An empirical formula derived from two sets of field data on tubing corrosion gives a satisfactory description for two different oil fields of the influence on corrosion of the API gravity of the oil and its watercut. A remarkably good level of agreement was found between predicted corrosion rates using this formula and field corrosion measurements. It reproduces the general concept that heavier oils are more protective than light ones, and that very light oils give hardly any protection at all. It also reflects the likelihood of various modes of corrosion associated with competitive wetting of the steel by water and oil arising from different modes of water entrainment. The link between API gravity, emulsion stability and water wetting of steel by an oil-water mixture is provided by considering the changes in interfacial tensions in the oil-water-steel system.
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