Duan C.G., Karelin V.Y. Abrasive Erosion and Corrosion of Hydraulic machinery
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404 Abrasive Erosion and Corrosion of Hydraulic Machinery
Figure 7.45 SEM micrographs of etch pits in characteristic surfaces of
(a) polished surface, (b) displaced surface, (c) lip surface,
(d) cut surface, impact direction, left to right
7.4.4 Parameters Affecting the Mutual Interaction Mechanism
(1) Effect of impact angle on dissolution rate:
The above discussion regarding the mechanism of the mutual interaction of
erosion and corrosion are totally based upon the behavior of relatively large
craters formed under special conditions. Accordingly the details of the
mechanism as obtained of real situations. However, the different mechanisms
described so far are able to satisfactorily explain the experimental results of
Figure 7.36.
Let us first consider the effect of impact angle on the amount of mutual
interaction, that is, the relationship between
W
t
-(W+F)
and a in Figure
7.36. The mutual interaction becomes more inhibitory with a decrease in
impact angle. It has been made clear that the lower the impact angle, the more
likely are the crater surfaces to be of the characteristic surface types (?) and
Corrosion on Hydraulic Machinery
405
®. For these surface types the dissolution rate is decreased, while it has been
shown that AW is smaller at lower impact angle (see Figure 7.37). Thus the
tendency in Figure 7.36 is reasonably understood.
Secondly, we consider the effect of the pH of slurry. In Figure 7.36 the
mutual interaction becomes more inhibitory with a decrease in pH. A decrease
in pH results in an increase in the corrosion intensity, and consequently
increases AF, while AW possibly remains within a certain value. Thus AF
dominates the mutual interaction term AW
+
AF . Since AF is negative for
all impact angles as shown in Figure 7.36, the values for W
t
—(W
+
F) in
Figure 7.36 are reduced as the corrosion intensity increase.
(2) Effect of the relative magnitude of the erosion and corrosion rate:
It has been made clear that the impact angle, actual rates of erosion and
corrosion as well as the relative intensities of erosion and corrosion are
important parameters which determine the extent of the synergistic
interaction. These results may be compared with the previous investigation
(see Oka and Matsumura, 1987) for conditions where erosion is predominant.
It is noteworthy that the synergistic-controlling component was erosion, and
the mechanism of the acceleration was quite different; it involved hydrogen
embrittlement. The effect of hydrogen was also examined in this investigation,
but no influence on the erosion rate was observed. A conclusion which can be
drawn from this comparison is that the relative magnitude of the erosion and
corrosion rates determined not only the amount of synergistic interaction but
also its mechanism.
The influence of relative magnitude of erosion and corrosion rates may
explain the cause of
the
often-encountered annoying problem in evaluation of
material for practical use; results of laboratory tests are not always in
agreement with experience in the field. A laboratory test conducted in
conditions of, for example, accelerated erosion naturally brings about an order
of merit of materials totally different from that in the field. This is simply
because the relative magnitude of erosion and corrosion rates in the test was
different from that in the field and the mechanism of damage was also
different.
406
Abrasive Erosion and Corrosion of Hydraulic Machinery
References
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McGraw-Hill, New York.
7.2 Haruyama, S., Tsuru, T. and Anan, M, (1978). 'Impedance of
Corroding Electrode', Boshoku Gijutsu, vol.27, No.9, September, pp.
49-456.
7.
3 Katoh, M. and Kayama, M, (1985). 'Improvement of the Polarization
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No.
4, April, pp.
215-221.
7.
4 Kitashima, N., Ichikawa, K., Kinoshita, K. and Miyasaka, M., (1986).
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5 Laque, F. L, (1972). 'Effects of Specimen size on Results of Corrosion
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7.
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7.
7 Oka, Y., Matsumura, M., Ohsako, Y. and Yamawaki, M, (1984).
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Impact, Cambridge, pp. 391 - 398.
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Corrosion on Hydraulic Machinery
407
7.11 Uhlig, H. H., (1971). 'Corrosion and Corrosion Control', John Wiley
& Sons Inc., New York.
7.12 Whitman, G. W., Russel, R. P. and Altier, V. J., (1924). 'Effect of Ion
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*&f*
SERIES ON HYDRAULIC MACHINERY - VOL 2
liECBSHM)
This book gives a systematic exposition of
abrasive erosion and corrosion of hydraulic
machinery in both theory and engineering
practice, and is the first comprehensive volume
to cover this area in depth.
All the important subjects are discussed
including fundamentals, calculation, analysis
and numerical simulation of liquid-solid flow
design,
erosion-resistant materials, interaction
between cavitation and abrasive erosion, and
corrosion of hydraulic machinery.
Imperial College Press
www.icpress.co.uk
P272 he
ISBN 1-86094-335-7
9 '781860"943355
Q)
Corrosion
of
Hydraulic Machinery