The Evaluation of the interphase capacitance by impedance technique

Abstract

Examination of the behaviour of alloys in their free-corrosion state and of the thermal stability and effectiveness of organic inhibitors for acid media in restraining the kinetics of corrosive processes IS performed using the impedance technique and the value of the differential capacitance of the metal-solution interphase at the open circuit potential. The physical meaning of this quantity and its relationship with the interphase structure are briefly discussed. The problem of the choice of the frequency interval where experimental measurements should be carried out is examined. Mention is then made of some mathematical methods that are useful for processing experimental data in order to compute the values of R,, Rt and C.The main advantages of performing surface-impedance determinations using the galvanostatic mode and specific software for driving commercial systems and of processing experimental data in real time are stressed. Lastly some experimental results, relating to the frequency interval [7, 131 kHz, are reported which refer to the behaviour of some copper alloys in artificial sea water at pH 8.2, 35 "C and the three velocities of 1.0,1.5 and 2.0 ms-1 and of Armco iron in 5% HCI solutions, containing a commercial inhibitor at various concentrations, at temperatures between 35 and 90 "C. These results demonstrate the validity of the proposed methods, which from practical purposes are equivalent, in the high frequency region where the response of the metal-solution interphase to an external sinusoidal excitation is described with a good accuracy by simulating its behaviour with a simple electric network composed by R and C elements in parallel.Untersuchungen zum Verhalten von Legierungen im Zustand der freien Korrosion und zur thermischen Stabilitat und Wirksamkeit yon organischen Inhibitoren fur saure Medien hinsichtlich der Veranderung der Kinetik der Korrosionsprozesse wurden mit Hilfe der Impedanztechnik und der Differentialkapazitat der Metall-Losung-Grenzflache beim Potential im offenen Stromkreis durchgefuhrt. Die physikalische Bedeutung dieser GroBe und ihre Beziehung zur Grenzflachenstruktur werden kurz diskutiert. Das Problem der Wahl des Frequenzintervalls, in dem die experimentellen Messungen durchgefiihrt werden sollten, wird untersucht. Anschlieaend werden einige mathematische Methoden vorgestellt, die fur die Bearbeitung der experimentellen Daten nutzlich sind, um die Werte von R,, R, und C zu berechnen.Die Hauptvorteile der Durchfiihrung von Oberflachen-Impedanz-Bestimmungen mit Hilfe der galvanostatischen Arbeitsweise und spezifischer Software zum Betrieb kommerzieller Systeme sowie der Aufarbeitung der experimentellen Werte in Echtzeit werden genannt.AbschlieBend werden einige experimentelle Ergebnisse, bezogen auf das Frequenzintervall [7, 131 kHz, vorgestellt, die das Verhalten verschiedener Kupferlegierungen in kunstlichem Meerwasser bei pH 8,2, 35°C und Geschwindigkeiten von 1,0, 1,5 und 2,Oms-I sowie Armcoeisen in 5% HCI Losungen, denen ein handelsiiblicher Inhibitor in verschiede...

The application of the transform of Kramers- Kronig for computing the polarization resistance

“…The value of this root was usually positive. In those cases where this root took negative values the extrapolation method [22] was used.…”

The corrosion-trend monitoring with the impedance technique

“…As regards the characterization of corrosion inhibitors for acid media at different temperatures, the program SOFTCOR-AC-G1 [16], running at present on IBM compatible PC, has been developed. This program drives the EG & G potentiostat mod.…”

“…At last, it is useful to recall that a useful discussion, concerning the use of elementary mathematical tools for processing experimental data, belonging to a high-frequency interval, in order to compute the capacitance C, can be found in [16] where various schemes are reported.…”