Duan C.G., Karelin V.Y. Abrasive Erosion and Corrosion of Hydraulic machinery

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344 Abrasive Erosion and Corrosion of Hydraulic Machinery

+2°.

It became weaker at

<p =

0°, whereas at

<p =

-3° is totally absent. At

q> =

+2°,

when Ah is reduced from 8.84 to 8.05 m, the decrease of hydroabrasive

erosion in the impeller is

24%.

For

<p =

0° the hydroabrasive erosion intensity

decreased by

17%

when

was reduced form 8.5 to 7.74 m, whereas at

= -

3° the reduction of Ah caused a continuous growth of erosion as observed in

the impeller of a pump.

Therefore, for large

values, when the difference in the angle formed

between the blade axis and the axis of the suction piping is the least, the

reduction of Ah is accompanied with formation of cavitation zones both on the

suction and on the pressure sides of the blade, which results in decrease of the

hydroabrasive erosion area within the pressure surface

At small

values

the suction surface of a blade faces the suction tube to a greater extent than

the pressure surface. Therefore, the increase of vacuum at the inlet to the

impeller causes drop in pressure and development of cavitation on the rear

surface only. This together with the hydroabrasive erosion of the pressure

surface, constitute a continuous growth of the cavitation-abrasive erosion

within the impeller blades.

AG/Q(g*s/m

)

Figure 6.20 Effect of cavitation level on erosio

at different impeller blade angles

Figure 6.21 illustrates changes in fluctuation of pressure 2A' acting on

the case walls and

the

erosion in the impeller

AG,

depending when

= 0, n =

960 rev/min and Q =

0.328

/s. The fluctuation changes in pressure 2A',

correspond to the pattern of changes of cavitation-abrasive erosion AG taking

Interaction between Cavitation

and

Abrasive Erosion Processes

345

place in the pump impeller. These results demonstrate again that the reduction

of the pressure fluctuations in the passages of a pump, leads to a decline of

the erosion rate within the pump impeller, with the decrease of hh.

AG(mg)

"^Sw*

^S~

Aff

2500

2000

2A*(ta)

6J 12 7.6 8.0 Ah(m)

1 - in blade leading edge region; 2 - in middle of blade

Figure 6.21 Pressure fluctuation and cavitation-abrasive erosion

as a function of cavitation margin

Figure 6.22 illustrates the mechanism of erosion at different regions of the

impeller blades, at

= 0, n = 960 rev/min, Q = 0.336 m

/s, p

10 g/1 and d

= 0.34 mm. With reduction of Ah to approximately 7.8 m, the intensity of the

total loss AG decreases due to reduction of the erosion area F

, caused by the

protective effect of the cavitation zones being formed on the blade pressure

surface. Further decrease of Ah is accompanied by enlargement of erosion

areas,

including the pressure and suction sides (F

andF

respectively) of the

blade surface. With decrease of Ah, the radius of curvature R at the blade

inlet region adjacent to the end face edge, is reduced as well; the angle of

attack a, defining the incidence of solid particles, is changed insignificantly in

this case as determined by the erosion angle formed at the inlet edge of the

blade. Based on the above description, it is necessary to note that the greater

the angle

is, the faster the intensity of hydroabrasive erosion declines will

be,

with the decrease of

Ah.

346

Abrasive Erosion and Corrosion

Hydraulic Machinery

6* IB 7.2 U 8.0 Ah(m)

Figure 6.22 Change of erosion geometric parameters in pump impeller

blades against cavitation margin

The results of the laboratory research study, aimed at the analysis of the

cavitational-abrasive erosion process taking place in axial-flow pumps, are in

good agreement with the data obtained during the full-scale observations

carried out in a number of pump-houses. The results obtained confirm that the

impeller blades, in the unified pumps 30 IIpB-60 installed in one of

the

pump

houses, had different erosion patterns and zone shapes, depending on the

operational conditions in which the blades were used. The first pump had the

greatest erosion at the blade inlet region adjacent to the end face edge, which

was not the case with the second pump. Moreover, in the second pump, at the

blade-trailing surface, the cavitation deterioration was predominant, and the

worn-out surfaces, as a result, had a typical sponge-like structure.

When comparing operational modes of these units, it was found that the

first pump station was used mostly in the spring and summer, when the level

and silt content of the river water was the highest; the second station operated

at lowered levels and silt content of the river water. This means that the first

pump was more subjected to hydroabrasive erosion, whereas cavitation

deterioration was predominant for the second unit. The worn surfaces in the

impeller blades of the axial-flow pumps fabricated alloy steel, IX18H9TI1

Interaction between Cavitation and Abrasive Erosion Processes 347

resembles a uniform wavy and scaly structure, incorporating grooves. There

can be found, on the surface of the blades small local depressions. Also large

craters were formed after longer duration of operation, resulting from the

combined action exerted by cavitation and solid particles.

348 Abrasive Erosion and Corrosion of Hydraulic Machinery

References

6.1 Karelin V.Ya., (1975) 'Cavitation phenomenon in centrifugal and

propeller type pumps, Second edit., Machinery Engineering Press,

Moscow, pp.336, (in Russian).

6.2 Kozyrev S.P., (1971). 'Hydro-abrasion of metals from cavitatio',

Machinery Engineering Press, Moscow, pp. 240, (in Russian).

6. 3 Karelin V.Ya., (1978) 'Mamazhonov M., Abrasion of propeller type

pumps in silt-laden flows', Hydro-technical Production, No. 1 (in

Russian)

6. 4 Yufin A.P., (1974). 'Hydraulic mechanism', Journal of Construction,

pp.222,

(in Russian).

6.5 Karelin V.Ya., Novoderezhkin R.A., (1976). 'Proper operating

condition and suction head of centrifugal pumps for cavitation

abrasion', Vodosnabzheniye sanitarnaya teknika, No. 6, pp. 10 ~ 12,

(in Russian).

6. 6 Karelin V.Ya., Novoderezhkin R.A., Tsoy V.A., (1977). 'Variation of

the pump characteristics caused by hydro-abrasion', Hydraulic

Technology and Mechanics, No. 1, pp. 52 ~ 54, (in Russian).

6. 7 Lomakin A.A., (1966). 'Centrifugal and propeller type pumps', Second

edit., Machinery Engineering Press, Moscow, pp.364 (in Russian).

6.8 Du-tong. (1981). 'Cavitation and erosion on hydraulic machines',

Water Power and Dam Construction, April.

Chapter 7

Corrosion on Hydraulic

Machinery

M. Matsumura

7.1 Fundamentals of Corrosion

7.1.1 Corrosion Cell

Corrosion is an electrochemical process which takes place on metal subjected

to a certain environment. This process is due to both current flow and

chemical reactions. Electric current flows in a metal through the migration of

free electrons which are characterized by metallic bonding and their free

movement within the metal. In the environments (usually electrolyte) the

migration of ions causes the currents. Ions are atoms which have acquired

negative of positive electric charge by obtaining or losing one or more

electron. Electrolyte is water-containing ions. But the current crossing the

boundary between a metal and its environment cannot be explained in either

of the ways mentioned above. A more reasonable explanation is that the

charged states of ion and metal vary according to the electron exchange

between them. For example, a positive ion (action) becomes a deposit in the

form of gas or metal by obtaining electrons from metal. Or conversely, a

metal atom changes into ion form by losing electrons and dissolving into

electrolyte. In this dissolving process, any particular atom can be called

oxidized because it has lost electrons. This process is corrosion. An important

feature of the corrosion process is that the oxidation is accompanied by the

equal amount of reduction.

349

350

Abrasive Erosion and Corrosion of Hydraulic Machinery

The model figure of a corroding metal (iron, in this case) illustrates this

(Figure 7.1). The area where metal atoms dissolve into environments as ions

by losing two electrons (i.e. are oxidized) is called "anode". Here some

quantity of

metal

is lost. On the other hand, the area where the oxygen in the

electrolyte is reduced into hydroxide ions by acquiring electrons is called and

allows reactions between oxygen and water so that no change in the metal

itself

takes

place. In natural corrosion, the number of electrons generated at

anode must be equal to that of the electrons consumed at cathode. In other

words, electrons (negative charge) are moving from anode through the metal

to cathode, and from cathode though electrolyte to anode. This means that

positive current is flowing between the two electrodes (anode and cathode) in

the direction opposite to the negative charge flow as shown by the solid line.

This is called "corrosion cell".

1/20,

Cathode

20H" <- l/20

+ H

0 +

2e*

(7.1)

Anode

Fe->Fe

2e"

(7.2)

Environment

Figure

7.1

Electrochemical reactions occurring during the corrosion

of iron in an aerated electrolyte

According to Faraday's law, a relation exists between the quantity of

dissolved metal m [mol] and the amount of electricity migration through the

circuit Q [coulomb]:

= nFm

(7.3)

Corrosion on Hydraulic Machinery 351

where, n = the number of electrons which participate in the reaction [chemical

equivalent/mol] (in the case of iron in Figure 7.1, n = 2), F = Faraday

constant (96,500 coulomb/chemical equivalent).

Also a relation exists between the corrosion rate and the corrosion current

horr [ampere]:

dQ „dm

■-a

where, / = time

[sec].

7.1.2 Electrode Potentials

Environments contain usually both dissolved oxygen gas and hydrogen ions.

Oxygen gas is reduced in cathode in the manner expressed in equation (7.1) of

Figure 7.1. Hydrogen ions are reduced as shown in the following equation:

<-2i/

+2e~ (7.5)

Then, questions arise; which of the reduction reactions occurs to a metal

in a certain environment and which part of the metal becomes cathode or

anode? The answer depends on potential. Potential is the driving force works;

we place a piece of homogeneous metal (like iron) into an electrolyte (Figure

7.2). Because the metal piece is completely homogeneous, only the anode

reaction takes place across the whole surface of

the

iron. In other words, the

iron atoms turn into ions according to equation (7.2) and dissolve into the

electrolyte. At the same time, the equivalent number of electrons (an equal

quantity of electricity) is stored inside the metal. With the increase in stored

electrons, the dissolving rate of equation (7.2) gradually slows down

according to Le Chatelier's law until it finally reaches zero. At this point, the

dissolving appears to be occurring at a rate of zero, but in reality, this is a

dynamic equilibrium such that oxidation (action toward right in the equation

(7.6)) slows down and reduction (acting toward left) speeds up until both

processed are acting at the same speed.

Fe^Fe

+2e~

(7.6)

352

Abrasive Erosion and Corrosion of Hydraulic Machinery

&■

Figure 7.2 An iron in the equilibrium state in an electrolyte

The quantity of electrons stored in the metal when the reversible reaction

of equation (7.6) has achieved equilibrium depends on the degree of the

tendency with which the metal is ionized of

oxidized.

That is to say, the larger

the tendency towards ionization (oxidization), the more electrons have to be

stored. And, with the increase of electrons in the metal, there arises a tendency

for them to escape. Actually, as is explained in the beginning of this

paragraph, electrons cannot go directly out into the environment (electrolyte),

but there still exists the tendency toward certain ways of decreasing their

number. The tendency of electrons to escape into surroundings implies as well

the tendency of positive electricity to enter the metal. This means that the

potential of the metal is lower than its surroundings. The electrical position of

the metal in equilibrium with its surrounding is called "equilibrium potential"

or "electrode potential". The stronger the tendency of the reaction toward the

right in equation (7.6), the lower the equilibrium potential. Therefore, this

equilibrium potential may be calculated theoretically from the tendency of the

reaction to act toward the right in the equation.

The degree of reaction tendency in the natural system can be expressed in

the terms of a change in Gibb's free energy AG

[joul].

The greater the

difference in the amount of free energy before versus after the reaction, the

more likely it is that the reaction will take place. So according to this

principle, the equilibrium potential E

[volt] can be determined by the formula

below.

nFE

- -AG

(7.7)

Corrosion on Hydraulic Machinery

353

where n and F

the same as in equation (7.3)

The left side of

this

equation represents the electrical energy. The change

in free energy in

chemical reaction can be replaced by chemical potential,

Aju . Chemical potential of a compound has long been accurately calculated

from the heat of formation or of combustion of the compound. By using those

data, we can calculate the equilibrium potential as follows:

=-Aju/nF

(7.8)

The equilibrium potential also depends

the concentration,

accurately, the activity of metal ions in the electrolyte. Standard equilibrium

potential

defined

as the

equilibrium potential

metal

ion

activity

environments

the temperature

°C.

The equilibrium potential

metal exposed

to an

electrolyte

some other concentration

some other

temperature can be determined by the Nernst equation below.

=E°

+^ln[M

]

(7.9)

where,

[AF]

is concentration of metal ions in moles per 1000 g of water,

is standard equilibrium potential

[volt].

The potential generated between

metal and

electrolyte cannot

measured experimentally because though we can connect

leading wire from

a potentiometer

metal, we cannot connect the other leading wire

the

electrolyte. Thus, for practical purposes, we measure it by utilizing

standard

electrode which always indicates

constant potential. Figure 7.3 shows how

to measure

the

equilibrium potential

of an

electrode with

the use of a

hydrogen electrode as the standard. Hydrogen equilibrium potential refers

that of the electrode at equilibrium in the following reaction.

<->2#

+2e~

(7.10)