Anti-Crevice Corrosion Surfacing Process for Turbine Components

Literature Overview

The paper by Wang Changzhu and Zhao Yujang, published in Welding (焊接) in 2000, addresses a critical reliability issue in steam turbine maintenance: crevice corrosion at bolted or flanged joints in turbine components. Crevice corrosion is a localized corrosion mechanism that occurs in confined geometries where oxygen depletion and chloride accumulation create aggressive microenvironments. In steam turbine applications, where components are exposed to high-temperature steam, condensate, and potentially corrosive chemical species from fuel or combustion products, crevice corrosion can lead to catastrophic failure of critical components such as casing flanges, gland seals, and valve bodies.

Corrosion Mechanism and Engineering Context

Crevice corrosion initiates when a stagnant electrolyte becomes trapped in a gap or crevice. The mechanism proceeds as follows:

Differential aeration cell formation: Oxygen is depleted within the crevice, creating an anode relative to the oxygen-rich exterior surface (cathode).
Chloride migration and accumulation: Chloride ions migrate into the crevice to maintain charge balance, increasing local acidity.
Autocatalytic dissolution: Fe²⁺ ions produced by anodic dissolution hydrolyze, further lowering the pH and accelerating the corrosion rate.
Pit propagation: Once initiated, the corrosion pit grows rapidly, potentially leading to through-thickness failure.

In turbine service, the combination of high temperatures, high pressures, and the presence of dissolved salts in condensate creates conditions particularly favorable for crevice corrosion. The authors' surfacing approach aims to eliminate the crevice geometry or to provide a corrosion-resistant barrier that prevents the initiation of the corrosion cell.

Surfacing Process Design

Design Element	Specification	Purpose
Surfacing alloy	Nickel-based or austenitic stainless steel overlay	Provides corrosion resistance in aggressive environments
Welding process	GTAW or SMAW with appropriate consumable	Ensures clean, low-dilution weld deposit
Preheating	Moderate preheat to minimize HAZ sensitization	Prevents intergranular corrosion in HAZ
Post-weld heat treatment	Solution annealing or stabilization treatment	Dissolves chromium carbides and restores corrosion resistance
Surface finish	Smooth, burr-free finish	Eliminates potential crevice initiation sites
Inspection	PT and UT for surface and subsurface defects	Ensures integrity of the corrosion-resistant barrier

Process Implementation Considerations

The authors from Harbin Turbine Co., Ltd. brought substantial practical experience to this work. Turbine components are often large, thick-walled castings or forgings made from materials such as cast stainless steel, duplex stainless steel, or nickel-based alloys. Surfacing such components presents several challenges:

Thermal stress management: Thick sections develop significant residual stresses during welding. The authors likely employed techniques such as low-heat-input welding, intermittent welding sequences, and post-weld stress relief to control distortion and residual stress.
Dilution control: When surfacing onto dissimilar base metals, dilution can compromise the corrosion resistance of the weld metal. Multi-pass surfacing with careful monitoring of each pass's composition is essential.
HAZ sensitization: For austenitic stainless steel surfacing alloys, the HAZ is susceptible to sensitization if the welding heat input is too high. The use of low-carbon or stabilized grades (such as 321 or 347) mitigates this risk.
Post-weld inspection: The authors emphasized post-weld inspection, which is critical for ensuring that the surfacing layer is free of defects that could serve as corrosion initiation sites.

Quality Control and Inspection

The emphasis on post-weld inspection (焊后检验) in the paper's keywords is particularly important. For corrosion-resistant surfacing applications, the inspection requirements are more stringent than for general structural welding:

Visual inspection: All weld surfaces must be smooth, uniform, and free of undercut, porosity, or other surface defects.
Penetrant testing (PT): Essential for detecting surface-breaking defects in the surfacing layer.
Ultrasonic testing (UT): Required to detect subsurface defects such as lack of fusion and slag inclusions that could compromise the integrity of the corrosion barrier.
Hardness testing: Hardness profiles across the surfacing layer provide information about the microstructural condition and dilution level.
Corrosion testing: Immersion testing or electrochemical testing in simulated service environments verifies the corrosion resistance of the finished surfacing.

Study Insights and Implications

This paper addresses a problem that is often underestimated in turbine maintenance: the tendency of engineers to focus on mechanical strength and wear resistance while neglecting corrosion resistance. In reality, corrosion failures in turbine components often result in longer shutdowns and higher repair costs than mechanical failures, because corrosion damage is more difficult to detect and often requires more extensive repair. The authors' approach of providing a corrosion-resistant surfacing layer at crevice-prone locations is a proactive maintenance strategy that shifts from reactive repair to preventive protection.

The choice of nickel-based or austenitic stainless steel overlays reflects the state of the art in corrosion-resistant surfacing technology at the time of publication. Modern alternatives such as amorphous metallic coatings and high-entropy alloy overlays offer additional options, but the fundamental principle of providing a corrosion-resistant barrier at crevice sites remains valid. The paper's emphasis on comprehensive post-weld inspection underscores the importance of quality assurance in corrosion-critical applications, where a single undetected defect can lead to component failure and significant production loss.