25-06-2013, 02:39 PM
Thermal Spraying between erosion and corrosion challenges
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Introduction
Corrosion damage and erosive material losses need adequate
overhauling strategies for boiler operators. Thermal
Spraying is one of the methods that should generally
be taken into closer consideration. No distortion even
on already damaged pipe walls and a comparatively
high coating performance are among its advantages.
As demands on protective measures in the boiler sector
have been changing, the demands on thermal spray
coatings have also increased. Initially the focus was on
those forms of damage characteristic in coal-fired power
plants but nowadays it is mainly centred on corrosioninflicted
strain. This is initiated by the increasing chlorine
content in the exhaust fumes and presents a significant
challenge for preventive maintenance methods like
Thermal Spraying.
Characteristics of thermally sprayed protective
coatings
Thermal Spraying is a method for mechanical bonding
of coatings composed of single particles, without intensively
heating the substrate. In order to achieve sufficient
bonding the particles need to have a high energy level
at the moment of their impact on the substrate. On impact
upon the substrate which has to be protected this energy
is transformed into a deforming force which ultimately
determines the adhesive strength of the coating being
applied.
Hence the structural composition of a thermally applied
coating differs significantly from that of casted materials.
The latter are heterogeneous in their structure and possess
a residual pore volume. As protective coatings, they are
not capable of forming a completely gas-proof bond and
will eventually be penetrated by combustion atmosphere.
Quality relevant process factors
Thermal Spraying is based on the mechanical anchoring
of highly heated or highly accelerated particles on tube
material without heating the substrate itself during the
process. Thermal Spraying avoids inducing tension into
the construction, which is a crucial motivation for boiler
applications. The layer is formed by successive attachment
of the sprayed particles whereby velocity and temperature
at the moment of impact on the substrate determine
adhesion and density respectively.
Characteristic data for the above mentioned processes
are cited in table 1.
Thermal Spraying for corrosion protection
Boiler components in the corrosive environment of refuse
and residual waste incineration are subject to accelerated
damage due to chlorine induced corrosion. With rising
temperatures of the water walls damage is increased.
Raised expectations towards thermally sprayed coatings
add also to this fact. Comparative analyses of thermally
sprayed coatings under realistic conditions in a superheater
area (refuse incineration) gave good support
[2]. A variety of thermally sprayed coatings produced
by arc and high velocity processes were applied in the
same superheater environment. Spray and fuse coatings
were also examined in this cycle of tests as alternatives.
Coatings of this type are subject to a high temperature
treatment following the coating, which leads to gas-tight
variants. They have been developed especially for superheater
tubes (illustration 3). This technology is only
suited for pre-coated tubes and cannot be used for the
reconditioning of damaged ones.
Erosion protection
Erosion protection by means of thermally sprayed coatings
requires above all appropriate resistance against
mechanically acting surface damage as well as sufficient
corrosive resistance adapted to the boiler atmosphere.
With these applications damage to boiler tubes always
occurs in the form of substantial abrasion rates, thereby
reducing the tube walls’ thickness. Showcases are various
applications with circulating and stationary fluidised bed
furnaces [4] as well as water walls above linings in grate-
fired plants, zonal effects on soot blowers, flow-sided
tube damage due to large dust loads coupled with high
flow velocities.
Summary
The disadvantage of the low penetration resistance of
thermal spray coatings is a common issue with high chlorine
content. Therefore, only dense coatings can succeed
at increased temperatures as they occur in superheater
applications for instance. High velocity spraying comes
close to fulfilling this demand but with plasma sprayed
coatings it is hard to meet. With increased thickness the
danger of penetration declines regardless of the process
being used. Coating thicknesses of under 500 μm for
high-energy spray processes and below 800-1000μm
for arc spraying have established themselves as economic
standards. Certainty about complete gas-proofness can
only be achieved by treating the coatings with an additional
thermal sintering process. Coatings of such quality
are mainly used for corrosion protection on superheater
pipes.