Fluid Corrosion Localized

The corrosion conditions can be different at the fluid line from the bulk condition. Aqueous liquids have a concave meniscus, which creates a thin film of liquid on the vessel wall immediately above the liquid line. Some corrosion processes, particularly the diffusion of dissolved gases, are more rapid in these conditions. Additionally, the concentration of dissolved gases is highest near the liquid surface, especially when agitation is poor. Locally high corrosion rates can therefore occur at the liquid line, leading to thinning in a line around the vessel. This effect is reduced if the liquid level in the vessel varies with time. Any corrosion tests undertaken as part of the materials selection procedure should take this effect into account. 

Additionally, specific environmental conditions can induce localized corrosion such as temperature, conductivity of the corrosive fluid, or thickness of the liquid corrosive film in contact with the metal. In some cases, both metallurgical and geometric factors will influence behavior, such as in stress-corrosion cracking. Preferential weldment corrosion of carbon steels has been investigated since the 1950s, commencing with the problems on icebreakers, but the problem continues today in different applications. (Bond)5... 

Fig. 1. Interrelationships between the fluid flow and localized corrosion phenomena.

In developing the arguments that are presented later in this review, it is necessary to keep in mind the relative scales (dimensions) at which each phase occurs. This is important because the effect of flow on localized corrosion is largely (though not totally) a question of the relative dimensions of the nucleus and the velocity profile in the fluid close to the surface. However, the velocity profile is a sensitive function of the kinematic viscosity, which in turn depends on the density and the dynamic viscosity. Because the kinematic viscosity of water drops by a factor of more than 100 on increasing the temperature from 25 °C to 300 °C, the conclusions drawn from ambient temperature studies of the effect of flow on localized corrosion must be used with great care when describing flow effects at elevated temperatures. 

All of the reactions in acidic and basic solutions are generally controlled by diffusion of the reactant through the boundry layer existing on the exposed surfaces of the aggregate or bond phase. Elevated temperatures usually increase the reaction rate. Thus, elevated temperatures and high local fluid velocities tend to increase the corrosion rate of silicon carbides as the corrosion products are swept away from the active surface sites. 

Heat transfer is a particular promoter of the development of local chemistries that are very different from the bulk fluid chemistry, in particular, where phase changes occur. Early condensates from acid gas/vapor streams can be very concentrated and corrosive relative to bulk condensates as in the cases of carbonic acid from steam, sulfuric acid from flue gases, and hydrochloric acid from refinery overhead streams. Initially, benign condensates or cooling fluids can concentrate due to intermittent contact with surfaces hot enough to promote concentration or dryout, as in the... 

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