Significance and Use
5.1 In the absence of water, the crude oil is noncorrosive. However, trace amounts of water and sediment have the potential to create corrosive situations during crude oil handling or transport if such materials accumulate and persist on steel surfaces. Test Methods D96, D473, D4006, and D4377 provide methods for determination of the water and sediment content of crude oil.
5.2 The potential for a corrosive situation to develop during the handling and transport of crude oil that contains water can be determined by a combination of three properties (Fig. 1) (1)6: the type of emulsion formed between oil and water, the wettability of the steel surface, and the corrosivity of water phase in the presence of oil.
5.3 Water and oil are immiscible but, under certain conditions, they can form emulsion. There are two kinds of emulsion: oil-in-water (O/W) and water-in-oil (W/O). W/O emulsion (in which oil is the continuous phase) has low conductivity and is thus less corrosive; whereas O/W (in which water is the continuous phase) has high conductivity and, hence, is corrosive (2) (see ISO 6614). The percentage of water at which W/O converts to O/W is known as the emulsion inversion point (EIP). EIP can be determined by measuring the conductivity of the emulsion. At and above the EIP, a continuous phase of water or free water is present. Therefore, there is a potential for corrosion.
5.4 Whether water phase can cause corrosion in the presence of oil depends on whether the surface is oil-wet (hydrophobic) or water-wet (hydrophilic) (1, 3-5). Because of higher resistance, an oil-wet surface is not susceptible to corrosion, but a water-wet surface is. Wettability can be characterized by measuring the contact angle or by evaluating the tendency of water to displace oil from a multi-electrode array by measuring the resistance (or conductors) between the electrodes (spreading methodology).
5.4.1 In the contact angle methodology, the tendency of water to displace hydrocarbon from steel is determined by direct observation of the contact angle that results when both oil and water are in contact with the steel. Although this contact angle is determined by the interfacial free energies of the phases involved, there is no standard method to determine the steel-oil or steel-water interfacial free energies.
5.4.2 In the spreading methodology of determining wettability, the resistance between isolated steel pins is measured. If a conducting phase (for example, water) covers (wets) the distance between the pins, conductivity between them will be high. If a non-conducting phase (for example, oil) covers (wets) the distance between the pins, the conductivity between them will be low.
5.5 Dissolution of ingredients from crude oils may alter the corrosiveness of the aqueous phase. A crude oil can be classified as corrosive, neutral, or inhibitory based on how the corrosivity of the aqueous phase is altered by the presence of the oil. Corrosiveness of aqueous phase in the presence of oil can be determined by methods described in Test Method D665, Guide G170, Practice G184, Practice G185, Test Method G202, and NACE TM0172.
Scope
1.1 This guide presents some generally accepted laboratory methodologies that are used for determining emulsion forming tendency, wetting behavior, and corrosion-inhibitory properties of crude oil.
1.2 This guide does not cover detailed calculations and methods, but rather covers a range of approaches that have found application in evaluating emulsions, wettability, and the corrosion rate of steel in crude oil/water mixtures.
1.3 Only those methodologies that have found wide acceptance in the industry are considered in this guide.
1.4 This guide is intended to assist in the selection of methodologies that can be used for determining the corrosivity of crude oil under conditions in which water is present in the liquid state (typically up to 100°C). These conditions normally occur during oil and gas production, storage, and transportation in the pipelines.
1.5 This guide is not applicable at higher temperatures (typically above 300°C) that occur during refining crude oil in refineries.
1.6 This guide involves the use of electrical currents in the presence of flammable liquids. Awareness of fire safety is critical for the safe use of this guide.
1.7 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
