This literature review summarizes several studies on slurry erosion performance and resistance of various coated and treated steels. Several studies investigated the effect of parameters like particle size, speed, concentration, and impact angle on erosion rate. Coatings like WC-Co, Cr2O3, and Al2O3 were found to improve erosion resistance compared to uncoated steels. Laser surface treatments using powders also increased hardness and erosion resistance. Erosion mechanisms for coated materials included cracking and removal of material, while uncoated steels experienced plastic deformation and cutting.

1. LITERATURE REVIEW
Bhandari et al. (1) studied slurry erosion performance study of detonation gun sprayed WC - lOCo-4Cr
coatings on CF8M steel under hydro accelerated conditions. They investigated the erosion behavior of
coated and bare steel on high speed erosion test rig. Silt used as abrasive media collected from actual
working conditions of power plant and then sieved and grains of 100 urn and 300 urn were obtained.
Parameters like rotational speed, particle size, and concentration were studied. Effect of slurry
concentration was pronounced in bare CF8M steel due to less hardness compared to slurry particles.
However in case of average particle size, effect was more pronounced in the coated steel. Also specific
mass loss increases with increase in rotational speed. Eroded samples were analyzed using SEM and
XRD.They concluded that most of erosion in CF8M steel was due to cutting/ploughing whereas in coated
steel erosion was due to brittle fracture.
Goyal et a1. (2) Compared slurry erosive wear of HVOF spray Cr203 coatings on CF8M and CA6NM
turbine steel of yield strength 205 MPa and 550 MPa respectively. Silt used in slurry was of irregular
shape and mainly Si02 particles of 1161 HV. The average particle size used was 100 μm and 300 μm. A
High speed erosion test rig consist of erosion chamber, rotor to rotate work piece up to 5000 rpm was used
for investigation. Effects of parameters namely, average particle size, speed (rpm) and slurry concentration
were investigated during experiment. Specimens surface morphology were studied using SEM before and
after of experiment to study the erosion mechanism. HVOF- sprayed Cr203 coating reduced the slurry
erosion rate of steels Because of high hardness in comparison with uncoated steels. Slurry erosion
resistance of both coated steels were same thus it is independent of base metal but among CF8M and
CA6NM, CA6NM had better erosion resistance because of high hardness and yield strength. Among all
the factors rotational speed i.e. rpm was most dominating factor in increasing the erosion rate. The main
mechanism for material removal in uncoated steels was crater and lip formation, cutting and ploughing,
plastic deformation and in coated steels was micro chipping at splat boundaries.
Saleh and Ahmed (3) investigated slurry erosion-corrosion of carburized AISI 5117 steel. Slurry used for
investigation was composed of sand particle and water or 3% NaCI solution. Erodent used was mainly
silica and sieved to size of 250 um -355 urn. Erosion test was performed on

2. whirling arm ring. Two specimen's holders were mounted at the end of the arm which was attached to
the rotor. Slurry stream velocity was 1.6 m/s at any preset angle between 0°-90°. Microstructure of
carburized steel was investigated using SEM. which shows microstructure was completely martensit ic
near the surface due to high carbon content and case depth was 0.65 mm. They concluded that maximum
erosion was at impinging angle of 45° and erosion-corrosion resistance was improved by 40-60% by
carburizing. Both treated and untreated material behaves as ductile material under erosion test. SEM
showed that erosion tracks were wider and deeper for untreated specimens and shallower for treated
ones.
Grewal et al. (2012) compared the slurry erosion of hydro turbine steels (I6Cr5Ni and 13Cr5Ni) with
and without detonation gun spray coatings using Taguchi technique. Samples of both steels were coated
with WC-lOCo-4Cr using detonation gun with a particle size of 15 to 45 μm. SEM/EDS. XRD techniques
were used for an in depth characterization of coated samples. SEM indicated laminar splat like
morphology of coated samples and XRD showed the WC as primary phase along with W2C and C06W6C
as secondary phase. Hardness of coatings was found to be 4 to 5 times of base metal. Slurry erosion test
was conducted using jet type test rig to ascertain the slurry erosion behavior of coated and bare steel
samples. The effect of parameters like velocity, impact angle, concentration of slurry and particle size
was studied. Erodent used for testing was collected from hydropower plant to replicate the actual
conditions. They concluded, in case of bare samples velocity and particle size was dominating factors
but in case of coated samples velocity was the dominating factor. Also bare 16/5 steel had better slurry
erosion as compared to 13/4 steel. SEM images showed for bare steel erosion mechanism was plastic
deformation and for coated steels removal of material was due to formation and interlinking of cracks.
Basba et al. (2013) improved the slurry erosiom wear resistance of 16Cr-5Ni martensite steel by laser
surface alloying (LSA) and laser transformation hardening (LTH). Co based Wallex-50 and Ni-based
Tribaloy- 700 powders were used for coating of steel by LTH and LSA respectively. Microstructures of
samples were observed using SEM, Tribaloy-700 coated showed hard intermetallic laves phase dispersed
in nickel eutectic and Wallex-50 was composed of needle of alpha solution distributed in beta solid
solution. Erosion test was performed on jet type test rig with abrasive size of 200 μm. The effect of
parameters like angle of impingement and velocity were studied. They concluded cumulative loss was
higher for 16Cr-5Ni steel as compared to

3. laser treated steel for impingement angle of 30°. Laser treated steel had high resistance to wear by 1.5-3
times as compared to substrate 16Cr-5Ni steel for all angles of impingement because of high hardness.
The erosion mechanism for both Tribaloy-700 and Wallex-50 was plastic deformation while for substrate
crater lips and cutting action were main mechanisms.
Bhandari et al. (2011) investigated the slurry erosion behavior of detonation gun sprayed Al203 and
A1203- 13Ti02 coating on CF8M steel under hydro accelerated conditions. Hollow cylindrical specimens
were used for erosion test. They studied the effect of parameters like average particle size, concentration
of slurry and rotational speed. Slurry was collected from hydro power plant to achieve actual replica of
slurry erosion condition. They concluded slurry concentration and average particle were dominating factor
in the slurry of Al203 coated steel while rotational speed was dominating factor in A1203-I3Ti02 coated
steel. A1203-13Ti02 coated steel result much better resistance to erosion as compared to Al203 because
of high toughness of Al203- I 3Ti02 coated steel. Erosion rate of both coatings increased was increased
with change in slurry concentration from 10,000-30,000 ppm.
Lynn et al. studied the effect of slurry particle size on slurry erosion using a pot tester at a constant velocity
18.7 m/s on steel specimen. Slurry was prepared by mixing 1.2% by weight of silica carbide in oil for
different diameter ranging from 2011 t050Il. They conclude that for particles sizes greater than about
10011 the erosion rate was proportional to the kinetic energy dissipated by particles during impact but for
particles size less than 10011 other metal removal mechanism become increasingly significant. Both
collision efficiency and impact velocity of particles decreased with decreasing particles size.
Lin and Shao studied the effect of velocity on erosion wear in a newly designed erosion tester using
equisized sand-water slurry. The velocity was varied between 10 m/s and 70 m/s and impact angle between
15° and 90°. Four specimens (pure aluminium, 1020 steel, high chromium and cast iron) were tested at a
constant concentration of Cw = 5%. They found that the velocity exponent increase with increasing
impingement angle and decreased with increasing hardness. Velocity exponent in the ranges of 1.87-2.48
were observed for different impingement angles and different materials. At 90° impingement angle, the
indexes of velocity for erosion wear for aluminium was2.48 where for the chromium it was 2. I 7. This
implies that with increase in hardness of the test material, the exponent of velocity decreases.

4. INTRODUCTION
Slurry erosion is defined as degradation of surfaces upon interaction with solid particles entrained in liquid
medium. This erosion occur either material moves through slurry or slurry impinges on the material at
certain velocity. Slurry erodes by the action of abrasive particles in liquid which result in the action of
wear of material depend upon the conditions to which the system is exposed. Slurry erosion is major
problem for the industries which deal with liquids having abrasive particles. When such a mixture of liquid
and sand particles comes in the contact of material surface, surface get eroded by these particles.
FACTORS AFFECTING SLURRY EROSION
There are several factors which contribute towards the degradation of the surface and removal of material
due to slurry erosion. These factors decide which type of erosion mechanism will be the dominating factor
in the material removal process. Several types of factors are:
 Factors depending on the operating conditions are velocity, concentration, impact angle,
temperature etc.
 Factors that depend on erodent's are particle size, particle shape, hardness etc.
 Factors that depend on the properties of target surface are hardness, microstructure, mechanica l
properties etc.
EROSION MECHANISMS IN SLURRY EROSION
Various erosion mechanisms responsible for the loss of material in slurry erosion are:
 Cutting erosion
 Ploughing erosion
 Platelet mechanism
 Subsurface deformation and cracking
SLURRY EROSION IN HYDROPOWER PLANTS
The hydro power plants suffer fi'OI11 a very big problem of slurry erosion as the sand particle flow along
the water. The sand particles not only reduce the efficiency of the turbines but also destroy the turbine

5. components. In India, hydropower plants located in the Himalayan region are severely affected by slurry
erosion. India alone faces the loss of around US $120-150 mil lion due to silt erosion. Hydropower plant
of China and Nepal also suffers the higher degree of damage due to slurry erosion. Problem of erosion
becomes severe in rainy season when the concentration of sand particles becomes 3, 00,000-1,000,000
ppm. In such conditions power plant does not work and shut down during high concentration period.
Hydropower plants normally work up to the concentration of sand around 5,000 ppm. The various parts
of hydropower plant which are affected by slurry erosion are impellers, guide vanes, buckets, nozzles,
spears and labyrinth seal. The material mainly used for hydro turbine are CA6NM, CF8M, 13Cr J Ni and
16Cr5Ni steels. Among these CA6NM is preferably used, owing its good resistance against corrosion,
cavitation erosion, high impact and fracture toughness. Commonly used material for different parts of
turbine are:
S.NO. COMPONENTS MATERIALS
1. Runner A S T M 7 4 3 -13Cr5Ni, SEW 410-16Cr5Ni
2. Labyrinth seal A S T M 7 4 3-J3Cr5Ni, 18CrIONi, SEW 410-J6Cr5Ni
3. Guide vane ASTM743-13Cr5Ni
4. Liner ASTM743-13CrSNi
5. Rubber seals Neoprene synthetic rubber
6. Draft tube SEW 41 0-16Cr5Ni, W-1.0566
The problem is not only the erosion, but many other kinds of factors which harm the material in one way
or the other. The material gets corroded very readily which weakens it and with successive flow of slurry
it erodes the material. So the need of the day is to look after this serious problem and take necessary
protective measures against the same. The material used should have proper strength and should not erode
readily. Some features that the material should possess are high tensile strength, high hardness, resistance
to corrosion, etc. If the material alone is not enough, a coating of suitable material should be applied on
the substrate material to avoid erosion. Coating is a common measure taken into consideration for
manufacturing of any part of even a simplest machine. Before coating a material it should be taken into
consideration, for what purpose the coated material is to be used. If it has to be subjected to some solid
particle erosion it should have high strength and if it has to be subjected to some corrosive environme nt
the coating done must be of corrosion resisting features.