A Hydrogen Effusion Method for the Determination of Corrosion Rates in Aqueous Systems at Elevated Temperature and Pressure

Abstract

This paper analyzes the limitations of weight change techniques at elevated temperature and pressure, and presents a description of a new technique based on the measurement of the hydrogen generated by the corrosion reaction. This H, which diffuses through the walls of a test specimen containing the aqueous solution, is collected and measured in a vacuum system of known volume which surrounds the test specimen; the corrosion rate is calculated from the hydrogen effusion rate. Results obtained with the technique show good reproducibility and compare well with results obtained by an independent method.Corrosion rates of metals in aqueous solutions at elevated temperature and pressure have been obtained almost exclusively by weight gain or weight loss measurement of specimens subjected to corrosion in autoclaves of various types (1-4). Such measurements are inherently difficult. Loss of corrosion products by spalling or dissolution introduces serious errors into weight gain data. Weight loss data are reliable only if complete removal of corrosion products without attack on the uncorroded metal beneath can be obtained. Galvanic effects between autoclave and specimens may affect the results obtained if the two differ in composition or treatment history prior to the test. One of the most serious limitations is that from a single specimen only a single value of total corrosion can be obtained from which only an "average" corrosion rate over the period of exposure can be calculated. In the usual case, however, where corrosion resistance depends on the type of protection provided by corrosion product films, such average rates are misleading. The information needed in such cases is the differential corrosion rate. Using weight gain and weight loss methods, differential rates can be obtained only by corroding a large number of samples under the given conditions for varying times, the rates being obtained by differences between the gains or losses of the specimens at the different time intervals. Since reproducibility from sample to sample determines the accuracy of any rates calculated by difference, and since reproducibility using these techniques is usually not good, a large number of samples must be run simultaneously to obtain dependable average values for such calculations. These limitations of weight gain and weight loss methods are particularly serious when the corrosion rates are low.To overcome the difficulties enumerated, a method was developed in which there is no error due to loss of oxide or of metal in the measuring process, the sample constitutes its own autoclave, samples need not be destroyed in the process of measurement, and observation can be continuous on a single sample so that a differential corrosion rate can be determined using only one specimen. Theoretical ConsiderationsExamination of the reaction Metal ~-H~O-~ Metal Oxide + H.~ (I) indicates four possible methods of measuring corrosion rate, i.e., measurement of (a) metal consumed, (b) water consumed, (c) metal oxide formed, and (d) hydrogen form...

Steel Corrosion Mechanisms

“…Berl and Van Taack, who heated small samples of steel powder in a static autoclave for 7.5 hr at 310~ obtained data indicating a minimum in the corrosion rate at a concentration of 0.7 g/1 which corresponds to a pH of 12.24. Some measurements at 316~ by the authors using a more precise static technique, the hydrogen effusion method (9), showed a drop in total corrosion in the region of pH 12.24, and more recent measurements (Table I) confirm the actual minimum in total corrosion at this pH as reported by Berl and Van Taack. Both of these recent studies, however, show that the phenomenon is transitory, and after 25 days the corrosion rate increases monotonically with pH (16).…”

“…The hydrogen effusion technique employed has been previously described (6,9). It uses as a specimen a small metal capsule filled with the corrosive medium.…”

Growth Characteristics of Iron Oxide Films Generated in Dilute Lithium Hydroxide Solution at 300°C

“…Experimental Most of the oxide films examined were formed on the interiors of sealed mild steel capsules filled completely with pH 11 LiOH aqueous solution following the technique devised by Bloom and Krulfeld (10), and heated at 300~ in a circulating air oven. The periods of heating ranged from 30 min to 12 days.…”

Strain/Oxidation Interactions in Steels and Model Alloys

“…This view is also strengthened by Pearse and Afzulpurkar's (73) continuous measurements of corrosion rates by the hydrogen effusion technique (74); these showed a series of peaks in corrosion rate of almost equal magnitude, suggesting that extensive exfoliation of thc 23. This view is also strengthened by Pearse and Afzulpurkar's (73) continuous measurements of corrosion rates by the hydrogen effusion technique (74); these showed a series of peaks in corrosion rate of almost equal magnitude, suggesting that extensive exfoliation of thc 23.…”