Mo-Mediated Corrosion Behaviour of 1 Cr - Carbon Steel in Sweet Medium Under Hydrodynamic Control

ABSTRACT

The effects of micro-alloying of plain carbon steel with Mo (0.7 wt.%) in the presence of 1 wt.% Cr on the corrosion behaviour and scale protectiveness in CO₂ saturated (sweet) brine (0.5 M NaCl) environment, under hydrodynamic conditions, at 80°C in a slightly acidic environment (pH 6.6) were investigated. Potentiostatic current transients suggest that there exists a synergistic interaction of Cr and Mo, which induces more rapid scale crystallization compared to the Mo-free steel. The presence of Mo also suppressed the current passing through corrosion scale. SEM images suggested that 1Cr.0.7Mo steel induced formation of thinner scale with better protectiveness compared to their non-Mo counterparts. From the mechanistic perspective, we suggest that the addition of small amounts of Mo induces formation of a crystalline scale at short times and then it accelerates the growth of that crystalline layer, by modifying the local environment at the steel surface. Modeling of this hypothesis is currently in progress.

INTRODUCTION

The development of high durability and low cost materials able to operate in a broad range of increasingly aggressing exposure conditions is critical for the oil and gas industries. Of the practical exposure conditions in the oil-field and pipelines, acidic pH (constituted of aqueous CO₂ and/or H₂S) and region specific elevated temperatures are common. In these environments scales are typically formed on the surface of the steel - sulfide or carbonate based; these scales can have varying degrees of protectiveness to the steel surface. The steel can potentially be made more durable therefore, by enhancing the formation of a protective, adherent, non-porous crystalline scale on the surface of the corroding steel. This type of protective scale could in principle be encouraged to form by controlling the most important exposure conditions such as pH and temperature: however as temperature is more or less fixed for a particular location, it is hard to find any practically feasible technique of controlling the exposure temperature. However, if one could control local interfacial pH to be modulated at the surface of the steel, thus enabling precipitation of a protective crystalline scale may be induced. One of the most interesting techniques that can be considered for modifying the local pH is micro-alloying: i.e. incorporating small amount of specific element in a base materials in purpose of achieving the desired properties.