Erosion-corrosion is a general term that refers to a corrosion process enhanced by the action of flowing fluids. The process can also be characterized by whether the fluid contains solid particles, is in the form of impinging droplets, or is undergoing cavitation. Cavitation is the formation and sudden collapse of vapor bubbles in a liquid.
Manifestation and Quantification
Erosion-corrosion can result in general corrosion that occurs at a higher rate than would be expected under stagnant conditions. In this case the measure of erosion-corrosion would be the rate of surface waste or the rate of penetration. The more usual effect of erosion-corrosion, however, is localized attack, which can appear as grooves, gullies, waves, rounded holes, etc., and usually exhibits a directional pattern that correlates with the direction of flow.
Erosion-corrosion is illustrated in Fig. below.
The appearance of cavitation damage is similar to pitting. However, the pitted areas are closely spaced and the surface is usually considerably roughened. Severe cavitation damage can completely remove sections of metal.
Cavitation damage that results from fluid movement relative to the metal surface can usually be correlated with the direction of flow A condition conducive to erosion corrosion is the flow of corrosive fluids (gas or liquid) relative to a metal surface.
The rate of corrosion depends upon the flow rate of the fluid. Turbulent flow results in much higher corrosion rates than Iaminar flow. Hard particles entrained in the flowing fluid can enhance erosion corrosion.
Cavitation damage is caused by flow discontinuities that result in the formation and subsequent coUapse of vapor bubbles on a metal surface. High-velocity drop impingement, such as raindrops on a helicopter rotor, can also result in rapid surface damage.
Erosion-corrosion implies that the fluid medium is potentially corrosive to the metal. Erosion facilitates the corrosion process. This fact distinguishes erosion-corrosion from pure erosion or mechanical wear. Erosion corrosion influences the rate of corrosion by changing the conditions of local cell action. The corrosion process is accelerated if the fluid speed is sufficient to remove weakly adhered corrosion products from the surface.
Removal of these products reduces their polarizing or inhibitive effect.
At the breakaway speed the fluid begins to remove the corrosion film and the corrosion rate increases. A steady corrosion rate is achieved at the speed at which the fdm is completely removed. Fluid flow also maintains a uniform concentration of corrodent at the metal surfaces.
Impingement of suspended hard particles can accelerate the damage to the protective film. and can cause mechanical damage to the underlying metal.
Application Constraints and Protection Approaches
Several methods for preventing or minimizing damage resulting from erosion-corrosion are available.
1. Material selection. Select materials with better resistance to erosion-corrosion.
2. Design considerations. Streamline the flow, avoid designs that create turbulence. Minimize abrupt changes in flow direction. Introduce smooth aerodynamic or hydrodynamic flow channels; avoid roughly textured surfaces. Carefully align pipe sections. Avoid flow obstructions in design or obstructions that can arise under operations, increase the thickness of material in vunerable areas, install renewable impingement plates or baffles, and design for easy repair by using interchangeable parts.
3. Aherarion of environment. Decrease fluid stream speed to achieve laminar flow, regulate the concentration of dissolved oxygen in the environment to achieve optimal film-forming characteristics, provide falters for removal of suspended solids, and provide condensed moisture traps in vapor lines.
4. Specification of suitable coatings or linings. Use of hard-facing may be helpful in some situations and resilient barriers may be helpful in others, e.g., cavitation.
5. Cathodic protection. Provide cathodic protection whenever possible.
Source : MIL-HDBK-735