The following paper is condensed from "Materials Selection for Corrosion Control" S.L. Shawla, R. K. Gupta, published by ASM International, 1993
Flow velocity is an important environmental factor. Its effect is especially pronounced in chemical processing, petroleum, marine, and power plants, which handle fluids of all kinds. Sometimes it proves so deleterious as to require the redesign of equipment. The subject is complicated by the fact that velocity influences other factors, such as oxygen depolarization and the film thickness, and thus the resistance, of surface films, which is reduced by high flow velocities, such that the distance through which dissolved gases or corrosive ions must diffuse to reach the metal surface is also reduced. Past a certain velocity, which may be called the critical, or breakaway velocity, surface film resistance almost vanishes, oxygen depolarization is complete, the products of corrosion and protective film are continuously swept away, and continuous corrosion occurs. This type of corrosion has been observed on brass condenser tubes through which a medium flows at a high velocity, and it is severe at places where the velocity changes rapidly, such as at bends and in valves.
Erosion Corrosion
This form of corrosion is deterioration of metal caused by the combined action of mechanical erosion and electrochemical attack The erosion is caused by relative motion between the corrosive processing medium and the metal surface. The electrochemical attack is caused by the surface condition and inherent nature of the bare metal vis-a-vis the corrosive fluid. The protective film on the metal surface is swept away by rapid movement of the processing fluid. Either the film, or the mechanical impact of the rapidly flowing stream, or both, are not uniform, with the result that the metal surface becomes heterogeneous. This form of corrosion damage is characterized by grooves, gullies, waves and valleys, which usually have a directional pattern. As expected, the effects are more pronounced at bends and corners where the velocity is non-uniform, and consequently the corrosive attack is more severe.
Any type of equipment or machinery exposed to moving fluids is vulnerable to erosion corrosion. In the process industry, there are complex piping systems and their accessories, such as bends, elbows, tees, etc. The devices that impart movement to fluids, e.g., pumps, blowers, propellers, impellers, centrifuges, and agitators are subject to severe erosion corrosion.
Because the accelerated chemical attack is caused by the flowing environments, the nature of these environments, their velocity, and the nature of the material are important factors. Therefore, erosion corrosion is more severe in multiphase flow systems. An examination of stresses developed by flow on a metal surface shows that shear stress predominates in a single phase flow system. Its value is usually less than a few Pascals, while in a multiphase system, a second phase, which can be solid particles or liquid drops can be brought abruptly to rest, when it hits a wall, generating high local stresses at vulnerable points. Two mechanisms have been proposed for erosion corrosion: (1) convective mass transfer; and (2) phase transfer.
Convective Mass Transfer. Here the rate of corrosion is controlled either by the removal of the protective layer of initial corrosion product from the metal surface or by transport of corrosive species from the bulk solution towards the metal-solution interface. The former is more common, and the latter is a second step in conjunction with it. The low value of shear stresses developed in this process cannot be responsible for the removal of corrosion product from the surface. This can happen only when the protective film is weak in tension or non-adherent, and the surface is rough.
Convective mass transfer controlled corrosion can be reduced by adding high molecular weight polymers, which have the effect of decreasing mass transfer coefficient, by affecting an increase in boundary layer thickness across which the transport of corrosive species and corrosion product occurs to and from the metal surface, respectively. Surface roughness also plays an important role in convective mass transfer, both in the laminar and the turbulent flow region. In the former it increases the effective area and in the latter decreases the both the turbulence and the rate of flow required for its onset.
Phase Transport. Very small amounts of aqueous phase dispersed in an innocuous hydrocarbon phase are a common cause of corrosion problems in most petroleum and petrochemical process industries. Corrosion occurs when the aqueous phase is transported over the metal surface. This takes place preferentially at the bottom surface, whether in a pipeline or a tank The factors which determine the minimum and maximum velocity for transport of water droplets to the surface are: (1) hydrocarbon and water densities, (2) hydrocarbon-water interfacial tension, (3) hydrocarbon viscosity, (4) fluid velocities, (5) pipe or container diameter, (6) water droplet size, and (7) surface roughness, including depressions that capture droplets of aqueous phase.
Factors Influencing Erosion Corrosion
The primary factors which influence this type of corrosion are: (1) material and environmental composition and their interaction, and (2) flow characteristics. The secondary factors that determine the nature of flow are geometry and surface roughness.
Material-Environment Interactions. This determines the nature of surface film. A continuous, hard, dense, impervious, and adherent film on a metal surface provides better protection, is not easily swept away by a flowing stream, and offers resistance to the impact of slurry. These condition occurs in corrosion-resistant materials, such as stainless steel, and more so in super stainless steels, Hastelloys, etc.
Flow Characteristics. Velocity exerts an overriding influence on the behavior pattern of the protective film. Its influence has already been dealt with to some extent. Each system has a particular value of critical velocity below which there is no attack and above which corrosion is rapid as the protective film is swept away. Where suspensions are present, moderately high velocity may reduce corrosion by preventing deposits of slit or dirt, which can cause crevice corrosion and other fouling problems. On the other hand, a very rapid flow of hard solid suspensions has a scouring effect that destroys the protective film.
Turbulent Flow. The greater agitation of the processing fluid produced by this type of flow is such as to cause more intimate contact between it and the metal surface. A well known example of turbulent flow attack is inlet tube corrosion, which is confined to the first few inches at the inlet ends of heat exchanger tubes.
Cavitation Corrosion. Cavitation is a special case of erosion corrosion caused by the formation and collapse of vapor bubbles in high-velocity fluid flow near a metal surface. This produces surface cavities and causes the surface to appear spongy. The bubbles, caused by the conjoint influence of high velocity and geometry of the flow path, which induces hydrodynamic pressure differences in the flowing stream, subsequently collapse with considerable impact at the metal-liquid interface sufficient for plastic deformation of some metals. It also disturbs any protective film that may exist on the metal surface. This type of damage has been observed on water-turbine blades, ship and boat propellers, pump
impellers, pipes carrying fluids at high speed and pressure, water cooled sides of internal combustion engines, etc. Cavitation damage can be prevented or considerably reduced by changing design to reduce hydrodynamic pressure difference in process streams as much as possible; using superior materials of construction, such as stainless steels; coating vulnerable components with resilient coatings of rubber or plastics; operating the pump at a speed and head that minimizes bubble formation.
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