ABSTRACT

Corrosion inhibitors are known to adsorb to a variety of corrosion product layers. Under conditions of high shear, the fracture of corrosion product scale is one important aspect affecting corrosion. In this paper, molecular mechanics is used to calculate the Young's Modulus of a ferrous carbonate scale. This information is used with existing theory for the fracture mechanics of corrosion product scales to understand the destruction of corrosion byproduct layers under conditions of high shear.

INTRODUCTION

Measured uninhibited corrosion rates for mild steel in water containing carbon dioxide increase with temperature to a maximum near 200 °F and then fall to lower values. Conditions of higher flow rate can increase the corrosion rate of mild steel at a given temperature. The increase of corrosion rate with flow rate for uninhibited systems is associated with two major effects namely 1) increased mass transport of corrosive species to the pipe wall surface and 2) destruction of corrosion byproduct layers.

The destruction of corrosion product by flow induced corrosion has been seen in flow loop experiments with visual techniques. Studies in flow loop s and jet impingement apparatus using conditions representative of both sweet and sour environments have shown that there is a critical shear stress at which corrosion rates will dramatically increase. Corrosion rates of mild steel have been observed to increase in multiphase slug flow from full pipe flow with a reduction in corrosion product thickness in 1o.ll the observed coupons.

The role of FeCO3 as a protective layer retarding CO2 corrosion has been considered by several workers. The rate of precipitation of FeCO3 is a function of the supersaturated concentrations of Fe 2+ and CO32- ions in solution and is kinetically controlled. The level of protection that can be created with the corrosion product depends upon the formation of the layer, the thermodynamic stability of the layer, its mechanical adherence to underlying metal and the ability to repair damaged scales. In conditions of high shear stress, high corrosion rates have been observed in systems that form FeCO3. TM The adherence of corrosion product to underlying metal has been investigated in recent work. The scales are ceramic materials and do not have the ductility of the original metals). The growing scales are filled with irregularly spaced defects. A fracture mechanical theory of the adherence of an oxide scale to a metal has been developed. 192° The mode of failure of the composite scale and metal system was related to tensile loading, strain, surface fracture energy and Young's modulus.

In cases of flow-induced corrosion, inhibitors have been observed to raise the critical shear stress at which corrosion rates dramatically increase. Components in an effective corrosion inhibitor formulation can serve several roles. One of which is adsorption onto an active anodic or cathodic site that can "poison" or block the corrosion reaction. Adsorption onto a porous corrosion product layer can make the pore hydrophobic minimizing transport of corrosive reactants. In earlier work using molecular modeling, the adsorption of corrosion inhibitors to magnetite and siderite surface was studied. Magnetite is an electrically conductive oxide located above cathodic reaction sites. In this paper, theories of fracture mechanics will be employed with molecular modeling to construct a porous FeCO3 scale containing water and corrosion inhibitor. The onset of scale cracking caused by mechanical forces will be calculated as a function of Young's modulus, porosity, and inhibitor.

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