Zhuojin Pipe Fitting Co., LtdCorrosion-Wear Properties of Corrosion-Resistant Overlay Layers
Literature Overview
Xu Xiaofeng, Zhang Keke, Zhang Yongzhen, Chen Darou, and Chen Weizhen from Luoyang Institute of Technology and Luoyang Tractor Research Institute (1995) conducted systematic corrosion-wear testing of overlay layers produced with different electrode compositions under various corrosive media conditions. Published in "Materials Development and Application," this study employed a custom-built corrosion-wear testing machine to evaluate the synergistic effects of chemical corrosion and mechanical abrasion on overlay layer performance. The research is particularly relevant to the agricultural machinery sector, where components are subjected to both abrasive soil and corrosive chemical environments.
Core Technical Findings
The study compared overlay layers deposited using austenitic electrodes and low-carbon steel electrodes under acidic, neutral, and alkaline corrosive environments. The key findings are summarized below.
| Overlay Type | Acidic Medium Wear Rate | Neutral Medium Wear Rate | Alkaline Medium Wear Rate |
|---|---|---|---|
| Austenitic electrode overlay | Low (best) | Low (best) | Moderate |
| Low-carbon steel electrode overlay | High | Moderate | Low (best) |
| Medium chromium steel overlay | Moderate | Moderate | Moderate |
An important additional finding was that increasing welding current improved the wear resistance of the overlay layer. This effect is attributed to the increased dilution and deeper penetration, which modifies the microstructure through enhanced austenite-to-martensite transformation and carbide precipitation during cooling.
Technical Interpretation
The corrosion-wear phenomenon is fundamentally different from pure mechanical wear because the chemical environment modifies the surface properties of the overlay material during sliding contact. The mechanism involves:
- Chemical softening: Corrosive media dissolve protective oxide films and attack the base metal, reducing the effective hardness of the surface in contact with abrasive particles.
- Oxidative wear: In the presence of oxygen and corrosive ions, the wear debris oxidizes, forming hard oxide layers that may either protect or accelerate wear depending on the oxide morphology.
- Galvanic coupling: When dissimilar materials are in contact in an electrolyte, galvanic currents accelerate the corrosion of the less noble material, creating a coupled corrosion-wear mechanism.
- Tribochemical reactions: The combination of mechanical energy and chemical environment produces reaction products not observed in either pure wear or pure corrosion conditions.
The superior performance of austenitic overlays in acidic and neutral media can be explained by the stability of the chromium oxide passive film (Cr2O3) in these environments. Austenitic stainless steel overlays typically contain 18–20% Cr and 8–10% Ni, providing excellent passive film stability across a wide pH range. The face-centered cubic (FCC) austenitic structure also offers superior deformation capacity, allowing the surface to accommodate wear without crack initiation.
In alkaline media, however, the passive film stability of austenitic materials decreases, particularly in hot concentrated alkali solutions where chromium dissolution is accelerated. Low-carbon steel overlays, while lacking inherent corrosion resistance, benefit from the formation of protective iron oxide films (Fe3O4, Fe2O3) that are stable in alkaline conditions. The Boudouard reaction and related iron oxide equilibria favor the formation of a dense, adherent oxide layer in pH > 10 environments.
Welding Current Effects
The observation that increased welding current improves wear resistance deserves detailed explanation:
| Current Level | Dilution Ratio | Microstructure | Hardness | Wear Resistance |
|---|---|---|---|---|
| Low current | Low (10–15%) | Predominantly austenitic | HRC 25–30 | Moderate |
| Medium current | Medium (20–30%) | Mixed austenite + martensite | HRC 35–45 | Good |
| High current | High (35–50%) | Predominantly martensitic + carbides | HRC 45–55 | Excellent |
Higher current increases base metal dilution, introducing more iron and carbon into the weld pool. The increased carbon content promotes carbide precipitation during solidification and subsequent cooling, while the iron dilution shifts the weld metal composition toward the martensite-forming region of the Fe-Cr-Ni phase diagram. The resulting microstructure contains harder phases (martensite, carbides) that resist abrasive wear more effectively than the soft austenitic matrix.
Engineering Applications and Recommendations
For agricultural machinery applications—specifically tractor components, plow bodies, and harvesting equipment—the selection of overlay electrode type must be guided by the dominant service environment:
- Acidic soil conditions (pH < 6, common in tropical and subtropical regions): Austenitic overlay electrodes (e.g., E309, E310 type) provide superior corrosion-wear resistance.
- Neutral soil conditions (pH 6–8): Austenitic overlays remain the preferred choice due to their balanced corrosion and wear resistance.
- Alkaline soil conditions (pH > 8, common in arid regions): Low-carbon steel overlays with appropriate hardening heat treatment may outperform austenitic alternatives.
The study also highlights the importance of multi-layer overlay strategies. A single layer may not provide adequate protection, and building up 2–3 layers with alternating compositions can optimize both corrosion and wear resistance. For example, a base layer of austenitic composition followed by a surface layer of medium chromium composition can combine corrosion resistance with enhanced wear resistance.
Key Reflections
This 1995 study, while methodologically limited by the testing equipment available at that time, established fundamental principles of corrosion-wear behavior that remain valid today. The custom-built corrosion-wear tester represents an early attempt at simulating real service conditions, where chemical and mechanical degradation occur simultaneously. Modern tribocorrosion research employs more sophisticated apparatus including reciprocating sliding testers with electrochemical monitoring, but the qualitative conclusions remain consistent.
From an engineering practice perspective, the most important takeaway is that overlay material selection cannot be based on pure wear resistance data alone. The service environment's chemical characteristics fundamentally alter wear mechanisms, and overlay specifications must account for the actual corrosive media encountered in service. This principle applies equally to modern pipeline applications where corrosion-resistant alloy (CRA) overlays are applied to carbon steel pipe surfaces in sour service environments.