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Materials technology for seawater pumpsCorrosion is the greatest problem encountered in stainless steel structures widely used in seawater pumps – even specialised brine pumps such as those used in desalination, oil & gas, chemical, thermal and nuclear power plants. Here, M Miyasaka and H Yakuwa of Ebara discuss the solutions currently on offer for surmounting crevice corrosion and take a look at the latest advances in this field.

Dissolved chlorides and other salts contained in a great quantity in

seawater cause localized corrosion of stainless steels and other passive metals. The corrosion can take the form of pitting, crevice and intergranular corrosion, and, in

high temperature seawater, stress corrosion cracking (SCC). The chemicals also promote corrosion of steels and copper alloys, particularly on the high flow-rate side. The high electrical conductivity of seawater promotes macro-cell corrosion such as galvanic corrosion and differential- aeration-cell-corrosion, including differential-flow-rate-cell-corrosion. If you fail to protect your seawater pumps from corrosion, they will corrode and become damaged in a short time, resulting in shutdown or critical safety problems.

Ebara’s experience in solving these varied corrosion problems include corrosion experiments in the seas around Japan, the Arabian Gulf, the Red Sea and other seawater environments of the world. This is in addition to experimental and theoretical studies based on materials science and electrochemistry in our laboratories to clarify corrosion mechanism of materials used and develop highly marine corrosion resistant materials and advanced corrosion protection technologies. One achievement is the world’s first elucidation of the SCC mechanism in the Ni-Resist (austenitic Ni) cast irons and development of effective countermeasures against it, which was attained in the middle of the 1980s1.

Countermeasures

Stainless steels, which are the most widely used seawater pump material, are sensitive to crevice corrosion and

require effective countermeasures against it. Figure 1 shows an instance of crevice corrosion that occurred on the flange of a Type 316 Stainless Steel (SS) seawater pump that was in service for five years.

The countermeasures available are grouped into two methods. One method is to use relatively economical materials such as Type 316 SS; the other method is to use highly corrosion resistant materials such as Super Duplex SSe and Super Austenitic SSa. The former group includes filler used in the crevice, crevice corrosion resistant alloys overlaid on the crevice surface2, and cathodic protection applied3. The selection of materials to be used should be made with the cost, maintainability, and effective life span taken into account.

Coating technology

Ebara has developed and commercialized a crevice corrosion resistant overlaying alloy, ‘Crevelloy’ (Ebara’s trade name)2. This material utilizes the highly corrosion resistant alloy Ni-Fe-30Cr-10Mo series overlaid on the base metal of relatively low priced Type 316 SS, which helps prevent crevice corrosion. This product has been supplied in seawater pump’s casings, shafts, sleeves, and pipes for more than 20 years. In particular, no other alternatives can be taken for the variable vane type pump in which the seal portion of the vane and the hub constitutes a sliding

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Figure 1. Instance of crevice corrosion on Type 316 SS.

Figure 2. Potential Distribution inside vertical seawater pump analyzed by boundary element method.

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crevice. All the pumps of this type use this technology. Crevelloy is approved in AWS (American Welding Society) Standard as SFA5.14 ERNiCrMo-16 and EN (Europeen de Normalisation) Standard as Numerical No.6057, Chemical NiCr30Mo11.

BEM analysis system

Crevice corrosion can also be prevented by cathodic protection, in which efficient protection is achieved by attaching an anode at a right position and predicting the consumption rate of the sacrificial anode and the current that will arise. Such predictions have usually been made in accordance with rules of thumb. Ebara uses an established proprietary technology for making quantitative cathodic protection design by numerical analysis. Ebara in collaboration with the Tokyo Institute of Technology (TIT) applied the boundary element method (BEM) to the corrosion protection analysis for the first time. The Ebara-TIT Group continue to have the world’s most advanced technology for corrosion prevention analysis 3.

Figure 2 illustrates an example of the analysis of potential distribution inside a Type 316 SS vertical seawater pump to which cathodic protection has been applied by a Zn sacrificial anode. The range the effect of the cathodic protection extends over is known by the potential distribution, and the consumption rate of the sacrificial anode is known by analysis of the current density distribution.

High grade steel

If long life or maintenance-free pumps are desired, or for pumps intended for use in highly corrosive sea areas such as Middle East, high grade stainless steels are required. Stainless steels with a Pitting Resistance Equivalent (PRE) of 40 or higher are called Super SS9 and have high crevice corrosion resistance. Ebara recommends Super Duplex SS9, which combine high

corrosion resistance and remarkable strength properties.

The water of the Arabian Gulf contains a high level of Total Dissolved Salt (TDS), and the water’s temperature is relatively high. The Red Sea has average salt consistency, but is known to have

high average annual temperature. It is expected that seawater in the Middle East is more corrosive than seawater of any other areas of the world, but the corrosion resistance of varied materials in seawater in the Middle East have not been quantitatively compared with each other. In

cooperation with Saudi Arabia’s Saline Water Conversion Corporation (SWCC), Ebara carried out immersion tests of varied stainless steels in the Middle East seawater (the Arabian Gulf and the Red Sea). The results demonstrated that Super Duplex SSs provide excellent crevice corrosion resistance in these areas. The test results will be presented at the MEMEC 2007, scheduled for November 2007 in Bahrain.

In the manufacture of Super Duplex SS pumps, it is critical to use adequate technologies to attain and maintain the corrosion resistance of the heat affected zones (HAZ) and the strongly cold worked zones. Ebara is has much experience in Super Duplex SSs working and welding technology, and has supplied to Middle East customers many Duplex and Super Duplex SS large seawater pumps manufactured using such technologies depending on the type of the materials used and the cost and the intended use of the products (Figure 3). ■

References

[1] Matsuho Miyasaka and Naoaki Ogure: Corrosion, 43,[10],pp.582-588 (1987).

[2] Matsuho Miyasaka, Shuhei Nakahama, Juichi Ishiguro: 51th Annual Assembly of the International Institute of Welding, Hamburg, Germany, pp.29-43 (1998).

[3] Matsuho Miyasaka, Hirokazu Takayama, Kenji Amaya, Shigeru Aoki: Proceedings of the 17th International Pump Users Symposium, Houston, USA, pp.121-129 (2000).

CONTACTTakafumi MaeharaGeneral manager Project Planning Office Fluid Machinery & Systems CompanyEbara CorporationTel: + 81-3-3743-6363F ax: +81-3-3741-4966E-mail: maehara.takafumi@ebara.com Web: www.ebara.com

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Figure 3. Reference of large size vertical pump for Arabian Gulf.