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

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384

Abrasive Erosion and Corrosion of Hydraulic Machinery

between the two metals. The results are given in Table 7.5. The potential of

the deposit metal was shifted basic with the lapse of time, markedly for the

case of immersion after the emery paper polishing, and considerable current

flow was also noted. Considering the exposure area ratio of the base metal

and the deposit metal in the real pump, the corrosion rate predicted from the

experiment was in relatively close accordance with the damage in the field.

(a) (b)

Figure 7.27 (a) Corrosion at weld repaired portion of impeller made of

SCS 13(1 year operation); (b) Cross-sectional micro-structure

of the repaired weld metal.

Table 7.4 Chemical compositions of base metal and deposited metal (wt.%)

C Si Mn P S Ni Cr Mo

Base metal 0.064 1.32 1.14 0.025 0.019 8.93 19.57 0.67

Deposit metal 0.064 0.34 1.06 0.019 0.100 10.57 20.23 0.06

Stainless steel castings which contain up to 0.5% Mo are not rare among

so-called 304 type stainless steels. Such excess in alloy element content is

usually disregarded because it dose not reduce but improve corrosion

resistance. However, when Mo containing stainless steel were brought into

Corrosion on Hydraulic Machinery 385

contact with a Mo free stainless steel under some critical conditions such as

high temperature, concentrated solution or erosive slurry, considerable

corrosion may occur to the Mo-free material.

Table 7.5 Potential of base metal and deposited metal and

short circuit current density between them

Tempera-ture

of solution

Oc)

90 (at start)

i slow cool

70 (after 2h)

Immersion

time before

measurement

(h)

just after

immersion

Potential

(mV vs SCE)

basic

metal

-270

-280

-310

-263

deposited

metal

-257

-351

-210

-710

Short circuit

current

density

(uA/cm)

Note

immerse after

2 4h from

polishing by

emery paper

immerse just

after polishing

by emery

paper

Chemical composition of CaS0

: 0.17; CaCl

: 2.95; MgCl

: 13.33

test solution (%)

l 8.06; NaCl: 12.45; S.S: 24;

Figure 7.28 is another example of corrosion which occurred at a weld

repair section of a seawater pump made of aluminum bronze (JIS AIBC 3). It

had been strictly specified that a similar or higher grade welding metal had to

be used for the repair welding of AIBC 3 castings. But small defects were

carelessly repaired with a lower grade welding metal by an inattentive worker.

If the specification had been completely controlled, such corrosion would have

been avoided.

(3) Corrosion of hard facing

Ni-or Co-baxed high carbon alloys are often used for overlay welding on the

shaft sleeve surface where a high resistance against abrasion is required.

These alloys were applied to seawater pump shafts made of stainless steel and

brought into contact with seawater. A result is shown in Figure 7.29: the hard

facing layer fell off due to the inter-granular corrosion in the sensitized area

beneath the overlay. The sensitization was caused by the diffusion of carbon

386 Abrasive Erosion and Corrosion of Hydraulic Machinery

from the hard facing alloy to the base stainless steel. Dr. Kitashima and his

collaborators succeeded in avoiding such corrosion by using a Co-based low-

carbon type hard facing alloy as shown in Figure 7.29 (c).

Figure 7.28 Corrosion occurred at weld repair portion of

aluminum bronze casing

(a) (b) (c)

(a),

(b) Hard facing layer fell off by corrosion in the boundary

(after immersed for 3.5 month in seawater);

Figure 7.29 Corrosion of the hard facing sleeve.(base metal; SUS 316L)

Corrosion on Hydraulic Machinery

387

7.4 Interaction of Corrosion with Erosion

The combined attack of erosion and corrosion to metals has recently become

serious in various engineering applications. The slurry erosion-corrosion

occurring on the materials containing coal slurry in its liquefaction process is

a well known example. Another example is the pump impellers in the lime

slush-lime stone process for exhaust gas desulfurization which circulate the

process liquid of high acidity containing solid particles of gypsum and fly ash.

In such cases, both the corrosion and erosion processes probably assist each

other to bring about a larger amount of damage than the simple sum of the

damage caused by each process separately. Any general law, however, cannot

be expected to exist for estimating the total amount of erosion-corrosion

damage to a metal, because the mechanism of mutual interaction between

erosion and corrosion is quite liable to variation depending on the conditions

of the situation. You can easily understand this when you remind yourself of

how strongly the extent and nature of the damage caused by the solid particles

impact on a surface depend on the conditions such as impact angle, the shape

of the particle, the impact velocity and so on, as repeatedly discussed in

Chapter 2. In addition to those physical or mechanical conditions,

electrochemical factors inevitably have a great influence on the mutual

interaction to result a different mechanism and a different damage estimating

equation for each case.

The following is the result of an investigation conducted by

Prof.

Matsumura and Oka (1987) which shows an example of the mutual

interaction of erosion and corrosion and discusses a simple fundamental

equation relating the total damage with each component.

7.4.1 Experiment on Slurry Erosion-Corrosion

(1) Testing apparatus

A schematic diagram of the fountain-jet apparatus is shown in Figure 7.30.

The main tank was made of transparent polyvinyl chloride resin and consisted

of two sections placed one above the other. In the lower section, of smaller

diameter, a fluidized-bed was formed in which solid particles were suspended

uniformly in an upward flow of liquid. In the upper section of the tank, the

slurry which flowed from the test section was separated into solid particles

and clear liquid. The solid particles were allowed to fall into the fluidized-bed

388

Abrasive Erosion and Corrosion of Hydraulic Machinery

below. The bulk of the clear liquid was circulated by a pump into the test

section and the rest into the distributor below the fluidized-bed.

Settling

Separator*^-,

Fluidized-bed

Figure 7.30 Schematic diagram of the fountain-jet testing apparatus.

By means of an ejector located at the top of the test section a stream of

slurry (particle concentration of 60%) is accelerated by the reducer nozzle and

allowed to impinge on the test specimen at a velocity of

m/sec, Figure 7.31.

The test specimen, a cylinder 5 mm in diameter and 5 mm in length was held

between two matching sections of silicon rubber which were held together on

a bolt. Only the top flat surface was exposed to the impinging slurry since the

bottom as well as the side-wall of the specimen was entirely covered by vinyl

tape.

The impact angle of the slurry stream on the specimen surface was

regulated by turning and then clamping the support bolt. The impact angle of

the slurry stream on the specimen surface was regulated by turning and then

clamping the support bolt. The particle impact angle was uniform all over the

test surface; it was equivalent to the angle between the slurry stream and the

surface of the specimen. To ensure that direction of impingement was uniform

it was necessary to restrict the test surface to a small area where the impact

conditions were uniform.

Corrosion on Hydraulic Machinery 389

Water

Reduced Nozzle

\Silicone Rubber

Detail of Test Specimen

Slurry

Figure 7.31 Schematic diagram of the test section of the fountain apparatus

The details of the measurement of the particle impact conditions for those

apparatus have been reported elsewhere (see Oka et. al. 1984), and only the

results of the measurements are listed in Table 7.6.

Table 7.6 Impact conditions of solid particle in the fountain-jet facility

Impact Velocity

(m/s)

Impact Angle

(deg)

Impact Frequency

(l/cm

«s)

3.0

10-80

ca«10

(2) Test results

Pure erosion tests on commercially pure iron were conducted in a slurry of

silica sand and deionized water. The results obtained are shown in Figure 7.32

which shows the relationship between erosion rate and particle impact angle

390 Abrasive Erosion and Corrosion of Hydraulic Machinery

a. The erosion rate in mg/cm

Tnin (solid line in the graph) was divided by the

flow rate of impacting particles to obtain an alternative erosion rate expressed

in mg/kg (dashed line). The shift of the peak of the curve results from the fact

that the flow rate of impacting particles depends on the angle, a between the

specimen surface and the slurry stream. In this paper the erosion rate

expressed in mg/cm

«min is adopted since corrosion rates are also expressed

using the same units.

*10-

I 0.15

5 0.10

§ 0.05

111

Figure 7.32 Effect of impact angle on erosion rate for iron specimens

in the fountain-jet facility; jet velocity, 3.0 m/s;

slurry, deionized water and silica sand of

wt%, 40°C

Pure corrosion tests were conducted by circulating aqueous solutions

without solid particles. As shown in Table 7.7 the average corrosion rate for

an hour period was found to depend on the pH of the test liquid but to be

independent of the angle the specimen was set at. A comparison between

Table 7.7 and Figure 7.32 makes it clear that corrosion rates and erosion rates

were of the same order of magnitude except for the corrosion rate at pH12.

Combined erosion-corrosion tests were carried out in slurries of silica

sand and corrosive liquids. The results are shown in Figures 7.33 through

7.35.

The solid line in each graph represents the erosion-corrosion rate, W

30 60

a (dea)

Corrosion on Hydraulic Machinery

391

as a function of particle impact angle. The broken line represents the pure

erosion rate W, the dotted line the pure corrosion rate F, and the dashed line

the sum of the pure erosion and pure corrosion rates, that is W

F .

Table 7.7 Corrosion rate of iron specimens in the fountain-jet facility

Corrosiveness Corrosion rate (mg/ cm

«min)

pH2 0.07

pH4 0.025

pH12 0

In the slurry of pH12 (Figure 7.35), the sum of

the

erosion and corrosion

rates (W

F) is essentially identical with the erosion rate (W); this results

from the fact that the pure corrosion rate (F) is essentially zero for all

impact angles. Nevertheless, the erosion-corrosion rate (W

) is higher than the

sun of the erosion and corrosion rates. Thus some mutual acceleration of the

rates of erosion and corrosion must be occurring. In other words, the sign of

the mutual interaction is positive.

In the slurry of pH4 (Figure 7.34), W

is still higher than W

F at large

impact angles. At low impact angles, however, a change impact angles. At

low impact angles, however, a change in behavior occurs and thereafter W

consistently lower than

W +

F. This is interpreted to mean that the erosion

and corrosion rates are inhibited when they occur together, or that their

mutual interaction is inhibitory. The same situation is observed in slurry of

pH2 (Figure. 7.33), but the change in behavior occurs at a larger angle.

In order to illustrate the synergistic effect more clearly (W

F) was

subtracted from W

. The relationship between this parameter and the impact

angle is given in Figure 7.36. The synergistic effect tends to become negative

as the particle impact angle decreases and the corrosion intensity increases.

392

Abrasive Erosion and Corrosion of Hydraulic Machinery

0.20

30 60

a ( deg)

Figure 7.33 Effect of impact angle on erosion-corrosion rate for

iron specimens; Jet velocity, 3.0 m/s; slurry, Hcl acid solution of pH2

and silica sand of

wt%, 40°C

BO.

'55

go.

lii 0

pH4 r.

/\\

//s~^ \

* F

30 60

a (deg)

Figure 7.34 Effect of impact angle on erosion-corrosion rate for iron

specimens; Jet velocity, 3.0 m/s; slurry, Hcl acid solution of

pH4 and silica sand of

wt%, 40°C

Corrosion on Hydraulic Machinery

393

0.15

0.10

"05

2 0.05 -

'to

uj 0

pH12

/ ,<f

i i i

=*v. \

\ \

] 1 1 1

F ■

30 60

a (deg)

Figure 7.35 Effect of impact angle on erosion-corrosion rate for iron specimens;

jet velocity, 3.0 m/s; slurry, NaOH solution of pH12 and

silica sand of 60 wt%, 40°C

0.10

30 60

a (deg)

Figure 7.36 Effect of impact angle on the amount of synergistic effect of

erosion and corrosion for iron specimens; jet velocity, 3.0 m/s;

slurry, corrosive liquids and silica sand of

wt%, 40°C