Study Note on Repair of Salt Circulation Pump Impeller by Weld Overlay

Literature Overview

The repair of a salt circulation pump impeller by weld overlay is a representative case of corrosion-resistant surface engineering in the chemical and petrochemical industries. Salt circulation pumps in chlor-alkali and salt processing plants operate in highly aggressive chloride environments, where pitting corrosion, crevice corrosion, and stress corrosion cracking (SCC) are the dominant failure mechanisms. The impeller, as a rotating component subjected to both hydraulic loading and chemical attack, presents unique repair challenges that require careful consideration of material selection, welding procedure, and post-weld treatment.

Failure Analysis and Repair Requirements

The salt circulation pump impeller in this case had experienced severe pitting corrosion and material loss at the leading edges of the vanes and at the hub-vane junction. The original material was a duplex stainless steel (2205 or equivalent), which had been selected for its high chloride resistance. However, after prolonged service in concentrated brine solutions at elevated temperatures, the material had experienced localized corrosion that compromised both the hydraulic efficiency and the structural integrity of the impeller.

The repair requirements were:

Weld Overlay Material Selection

The selection of the weld overlay material for this application required careful evaluation of several competing factors. The primary requirement was to match or exceed the corrosion resistance of the original 2205 duplex stainless steel. The following materials were considered:

The selected material was Inconel 625 (UNS N06625) for the following reasons:

1. Superior corrosion resistance: Inconel 625 exhibits excellent resistance to chloride pitting and crevice corrosion, even at elevated temperatures, making it suitable for the aggressive salt brine environment.
2. Excellent weldability: The nickel-based composition provides excellent resistance to cracking during welding, which is critical for the repair of a thin-walled rotating component.
3. Low dilution sensitivity: Unlike austenitic stainless steels, Inconel 625 maintains its corrosion resistance even with moderate dilution from the base material, providing a safety margin against over-dilution.
4. Good balance with the base material: The thermal expansion coefficient of Inconel 625 is compatible with duplex stainless steel, minimizing residual stress and distortion during welding.

Welding Procedure and Process Parameters

The welding procedure was designed to minimize heat input and distortion while ensuring full penetration and adequate overlay thickness.

The GTAW process was selected over SMAW or SAW due to the following considerations:

• The thin section thickness of the impeller vanes (typically 8–15 mm) requires precise heat input control to prevent burn-through.
• The complex geometry of the impeller (curved vanes, hub, shroud) requires excellent welder control and accessibility.
• GTAW produces clean, defect-free welds with minimal spatter, which is essential for maintaining the hydraulic efficiency of the impeller.

Critical Process Steps

1. Surface preparation: The corroded surface was prepared by grinding down to sound metal, followed by acetone cleaning to remove all contaminants. The prepared surface was inspected by MT to confirm the removal of all corrosion-related defects.
2. Root pass welding: The root pass was applied using a low current setting (80 A) with a short arc length to minimize dilution and ensure proper fusion. The welder maintained a consistent travel speed to achieve a uniform bead width and penetration.
3. Filler and cap passes: Subsequent passes were applied with slightly higher current (100–120 A) to increase deposition rate while maintaining control over the weld pool. The cap pass was applied with a weaving technique to ensure full coverage of the weld groove.
4. Post-weld solution annealing: The welded impeller was subjected to solution annealing at 1050 °C for 1 hour followed by rapid water quenching. This treatment dissolved any precipitated intermetallic phases (such as the σ-phase) and restored the full corrosion resistance of the Inconel 625 overlay.

Quality Control and Testing

The quality assurance program for this repair was comprehensive, reflecting the critical nature of the component in the salt processing circuit.

Integration with Engineering Practice

This case study highlights several important engineering considerations for the repair of rotating components in corrosive environments. First, the selection of a nickel-based overlay material (Inconel 625) over a duplex stainless steel overlay material, despite the higher cost, was justified by the superior corrosion resistance and weldability of the nickel alloy. This decision reflects the principle that the total cost of ownership — including the cost of unplanned downtime and replacement — often favors the more expensive material.

Second, the post-weld solution annealing step is not optional but essential for ensuring the long-term corrosion resistance of the Inconel 625 overlay. Without solution annealing, the precipitation of intermetallic phases during the welding thermal cycle can significantly reduce the corrosion resistance, particularly in chloride-containing environments.

Third, the balance test after repair is a critical step that is sometimes overlooked. The addition of weld metal to one side of the impeller creates an imbalance that must be corrected by trimming or adding counterweights. Failure to achieve the required balance quality can lead to excessive vibration, bearing failure, and premature component damage.

Key Questions and Reflections

A significant question arising from this case is whether the weld overlay repair provides a permanent solution or merely extends the service life of a component that is fundamentally unsuitable for the service environment. If the original material (2205 duplex SS) was correctly specified for the service conditions, then the corrosion damage may have been caused by abnormal operating conditions (such as elevated temperature or contamination). In this case, the root cause must be identified and corrected to prevent recurrence. If, however, the original material was underspecified, then the repair with Inconel 625 provides a more robust solution but also indicates a specification error that should be addressed in future procurements.

Another reflection concerns the applicability of this repair approach to other rotating components in similar service. The same methodology — Inconel 625 GTAW overlay with post-weld solution annealing — can be applied to pump casings, valve bodies, and other components in chloride-containing environments. The key is to ensure that the welding procedure is qualified for the specific geometry and thickness range of the component.

Study Insights and Implications

The salt circulation pump impeller repair case demonstrates the effectiveness of weld overlay technology in restoring the service life of corrosion-damaged rotating components. The selection of Inconel 625 as the overlay material, combined with a carefully designed GTAW welding procedure and post-weld solution annealing, provides a reliable repair solution for components operating in aggressive chloride environments. The case underscores the importance of integrating materials selection, welding procedure design, and post-weld treatment into a coherent repair strategy. For engineers working in chemical processing and petrochemical applications, this case provides a practical template for the repair of corrosion-damaged rotating equipment, emphasizing that the repair must address both the immediate material loss and the underlying corrosion mechanism.