Coatings resistant to erosive/corrosive and severe environments

Prakash

2007

Oxid Met

Nickel-and Cobalt-base coatings were developed on boiler-tube steels by the argon-shrouded, plasma spray process. The cyclic behavior of bare and coated boiler-tube steels has been studied in air at 900 8C. Four types of coatings were used: Ni-22Cr-10Al-1Y (NiCrAlY), Ni-20Cr, Ni 3 Al and Stellite-6 (St-6). The NiCrAlY has also been used as a bond coat of approximately 150 mm thick before the final coating in all the cases. Oxidation products analyzed in the scale are mostly oxides of the elements present in the coatings and substrate steels. NiCr 2 O 4 and CoCr 2 O 4 spinels are the most-common observed phases in the scale for nickel-and cobalt-base coatings. The internal oxidation of coatings and diffusion of iron from the base steel to the upper scale occurred during the study. Cracks formed in the scale and coating and may be due to differences in composition of coatings, bond coat, substrate and oxides formed.

Section: Coating Alloy/powdersmentioning

confidence: 99%

Analytical Studies on the Behavior of Nickel- and Cobalt-Base Shrouded Plasma Spray Coatings at Elevated Temperature in Air

Prakash

2007

Oxid Met

“…Many MCrAlY coatings are secondary aluminium oxide formers, that is, they initially form a chromic oxide external scale when exposed to an oxidising environment at high temperature, then aluminium oxide forms under the chromic oxide layer [39]. Also the MCrAlY coatings used as bond coats in many applications provide a rough surface for mechanical bonding of the top coat, protect the underlying alloy substrate against high temperature oxidation corrosion and minimize the effect of the coefficient of thermal expansion mismatch between the substrate and the top coat materials [40,41]. NiCrAlY bond coats may also help in the formation of metallurgical bonds between the mating surfaces.…”

Section: Mcraly Coatingsmentioning

confidence: 99%

Use of plasma spray technology for deposition of high temperature oxidation/corrosion resistant coatings – a review

Singh¹,

Puri

et al. 2007

Materials & Corrosion

109

High temperature oxidation is one of the main failure modes of the hot-section components in gas turbines, boilers, waste incinerations, diesel engines, coal gasification plants, chemical plants and other energy generation systems. In such applications the use of Fe-, Ni-and Co-based alloys, especially of superalloys is well known. The superior mechanical strength and good corrosion resistance of the superalloys at high temperature make them favorites for such applications. However, the presence of combustion gases constitutes an extreme environment and hot corrosion is inevitable when superalloys are used at high temperatures for long durations of time. Therefore these alloys need to be protected against this type of oxidation. Several countermeasures have been suggested in the literature to combat the same. One such countermeasure against hot corrosion and oxidation constitutes the deposition of protective coatings on these alloys. Among the various techniques used for deposition of coatings, plasma spraying is a versatile technology that has been successful as a reliable cost-effective solution for many industrial problems. It allows the spraying of a wide range of high performance materials from superalloys and refractory intermetallic compounds to ceramics with continuously increasing commercial applications. Furthermore it does not cause deterioration of the substrate alloys, and relatively thick coatings can be formed with high deposition rates. In this paper the technique of plasma spraying has been detailed and the role of plasma sprayed coatings to arrest high temperature oxidation has been discussed with the help of a comprehensive literature survey. The main focus of this investigation is the studies related to plasma sprayed NiCrAlY, Ni-Cr, Nickel aluminide and Co-based coatings.

“…[1][2][3] Thermal-sprayed coatings are economical, can be produced by means of relatively simple techniques, and offer excellent corrosion and wear protection. 4 Nickel aluminide-type protective coatings are of relatively recent origin.…”

Section: Introductionmentioning

confidence: 99%

Effect of Laser Remelting on Hot Corrosion Behavior of Ni₃Al and Ni-20Cr Plasma-Sprayed Coatings

Prakash

2005

CORROSION

The hot corrosion behavior of Ni 3 Al and Ni-20Cr plasmasprayed coatings after laser remelting has been identifi ed in a sodium sulfate-60% vanadium pentoxide (Na 2 SO 4 -60%V 2 O 5 ) environment at 900°C under cyclic conditions. Complete areas were melted by making a series of parallel overlapping tracks using the 300-W Nd:YAG Laser at the Atomic Fuel Division (AFD), Bhabha Atomic Research Centre (BARC), Trombay, India. At the end of each cycle the samples were subjected to weight-change measurements after a visual examination. Finally the scale was analyzed using scanning electron microscopy/energy-dispersive x-ray analysis (SEM/EDAX) and electron probe microanalysis (EPMA) techniques. A slight improvement has been observed in the hot corrosion behavior after laser remelting for the Ni 3 Al coating, whereas a slight decrease in the hot corrosion resistance of the Ni-20Cr coating after laser remelting has been experienced. The lower resistance to hot corrosion, as indicated by Ni 3 Al coating, compared to that of Ni-20Cr may be due to the sulfi dation of nickel.