High wear and corrosion of parts result in increase of operating
costs at thermal power plants. The arc spraying of protective coatings is an
effective solution to this problem. The wear & heat resistance of the
coatings were tested via a two-body wear test accompanied by microhardness
measurement and the gravimetric method respectively. Cored wires of the base
alloying system Fe-Cr-C were used as a feedstock. Rise of wear resistance and
heat resistance of the coatings was achieved by additional alloying with Al, B,
Ti and Y. A high temperature corrosion test was performed at 550°C under KCl
salt deposition. The porosity and adhesion strengths of the coatings were also
evaluated. The microstructure was investigated with a scanning electron
microscope (SEM) unit equipped with an energy dispersive X-ray (EDX)
microanalyzer and the phase composition was assessed by X-ray diffractometry.
The test results showed the positive influence of additional alloying with Y on
the coating properties. A comparison with commercial boiler materials showed
that the coatings have the same level of heat resistance as austenite steels
and are an order of magnitude higher than that of pearlite and
martensite-ferrite steels. The coatings can be applied to wear-resistant and
heat-resistant applications at 20°C-700°C.
Thermal spray coating is a suitable method to improve corrosion
resistance of low carbon steels. The corrosion behavior of wire-arc-sprayed
coatings on C35 steel was undertaken. Characterization techniques including
metallographic and corrosion resistance of different coatings were
investigated. Electrochemical tests of arc sprayed stainless steel, Inconel and
composite coatings were performed in 3, 5% NaCl solution. The results
comparison showed an excellent corrosion resistance of the stainless steel
coating.
Nickel-basedalloys are used for Arc Spray Wire in numerous applications due to
their outstanding wear and corrosion resistance at high temperatures and their
relatively low cost. These alloys have high strength, hardness and excellent
corrosion resistance due to addition of chromium.
The oxidation and sulfidation can cause problems for the materials
exposed depending on the local activities of oxygen and sulphur in coal fired
boilers used for power generation. Burning fuels with significant amounts of
chlorine or elements such as potassium, zinc, sodium, vanadium and lead can
cause the formation of ash & salt deposits with very low melting points. Corrosive
conditions develop with the molten salts fluxing the protective oxide scales or
directly dissolving the metal if the temperature of the metal surface is above
the melting point of the salt’s eutectic composition. This corrosion mechanism
is often encountered in waste incinerators, fluidized bed boilers burning fuels
with higher chlorine contents (e.g. solid waste), black liquor recovery boilers
in the pulp & paper industry and engine exhaust systems when oils with
higher amounts of vanadium are burnt.
There are numerous countermeasures that are developed to avoid
excessive material damage corresponding to the variety of corrosive
environments. Alloy selection is an essential factor wherein a large number of
Fe-, Co- and Ni- based alloys exist today specially designed for excellent
resistance to sulfidation, oxidation and corrosion by ash/salt deposits.
Designs aspects improve temperature distribution (avoid hot spots), avoid
excessive deposition of ash and slags by use of soot blowers, rapping, screens.
Chemical additives such as neutralization of corrosive components in the flue
gases by injecting additives such as dolomite or limestone. Shielding such as SiC tiles in waste incinerators and
other types of refractory linings. Different coating techniques are applied to
protect critical surface areas from corrosive gases including co-extrusion,
weld overlay, chromizing and thermal spray coatings.
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