Using an Inhibitor to Combat Erosion-Corrosion in Carbon Steel Piping - How Much Does it Help?
Authors: Shadley J.R. , Shirazi S.A. , Dayalan E. , Rybicki E.F. 
Published: October 2008

Abstract
When sand, water, and COZ gas are produced along with other production fluids, erosion-corrosion can occur in carbon steel piping and fittings, Metal loss rates in erosion-corrosion can be extremely high. The term 'threshold velocity' is sometimes used to define a limiting flowstream velocity above which erosion-corrosion takes place. Threshold velocities depend on a number of factors such as temperature, solution composition (including PI-I), sand size, and pipe size. Sometimes desired production velocities are above threshold velocities and costly erosion-corrosion is a risk. But, in some cases, using a corrosion inhibitor can prevent erosion-corrosion damage by shifting threshold velocities to above production velocities. This paper describes how a laboratory flow loop and an erosion prediction model have been used to quantify the effect that using a corrosion inhibitor can have on the threshold velocity. Several examples of how the results of this research can be applied to oilfield problems are provided.

Introduction
The process termed 'erosion-corrosion' involves both erosion and corrosion mechanisms. In this paper, the term is applied to a process in an environment/metal system in which a protective corrosion product scale can form on the surface of the metal. At locations where the protective scale is prevented or removed by the erosive effects of the flowing medium, corrosion is again enabled at bare metal loss rates. Ellison and Wen argued that erosion-corrosion is generally most severe in two-phase (gas-liquid, or liquid-solid) flows or multiphase flows. In single-phase flows, only shear stresses of rather small magnitude can be impressed on the wale covering the pipe wall. By contrast, in two-phase flows, stresses on the scale can be quite high when a solid particle or slug of liquid (in gas-liquid flows) impinges the wall. According to Levy2 in elevated temperature air streams, such as those occurring in fossil fuel combustion chambers, the morphology of the corrosion product scale that forms on carbon steel surfaces has a major effect on the loss rate of the material. Removal of scale can take place under both ductile and brittle erosion mechanisms depending on the characteristics of thescale and of the particles.
For two-phase liquid/particle flow, Lot recognized that sand may cause severe erosion and erosion-corrosion attack in the production of oil and gas, and suggested an erosion rate, of the form:
R= Constant .Vn. .. .. .. .. ... .(1)
where V is the flow velocity, and the power n is in the range of 1 to 3 depending on whether corrosion (closer to 1) or erosion (closer to 3) is the rate controlling process.
In erosion-corrosion where the erosion component is provided by the presence of solid particles in the flow, the efficiency of the particles in removing the protective corrosion product scale plays an important role in determining the metal loss rate. Shirazi, et al, developed a procedure for predicting erosion rates in piping systems.