Studying corrosion in the vane wheel of a submersible pump

S t u d y i n g corrosion in t h e v a n e w h e e l of a s u b m e r s i b l e p u m p Although they provide essential services in a wide range of industries, submersible pumps are perhaps most commonly used to supply cooling water or circulate water. The main reason given for the failure of a submersible pump is usually corrosion, wear and component fracture. In addition, operating efficiency is greatly decreased once the vane wheel begins to corrode. Usually, it is said that "cavitation corrosion" and "abrasion corrosion" on the surface of the vane under conditions of high-speed rotation is the major corrosion behaviour. In studies by Sun Zhi, Wang Wenyin, Dong Xiaowen and LU Chengliang, the corrosion of the vane mainly appeared on the surface of the root and pilot column while there was less corrosion on the surface of the vane body. The electrochemical reactions behind this are analysed here, and the possible protective processes discussed.

n this project, the corrosion behaviour of a cast iron vane wheel .working in ordinary water was examined. The corroded surface and corrosion products were observed with an electron microscope, and their components and microstructures analyzed using XPS and X-ray techniques. The results showed that corrosion of the vane wheel mainly occurred on the surface of its root and pilot column with much of pitting damage. There was significantly less corrosion on the surface of vane body.

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In addition to Fe and Fe304, the corrosion products contained a lot of FeCO 3 and CaCO 3. The separation and accumulation of HCO 3" in areas

where the water flowed relatively slowly appeared to be the main factor behind the great increase of the corrosion rate of iron on the surface.

Working conditions of the vane wheel The submersible pump used in the study was an electrical unit with a lift of 60 m. Its nominal rotational speed was 3000 rpm. The material of the vane wheel is HT200 cast iron and its chemical compositions are shown in Table 1. The depth of the well in which the pump worked was 60 m and the normal water level is 20 m. Water

quality analysis showed the water composition to be CI "1 @ 40 ppm, CaCO 3 @ 210 rag/l, Ca(HCO3) 2 @ 114 rag/l, SO 3" @ 236 ppm, and that it contained Fe, Na, K and Ca ions as well.

Corrosion macrofeatures The corrosion behaviour of the vane wheel is shown in Table 2. The depth of corrosion in the vane wheel had reached over 3 mm after only 4 - 5 months of operation and then it suddenly broke. The broken wheel vane is shown in Figure I. The corrosion characteristics were that:

rABLE 1: THE CHEMICAL COMPOSITION OF CAST IRON (0~

C

Si

Mn

S

P

Note

Vane body Vane root

rABLE 2: CORROSION BEHAVIOUR OF THE VANE WHEE

Working Time

Corrosion depth (ram) Vane root Vane body Pilot column

Failure Taper

Pump 1 Pump 2 Pump 3

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FEATURE •

• The most serious corrosion occurred on the surface of the top and bottom of the pilot column and the central circular cone • There were two dead areas in the root of the upper side and the central part of the underside of the vane wheel and in the location of the local serious corrosion • There was less corrosion on the surface of the wheel vane body impacted by flowing water which still kept its original geometric shape. It shows that the corrosion of vane wheel was closely related to its geometric structure - the degree of corrosion was related mainly to the shape of the vane wheel or to the rate/type of water flow in a specific area. The corrosion was much more serious on the surface of "dead areas" where the velocity of flow water was slower. The effects are obviously different to cavitation or abrasive corrosion.

Close.up on the corroded surface of the v a n e w h e e l O n the metal surface of the corroded areas, the depositions were about 2 - 3 mm in depth. The appearance of the metal surface was honeycomb-like, with well-distributed holes at a typical diameter of 0.7 mm. Figure 2 shows their appearance under electron microscopy, at several different magnifications. Many studies have illustrated [2'31 that there are plastic deformations on the corroded surface of vane

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wheel of submersible pump because of the force of the water and the effect of cavitation corrosion. In this case, however, it shows that the surfaces of serious corrosion had not been as heavily altered by the force of water. The formation of the corroded areas described was mainly caused by the deposition of corrosion products and other dirt, and by graphite corrosion, which will be discussed later.

M i c r o f e a t u r e s of the corrosion products The corrosion products still remaining on the vane wheel were ground and

polished. Their microfeatures were then observed with a scanning electron microscope. The results are shown in Figure 3. The images show that the corrosion products basically maintained the microstructural characteristics of the original metal (grey cast iron). The plate graphite and the others were unaffected but the ferrite surface became loosened by particulate corroded products with much pitting. It is well understood how local corrosion occurs in cast iron. The iron is speedily corroded as an anode and the graphite is protected as a cathode. So it is suggested that the graphite in the cast iron was an important cause of

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serious local corrosion on the surface in certain areas. It's unclear, however, whether this was the main cause. Generally, the analysis shows that during the corrosion process the mechanical force imparted by the water flowing at higher speed didn't have a particular effect on the surface of the metal.

with the remaining Fe(OH) 2 according to the following reaction... Fe(OH) 2 + 1,402 + ½H20 = Fe(OH)3 Fe(OH) 3 + Fe(OH) 2 = Fe304 + 2H20 (2)

Phase structure of the corrosion products

which is, in fact, black rust. At the same time, because a lot of CaCO 3 separates into H C O 3" ions, the H C O 3" and Fe(OH) 2 are neutralized by flowing reaction I61

The chemical composition of corrosion products was analyzed. The results showed that it contained the normal elements in cast iron metal such as Fe, Si, S and AI etc, as well as Ca 2+ that greatly exceed the normal content. X-ray diffraction analysis (see Figure 4) of the phase structure of corrosion products shows that the corroded products contained a lot of FeCO 3 and CaCO 3 (along with Fe, Graphite, and Fe304 as would be expected).

H C O 3" + Fe(OH) 2 + H + = FeCO 3 + 2H20

The reactions mentioned above would indicate that some of the Fe(OH) 2 from the reaction of (1) was further oxidized into Fe(OH) 3, and then transformed into Fe30 4 deposits. Actually, most of the Fe(OH) 2 was neutralized into FeCO 3 depositing the H C O 3 separated from water.

The corrosion mechanism of vane wheel Iron of course is readily oxidized and corrodes easily in normal water. The corrosion process of iron in water may be described as...

Because H C O 2" accelerated the transformation of Fe(OH) 2, the equilibrium of reaction (1) was upset, so the reaction could be continuous and the corrosion rate of iron rapidly increased.

Anode reaction Fe + 2H20 = Fe(OH) 2 + 2H ÷ + 2e Cathode reaction 2H + ½02 + 2e = H 2 0 General reaction Fe + ½02 + H20 = Fe(OH) 2 (1)

Some sectors in the root of the vane wheel became "vacuum areas" as the water flowed more slowly there due to the geometric structure of vane wheel. CaCO 3, dissolved oxygen, and CO 2 easily separated and decomposed in these areas in line with the depressors principle. This resulted in a greatly increased local content of HCO3". Therefore, in a mixed-ion environment with a higher proportion of

Because the vane wheel works well below the surface of the water, the oxygen content at this depth is relatively low - thus only part of the Fe(OH) 2 can be oxygenated to Fe(OH) 3. Some of this Fe(OH) 3 reacts

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FEATURE •

Deposition

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Figure 5. The corrosion mechanism of iron under deposits.

HCO 3" and dissolved oxygen, the rate of reaction (1) and (2) could rapidly increase.

increased due to the role of oxygen, Fe + and HCO 3" deposition.

Conclusions The role of CaCO 3 in corrosion It is generally accepted that rust deposits containing CaCO 3 can resist the further development of iron corrosion in ordinary water, especially in locations like a tube or boiler [7], if the rust is uniform and close. In the present study, the analysis shows that rust deposits in the vane wheel were, in fact, composed of loose particulate with much surface pitting. Thus it failed to resist further corrosion, as described in Figure 5, and did not perform a protective service at all. So the areas under the deposition were rapidly corroded because the diffusion of oxygen and other ions through the rust layer was not resisted. On the other hand, areas around the deposition were protected, since they acted as a local cathode. Local corrosion occurred in the bottom of the areas of deposition and produced corrosion pits. The corrosion ratio of iron greatly

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Serious local corrosion occurred on the surface of the vane wheel's root and pilot column while there was less corrosion elsewhere. This corrosion was mainly determined by the geometric character of the vane wheel and the rate of water flow.

• The electrochemical process of corrosion on the vane wheel is that the iron was oxidised into Fe2+ and further into FeCO 3 deposits. This was caused by the presence of HCO 3 which separated from the water.

References [1] Jiang Nanchang, Pumps and Their Station (in Chinese), China Press of Construction Industry, Beijing, 1980 [2] Huang Jitang, The Principle and Application of Cavitations (in Chinese), Qinghija University Press, Beijing, 1991 [3] Li Shihuang, Pumps (in Chinese), Mechanical Industry Press, Beijing, 1990 [4] Ulick R. Evans, An Introduction to Metallic Corrosion, Third Edition, Edward Arnold, London, 1981 [5] H. Kaesche, Die Korrosion der Metalle, (Translated into Chinese by Wu Yishun Chemical Industry Press, Beijing, 1984

The separation and accumulation of HCO 3" in areas where water flows more slowly was the main factor in the great increase of corrosion rate on the surface of vane wheel.

[6] S.Powell, H. Bacon, J. Lill, Ind. Eng. Chem. 3 7,842(1945)

The rust deposition contained CaCO3, but being loose and particulate, it could not help resist the further development of corrosion.

AUTHORS Sun Zhi, Wang Wenyin, China University of Mining & Technology, P.R. China & Dong Xiaowen, LU Chengliang, XuZhou Power Plant, P.R. China

[7] Gosta Wranglen, An Introduction to Corrosion and Protection of Metals, Chap Hill, London/New York, 1985 •

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