The impact of salts and various ions on oil well pipes

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Dissolved salts in oilfield produced water have a significant impact on the corrosion of oil wells. Corrosion caused by acidic salts (salts that produce an acidic solution upon hydrolysis) is mainly due to hydrogen depolarization; such salts include AICl3, NiSO4, MnCl2, and FeCl2. The presence of Ca2+ and Mg2+ ions increases the mineralization and ionic strength of the solution; generally, under identical other conditions, these two ions exacerbate the severity of local corrosion. At low concentrations, HCO3 ions promote corrosion; the mechanism lies in the fact that HCO3 can act as a cathodic depolarizer ； At high concentrations and in the presence of C1-, HCO3 leads to localized corrosion ； HCO3 can not only be converted into CO2, but also dissociates to produce H+ and CO3-2; the former accelerates corrosion, while the latter forms scale with Ca2+. When the concentration of Fe2+ in the solution is high, the permeability of the membrane is higher, and the rate at which the membrane grows exceeds the rate at which it dissolves, allowing the membrane to continue growing. Therefore, when the concentration of Fe2+ in the solution is high, the corrosion rate increases significantly. C1- ions in the medium promote local corrosion of metals or alloys such as carbon steel and stainless steel. In chloride environments, iron and its alloys can suffer from pitting corrosion; the presence of chloride ions accelerates the corrosion process of metals. When the C1- concentration is high, it leads to the spread of general pitting in the anodic area. On the other hand, due to their small size, C1- ions can easily penetrate the protective film, thereby intensifying corrosion and causing local corrosion. As the C1- concentration increases, the pitting potential shifts negative, indicating that as the concentration of aggressive ions rises, the stability of the passivation film on the steel surface decreases. Therefore, C1- is the anion that has the greatest impact on the corrosion of carbon steel. Generally, the main factors affecting chloride corrosion include alloy composition, medium temperature, medium flow rate, and the type of deposited salts. Carbon steel and low-alloy steel tend to have high rates of chlorination corrosion, while A1-containing nickel-based alloys exhibit good corrosion resistance under chlorinated conditions ； In oil production systems, the surfaces of structural materials are often covered with chlorides and sulfates of alkali metals (Na, K), alkaline earth metals (Ca, Mg), etc. These deposited salts may react with oxide films to form ferrites, thereby exacerbating corrosion ； Generally, as the temperature rises, chlorine corrosion increases, but above 300°C, the corrosion rate of stainless steel actually decreases.