FIELD OF THE INVENTION:
This invention relates to a method for evaluation of high temperature corrosion
resistance of coating material at simulated boiler conditions.
The present invention generally relates to a method for obtaining a coating
material for boiler which has high temperature corrosion resistance with
different Indian fly ash, at simulated boiler conditions. Wire Arc Spraying
process is taken and Ni-5Al is used as coating material. More particularly the
invention relates to a method for evaluation of coated material having high
temperature corrosion property with Indian fly ash produced in boiler
materials.
BACKGROUND OF THE INVENTION
In coal-fired power stations, about 20% of the ash produced in the boilers is
deposited on the boiler walls, economisers, air-heaters and super-heater tubes.
This deposited ash is subsequently discharged as slag and clinker during the
sootblowing process. The rest of the ash is entrained in the stream of flue gas
leaving the boiler. These ash particles collide with the boiler steel components
and cause extensive surface corrosion. In advanced stages of corrosion, the
components get perforated, and may fail once they lose their structural
integrity. Such corrosion, together with the processes of blocking, fouling and
erosion, shortens the service life of boiler components. Once this happens, the
power station unit has to be shut down in order to replace the damaged
components. The resulting penalty is not only the cost of replacing the
components but also the cost of stoppage of power production. It is desirable,
therefore, to use coated components and be able to predict the rate of corrosion
of the coated coal-fired boiler components in order to plan systematically for
the maintenance or replacement of these components and avoid forced
outages.

Faced with deregulation, increasing retail competition and pressures to keep
boilers online, many coal-fired power generating stations have adopted
strategies centred on increasing unit availability, reliability and increasing the
operational life of the critical equipment. However, boiler tube failures continue
to be the number one cause of forced outages in fossil plants today. These
costly forced outages are responsible for an estimated six percent overall loss of
unit availability. One of the major causes for premature tube failure is
excessive fireside boiler tube corrosion caused by the impact, cutting action
and abrasive wear of fly ash entrained flue gases undercutting the area they
strike.
Power generation utilities and holding company goals are to extend times
between major planned boiler outages. Systems types and configurations, the
age of the plant, their specific plant operating demands and both preventative
and general maintenance philosophies can dictate the accomplishment of these
goals. Extending time between major outages two, four and even five years is
resulting in increased forced outages due to tube failures. An estimated
seventeen causes of tube leaks have been sited. However, one of the most
problematic and hardest task is to predict corrosion-caused failures.
Corrosion is defined as a process by which material is removed from the layers
of a surface impacted by a stream of abrasive particles. Fly-ash particles
entrained in the flue gas from boiler furnaces in coal-fired power plants can
cause serious corrosive wear on steel surfaces along the flow path, thereby
reducing the operational life of the mild-steel heat transfer plates that are used
in the rotary regenerative heat exchangers.
In India, thermal power accounts for about 65-70% of the total installed power
generating capability and about 75-80% of the total electrical energy that is

generated annually. In the field of thermal power wherein fossil fuels are used,
coal based power generation dominates the scenario, accounting for about 85
to 90% of the total thermal power generation. India has huge reserves of coal
deposits (estimated at around 180-200 billion tons); due to the abundant
availability and the low cost coal will continue to be the main fuel for electrical
power generation for many more years.
The wire-arc spray process had become one of the most successful techniques
for thermal spray applications due to lower operating cost, higher material
output per hour, production of a more coarse coating and cost efficient
corrosion protection. Also, it is an easily repeatable process that allows for a
tremendous variety of highly reliable and material coating solution. Minimizing
the porosity and oxygen-content in the coating and a defined splat formation at
the substrate are major challenges for a further process improvement which
leads to a deep analysis of influencing parameters on the wire arc spray
process. However wire arc spraying process have proven effective at extending
service life of boiler tubes by preventing the wear factors spraying protective
coating is a proven cost effective solution that will extend the boiler lifecycle
and decrease the down time.
In Indian coal-fired power plants, corrosion/erosion induced by fly ash has
been a perennial problem. This has long-term techno-economic implications on
the life cycle management of the power plant components. It has been observed
that in coal-fired power stations, about 20% of the ash produced in the boilers
is deposited on the boiler walls, economizers, air-heaters and super-heater
tubes.
Although aluminum protects itself against corrosion by forming a natural oxide
coating, the protection is not complete. In the presence of moisture and

electrolytes, aluminum alloys corrode much more rapidly than pure aluminum.
Accordingly, there is a need to treat aluminum alloy substrates with
pretreatments or other chemicals that provide improved corrosion resistance as
well as good adhesion.
Various investigators have addressed the problem of solid particle corrosion
but the work has remained confined to high temperature investigations. Many
parameters are now known to influence corrosion behaviour. The magnitude
and direction of an ash particle’s impact velocity relative to the target metal
surface constitute essential data needed for evaluating corrosion of the surface
due to particle impact. Magnitude and direction of a particle’s rebounding
velocity depend upon the conditions at impact and the specific particle–surface
material combination.
US 6020030 A for coating an aluminium alloy substrate, describes an
aluminium alloy substrate which is pre-treated with an aqueous solution
containing an organophosphorus compound, preferably a vinylphosphonic
acid-acrylic acid copolymer, before coating the substrate with a polymer.
Passing the substrate through the solution contaminates it with aluminium
and other elements.
US 6375726 B1 relates to corrosion resistant coatings for aluminium and
aluminium alloys. This invention describes the protection and surface
treatment of aluminium, aluminium alloys and coated aluminium substrates
against corrosion. The aluminium substrates are treated with an acidic
aqueous solution containing small but effective amounts of at least one
trivalent chromium salt such as a trivalent chromium sulphate. The corrosion
resistant aluminium substrates of this invention have improved adhesion for
overlaying coatings e.g. paints and a lower electrical resistance contact.

EP1431416 for protective Ti-Al-Cr-N coating, relates to coatings for the
protection of substrates operating at moderately elevated temperatures, and,
more particularly, for the protection of titanium-alloy aircraft and stationary
gas turbine components as well as engine components for automotive
applications, articles having such coatings and a method for their production.
US 5203985 A discloses process for manufacturing galvanized steel sheet by
nickel pre-coating method, comprising steps of coating a steel sheet with nickel
in an amount of from 0.2 to 2 g/m2, heating the steel sheet thus coated to a
temperature within the range of from 420°C to 500°C in a non-oxidative
atmosphere, and dipping the nickel coated steel sheet into a molten zinc bath
containing aluminium at a content of from 0.1 to 1% without contract with air,
wherein the steel sheet is dipped into the molten zinc bath within 15 seconds
after the temperature of the steel sheet extends 350°C in the heating step.
US 20060121302 A1 for a wire-arc spraying of a zinc-nickel coating, refers to a
zinc-nickel substrate which is applied to a component by using a zinc-nickel
wire formed with between about 8 to 20 percent nickel and between about 80
to 90 percent zinc. This formed zinc-nickel wire is held in a two wire electric arc
spray system to spray the zinc-nickel substrate on the component, such as an
aircraft part, while operating the two wire electric arc spray system within a
specific range of operating parameters.
In US 7,585,568B2, the disclosure provides an improved solar selective
multilayer coating having higher thermal stability and a process therefor.
According to the disclosure, a tandem stack of three layers of TiAlN, TiAlON
and Si3N4 is deposited on metal and non-metal substrates using a planar
reactive magnetron sputtering process. The solar selective coatings exhibit high
hardness, high oxidation resistance, chemical inertness and stable
microstructure.

US 6020030 A for coating an aluminium alloy substrate, describes the process
how an aluminium alloy substrate is pre-treated with an aqueous solution
containing an organophosphorus compound, preferably a vinylphosphonic
acid-acrylic acid copolymer, before coating the substrate with a polymer.
US 08632940 B2 for Aluminum Substrates and Lithographic Printing Plate,
gives electrochemically grained and anodized aluminum supports are treated
with a post-treatment coating solution containing a polymer derived at least in
part from vinyl phosphonic acid and phosphoric acid.
WO 2015034985 A1 for wire alloy for plasma wire arc coating, describes a
method of depositing a corrosion resistant material via a plasma transferred
wire arc (PTWA) thermal spray method on the cylinder surface of heavy duty
diesel internal combustion engines. The PTWA process uses a stainless steel
hollow core wire that is filled with a metal oxide or carbide powder. The powder
can be 100% chromium carbide.
In US 5294462 A for electric arc spray coating with cored wire, a method is
disclosed for the electric arc spraying of powder-filled cored wires to apply
hard, wear-resistant coatings to various substrates. Inert gas, preferably
nitrogen, is supplied to the arc spray gun such that the mass ratio of the wire
feed rate to the gas feed rate is preferably between about 0.07 and about 0.11.
Operation in this range yields an optimum combination of coating hardness
properties and arc spray gun operating characteristics
Indian Patent Number: 243606, relates to a Zinc-Aluminium pseudo-alloy
coating to enhance the corrosion resistance of steel structures. The coating
consists of pseudo-alloys of Zinc and Aluminium where the total weights of

Zinc and Aluminium are maintained close to 73 percent and 27 percent
respectively, which has been tested to provide the best protection for the steel
structure underneath. A convenient way of obtaining this is by the use of a
Twin Arc Spraying method.
US 4451304 A describes a method of improving the corrosion resistance of
chemical conversion coated aluminium. A method for treating aluminium prior
to applying a chemical conversion coating for improving its corrosion resistance
especially when rigid requirements need to be met. The aluminium is alkaline
cleaned and deoxidized, preferably with a non-chromated deoxidizer. A
supplementary treatment consists of immersing the deoxidized aluminium in a
potassium of sodium nitrite solution. The aluminium is then coated with a
standard chemical conversion coating, and dried at a temperature between
110°F and 130°F. The resulting product is able to pass salt spray test
requirements as outlined in the military specification.
US 3,958,046, titled Coating for corrosion resistance, refers to Aluminium
coatings to reduce corrosion of steels and the like, are very effective when
applied by pack diffusion below 1000.degree.F using a retort cup not over
fifteen inches high, with anhydrous or hydrated energizer in a layer on top of
the pack and out of contact with the work pieces.
US Patent No. 4,123,290, refers to a chromium-containing coating
composition, also containing pulverulent metal, will provide a coating of
excellent corrosion resistance when the composition contains urea as at least a
part of the reducing agent, and when the composition also contains particular
pH adjusting agent. The reducing agent of the composition is used in regard to

the hexavalent chromium contained in the chromium portion of the
composition. The pH adjusting agent should be a compound of zinc. In addition
to achieving excellent corrosion resistance, resulting coatings can provide the
other desirable characteristics for coated substrates, such as topcoat adhesion
and formability, without deleterious effect.
US 4537837 A relates to a corrosion resistant metal composite with metallic
undercoat and chromium topcoat. A coating composite provides extended
corrosion resistance for substrate metals. The thin metallic undercoat of the
composite contains combined metals. The heat curable and substantially resin
free topcoat is established from composition containing chromium in non-
elemental form. In addition to outstanding corrosion resistance, the composite
can retain substrate weldability as well as achieving formability.
WO 2010112914 A1 refers to a process for the enhanced corrosion protection
of valve metals.
A process for the corrosion protection of metals such as magnesium,
aluminium or titanium, where at least two steps are used, including both
plasma electrolytic oxidation and chemical passivation. The combination of
these two processing steps enhances the corrosion resistance performance of
the surface beyond the capability of either of the steps in isolation, providing a
more robust protection system. This process may be used as a corrosion
protective coating in its own right, or as a protection-enhancing pre-treatment
for top-coats such as powder coat or e-coat.
WO 1996012052 A1describes corrosion resistant aluminium and aluminium
coating, aluminium or an aluminium alloy which has not been treated with

chromium and which is protected with a non-chromium aluminium conversion
coating composition which has as the essential ingredients an alkaline metal
permanganate and aluminium nitrate and a pH of about 2.5 to about 4.0. The
composition is effective in protecting aluminium and aluminium alloys for more
than 168 hours in salt fog at 95 °F according to standard ASTM method B-117.
US 4878963 A describes Corrosion resistant aluminium coating composition.
In this an aluminium conversion coating composition for aluminium or an
aluminium alloy. The composition has as essential ingredients alkali metal
permanganate, and alkali metal chloride, a pH of 7 or over, and a phosphorous
compound selected from phosphorous acid and alkali metal phosphate.
US 6074464 A, describes Phosphate bonded aluminium coatings. A bonding
composition for a heat curable overlay coating for coating a substrate is
provided. The bonding composition comprises phosphate ions, permanganate
ions, at least one species of metal cation having a valence greater than +1, and
water. The bonding composition has a pH in the range from about 1.5 to about
4.5 and is substantially free of chromate ions and molybdate ions. Aluminium
ions are a preferred species of metal cation in the bonding composition. Overlay
coating compositions are also provided containing metallic aluminium particles
dispersed in the bonding composition of the invention.
Patent US 5520750 A, titled an anti-corrosion treatment of an aluminium or
aluminium alloy surface comprising the steps of treating the surface with a
solution having a pH of from greater than 0 to less than 2 and comprising at
least one metal oxo ion completed with phosphorus (V) wherein the metal oxo
ion is vanadate or tungstate or a mixture thereof.

US 7135075 B2, titled Corrosion resistant coating with self-healing
characteristics, discloses an aqueous solution for depositing an inorganic
corrosion resistant coating on a metal substrate is disclosed. The aqueous
solution comprises a film-forming agent, a supplemental anion, and a
substrate activator. The film-forming agent is a vanadate salt that forms the
corrosion resistant coating. The supplemental anion accelerates the rate at
which the corrosion resistant coating is formed. The substrate activator serves
to remove any existing oxides from the metal substrate prior to the formation of
the corrosion resistant coating. The present invention additionally covers
objects so coated and methods of application.
Corrosion resistant metal composite with metallic undercoat and chromium
topcoat is disclosed in patent No. US 4537837 A. It gives a coating composite
provides extended corrosion resistance for substrate metals. The thin metallic
undercoat of the composite contains combined metals. In addition to
outstanding corrosion resistance, the composite can retain substrate
weldability as well as achieving formability, while further enhancing
paintability and weatherability.
In US 4314893 A for production of multiple zinc-containing coatings, markedly
superior corrosion protective properties are imparted to metal-bearing surfaces
at substantially reduced costs by depositing upon said surfaces one or more
superimposed coatings containing zinc alloying elements located intermediate
the metal-bearing surfaces and a sacrificial layer of generally pure zinc.
OBJECTS OF THE INVENTION:
It is therefore an object of this invention to propose a method for evaluation of
high temperature corrosion resistance of coating material under simulated
boiler conditions.

It is a further object of this invention to propose a method for evaluation of
high temperature corrosion resistance of coating material under simulated
boiler conditions, which is simple and easy to operate.
It is another object of this invention to propose a method for evaluation of high
temperature corrosion resistance of coating material under simulated boiler
conditions, which is cost effective.
Yet another object of this invention is to propose a method for evaluation of
high temperature corrosion resistance of coating material under simulated
boiler conditions, which uses easily available materials and is eco-friendly.
These and other objects and advantages of the invention will be apparent from
the ensuing description, when read in conjunction with the accompanying
drawings.
SUMMARY OF THE INVENTION:
High temperature corrosion is an unexpected failure in a boiler material, is
caused by corroding environment with different coal/fly ash and at high
temperature. Hence there is a necessity that the high temperature corroding
properties of a materials has to be evaluated particularly when a new ultra
super critical boiler materials/weld are involved. There is no conventional
method to eliminate high temperature corrosion for boiler material with Indian
coal ash/fly ash at very high temperature (simulated boiler condition). Coating
with Ni-5Al by wire arc spraying of T23 base material as well weld material
were done .After coating the coated base material and coated weld material
were taken for evaluation. Initially the specimen was weighed and its
dimensions were measured. The specimen is loaded with simulated boiler

environment. Synthetic fly ash was prepared with different combinations of
sulphate/Chloride and actual coal ash/fly ash. The temperature experienced
by the specimen is measured in the furnace with thermo couple. The high
temperature corrosion experienced by the specimen is computed using
quantitative measurements as well as dimensional measurements. High
temperature corrosion resistance was calculated for each specimen with
definite time duration in the present invention with the particular engineering
material.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1–Shows flow chart of the methodology for evaluation of high
temperature corrosion resistance.
Figure 2–Shows the methodology for Wire arc coating process with the Ni–5Al
coated material with T23 base metal/weld specimen.
Figure 3–Shows the methodology for evaluation of high temperature corrosion
resistance in the Ni–5Al coated/uncoated and base/weld specimen in
simulated boiler condition with Indian coal ash/fly ash.
DETAILED DESCRIPTION OF THE INVENTION:
Thus according to this invention is provided a method for evaluation of high
corrosion resistance of coating material at simulated boiler conditions.
In accordance with this invention, is proposed, method to evaluate the high
temperature corrosion resistance in Ni-5Al coated base material and Ni-5Al
coated weld material.
Coal-fired power plants, the high temperature side of the boiler contains ash,
alkali sulfates and NaCl, which at higher temperature of range 650°C-800°C

creates the corroding environment. This mixture leads to corrosion of the base
metal as well as weld area. The corrosion rate depends mainly on the chemistry
of the exposed area (base/weld apart from temperature, environment and time.
Coating is done for both base and weld specimens to prevent corrosion.
Coating is done thru Wire arc coating process with Ni-5Al wire on T23 base
material and weld material. Thus in present investigation, the coated base
specimen/coated weld specimen is taken and heated in the range of 600°C-
800°C, to analyse the corrosion rate at higher temperature range. Experiments
are conducted to investigate the high temperature corrosion resistance on T23
base material and its weldment, coated as well as uncoated. The Ni–5Al
coatings on SA213 T23 has been made with wire arc spraying system and Ni–
5Al coating was done on both weld and base metal. Investigation on high
temperature corrosion behaviour under cyclic conditions at about 9000C for
about 100 hours in an aggressive molten salt environment was made and
compared with uncoated material. After several heating cycles the quantitative
evaluations are carried out. A methodology for evaluating high temperature
corrosion resistance with different media and different high temperature boiler
materials and their coated base/weldments is invented. Initially the specimen
was weighed and its dimensions were measured. The specimen is loaded with
simulated boiler environment. Synthetic fly ash was prepared different
combinations of sulphate/Chloride and actual coal ash/flyash. The compounds
employed were sodium chloride, potassium chloride, sodium sulphate,
potassium sulphate, and vanadium oxide. The temperature experienced by the
specimen is measured in the high temperature furnace with thermo couple.
The high temperature corrosion experienced by the specimen is computed
using quantitative measurements as well as dimensional measurements. High
temperature corrosion resistance was calculated for each specimen with
definite time duration. The coated specimens show better corrosion resistance
compared to uncoated specimens.

Coating of Ni-5Al material on T23 base/weld material by wire arc coating
process is done. Evaluation of high temperature corrosion resistance of coated
base material and coated weld material is done and compared with high
temperature corrosion resistance of uncoated material.
The method for evaluation of high temperature corrosion resistance in the Ni-
5Al coated base/weld metal as well as uncoated specimens in simulated boiler
condition with Indian coal ash/fly ash is given in Figure-1. The methodology
has multiple steps for the evaluation of corrosion resistance of Ni-5Al coated as
well as uncoated specimens.
The methodology of wire arc coating with Ni-5Al powder has 9 parts, given in
Figure-2, the wire (9) is sent through push system(8) with compressed air(7).
Both wires (4) melted by arc thru contact nozzle (6). The wire arc spray jet (3)
coats the substrate the T23 weld metal/base metal (substrate) of size (12 X 11
X 5mm) (5) were adhered in a plate by adhesive. The coated (2) material gets
separated out from over spray (1). Prior to the coating deposition, the substrate
surfaces(5) were grit blasted using garnet particles of 300-400 grate size
particles to increase the roughness of the surface to enhance the adhesion
strength between coating and substrate. Over spray(1) where Ni-5Al wire(9) is
fixed. The process parameters are spraying distance (130 mm), transverse
movement (100mm/sec), air pressure (3-5 MPA) coatings with 6 layers (2). After
the coating process, samples are removed from the plate and ultrasonically
cleaned with acetone and dried with warm air.
The set-up for evaluation of high temperature corrosion resistance in the Ni–
5Al coated/uncoated and base/weld specimen in simulated boiler condition
with Indian coal ash/fly ash given in Figure-3, has 7 parts. A tubular furnace

(1) where a pre weighed test (Ni-5Al coated base metal/weld metal or uncoated
base metal/weld metal) (2) specimen is taken in a boat(3) and exposed to
synthetic fly ash of different composition of Indian fly ash/coal ash(4).
Thermocouple (5) is inbuilt with the furnace to record the temperature. The pre
weighed and known dimensions and known thickness, specimen is kept in the
furnace through the inlet with leak proof gasket (7). Now the specimen is
exposed to high temperature corrosion environment, so that specimen is under
high temperature and corrosion environment. The specimen is kept for a
specific duration depending on the material. The specimen is removed through
the outlet (6).
COATING PROCESS
The weld metal and base metal of size (12 X 11 X 5mm) were adhered in a plate
by adhesive. Prior to the coating deposition, the substrate surfaces were grit
blasted using garnet particles of 300-400 grate size particles to increase the
roughness of the surface to enhance the adhesion strength between coating
and substrate. Different parameters of the wire arc spraying technique are
optimized, providing a set of conditions that are acceptable in terms of bond
strength and presence of defects in the coatings, such as oxides or porosity.
The process parameters are spraying distance (130 mm), transverse movement
(100mm/sec), air pressure (3-5 MPA) coatings with 6 layers are fabricated in
order to obtain a more adequate thickness for the application. After the coating
process, samples are removed from the plate and ultrasonically cleaned with
acetone and dried with warm air.

PROCEDURE FOR EVALUATION OF CORROSION RESISTANCE
A methodology for finding high temperature corrosion resistance through high
resolution gravimetric analyser is invented. The specimen of coated base
material is loaded high temperature environment with gravimetric
specifications. The high temperature experienced by the coated specimen is
measured with the thermocouple. The high temperature experienced by the
loaded coated specimen is measured using the high resolution gravimetric
analyser. (Mass balance with accuracy of 0.1mg) Now the coated specimen is
under known temperature and time. In this condition the coated specimen is
corroded in the known temperature and the corroding media/environment.
After the tests, the samples were washed with boiling distilled water to dissolve
the remains of sulphate/chloride and to remove the loose corrosion products
with hydrochloric acid. After specific time duration, the specimen is removed
from the furnace and it dimensional variations as well as gravimetric changes
are measured and are computed to measure its high temperature corrosion
resistance. Both of the coated specimens before and after the tests are
measured using an electronic balance with an accuracy of 0.1 mg.
The following table is given to show typical the high temperature corrosion rate
(CR) measured for coated as well as uncoated base/weld specimens with
different media and at 9000C with the duration of 100 hours


It is readily apparent that the coated specimens show better corrosion
resistance compared to uncoated samples.

WE CLAIM:
1. A method for evaluation of high temperature corrosion resistance of Ni-
5Al coated specimens, comprising the steps of preparing specimens of
T23 metal/weld metal, followed by coated said specimens with Ni-5Al
powder by wire arc spraying to obtain the Ni-5Al coated base metal/weld
metal,
obtaining the initial weight and dimension of the coated base metal/weld
metal specimens,
subjecting said coated base metal/weld metal specimens to simulated
boiler conditions and measuring the temperature and time experienced
by the coated metal/base metal,
computing the difference in weight and dimension of the coated
metal/base metal,
subjecting uncoated specimens of base metal/weld metal of the same
dimensions to identical conditions of simulated boiler environment
followed by computing the difference in weight and dimensions of the
uncoated specimens, and evaluating the high temperature corrosion
resistance of the coated Ni-5Al and uncoated N-Al base metal and weld
metal specimens.
2. The method as claimed in claim 1, wherein said base metal and weld
metal specimens are adhered to a plate, and the surfaces are grit blasted
prior to coating.
3. The method as claimed in claim 1, wherein said coated Ni-5Al specimens
have a coating hardness of 162-172HV, and bond strength of 60 MPa or
above.

4. The method as claimed in claim 1, wherein said Ni-5A1 specimens are
coated at a temperature of max 8000°C.
5. The method as claimed in claim 1, wherein said simulated boiler
conditions include typical high temperature of about 900°C, duration of
about 100 hours in the presence of combinations of compounds such as
chlorides, sulphates and oxides of metals.
6. The method as claimed in claims 1 & 5, wherein said compounds are
selected from sodium chloride, potassium chloride, sodium sulphate,
potassium sulphate, vanadium oxide.