Since they are the key components of turbomachinery, compressors, and pumps, impellers work in the long term in complex conditions with corrosive and solid-laden media and are thus highly susceptible to erosion and corrosion combined. Erosion is a mechanical wear phenomenon caused by solid particles in high-speed fluids, and corrosion is due to continuous attack by chemical or electrochemical reactions on materials. In the majority of engineering applications, both these effects simultaneously happen and cause each other, resulting in a coupled damage mechanism that gravely compromises the structural integrity and service life of impellers.
This paper elaborates extensively on the basic mechanisms, coupled action modes, and key influencing factors of corrosion and erosion, and introduces efficient measures to improve damage resistance from the viewpoints of material choice, structural design, and protection of the surface, with a view to providing theoretical support and engineering guidance for guaranteeing steady operation of impeller products in severe environments.
Introduction
Impellers find broad use in a range of industrial processes such as energy, chemical engineering, metallurgy, and environmental conservation. Their uses in operation terms typically involve high rotational speeds, elevated temperatures, highly corrosive mediums, or media containing particles, which couple to accelerate the service life degradation of the material considerably. For engineering historical experience, failures of the impeller are largely of composite multi-factor type, among which the erosion-corrosion interaction is recognized as a leading damage mechanism. Erosion not only causes high-velocity stripping of the surface material but also degrades the passivation layer or protective coating layer on the metal surface, forming active corrosion reaction sites; corrosion makes materials susceptible to mechanical wear by leading to metal dissolution and structural weakening, eventually causing local perforation, fatigue crack growth, and even component-wide failure.
Unlike common single wear or corrosion models, the erosion-corrosion synergistic effect is more complex in mechanism, and the range of its influence is not only on the material surface but deep effects on altering the microstructure and macroscopic stress distribution of materials. Therefore, regular studies of this coupled degradation process not only provide help with the revelation of causes of diminished impeller lifespan but also provide a theoretical basis for the development of a new generation corrosion- and wear-resistant materials and coatings.
Basic Mechanisms of Erosion and Corrosion
Erosion mainly occurs in high-speed solid particle-concentrated fluid media. When particles with high speed impact on the surface of the impeller, their kinetic energy is converted into local micro-cutting, plastic deformation, and stripping of material and thus forms surface micro-pits and crack sources. In the early stages of erosion, the surface of the material has a certain resistance; but as the impact frequency and crack accumulation increase, micro-damage evolves into macroscopic spalling step by step.
Corrosion, on the other hand, is an electrochemical reaction-controlled metal dissolution process. For impellers operating in acidic, alkaline, or salt water environments, their surface will have a tendency to create anodic reaction zones, leading to local corrosion or even perforation. Common forms of corrosion are uniform corrosion, crevice corrosion, pitting corrosion, intergranular corrosion, and stress corrosion cracking (SCC). These damages are usually associated with local potential changes in materials or with the presence of stress concentration areas and therefore degrade rapidly under multiaxial stress or cyclic loading.
Although the two mechanisms are essentially distinct, they often interact with each other in actual operation: erosion penetrates into passivation film and triggers corrosion; corrosion lowers the surface strength and promotes erosion. Such an intertwined damage behavior makes failure behavior of impellers highly uncertain, making life assessment and condition monitoring even more challenging.
Typical Characteristics of Erosion-Corrosion Coupling
Impellers in extremely corrosive environments, i.e., seawater pumps, chemical catalytic cracking expanders, or slurry pumps, not only are subjected to extreme impact flow fields in operation but also are worn out by various active ions, i.e., Cl⁻, SO₄²⁻, etc. Erosion will assail the passivation layer on the metallic surface or the surface coating and reveal the new metal to the corrosive environment; corrosion makes the material susceptible to erosion by developing a brittle corrosive product layer or inducing debonding at grain boundaries. This is a case of a positive feedback effect typical of the erosion-corrosion coupling effect.
Take the catalytic cracking hot gas expander as an example. The device will generally operate in an environment of high temperature (>600°C) and high particle concentration flue gas. The blades inside internally suffer from intense erosion, while the sulfides and oxide materials in the flue gas react with the material constantly, presenting a non-protective oxide layer on the surface or even metal loss. Appropriate CFD simulations and failure analyses revealed that the leading edge of the impeller, tail of the guide vane, and outer ring of the stator are common high-risk regions, where manifest pits, grooves, and perforated structures are frequently found, indicating a significant synergistic damage effect.
In engineering years of use, the erosion-corrosion coupling effect has been more recognized as not being a simple superposition but having time-dependent complexity, space selectivity, and material sensitivity. The failure rate in the coupling region is much higher than that under single erosion or corrosion, and thus classical life estimation methods based on empirical coefficients become more (fail). This problem has to be met by the joint development of higher-resolution test systems, higher-fidelity simulation equipment, and multi-scale failure models.
Material Selection and Structural Optimization Strategies
Material Response Strategies
The inherent characteristics of erosion-corrosion resistant materials are excellent high hardness, high strength, excellent chemical inertness, and high-temperature stability. Common materials currently utilized are:
- Austenitic stainless steel (such as 316L):It has satisfactory corrosion resistance but is missing necessary hardness and overall erosion performance;
- Duplex stainless steel (such as 2205): It is corrosion resistant and strong and performs very well in particle-containing seawater environments;
- Hastelloy: It is very resistant to corrosion in high-acid and chloride environments and suitable for extreme corrosion conditions;
- Ceramic composites: They are very hard and very erosion-resistant but very brittle and difficult to resist alternating stresses.
In addition, composite materials and functionally gradient materials (FGM) will become study focuses in the future. By realization of performance transition inside the material, both surface erosion resistance and crack growth resistance in the matrix can be (balanced).
Surface Protection Technologies
Surface engineering technology is an important means to improve erosion-corrosion resistance, including:
- Thermal spraying (such as HVOF, plasma): It is used to deposit high-hardness wear-resistant layers;
- Electroless nickel plating, PVD/CVD coatings: They form dense, low-defect-rate anti-corrosion protective layers;
- Ceramic coatings (such as TiN, Al₂O₃): They provide extremely high hardness and chemical stability.
Films have been discovered in experimental works to extend significantly the failure process, especially in high-frequency erosion or high-corrosion-rate environments.
CFD-Assisted Structural Optimization
CFD simulation can be used to identify the impact energy distribution and corrosion-susceptible areas in the impeller. For example:
- Modification of blade angles and channel widths to reduce particle coalescence zones;
- Changing the guide vane location to avoid local turbulence formation;
- Establishing internal cavity buffer structures to minimize high-velocity particle impact kinetic energy.
The results of CFD are highly correlating with experimental wear maps and are a critical basis for quantitatively predicting failure patterns and design optimization.
Conclusion
Erosion and corrosion are the two main competitors restricting the lifetime of impeller materials, especially in high-speed, particle-existence, and severely corrosive conditions. The combined effect of the two will lead to rapid performance degradation and premature failure.
Therefore, during the designing and manufacturing stages, material dual tolerance must be considered in full, aided by scientific surface protection and running practices to provide long-term stable operation. Research in the future must focus more on the creation of high strength and toughness advanced material systems, erosion-resistant and corrosion-resistant material systems, and multi-functional composite coatings to meet the growing challenges of harsh working conditions.
