6/4E-AH Slurry Pump in Nuclear Power Plant Seawater Cooling Systems: Titanium Impeller Upgrade for Chloride Stress Corrosion Cracking Prevention

Introduction

The seawater cooling system in a nuclear power plant is responsible for transporting large volumes of seawater to condensers, equipment coolers, and other heat exchange devices – critical for the stable operation of both the nuclear island and the conventional island. The 6/4E-AH, a medium‑sized AH series slurry pump (150mm discharge, 100mm inlet), is widely used in coastal nuclear power plants for seawater lifting and transport due to its compact structure, wear resistance, and reliable mechanical seals.

However, the high chloride concentration in seawater (approximately 18,000–30,000 ppm Cl⁻) poses a severe corrosion threat to conventional metallic materials. Ordinary austenitic stainless steels (304, 316L) are highly susceptible to stress corrosion cracking (SCC) in seawater environments. SCC is a sudden, unpredictable brittle fracture: cracks initiate at the surface and propagate rapidly under tensile stress, often causing impeller fracture or casing rupture within months of operation. This can lead to cooling water interruption and even plant shutdown.

Hebei Xingou Machinery Equipment Co., Ltd. has developed a titanium impeller upgrade solution for the 6/4E-AH slurry pump for nuclear seawater cooling systems. Using TA10 (Gr.12) titanium alloy instead of 316L stainless steel or high‑chrome iron fundamentally eliminates the risk of chloride SCC. This article analyzes the mechanism of SCC in seawater, compares the corrosion resistance of titanium with other metallic materials, and describes the design, manufacturing, and field application results of the titanium impeller.

1. Demanding Requirements for Slurry Pumps in Nuclear Seawater Cooling Systems

Coastal nuclear power plants use raw seawater as the cooling medium. Seawater contains high concentrations of Cl⁻, SO₄²⁻, Mg²⁺, and other corrosive ions, along with sand, microorganisms, and marine organisms. The key requirements for slurry pumps in seawater cooling systems are:

The 6/4E-AH slurry pump is typically used in nuclear seawater cooling systems for:

• Seawater intake pump stations (from intake to forebay)

• Circulation pump forebay transfer

• Strainer backwash pumps

• Plant industrial seawater distribution network

2. Mechanism and Hazards of Chloride Stress Corrosion Cracking (SCC)

2.1 The Three Essential Conditions for SCC

SCC is a brittle fracture caused by the combined action of a corrosive medium and tensile stress. All three conditions must be present:

2.2 Typical SCC Characteristics of 316L Stainless Steel in Seawater

A coastal nuclear power plant experienced a 316L impeller fracture after only 8 months of operation. Fracture analysis revealed classic transgranular SCC morphology. The incident caused a 36‑hour interruption of cooling water to that circuit, with enormous economic losses.

3. Corrosion Resistance Comparison of Metallic Materials in Seawater

3.1 Key Performance Indicators

PREN (Pitting Resistance Equivalent Number) = %Cr + 3.3×%Mo + 16×%N. Higher PREN indicates better resistance to Cl⁻ pitting and SCC.

Unique advantages of titanium alloy:

• Forms a dense, stable TiO₂ oxide film in seawater with excellent self‑repair capability

• Insensitive to Cl⁻, no SCC tendency

• Excellent crevice corrosion resistance, especially suitable for areas prone to biofouling

• Low density (4.5 g/cm³, ~60% of steel), reducing centrifugal force

3.2 Material Selection Decision

For the 6/4E-AH pump impeller in nuclear seawater cooling service, Hebei Xingou Machinery recommends:

Conclusion: For critical nuclear plant equipment requiring 30‑40 year service life, a titanium impeller has a much lower life‑cycle cost than frequent replacement of 316L or 2205 impellers, despite the higher initial cost.

4. Titanium Impeller Upgrade Solution for 6/4E-AH Pump

4.1 Introduction to TA10 Titanium Alloy (Gr.12)

TA10 titanium alloy (ASTM Gr.12, Ti‑0.3Mo‑0.8Ni) is a low‑alloyed titanium grade specifically developed to improve crevice corrosion resistance. Its chemical composition and properties:

TA10 titanium alloy has a corrosion rate in seawater below 0.001 mm/year – 50 to 100 times lower than 316L – and no SCC risk whatsoever. Its crevice corrosion resistance is superior to unalloyed titanium (Gr.2), making it particularly suitable for seawater intake applications prone to biofouling.

4.2 Impeller Design and Manufacturing Requirements

⚠️ Critical notes: Titanium is exceptionally reliable in seawater, but two usage limitations must be observed:

1. Avoid direct contact with copper alloys: Galvanic corrosion will occur, with copper preferentially corroding. Use insulating gaskets or coatings to isolate.

2. Avoid dry running: The titanium oxide film is destroyed under dry conditions, dramatically reducing corrosion resistance.

4.3 Field Retrofit Procedure

5. Field Application Case

Background: A coastal nuclear power plant used a 6/4E-AH slurry pump for strainer backwash service in its seawater cooling system. The original impeller was 316L stainless steel. After 18 months of operation, the impeller developed multiple through‑wall cracks, the inlet edge fractured, pump vibration exceeded limits, and the pump was forced out of service.

Failure analysis: The fracture exhibited classic transgranular dendritic SCC morphology. Cracks initiated at stress concentration points on the impeller inlet edge. Seawater Cl⁻ concentration was approximately 22,000 ppm, water temperature 25°C – conditions favorable for SCC.

Upgrade solution: Hebei Xingou Machinery provided a TA10 titanium impeller, and also upgraded the liner and shaft sleeve to 2205 duplex stainless steel (to avoid galvanic corrosion with titanium).

Results:

Payback period: One‑time cost increase ~$600, annual savings ~$1,100 → payback <7 months. The pump has now operated for 6 years with the titanium impeller still in excellent condition, and an additional 24+ years of service life is expected.

6. Monitoring and Maintenance Recommendations

Conclusion

In nuclear power plant seawater cooling systems, ordinary 316L stainless steel impellers cannot resist stress corrosion cracking caused by high chloride concentrations. Impeller life is often less than 2 years. The TA10 titanium alloy (Gr.12) impeller – with its dense, stable passive film, insensitivity to Cl⁻, and no SCC tendency – fundamentally solves the SCC problem. Paired with 2205 duplex liners and shaft sleeves (to prevent galvanic corrosion), the impeller can achieve the same service life as the nuclear plant itself (30+ years). Although the initial investment is higher, the total life‑cycle cost is significantly lower than repeatedly replacing stainless steel impellers.

Hebei Xingou Machinery Equipment Co., Ltd. offers custom design, manufacturing, and field retrofit services for 6/4E-AH titanium impellers. For material upgrade assessments or custom quotations for nuclear seawater cooling systems, please contact our technical team.