THE EFFECT OF A SUDDEN FLOW EXPANSION ON TRANSPORT PHENOMENA AND CORROSION PROCESSES

Toni  Y. RIZK

This study investigates the transport phenomena, corrosion and inhibition in the first portion of a 9.5 equivalent pipe diameter length beyond an axisymmetric pipe sudden expansion. A flow rig, with a total tubing length of more than 20m, was designed and constructed for the purpose of simulating the hydrodynamic conditions in sudden expansion. Electrochemical tests were performed under turbulent flow conditions at 20,000≤Re≤100,000. Two different electrode/electrolyte combinations were used in the present work.

For the mass and momentum transfer studies, a combination of nickel electrodes with an equilibrium electrolyte of 5x10^-3 M of ferri-errocyanide in 0.5 M of sodium hydroxide was used. The electrochemical Limiting Diffusion Current Technique (LDCT) was implemented based on the cathodic reduction of the ferricyanide Icons and the supplementary oxidation of the ferrocyanide ions. In this part of the study, two different electrodes were prepared. A small local cathode of 0.5 mm diameter surrounded by a much larger active cathode was used for the mass transfer measurements. Dual cathodes, each of 1.0 mm² of area and electrically isolated, were used for the shear stress and turbulence evaluations.

For the corrosion and inhibition studies, polyphosphate, which is known commercially as calgon or hexametaphosphate, was used at an equilibrium concentration of 2x10^-3 M with zinc species dissolved in 0.5 M of sodium chloride and mild steel electrodes. The system was examined using in-situ electrochemical measurements e.g. potentiodynamic polarization, LPRM and electrochemical impedance, and after exposure by non-electrochemical techniques e.g. SEM/EDX and X-ray diffraction.

It was found that the sudden expansion enhances mass transfer and substantially affects the shear stress. A comparison of the calculated turbulence intensity profile with the observations of the fluctuations of the current from its mean value, provided a new insight into the significance of turbulence intensity in determining mass transfer rates. Empirical relationships are derived involving dimensionless parameters, for example Sherwood, Reynolds and schmidt numbers, and the expansion ratio.

The results obtained with the corrosion inhibitor were compared with the work of other investigators who previously studied corrosion enhancement in sudden expansion. The effect of flow rate and the exposure time of the metal with build up of corrosion product film in the flowing solutions were examined. Information about the corrosion processes encountered in this study was evaluated for the presence of the inhibitor species.