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

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364

Abrasive Erosion and Corrosion of Hydraulic Machinery

The result is shown in Figure 7.7. The corrosion activity can be classified

into three categories according to pH. In a range of pH5 to pHIO, the

corroded surface is covered with by a film of ferrous oxide through which

oxygen migrates to the surface of the mild steel and corrosion continues. On

the surface of the steel, the pH is always 7.5 regardless of the pH of the

electrolyte. Thus the rate of corrosion becomes constant independent of the

pH out side the film. In acidic environments where the pH is less than 4, the

ferrous oxide film on the surface is dissolved away and the steel is directly

exposed to the electrolyte. In addition to oxygen reduction, hydrogen gas is

generated and facilitates the cathode reaction, hydrogen gas is generated and

facilitates the cathode reaction. Thus, the corrosion rate is greatly increased.

In alkaline environments where the pH is greater than 10, a more stable and

fine protective layer is formed than in the neutral environments and corrosion

rate is slowed significantly.

Let us compare the experimental results of Figure 7.9 with the theory of

Pourbaix diagram (Figure 7.8). Firstly, as to the effect of pH on hydrogen gas

evolution the experimental result may be judged consistent with the theory

because the corrosion potential of a mild steel in neutral and acidic

environments is usually -0.4V to -0.5V, which is lower than the potential of

a hydrogen electrode in an acidic region. In such conditions, the theory

predicts hydrogen gas evolution as was already pointed out in the preceding

paragraph.

Secondly, the experimental result that the corrosion rate in an alkaline

environment declines due to the stable oxide film on the metal surface is well

illustrated in the Pourbaix diagram.

Thirdly, as to the effect of fluid flow velocity on corrosion rate,

Whitman's experiment described above was conducted in a flowing

electrolyte. Generally speaking, corrosion increases with an increase in the

velocity of the electrolyte flowing over the metal surface. Theoretically, the

velocity of flow does not influence equilibrium potential, so we cannot explain

with the Pourbaix diagram why a higher flow velocity causes larger

corrosion. The reason for that phenomenon is explained as follows in the

polarization diagram. The cathode polarization curve in Figure 7.10

represents the polarization of oxygen reducing reactions; the polarization

curve has two parts, one with a gentle slope due to the activation polarization

and the other with a steep one due to the effect of concentration polarization.

As explained in equation (7.14) presented earlier, the current cannot increase

Corrosion on Hydraulic Machinery

365

beyond the limiting current because the migrating rate of oxygen though the

boundary layer gets thinner and the rate of migrating oxygen increases. Thus

the limiting current moves to a higher value. With that, the current at the

intersection of anodic and cathodic polarization curves (i.e., the corrosion

rate) increases.

0.04

" 0.03

.2 0.O2

0.01

-—f

*■

H2 evolution

begins-sy

14 13 12 11 10 9 8 7 6 5 4 3 2 PH

Figure 7.9 Effect of pH on corrosion rate of iron in aerated soft water,

room temperature

Figure 7.10 Polarization diagram showing the effect of flow velocity

on corrosion rate

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Abrasive Erosion and Corrosion of Hydraulic Machinery

7.2.3 Cathodic Protection

The Pourbaix diagram in Figure 7.8 presents the possibility that iron at a

potential of -0.5V in an electrolyte of pH4 with an iron ion concentration of

10"

mol/1 at a temperature of 25°C is likely to be corroded, but it does not

tell us its corrosion rate. On the other hand, it does tell us that we can stop the

iron corrosion by lowering the potential below -0.62V. Then, the iron

becomes immunity and corrosion will cease. Figure 7.11 shows an

arrangement for lowering the potential of the test specimen while measuring

it, and Figure 7.12 shows the relationship between the current and potential

during the operation. In Figure 7.11, for measuring the potential of the test

specimen, a saturated colonel reference electrode is used rather than hydrogen

one because it is easier to handle. This electrode shows +0.24V at 25°C

(assuming the hydrogen electrode as the zero standard). In order to lower the

potential of the test specimen, an external current is applied to the test

specimen from the counter electrode made of inert material such as platinum

or graphite carbon. Before being supplied with current, the test specimen is

undergoing natural corrosion at a potential of

C00

and with the corrosion

current of

corr

On supplying the current to the test specimen, it flows in the

opposite direction of

the

anodic current which is due to the dissolving of iron,

and in the same direction as the cathodic current which is due to the reduction

oxygen.

Therefore, the external current supplied from the counter electrode

(through the electrolyte) to the test specimen decreases the anodic current and

increases the cathodic current. These changes in the currents at the cathode

and anode both lower the electrode potentials. Figure 7.12 indicates the

followings: when external current i

is applied, the cathodic current increases

from

icorr

to i

i, anodic current decreases from

corr

to /'„;. The potential lowers

itself from E

corr

to E

the

external current is increased further the potential

of the test specimen is lowered further to reach E

which is the equilibrium

potential of anode. At this point the anodic current drops to zero and

corrosion stops. This is the principle of cathodic protection.

When we express the external current using the same axis as is used for

the cathodic or anodic currents, we obtain Figure 7.13. In this figure, an

external anodic current is also shown. The thick lines are external polarization

curves. As the horizontal axis is scaled in logarithms, the external polarization

curve almost coincides with the inner polarization curve (represented by the

thin line) when the current exceeds the

corr

The slopes of these curves are

Corrosion on Hydraulic Machinery 367

particularly similar. As has been stated previously, we cannot measure inner

polarization curves in real metal because of the many micro-cells that are

generated on the surface. Therefore, we usually regard the inner polarization

curves to be the same as the external polarization curves. This approximate

method is accurate enough in comparing with the Tafel slopes in cathodic and

anodic polarization.

Potentiometer

Counter Working Saturated calomel

electrode electrode reference electrode

Figure 7.11 Arrangement for lowering the potential of

a test specimen while measuring it

logi

Figure 7.12 Relationships between

and i

during cathodic protection

368

Abrasive Erosion and Corrosion of Hydraulic Machinery

External polarization curve

Coir

logi

Figure 7.13 Relationships between internal and external polarization curves

7.2.4 Passivity

Another way to prevent the corrosion of an iron test specimen (in an

electrolyte of 10

mol/l iron concentration, 25°C and pH4) is to put the

specimen into a state of passivity by raising the potential to about +0.5 V (see

Figure 7.8). This method is diametrically opposite to the cathodic protection.

As has already been mentioned, ferrous oxide is stable in the state of

passivity. When this oxide uniformly covers the surface of the iron, the

movement of oxygen and Fe

on the iron surface is inhibited and corrosion

does not occur. The behavior of the external polarization curves on entering

the passivity is shown in Figure 7.14. When no external current is applied, the

potential of the iron test specimen is apparently at its natural corrosion

potential. But if a current is supplied by using the same arrangement but in

the opposite direction as in Figure 7.11, the potential of the test specimen is

raised. At the same time, the corrosion current is increased. With the increase

of current, when the potential reaches a certain level (passivation potential),

the current is suddenly and dramatically decreased. This is because a stable

and multi-ionic oxide is formed and a non-porous film of oxide covers the

surface. In this condition (passivity), ions are inhibited from passing through

the film and the corrosion current is extremely low. With the further raising of

potential, the current remains constant for a while, but at a certain potential

the current suddenly increases, this state is "transpassivity". Here the current

is consumed mainly for the evolution of oxygen gas from water.

Corrosion on Hydraulic Machinery 369

CCorr

icorr Jog i

Figure 7.14 Schematic diagram of anodic polarization curve

on entering passivity.

There are some ways, other than supplying an external current, to keep a

test specimen in the passive state. For example, a cathode reaction of a

sufficiently high equilibrium potential can be combined with the test

specimen. Oxide inhibitors, like chrome-oxide of nitric acid, prevent corrosion

by promoting the cathode reaction and pushing the metal potential into

passivity.

Materials with low passivation potential are more likely to become

passive. For example, the passivation potential of chromium is considerably

lower than that of iron, and the passivity current is also small. Therefore,

chromium is far more likely to become passive than iron is. The passivation

potential of chromium is lower than hydrogen, so even the hydrogen gas

evolution can be used to promote chromium passivity. We can get passivity

characteristics similar to chromium by adding 12 ~ 18% chromium to iron.

This is stainless steel.

7.2.5 Stainless Steel

There are three main classes of stainless steels: ferric, martensitic and

austenitic. The austenitic stainless steels are named after the austenite phase

^S*^ Transpassivity

Passivity

immunity

370

Abrasive Erosion and Corrosion of Hydraulic Machinery

which is of face-centered cubic lattice and exists stable for pure iron between

910 and 1400°C only. By adding more than 8% of nickel to it, this phase can

exist stable or metastable at room temperature. The typical austenitic stainless

steel is the AISI (and JIS) type SUS 304 which is sometimes called 18-8 as it

contains 18% Cr, 8% Ni and the Fe. The stainless steels of this class are the

most reliable materials because they have the highest general corrosion

resistance, excellent heat resistance and free from cold shortness. They have,

however, some weak points in regard to pitting corrosion, crevice corrosion,

intergranular corrosion and stress corrosion cracking.

Pitting corrosion is initiated by the adsorption of anions (negative charged

ions),

particularly chloride ions, on the defect sites in the oxide film such as at

slag inclusions. The chloride ions can break the oxide film under a certain

condition and cause local dissolution. Once a pit has been formed, a "passive-

active cell" is set up; the inside surface for the pit is anode and the whole

oxide film surface outside the pit is cathode, which results in a high current

density at the very localized anodic area, and consequently a high corrosion

rate.

Chloride ions migrate into the pit to form their concentrated solutions of

, M

, and O*

* chlorides with a high conductivity. A crest of hydrate is

formed at the pit mouth which limits 0

supply into the pit. Thus a differential

aeration cell is also developed. Crevice corrosion proceeds in nearly the same

procedure as pitting corrosion. In order to prevent pitting corrosion and

crevice corrosion from occurring, the nickel content of the material needs to

be increased. The austenitic stainless steel containing molybdenum (types

316,

317) is more resistant to this kind of corrosion.

Heating austenitic or ferritic stainless steels to certain temperature ranges

induces in them the susceptibility to intergranular corrosion. This is called

"sensitizing" and is due to the formation of chromium carbide (Cr

) at

grain boundaries, which brings about the reduction of Cr content there: When

the sensitized steel is exposed to corrosive environment corrosion proceeds

along the chromium depleted grain boundaries and in extreme cases it may

undermine the grains so that the metal is transformed to a powder. "Weld

decay" which occurs in sensitized zones along but some distance apart from

weld lines is certainly caused by the same process. Prevention of chromium

carbide formation is the principle behind any practical and effective protection

methods, some of which are noted below:

i) Dissolution of the chromium carbide by heating above the

sensitizing temperature range, and subsequent rapid cooling (It

Corrosion on Hydraulic Machinery

371

should be noted that the temperature range for the heat treatment is

different for the different classes of stainless steels).

ii) Lowering carbon content. Stainless steels of low carbon content are

designated by letter L, such as types 304L, 316L.

iii) Addition of carbide forming alloy elements such as titanium or

niobium, stainless steels of this type are called stabilized grades;

types 321 and 347 are of this case.

Stainless steels may crack under a tensile stress in certain environments.

This is called "stress corrosion cracking (SCC)". The stress may be an

applied or residual tensile stress, but compressional stresses do not cause the

damage. The cracks propagate leading to a perforation or to a brittle fracture

where the surface is virtually unattached and no contraction on the fracture

region occurs. It is characteristic of SCC that any metal or alloy is damaged

when it is exposed to a certain special corrosive environment. For austenitic

stainless steels there are two major damaging ions, hydroxyl and chloride.

High-pressure high-temperature water and polythionic acid are also harmful

to them. In order to prevent SCC of austenitic stainless steels, cathodic

protection may be effective, but this protection accelerates the failure of

martensitic and ferritic stainless steels. Atomic hydrogen formed by cathodic

reaction increases the tendency of those steels to crack. This is called

"hydrogen cracking". For details see the books of Wranglen, 1972; Fontana

and Greene, 1978; Uhlig, 1971.

7.2.6 Polarization-Resistance Method

The polarization-resistance method which was developed by Stern and Geary

in 1957 is used to monitor corrosion rate by means of the polarization curve.

The state of a test specimen in corrosion is expressed with E

corr

, and

corr

Figure 7.15, which is the intersection of two inner polarization curves. If a

very weak external current Ai

is supplied along with an arrangement similar

to that shown in Figure 7.11, the potential of the test specimen changes

slightly by the amount AE. Because these polarization curves can be

expressed as in equations (7.12) and (7.13), and because the external current,

the increments in anodic current as well as that in cathodic current can all be

related in the following equation,

= Ai

(7.19)

372 Abrasive Erosion and Corrosion of Hydraulic Machinery

we can obtain the next equation,

AE _. b'b

Ai„ ~

horr

(7.20)

provided that the amount of

the

external current is small enough to satisfy the

following equation,

log(l±-—)

±-—

corr corr

by using the constant K for the term b

*b/(b

'+b), we obtain.

(7.21)

(7.22)

Eo.

log i

Figure 7.15 Change in potential AE caused by small amount

of external current Ai

In other words, through the measurement of a slight potential change AE

caused by the application of external current Ai

, we can get the polarization

resistance R {^/

), and from this we can calculate the corrosion current

Corrosion on Hydraulic Machinery 373

or.

This method enables us to measure the corrosion rate electrically in a

short time. This is now an indispensable procedure for monitoring corrosion.

Many factors influence the polarization curves, so that a considerable error

remains in this method. But many recent improvements have increased the

accuracy to the point where we are now quite confident of this method (See

Katoh et. al., 1985, Haruyama et. al., 1978).

7.3 Corrosion of Pump Parts

Among the various different types of pumps in use for different purposes,

seawater pumps are mostly of conventional type and are apt to suffer

corrosion damage. The quality of the material used for seawater pumps has

been improved over the years. Recently the large-sized pumps, of which the

total weight can attain 10 tons, have been made of austenitic ductile cast iron

consisting of

20%

Ni and 2% Cr (ASTM A 439 Type D2) or of cast stainless

steel consisting of 20% Cr, 29% Ni, 2.5% Cu (JIS SCS 23) and so on.

Adoption of such materials is considerably effective in reducing corrosion

damage, but corrosion problems cannot be completely overcome even though

higher-grade materials may be used. It is, therefore, desirable from an

economic point of view to use common materials such as plain cast iron,

carbon steel or type 316 stainless steel in skillful combination in order to deal

with the problem of corrosion. Dr. Kitashima and his collaborators reported

in 1986 the following examples of corrosion damage in seawater pumps and

suggested methods to prevent it, in accordance with the above-mentioned

concept. They are expert corrosion engineers who have engaged themselves in

manufacturing of various kinds of pumps for a long time in one of the largest

factories in Japan.

7.3.1 Corrosion Caused by Velocity Difference

In almost every pump, the velocity of fluid flow is different from place to

place on the surface of the pump parts, which often brings about macroscopic

corrosion cells which promote corrosion. Figure 7.16 shows the damage from

this type of corrosion on the casing surface of a seawater pump made of cast

iron. The corrosion rate of cast iron in seawater at rest is usually about 0.2

mm/year and is increased with the increase in flow velocity. On the casing