layers showed a uniform composition across the track, similar to the initial powder composition, without damage or cracks. The mechanical properties were evaluated by hardness measurements and wear tests (ball-on-disk) at room temperature and without lubrication, in accordance with the ASTM G99-95a standard. The tribological couple was comprised by an alumina ball sliding against the clad layers. The wear rate varied with the different ball and disk combinations, and a considerable improvement in wear resistance was observed with the AISI M2 laser coatings compared with the substrate. © 2004 Elsevier B.V. All rights reserved.

Zinc alloys with additions of Ti and Cu present excellent creep resistance and an improvement in strength and hardness compared with pure Zn. However, these elements cause a decrease in localised corrosion resistance due to the presence of TiZn15 intermetallic particles. In the present work, the corrosion behaviour of the Zn-0.084 %Ti-0.18%Cu alloy in a NaCl solution, with and without cerium salts, has been studied. Polarization curves recorded in the non-inhibited solution reveal a corrosion mechanism under cathodic control with a corrosion rate of about 1.6×10-5 A/cm2. Localized attack can be observed in the rolling direction after immersion in the electrolyte. A three-dimensional layer is formed on the surface when inhibitor is added to the electrolyte. This layer is due to the precipitation of insoluble cerium hydroxide on the cathodic areas, which correspond to TiZn15 intermetallic particles. Inhibition percentages up to 98% after immersion for 4 h with 500 and 1000 ppm concentrations have been obtained. © 2004 Elsevier B.V. All rights reserved.

Vitreous coatings are widely used for protecting steel at temperature up to 600 °C in applications where high oxidation- and wear-resistance are required. However, the action of alkaline chemical agents such as household cleaning products can cause the devitrification and spalling of these coatings, with the consequent loss of their protective properties. In such cases, corrosion of the substrate is accelerated due to the heat treatment applied during the manufacturing process of the coating - tempering or curing. The present work characterizes the initial stages of this deterioration using the electrochemical noise technique (EN). When coatings are undamaged the current noise is very low, with current intensity values of the order of picoamperes. However, when defects appear as a result of chemical attack, the system electrical activity changes considerably and there is an increase in both current and potential noise. This work also establishes the loss of the coatings protective nature as a function of spectral noise resistance, Rsn, finding good concordance with values obtained by other more conventional techniques such as electrochemical impedance. © 2002 Elsevier Science B.V. All rights reserved.

Vitreous coatings can be used as a means for the corrosion protection of metals and alloys due to their excellent dielectric properties. Their use is of particular interest in the case of highly aggressive media (different pH levels, temperatures up to 600°C). However, ceramic coatings exposed to aggressive agents and thermal cycles can experience devitrification and spalling problems, with the consequent loss of their protective properties. The present study uses the electrochemical impedance spectroscopy technique to characterize, in a non-destructive way, the appearance of surface defects induced by the effect of temperature and chemical attack on a vitreous enamel deposited on steel. By means of analysis of the impedance diagrams obtained, a relationship is established between the loss of the coating's protective properties and the mechanism by which the corrosion process takes place. © 2002 Elsevier Science Ltd. All rights reserved.

Laser surface cladding is a well recognised technique to obtain wear and corrosion resistance coatings, or for repairing damaged parts. Recently, the use of high power diode laser (HPDL) has undergone rapid growth for materials processing. The present work describes the process for obtaining two boride-containing nickel-based hardfacing alloy coatings, primarily composed of Ni-Cr-B-Si on carbon steel using a HPDL. After laser treatment the microstructure is composed of a matrix of γ-Ni with different intermetallics depending on the initial composition of the powder. For the alloy with lower concentration of alloying elements, X-ray diffraction indicates the presence of nickel (Ni3Fe, Ni3Si,), boron (B4C, CrB) and chromium (Cr3Si) intermetallics. On the other hand, for the alloy with higher amount of C, B and Cr, the analysis revealed the presence of Ni3B together with lesser proportions of Ni3Fe and Ni3Si and the presence of chromium carbides Cr23C6 and (Fe·Ni)23C6. © 2002 Elsevier Science B.V. All rights reserved.

Aluminum-lithium alloys are widely used in the aircraft industry because of their low density and high elastic modulus. The Al-Li alloy 8090 has been well characterized from a mechanical viewpoint, but its corrosion mechanism has not been studied so completely. Recent work has focused on the use of electrochemical techniques to detect the early stages of localized corrosion and to assess the rate of corrosion taking place at specific locations in aircraft. Electrochemical impedance spectroscopy (EIS) has been used to measure and monitor the localized corrosion susceptibility of this Al-Li alloy, which is currently being used in aerospace applications, in solutions of 1 M NaCl+10 mL H2O2 (30 wt-%) at neutral pH. During the corrosion process, there is an increase in surface inhomogeneity which causes deviations from the ideal behaviour proposed by Randles. A tendency to behave as a porous electrode is evidenced in the Nyquist plot by the presence of a straight segment in the high frequency range. This response is the result of preferential dissolution at grain boundaries, which leads to the development of deep intergranular attack and promotes a non-homogenous signal distribution. At low frequencies a capacitive arc appears that may be related to the chloride adsorption involved in alloy dissolution.