Papers

ICMCTF2007 Session H3: Corrosion-Resistant Coatings: New Approaches, Techniques and Materials

Wednesday, April 25, 2007 1:30 PM in Room Tiki Pavilion

Wednesday Afternoon

Start	Invited?	Item
1:30 PM		H3-1 Characterisation of Corrosion Resistant Coatings Produced on Ti by Plasma Electrolytic Oxidation R.H.U. Khan, A.L. Yerokhin, A. Pilkington, A. Matthews (University of Sheffield, United Kingdom) The corrosion resistance of Ti alloys can be enhanced by oxide films produced using plasma electrolytic oxidation (PEO). In this work, titania films with thicknesses in the range from 5 to 10 micron were produced on commercially pure titanium by PEO in phosphate based alkaline electrolytes under direct current conditions. The coatings consisted of a mixture of nanocrystalline anatase and rutile phases. The rutile content was 3 wt% for the coatings produced using higher electrolyte concentration (15 g/l) whereas it was 20 wt % for a lower electrolyte concentration (5 g/l). Moreover, coatings produced at higher current density (20 A/dm²) have higher rutile content compared with the other current densities (5, 10, 15 A/dm²). Smaller crystallite sizes of anatase (25 nm) and rutile (22 nm) phases were found in the coatings produced using a lower electrolyte concentration (5 g/l). Surface morphology was observed using a Camscan SEM. Coating corrosion behaviour was determined in 3.5% NaCl solutions using electrochemical potentiodynamic and impedance spectroscopy techniques. The results of electrochemical examinations show that the oxide film produced by PEO process increases the corrosion resistance of titanium. Effects of internal stresses on corrosion properties are also studied. Residual stresses were measured using X-ray Sin²ψ method. PEO process parameters to produce corrosion resistant oxide coatings with minimal internal stresses were identified.
1:50 PM		H3-2 Oxidation and Hot Corrosion Behavior of Detonation Gun Sprayed Cr₃C₂-NiCr Coatings on Ni and Fe-Based Superalloys S Kamal, R. Jayaganthan, S. Prakash (IIT Roorkee, India) Detonation gun (D-gun)spray is widely used to deposit ceramic coatings on hydroturbine to protect it from erosion and wear during service conditions. D-Gun can be used to develop coatings that can withstand high temperature oxidation and hot corrosion at elevated temperature in gas turbines used in aerospace and power plants. Cr₃C₂-NiCr coating is a promising cermet coating find applications in fluidized bed boilers and coal-fired boilers due to its excellent erosion and corrosion resistance at high temperature. The literature on high temperature oxidation and hot corrosion behavior of D-gun sprayed Cr₃C₂-NiCr is scarce. Therefore, the present investigation is focussed to analyze the high temperature oxidation and hot corrosion resistance of D-gun sprayed Cr₃C₂-NiCr coatings on Ni-and Fe-based superalloys. The high temperature oxidation kinetics and hot corrosion behavior of Cr₃C₂-NiCr coatings in molten salt environment (Na₂SO₄ - 60% V₂O₅) at high temperature 900°#C are studied using themogravimetric technique. The corrosion products of the D-gun sprayed Cr₃C₂-NiCr coatings on superalloys are analyzed by using XRD, FE-SEM/EDAX, and EPMA to reveal their microstructural and compositional features for elucidating the corrosion mechanisms. It is observed that the bare Ni and Fe-base superalloys suffered accelerated corrosion under the aforementioned environmental conditions and showed spalling/sputtering of the oxide. The Cr₃C₂-NiCr coatings on Ni- and Fe- based superalloy substrates are found to be very effective in decreasing the corrosion rate in given molten salt environment and at high temperature air atmosphere at 900°C. It is found that the Cr₃C₂-NiCr coating serves as an effective diffusion barrier to prevent the diffusion of oxygen from the environment into the substrate superalloys. D-Gun sprayed coating exhibits less porosity when compared to Air Plasma Spray as calculated for the Cr₃C₂-NiCr coatings in the present investigation.
2:10 PM		H3-3 Application of Advanced Electrochemical Techniques to Evaluate the Corrosion Performance of Coatings R. Akid (Sheffield Hallam University, United Kingdom) There are several mechanisms by which coatings fail in service. Notwithstanding mechanical damage, coatings are subject to tribological wear and abrasion, interfacial adhesive failure and cracking due the effects of residual or applied stresses. One more common failure mechanism which, may or may not take place in conjunction with the above, is that of corrosion. Corrosion may take place uniformly over the surface of the coating or may be localized at a specific site on the surface. The corrosion rates associated with uniform and localized corrosion differ significantly and, subject to the type of coating, nature of the substrate and aggressiveness of the environment be orders of magnitude higher for local corrosion. Traditional methods of corrosion testing, such as weight loss and linear polarization resistance, allow an assessment of the uniform corrosion rate of a surface to be made. However where localized corrosion takes place these methods are inappropriate as they do take into account the actual corroding area and it is therefore necessary to adopt a technique(s) that will take into account the anodic and cathodic activities occurring at different sites on the surface. This has been achieved by the development of the new scanning electrochemical techniques. This paper will discuss the origins of localized corrosion on coated surfaces and illustrate examples of the various local scanning electrochemical techniques, namely the scanning reference and vibrating reference electrode technique (SRET, SVET), the scanning Kelvin probe (SKP) and the scanning droplet cell (SDC), that have been applied to understand the coating failures and quantify localized corrosion rates.
2:50 PM		H3-5 Effects of Plasma Assisted Electrolytic Treatments on Corrosion Resistance of High Strength Steels A.L. Yerokhin, D. Patel, A. Pilkington, A. Matthews (University of Sheffield, United Kingdom) Plasma assisted electrolytic treatment is an emerging environmentally friendly technology, involving electrolysis and electrical discharge phenomena that can be used to clean steel substrates prior to coating deposition. It possesses a number of advantages, including quick treatment times, good removal of both organic contaminants and inorganic films, excellent surface wettability, specific profile favourable to good adhesion with coatings, compressive surface stresses. This work reports on the effects of the plasma assisted electrolytic cleaning on electrochemical characteristics of 4340 steel. The cleaning was performed in a 10% solution of sodium bicarbonate operated at 70°C, with voltage and treatment times varied within 100 to 350 V and 0.5 to 5 min, accordingly. The methods of surface profilometry, optical microscopy, SEM, TEM and EDX analyses were employed, together with microhardness tests to evaluate the effects of the treatments on the characteristics of surface microstructure, morphology and chemical composition. XRD studies were also carried out to evaluate phase composition, grain size and stress state in the surface layer. A further corrosion studies, including potentiodynamic corrosion tests and electrochemical impedance spectroscopy analysis, were carried out to investigate changes in corrosion performance of metal surfaces due to the cleaning treatment. Enhanced capabilities of the plasma assisted technology compared to conventional cleaning treatments were demonstrated, showing substantial reduction of surface roughness and minimisation of detrimental effects of electrolytic plasmas on surface hardness. Optimal ranges of process parameters were identified.
3:10 PM		H3-6 Plasma Deposition of Siloxane Based Coatings at Atmospheric Pressure for Corrosion Protection of Galvanized Steel R. Dams, B. Verheyde, S. Paulussen, R. Rego, D. Vangeneugden (VITO Belgium); J. Bardon, J. Bour, H. Aubriet, D. Ruch (Centre de Recherche Public Henri Tudor (CRPHT), Luxembourg) Plasma deposited Polysiloxane coatings are presented as a more environmentally friendly approach to replace toxic chromates for the corrosion protection of galvanized steel. The coatings are formed by polymerizing hexamethyldisiloxane (HMDSO) into a dielectric barrier discharge working at atmospheric pressure. This is a dry technique without solvent waste. Furthermore, inline processing is possible. Coating properties were optimized by adjusting the process parameters and the reactor design. The use of a mobile high voltage electrode and the use of the galvanized steel substrates as an electrode, results in a more homogeneous deposition. Addition of oxygen to the nitrogen carrier gas, increases the inorganic content of the coating, which leads to a better corrosion protection. The chemical structure of the coatings was studied with Fourier Transformed InfraRed spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS). Coating roughness and thickness was measured with White Light Interferometry (WLI) and Scanning Electron Microscopy (SEM). Corrosion resistance was studied with polarization curves (corrosion current) and salt spray tests. These corrosion tests show that a corrosion protection equal to chromates can be reached.
3:30 PM		H3-7 Process Structure Properties Relationship During Formation of CrN and AlN Layers on H13 Steel A. Rojo, J. Acosta, O. Salas (ITESM, Mexico); J. Oseguera (ITESM-CEM, Mexico) AlN and CrN films were deposited on H13 substrates by reactive magnetron sputtering and their response to corrosion evaluated. As a tool to control the properties of the films, the relationship between microstructure-processing-properties was studied. Processing conditions and the resulting reactive atmosphere were monitored by optical emission spectroscopy and Langmuir probe measurements. The phases and microstructure present in the films produced were analyzed by scanning electron microscopy, energy dispersive x-ray microanalysis and x-ray diffractometry. The corrosion behavior of the films was studied by means of a potentiodynamic polarizations tests.
3:50 PM		H3-9 The Effect of Shot Peening on the Corrosion Performance of PEO Coated Al Alloys D.T. Asquith, A.L. Yerokhin, A. Matthews, J.R. Yates (University of Sheffield, United Kingdom) Recent work has highlighted the importance of the phenomenon of mechanical fatigue in surface treated aluminium [1]. The concept of duplex treatments, combining mechanical cold work with a hard surface layer, e.g. Plasma Electrolytic Oxidation (PEO) coating, has been demonstrated to significantly improve the fatigue resistance of an Al alloy over that of a single PEO treatment. A further consideration is that of corrosion resistance; PEO is used in many circumstances to reduce the susceptibility to corrosion whereas shot-peening is known in some cases to be detrimental, particularly with stress corrosion cracking. In this work, the corrosion resistance of three different surface treatments (i.e. single shot-peening and PEO as well as their combination) is investigated on a 2024-T351 aluminium alloy. Corrosion studies were carried out in a 3.5 % NaCl solution using open circuit potential, potentiodynamic corrosion scan and electrochemical impedance spectroscopy techniques. Surface microstructure and phase composition were studied by SEM, EDX and XRD analyses, respectively. The effect of the duplex treatment is discussed in comparison with that of the single treatments and as-received (mill finish) surfaces. [1]. D.T. Asquith, A.L. Yerokhin, J.R. Yates and A. Matthews, Effect of combined shot-peening and PEO treatment on fatigue life of 2024 Al alloy, Thin Solid Films (2006), doi:10.1016/j.tsf.2006.07.123.
4:10 PM		H3-10 In Vitro Corrosion Behavior of AZ91 Magnesium Alloy Treated by Metal Ion Implantation in Artificial Physiological Fluids C.L. Liu (City University of Hong Kong); Y. Xin (Tsinghua University, China); X. Tian (Harbin Institute of Technology, China); P.K. Chu (City University of Hong Kong) Degradable metal alloys constitute a new class of materials for load-bearing biomedical implants. Due to their good mechanical properties and biocompatibility, magnesium alloys are promising in degradable prosthetic implants. The objective of this study is to investigate the corrosion behavior of surgical AZ91 magnesium alloy treated by aluminum, zirconium, and titanium ion implantation in artificial physiological fluids. The surface characteristics of the ion-implanted layer in the magnesium alloys are examined. Our results disclose that an intermixed layer is produced and the surface oxidized films are mainly composed of aluminum, zirconium or titanium oxide with a lesser amount of magnesium oxide. The effects of metal ion implantation on the corrosion resistance and electrochemical behavior of the magnesium alloys are investigated in artificial physiological fluids at 37±1°C, using electrochemical impedance spectroscopy and potentiodynamic polarization techniques. Comparing the three sets of samples, zirconium ion implantation significantly shifts the open circuit potential (OCP) to a more positive potential and improves the corrosion resistance at OCP. Moreover, the implanted metal ions decrease filiform corrosion and breakdown of the magnesium alloys can be attributed primarily to pitting corrosion.
4:30 PM		H3-11 EIS Characterization of Alumina Coatings Produced by Frequency Swept Pulsed PEO E.V. Parfenov (Ufa State University for Aviation Engineering, Russia); A.L. Yerokhin, A. Matthews (University of Sheffield, United Kingdom) Plasma electrolytic oxidation (PEO) is an emerging environmentally friendly technology for producing corrosion and wear resistant coatings on aluminum. Currently new pulsed voltage techniques have been shown to be more effective than traditional DC and mains frequency AC approaches. However, the problem of optimal frequency and pulse shape has not been thoroughly investigated yet. One of the PEO process characteristics that could provide a justified solution for the problem is magnitude and phase frequency response. Therefore, the frequency response has been identified in-situ during PEO of aluminium, using frequency sweep of bipolar voltage pulses between 20 Hz and 20 kHz. As a result, magnitude and phase frequency response as a function of voltage and treatment time has been obtained. It was found that the impedance decreases with frequency growth from 10³...10⁴ to 10...10² Ω and phase angle decreases from 0 to -70...-80 degrees. This fact appears to be consistent with a hypothesis that the PEO process exhibits active-capacitive load behavior. To further verify this assumption, electrochemical impedance spectroscopy (EIS) of the samples after the PEO has been performed at open-circuit potential in the same electrolyte that has been used for the PEO. Indeed, the samples were found to exhibit active-capacitive load behavior, whereby the impedance decreases with the frequency growth from 10⁵...10⁶ to 10...10² Ω and phase angle varies between -20 and -80 deg. One of the most notable features is that both of the frequency responses, obtained from the frequency swept pulsed PEO and the EIS characterizations, show similar behavior within the frequency range 5-20 kHz. This approach helps to estimate the frequency response of the PEO micro discharges and to justify optimal frequency for the pulsed bipolar PEO.