P. Vassiliou

Vacuum heat treatment of electroless Ni-B coatings

Abstract Electroless nickel–boron plating with subsequent rapid heat treatment in vacuum has been tried on steel substrate in an effort to obtain hard engineering coatings. By selected conditions of heat treatment in a high vacuum environment (a type of rapid quenching), the plating can acquire chromium equivalent hardness without the effluents of the hard chromium plating process. The resulting surfaces were examined and characterized under an optical and a scanning electron microscope. XRD analysis was also performed to investigate re-crystallization effects. The fabricated materials were also tested under corrosion conditions and linear polarization measurements were performed. The results show that after a 5-min heat treatment in vacuum, the plating acquires a crystalline structure with very good adhesion to the substrate material, obtained so far by several weeks/months thermal treatment.

Wear and corrosion resistance of laser surface hardened structural steel

Abstract CO 2 laser surface treatment in solid state has been performed on a CK60 structural steel. The laser energy density was lower than the melting threshold and only caused surface hardening. The observed heat affected zone (HAZ) ranged in depths of 50–430 μm. Microstructure of the HAZ was composed of needle, fine-grained martensite and small amount of pearlite and retained austenite. Microhardness values changed in areas of overlapping successive laser passes. Wear tests revealed the improved wear resistance of the laser treated specimens in comparison with the untreated one. Specimens laser treated using 0 and 38% overlapping ratios exhibited improved wear resistance. The corrosion resistance exhibited a tendency to decrease with increasing overlapping ratios. Among all specimens studied the one, which was laser treated with 0% overlapping ratio presented the best wear and corrosion behaviour.

Sulfur Hexafluoride (SF6): Global Environmental Effects and Toxic Byproduct Formation

ABSTRACT This work provides information concerning possible global environmental implications and personnel safety aspects that should be considered during the commercial uses of sulfur hexafluoride (SF6). SF6 is an anthropogenically produced compound, mainly used as a gaseous dielectric in gas insulated switchgear power installations. It is a potent greenhouse gas with a high global warming potential, and its concentration in the earth atmosphere is rapidly increasing. During its working cycle, SF6 decomposes under electrical stress, forming toxic byproducts that are a health threat for working personnel in the event of exposure. Several precautions are recommended to avoid personnel exposure to toxic byproducts: oxyfluoride levels or other byproduct concentrations in the operating gas matrix should be traced to predetermine the overall gas toxicity; contaminants should be systematically considered during maintenance, chamber evacuation and system opening process; small SF6 quantities leaking into air or stagnated pollutant concentrations in the operating field should be analyzed and compared to the threshold limit values and permissible exposure levels. New system design rules (i.e., hermetically sealed gas compartments, gas recycling or disposal in the field area) and different handling policies—both during maintenance and final disposal—now should be considered globally to provide for environmental and personnel safety.

Thermal stability of SF6 associated with metallic conductors incorporated in gas insulated switchgear power substations

SF6 is generally treated as thermally stable and inert for applications below 500 °C. This work investigates the thermal stability of pure SF6 gas under 1.2 atm pressure between 200 and 450 °C in the presence of construction metals (Cu, Al), without any applied electric field. The obtained experimental results indicate that SF6 may react with metallic surfaces forming solid and gaseous by-products, either in the gas matrix or diffused in the metallic surfaces. The phenomenon is enhanced in the presence of adsorbed moisture. For copper surfaces, sulfide layers are formed. By-products are not formed for pure Al surfaces. However, when Al is covered by a few micrometres thick Al2O3 film, hot SF6 molecules have a structure change effect, i.e. reduce porosity in the oxide and in the substrate, provide smooth transition layers Al/Al2O3 and increase the Al2O3 layer width. In the presence of moisture this phenomenon is significantly intensified and a diffused overlayer of AlF3 also forms. The by-products in the gas matrix are mainly sulfur oxides for hot spot temperatures below 300 °C, while at higher temperatures oxyfluorides SO2Fx and HF are mainly formed. These by-products are either toxic or corrosive. Thus, the thermal stability issue of SF6 may have to be reconsidered.

Temperature dependent dielectric spectroscopy in frequency domain of high-voltage transformer oils compared to physicochemical results

The temperature dependent dielectric spectroscopy in frequency domain is compared to physicochemical and electrical tests that are periodically applied on highly insulating transformer oils. Complex permittivity and tandelta data are obtained as a function of frequency and operation temperatures, to demonstrate the polarization phenomena and the induced loss intensification by the thermally stimulated currents. The acquired dielectric data of oil samples collected from 201 operating high voltage (HV) power transformers, were correlated to their corresponding physicochemical ones in order to demonstrate the future potential employment of the proposed measuring technique in the field of high voltage engineering, towards reliability monitoring of oil filled electrical equipment (OFEE)

Nickel matrix composite electrocoatings as electrical contacts

The work presented in this paper explores the possibility of using composite nickel matrix electrodeposits prepared under direct current (DC) and pulse current (PC) conditions as electrical contact materials. The codeposition of liquid containing microcapsules (/spl mu/caps) along with the metal matrix from electrolytic baths leads to the development of new type of materials with prespecified properties: the liquid-containing metal electrocoatings. The incorporation, in the nickel matrix, of polymeric wall /spl mu/caps containing oil ensures a self-lubrication and cooling action during fretting or transient operation. The electrical properties of composite nickel electrodeposits have been investigated. Different preparation conditions have been employed. Their contact resistance and interfacial temperature have been monitored for a great number of operation cycles during the simultaneous application of mechanical and electrical fatigue. SEM and XRD analysis were applied toward investigating the structural characteristics of the inter-electrode surfaces.

Corrosion effects on the electrical performance of silver metal contacts

In this work, commercial silver metal contacts welded on top of silver plated brass or brass substrates have been exposed to air rich in NaCl. Scanning electron microscopy and energy dispersive analysis of the exposed contact surfaces were performed to identify the corrosion by‐products on top of the silver contacts, suspending wafers, and welding materials. Surface corrosion products were mainly found to consist of small spherules of Cu‐Zn or Ag‐Cu compounds which cover the surface of the contact proper with low adhesion properties. They mainly originate from the underplating wafer or welding materials. Electrical characterization of the contacting materials was based on dc temperature overheat tests, current switching cycle tests, and energy storage during ac current excitation. The experimental results display that the operating environment is indeed a very significant parameter determining the overall performance of the electrical contacts. New design rules as well as material selection properties may have to be systematically considered to allow for electrochemical induced degradation in saline operating environments.

Electroless Ni-B and Ni-P coatings with high-fretting resistance for electrical contact applications

Electroless nickel-phosphorus and nickel-boron plating have been tried on steel and brass substrates. By selected conditions of heat treatment in a high vacuum environment the plating can produce more than chromium equivalent hardness without the effluents of the hard chromium plating process. The resulting surfaces of Ni-P and Ni-B were examined and characterized under an optical and a scanning electron microscope. XRD analysis was also performed to investigate re-crystallization effects. The fabricated contact materials were also tested under corrosion conditions, and linear polarization measurements were performed. The electrical contact resistance of the deposits was also recorded during a 10,000 cycle test. The results show that after the heat treatment in a high vacuum environment the platings acquire a crystalline microstructure, with very good adhesion to the substrate materials (steel or brass). In addition, the microhardness of vacuum heat treated Ni-P coatings attained values of 1200 HV, while the nickel-boron coatings acquired an even higher microhardness, exceeding 1600 HV. The electrical performance of the fabricated layers showed that after thermal treatment the contact resistance remained almost unaffected. However, the Ni-B overlayers systematically exhibit lower contact resistance profiles. The above materials could be practically utilized whenever a good electrical performance of a hard engineering coating is required.

Plasma reduction of bronze corrosion developed under long-term artificial ageing

Reference Cu-based alloy with chemical composition and micro-chemical structure similar to that of ancient alloys has been used for carrying out the artificial long-term degradation test based on chloride enriched soil (chemical + soil) degradation. The results show that such degradation procedure produces natural like “patinas” as the ones grown on archaeological artefacts, from a chemical, structural and micro-morphological point of view. Glow discharge plasma technique has been employed for the treatment of the as-corroded bronze coupons. The gradual elimination of chloride-containing corrosion products in favour of the formation of more stable species and even the complete reduction back to copper has been observed. The chemical and metallurgical features have been determined by combined use of different analytical techniques such as scanning electron microscopy with energy dispersive X-ray micro-analysis (SEM-EDS), X-ray diffraction (XRD) and optical microscopy.

Corrosion resistance of composite nickel‐Al2O3 deposits

The corrosion resistance of composite nickel electrodeposits with alumina embedded in the nickel matrix prepared in sulfate and chloride bath has been studied. As corrosion environment salt spray and nitric acid solution have been tested. The chloride type of deposits were more resistant in the corrosive environments studied than the sulfate ones. The structure as a consequence of the preparation conditions affect the corrosion resistance behavior of the deposits. The composite deposits with aluminium oxide exhibited a higher corrosion resistance in all tested conditions.