Repair of Brine Circulation Pump Impeller by Weld Overlay

Background and Service Conditions

Brine circulation pumps in chlor-alkali electrolysis cells operate under extremely aggressive conditions: saturated brine at elevated temperatures (80–90 °C), high chloride concentrations (200–250 g/L NaCl), and continuous mechanical loading from the rotating impeller. Impellers are typically fabricated from duplex stainless steel (2205) or super duplex (2507) to resist chloride stress corrosion cracking (SCC) and pitting. However, even these advanced alloys can suffer from localized erosion-corrosion damage at the impeller vanes, hub, and discharge area, particularly where cavitation occurs or where the brine velocity exceeds critical thresholds.

The study of repairing such impellers by weld overlay is significant because replacement impellers are expensive, have long lead times, and often require custom fabrication. A well-executed weld overlay repair can restore dimensional accuracy and corrosion resistance while significantly reducing downtime.

Weld Overlay Process Selection

The selection of the overlay process and consumable is critical for brine pump impeller repair. The following table summarizes the key considerations:

Factor	Requirement	Recommended Approach
Base material	2205 or 2507 duplex SS	Match or exceed base alloy
Corrosion resistance	Resistant to 25% NaCl, 80–90 °C	2205, 2507, or Alloy 6
Mechanical properties	≥ 450 MPa yield strength	Full-penetration welds
Cavitation resistance	Hardness 25–35 HRC	Alloy 6 or Ni-based
Process	Low heat input, minimal HAZ	GTAW or GMAW
Preheat	Minimize to avoid sensitization	≤ 50 °C
Post-weld treatment	Stress relief without sensitization	Solution treatment or careful SR

GTAW (TIG) is generally preferred for impeller repair because it offers precise heat input control, excellent weld appearance, and minimal spatter. For thicker repair areas, a two-pass approach with GTAW root and GTAW or GMAW fill is common. The filler metal must be selected to avoid Cr-Mo carbide precipitation at the weld interface, which would create a corrosion-sensitive zone.

Repair Procedure and Quality Control

The repair procedure follows a structured sequence:

Inspection and damage assessment: Visual examination, ultrasonic testing (UT) for subsurface cracking, and dimensional measurement to determine the extent of material loss.
Surface preparation: Grinding back the damaged area to sound metal, ensuring a smooth transition from the overlay to the base material. The ground surface should be free of oxide and contamination.
Preheat: Light preheating to 30–50 °C to reduce cooling rates without promoting sensitization. Higher preheat temperatures risk reducing the ferrite content in duplex alloys below the acceptable 35–65% range.
Welding: Back-step welding sequence to minimize distortion. Interpass temperature maintained below 150 °C. Each pass should be ground flush before the next pass.
Post-weld treatment: Stress relief at 400–450 °C for 1–2 hours, followed by rapid cooling to avoid the "no-man's land" temperature range (600–800 °C) where sigma phase and chromium carbides form.
Final inspection: Dye penetrant testing (PT) of the overlay surface, UT for bond quality, and dimensional verification against the original impeller profile.

A critical quality control point is the ferrite content measurement of the weld metal. For duplex stainless steel overlay on a duplex substrate, the weld ferrite should be maintained in the 35–65% range. Ferrite below 35% increases susceptibility to SCC, while ferrite above 65% reduces toughness and increases the risk of intergranular cracking. The ferrite gauge (MAGNEX) should be used on every repair.

Common Defects and Countermeasures

Defect	Cause	Countermeasure
Cracking at weld root	Excessive cooling rate, hydrogen	Increase preheat, use low-hydrogen consumable
Porosity	Contaminated surface, moisture	Thorough cleaning, dry electrodes
Excessive dilution	Poor weld technique, too large wire	Smaller wire diameter, controlled deposition rate
Ferrite out of range	Incorrect heat input	Adjust travel speed and current
Distortion	Asymmetric welding sequence	Back-step or symmetric welding

Study Insights

The repair of brine pump impellers by weld overlay is fundamentally a balance between corrosion resistance restoration and mechanical integrity preservation. The engineer must recognize that the overlay is not merely a surface coating but a structural repair that must integrate metallurgically with the base material. The duplex stainless steel system presents unique challenges because the microstructure is highly sensitive to thermal history, and the two-phase nature means that even small deviations in heat input can shift the ferrite-austenite balance beyond acceptable limits.

A practical lesson from field experience is that impeller repairs should always include a root cause analysis of the original damage. If the damage was caused by cavitation, simply overlaying the surface without addressing the underlying hydraulic design issue will result in repeat failure. The overlay should be combined with geometric modifications, such as adjusting the vane angle or increasing the clearance between the impeller and the volute, to reduce cavitation intensity.

In summary, weld overlay repair of brine circulation pump impellers is a technically demanding but highly cost-effective approach when properly executed. The key to success lies in careful consumable selection, precise thermal management, rigorous quality control, and integration of the repair with a broader understanding of the pump's operating conditions and failure mechanisms.