EXPERIMENTAL AND NUMERICAL INVESTIGATIONS OF FLOW-ACCELERATED CORROSION DOWNSTREAM ORIFICES

Thesis Overview

INTRODUCTION1.1 BackgroundCorrosion is the degradation of materials by means of chemical or electrochemical reactions, occurring at the material surface, with the environment. The corrosion pattern can be categorized into either uniform corrosion or localized corrosion, depending on the resulting surface morphology. The first type involves uniform material loss from the surface while the second occurs at local points or areas on the surface.Many if not most cases of corrosion processes involve some relative motion between the corroding material surface and the environment. The rate of such motion and the type of corrosion mechanism depend on the exact nature of the environment (air, soil, water, etc.) in which the corrosion takes place. Corrosion due to electrochemical reaction involves mass transfer from the corroding material to the environment, which is usually a solution. An example of this type of relative motion is seen in the case of carbon steel corrosion in an aqueous environment, in which ferrous ions (Fe2+) are released into water at the (concentration) boundary layer where their concentrations become supersaturated.The rate of release of the ferrous ions is increased if the aqueous environment is flowing.This type of corrosion mechanism is known as Flow-Accelerated Corrosion (FAC).1.2 Flow-Accelerated CorrosionFAC is a form of corrosion that results in general reduction of wall thickness of carbon steel or low-alloy steel piping or fittings, by flowing water or wet-steam mixture over the surface. FAC frequently occurs over a limited area within a piping system where local high turbulence exists. The wall thinning occurs due to the mass transfer of dissolved corrosion products near the inner wall into the bulk of flowing fluid. This explains the two major processes responsible for FAC. The first process involves chemical reaction, which occurs at the metal surface, while the physical removal of dissolved corrosion products to the bulk fluid takes place through a flow dynamics process as explained by Uchida et al. [1]. They also explained that the chemical process is initiated by the existence of electrochemical potential difference between the metal surface and the bulk fluid, which leads to the formation of the normally protective magnetite (Fe3O4) film over the inner surface of the pipe. Then, the existence of concentration gradient between the protective film and the bulk fluid initiates the dissolution process of the film into the bulk fluid. This dissolved magnetite is carried away by the flow dynamics process, which creates potential for more magnetite formation followed by more dissolution, until equilibrium film thickness is attained where the dissolution rate equals the convection rate.