Electrochemical impedance spectroscopy of iron corrosion in H2S solutions

Abstract

Corrosion of iron exposed to H2S saturated solution at pH 4 was studied by electrochemical impedance spectroscopy, weight loss coupons and surface analysis. Hydrogen permeation was also used as indirect means of evaluating the intensity of the proton reduction reaction leading to hydrogen entry into the metal. Since corrosion in this type of test solution results in the rapid build-up of a conductive and highly porous iron sulfide scale, a specific contribution of the film has to be considered. An impedance model was thus proposed. The faradaic anodic impedance consists of a two-step reaction with charge transfer and adsorptiondesorption. An additional contribution, associated with the conductive and highly porous iron sulfide film was added in parallel. This contribution, mostly visible in the 2 low frequency domain, presents a 45° tail associated with a porous electrode behavior. This model was well adapted to describe impedance diagrams measured at various exposure times, up to 620 hours. Charge transfer resistance determined from impedance analysis allowed calculating the evolution with time of the corrosion current density. A very good correlation was found between this corrosion current density and the hydrogen permeation current density. As expected in our experimental conditions, a permeation efficiency close to 100 % is demonstrated. Corrosion rate of 490 µm/year was measured by weight-loss specimens, confirming the validity of the impedance analysis, which resulted in a calculated corrosion rate of 530 µm/year.

Corrosion and Hydrogen Permeation in H2S Environments with O2 Contamination, 1: Tests on Pure Iron at High H2S Concentration

“…EIS analysis required the development of a specific model, with all details provided in a previous work. 16 This model takes into account the build-up of a highly porous and conductive iron sulfide layer, represented by Z film . As a result of a distribution of pore sizes, the impedance of this film presents a dual behavior with two parallel contributions: (i) corresponding to a plane electrode (Equation [5]) and (ii) corresponding to a porous electrode (Equation [6]).…”

“…As shown in a previous paper, EIS diagrams are dominated by the impedance of anodic reactions. 16 As a consequence, a modified version of the Stern and Geary relationship was used, considering only the anodic Tafel coefficient (b a ). A value of 40 mV was used, as usually observed in H 2 S environments: 19 (8) The evolution of corrosion current densities with exposure time are compared in Figure 7 for the experiments conducted with and without O 2 pollution.…”

Electrochemical corrosion behavior of bronze materials in an acid‐containing simulated atmospheric environment

“…The analysis of Bode plots also indicates the existence of two resolved relaxation phenomena . In some cases, two capacitive loops appear, possibly due to the response of the deposited film or the diffusion of oxygen or the response of the corrosion products …”

“…[24] In some cases, two capacitive loops appear, possibly due to the response of the deposited film or the diffusion of oxygen or the response of the corrosion products. [26] The equivalent circuit given in Figure 3 is used to simulate the reaction circuit and fit the EIS data, where R s is the solution resistance, C a and R a are the capacitance and resistance of the surface film respectively, R ct is the charge-transfer resistance, and C dl represents the doublelayer capacitance, As there is a certain deviation between the interface electric double layer and the pure capacitance, n a represents the diffusion coefficient of C a , and the correspondence between n dl and C dl is the same, which reflects the depression in the shape of the capacitive loop. [27] The impedance spectrum parameters fitted according to Figure 2 are shown in Tables 1 and 2, where R ct is a parameter obtained based on the charge-transfer controlled Faraday process and R p is the sum of R a and R ct in the system, [28] which can be used to characterize the corrosion resistance of metals.…”

Study on stress corrosion behavior of C110 steel by electrochemical hydrogenation

“…Combining with figure 7(b) Bode map, it can be inferred that the reaction has two time constants. There is only capacitive arc in each curve, and the formation of capacitive arc may be related to the corrosion product film on the surface of metal matrix or to the double electric layer in corrosive medium [29,30]. From figure 7(a), it can be seen that the radius of capacitive arc loaded with tensile stress is nearly half smaller than that of capacitive arc without stress, and with the increase of stress, the radius is also gradually decreasing.…”