Assessment OF HVOF Coatings for Wet Corrosion Protection

Until now the use of thermal sprayed metallic coatings for wet corrosion protection is limited to applications where the coating (Al-alloys, Zn-alloys and their mixtures) acts as an anode to protect the substrate or special cases, where thick cold sprayed metallic layers give good results. Other atmospheric cold sprayed layers made of corrosion resistant Ni, Co, Cu or Fe base alloys have their limitations due to the process related discontinuities like pores and oxide films. In more aggressive environments thermal sprayed and fused layers made of so called self-fluxing Ni and Co based alloys are commonly applied. Also in some applications the use of specially designed gas shrouds or of spraying techniques running in inert gas atmospheres or vacuum can yield protective coating solutions. However, these techniques have high investment or service costs or the size of the parts to be coated is restricted. Recent developments in HVOF-spraying open new possibilities in applying cold sprayed coatings on site with good corrosion resistance. The aim of this paper is on one hand to give a comparative overview about the wet corrosion behaviour of thermally sprayed metallic coatings using different spraying techniques related to the performance of the corresponding bulk materials and on the other hand to demonstrate the potential of a new generation of coatings to protect or repair structures exposed to aggressive environments.

Oxidation Properties of NIAI Intermetallic Coatings Prepared by High Velocity Oxy-Fuel Thermal Spraying

A reaction-formed NiAI intermetallic compound (IMC) powder has been deposited as a coating onto low carbon steel test coupons by the High Velocity Oxy-Fuel (HVOF) process using both gaseous and liquid fuels. The microstructure of this coating has been examined using scanning electron microscopy and x-ray diffraction and was found to depend on spraying conditions. Oxidation tests on the coating in air, between the temperatures of 800°C-1200°C, revealed that an a-alumina (Al2O3) scale formed on the coating's surface. At 1200°C, a nickel spinel (NiO/NiAl2O4) and haematite (Fe2O3) phases were observed. Diffusion studies were performed to calculate an activation energy for iron ion diffusion in NiAl.

The Tribology of Amorphous Surfaces Formed by Wear of Thermal Spray Coatings

This paper describes the wear induced transformation of crystaline metal surfaces into amorphous and/or microcrystalline surfaces that exhibit gross changes in the fundamental properties of friction, wear, hardness and toughness. The coatings are applied using wire and powder feed to TWAS and HVOF equipment. Wear processes investigated include adhesive wear, low stress abrasion, grinding wear and galling. The effects of chemical makeup of the surfaces and the alloy structure are examined using microscopy and x-ray diffraction. The surface & underlying coating characteristics including roughness, microstructure, hardness and friction coefficient are determined. Results show that the surface structure is dependent on the wear vector. The structural transformation is a function of the chemical makeup and intrinsic wear resistance of the crystalline alloy coupled with the energy input of the wear process. High energy wear such as grinding wear can overcome the transformation. The results also suggest that the micro-welding that occurs between asperities in crystalline alloys is replaced by a flow process on the transformed surface. Coating structure, glass transition temperature, crystallization temperature and critical cooling rate of the transformed surface are much more significant than the chemistry of the alloy once the transformation takes place.

Latest Gas Turbine Repair Techniques Using Laser Technology

Over recent years, with the drive for new higher power, higher efficiency Gas Turbine engines, manufacturers have had to look at new alloys and new coating techniques to achieve and support the industry requirements. Repair technology has therefore had to keep pace with the OEM advances and much research and development has been undertaken in developing new repair processes. Many of the alloys now used are directionally solidified or single crystal, which until now have been deemed irreparable by traditional welding techniques. Recent developments in the use of lasers have not only rendered these alloys salvageable but have also reduced the overall repair time and therefore the cost. This paper looks at the use of laser technology as a repair process for gas turbine components, touching briefly on laser cutting and drilling but concentrating mainly on laser powder feed welding and its applications.

Influence of Surface Microstructure of Ceramic Coatings on the Initiation and Development of Scuffing Phenomena

This paper deals with findings of experiments concerning the scuffing phenomenon in case the frictional pair is embodying an element with a thermally sprayed ceramic coating. The progress of building up of seizure is related and evaluated for a set of ceramic coatings embodying a diversity of granulations of Al2O3TiO2 and Cr2O3 plasma sprayed on steel, the IHI8N9T steel and carbon-graphite when tested on a roller-block machine. The greatest antiscuffing resistance was recorded for Cr2O3/carbon-graphite pair. An analysis of regression approximating friction force was carried through.

Cored Wire Application Using Electric Arc Spray

The use of cored wires for thermal spraying is a relatively new development that is being rapidly utilized for arc spray in a wide variety of applications. This paper will discuss the existing applications and industries in which cored wire coatings are used. Additionally, this presentation will cover the effect cored wires have had on the use ofother types ofthermal spray equipment. The paper will close with a trend analysis that discusses the applications of the future.

Wear Evaluation of Plasma Sprayed Oxide and Carbide Coatings

Wear of plasma sprayed Cr2O3 TiO2, Cr3C2-NiCr and WC-Co coatings have been evaluated with a block-on-ring arrangement under dry and lubricated conditions. The results indicated that the wear of the coatings was interpreted in term of subsurface grain fracture which was related to the special microstructure of the coatings such as size, shape and distribution of pore as well as crack. Among the four kinds of coating, Cr2O3 coating possesses the lowest wear coefficient. Water accelerated cracking and fracturing and deteriorated the wear resistance of both Cr3C2-NiCr and TiO2 coatings. Ethanol reduced the wear of Cr3C2-NiCr coating, which was attributed to the formation of a smooth surface film mainly consisting of Cr2O3. Wear of Cr2O3, coating against an Al2O3 ball at high temperature has been also produced. The wear of Cr2O3 coating against Al2O3 ceramics decreased with increase in temperature and load. The reaction between Cr2O3 coating and Al2O3, ceramics at high temperature and the formation of a protective film consisting of Cr2O3, Al2O3 and SiO2 glass on the surface of Cr2O3 coating improved the wear resistance of Cr2O3 coating.

A Contribution to Methodology and Protocol Definition for Thermal Spray Coated Surface Compatibility Analysis in Food Processing Applications

The paper discusses the testing methodology and identifies the analytical protocols, with proper validation, in order to evaluate the compatibility of thermal spray coatings in the food production technology, according to EU and FDA applicable standards. A brief state-of-the-art analysis of the international standards on food additives and human health is given, namely on indirect food additives (as defined in 21 CFR 170.3(e)), that can migrate into the food during the process. An outline of the test protocols, based on contact between coating and food simulating solvents in a set time/temperature conditions, are presented, and the main phases for the proposed testing methodology, as the choice of the simulating solvent, the migration cell design and the time/temperature conditions, are discussed. Finally the proposed methodology and protocols are validated through a thermal spray coating for food process application test case.

Applications of PTA Powder Welding in Petroleum Chemical Industry

PTA welding is applied to deposit Ni-based and Co-based materials on the key parts in petroleum and chemical industry, for the purpose of surface modification. The test shows that this process is played an important role in upgrading the materials performance of anti-wear, anti-corrosion and high temperature resistance, prolonging service life, saving expensive materials and reducing production cost. Key words: PTA, Deposit, Alloy Powder, Petroleum Industry, Chemical Industry

Plasma Spraying Using Ar-He-H2 Gas Mixtures

In D.C. plasma guns used for plasma spraying, the properties of the plasma forming-gas control, to a great extent, the characteristics of the plasma jet and the momentum, heat and mass transfer to the particles injected in the flow. This paper deals with mixtures of argon, helium and hydrogen and the effect of the volume composition of these mixtures on the dynamic and static behavior of the plasma jet. Both were investigated from the measurements of arc voltage and gas velocity. Correlations between these parameters and the operating variables (arc current, gas flow rate, gas mixture composition) were established from a dimensional analysis. The results were supported by the calculation of the thermodynamic and transport properties of the ternary gas mixtures used in this study.