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.
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 coupes 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 vulnerable 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.
The atmospheric environment varies drastically with regards to corrosivity depending on the geographical location.
In atmospheric corrosion, the electrolyte is moisture from precipitation, fog or dew, sea spray, or other sources.
The three factors that have the most influence on the corrosivity of the atmosphere at a given site are:
(1) the amount of time that exposed surfaces remain wet at the site,
(2) the amount of chloride from the sea that reaches the surfaces
(3) the amount of industrial pollutants (mainly acids) that reach the surfaces.
In all atmospheric environments there is an excess of oxygen, thus the corrosion of most metals in atmospheric environments is not limited by the amount of oxygen present and can proceed rapidly when the electrolyte is present.
The corrosivity of the atmosphere varies drastically.
The corrosion rate of steel, for example, can vary by a factor of 100.
In general, the least corrosive atmospheres are found in dry inland (desert) sites and the most corrosive sites are industrial or industrial marine sites.
Moist tropical locations are very corrosive due both to the time of wetness and the high temperatures experienced.
However, local conditions and features of design have an influence on corrosive attack that often exceeds the differences experienced due to geographical conditions.
Thus, generalizations regarding specific site corrosivity based on the corrosion of a single metal at a single location at a given site can be misleading.
It is not prudent to ignore the possibility of corrosion at a dry inland site or to consider corrosion inevitable at a marine industrial site.
The rates of attack can vary at different sites, but the mechanisms of attack, features that cause accelerated attack and corrective measures for corrosion prevention and corrosion protection that can reduce the attack are similar.
Source : "Corrosion Control" NAVFAC MO-307 September 1992