Effectiveness of Corrosion Inhibitors in Retarding the Rate of Propagation of Localized Corrosion

• Published in: Corrosion (Houston, Tex.) Tex.), 2003-03, Vol.59 (3), p.250-257
• Main Authors: Turnbull, A., Coleman, D., Griffiths, A.J., Francis, P.E., Orkney, L.
• Format: Article
• Language: English
• Subjects: Applied sciences; Carbon dioxide; Contamination; Cooling; Cooling rate; Cooling water; Corrosion; Corrosion environments; Corrosion inhibitors; Corrosion prevention; Corrosion tests; Exact sciences and technology; Growth kinetics; Imidazoline; Inhibitors; Kinetics; Localized corrosion; Metals. Metallurgy; Oil production; Petroleum production; Propagation; Retarding; Uniform attack (corrosion)
• ISSN: 0010-9312; 1938-159X
• DOI: 10.5006/1.3277558

ABSTRACTIn many service applications, excursions in solution chemistry, temporary loss of inhibitor, or transient increases in temperature may give rise to localized corrosion in an otherwise inhibited system. It is important to demonstrate that inhibition will be effective in retarding the rate of propagation of localized corrosion when normal conditions, appropriate to the prevention of general corrosion, are restored. To test this requirement, the use of a ?pencil? type of artificial pit has been investigated using simple simulations of a cooling water and of an oil production formation water. The results demonstrate the effectiveness of this technique and show that nitrite, as an example of a cooling water inhibitor, can be effective in retarding the rate of propagation with the appropriate dosage. Tests in simulated oil production formation waters at 50 ?C demonstrated that an imidazoline-based inhibitor can decrease pit growth kinetics in nearly neutral carbon dioxide (CO2)-saturated solution but does not appear to be so effective with oxygen contamination of that solution or in mildly acidic CO2-saturated solution. INTRODUCTIONInhibitors for carbon steel (CS) are used in a wide range of applications, such as oil pipelines, domestic central heating systems, industrial cooling water systems, and metal extraction plants. Localized corrosion may initiate due to transient changes in the environmental conditions; for example, an increase in the chloride ion concentration, a fall in the level of inhibitor, or an increase in temperature. The requirement is to constrain propagation of corrosion when normal conditions are restored. At present, there is no standard for testing the effectiveness of inhibitors in retarding the rate of localized corrosion propagation once damage has developed. The recently published ASTM standard,1 G170-01, although excellent in considering a wide range of test methods for inhibitors, does not cover this issue. In a recent study,2 various methods including galvanostatic pre-pitting, a ?pencil? artificial pit, a differential flow method, and a sandwich crevice arrangement were evaluated.Of these, the pencil artificial pit proved most promising. The outline of the method is as follows. A two-compartment cell is used with an agar salt bridge to prevent solution mixing. One compartment contains a rotating cylinder electrode immersed in the inhibiting environment. A rotating electrode is used to increase the cathodic current density w