Investigations performed in recent years regarding alternating current (AC) corrosion on cathodically protected pipelines are reported and summarized. A model for the corrosion mechanism is shown and the formation of a rust layer is experimentally demonstrated. The use of the coulometric oxidation for detecting the degree of AC corrosion on coupons by oxidizing the Fe(II) in the rust layer is introduced and experimental results from field investigations are presented. The effect of the cathodic protection level on the corrosion rate is shown. In order to obtain data for optimizing the cathodic protection parameters, the corrosion rate was investigated at various on-potentials and interfering AC voltages. Based on the obtained results it was possible to demonstrate that the corrosion rate can significantly be decreased if the off-potential is more negative than-0.85 V vs. copper/copper sulfate (Cu/ CuSO 4) electrode (CSE) and the on-potential is in the range of-1.2 V CSE. Moreover, it was possible to demonstrate that the high AC corrosion rate on coupons can readily be decreased if the cathodic protection level is adjusted. To obtain an understanding of the processes involved and to clarify the influence of alkalinity on the corrosion rate, the effect of cathodic current density on the pH value on the metal's surface of coupons were analyzed with a new in situ pH measurement technique. The effects of pH, spread resistance, and electrochemical reduction are discussed with respect to the experimentally observed corrosion rate.

This paper represents the consensus reached during a panel discussion held at the CEOCOR International Congress in Stockholm, Sweden, June 2–5, 2015. It reviews the state‐of‐the‐art about the generally accepted understanding of the working mechanism of cathodic protection of ground buried steel structures. Based on this, limitations of existing protection criteria are discussed, particularly the difficulties related to measuring and interpreting current‐applied potentials, instant‐off potentials, and IR‐free potentials. A set of protection criteria and their interpretation in accordance with the understanding of the working mechanism of cathodic protection is proposed. The main idea is that it is distinguished between the following two situations: 1) conditions that permit an increase in pH at the steel surface as a result of the applied protection current (good bedding conditions inhibiting convection) and thus provide corrosion protection by achieving passivity; and 2) situations where this is not possible (e.g. in streaming soft water) or where it is unknown if this is possible, and thus effective corrosion protection can only be achieved by polarizing the steel into the immunity zone.

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An Umhullungsfehlstellen von kathodisch geschutzten erdverlegten Rohrleitungen wird bei Korrosionsschaden haufig die Anwesenheit sulfatreduzierender Bakterien (SRB) festgestellt. In Laborversuchen wurde das Korrosionsverhalten von Stahl St 37 im Potentialbereich (-03 V > Uc,,cuso, > -l ,27 V) und beim Ruhepotential in Gegenwart einer SRB-haltigen Mischkultur untersucht. Zur Simulation realer Verhaltnisse wurden die Versuche an Stahlproben in sandgefiillten Saulen durchgefiihrt, die kontinuierlich mit einer glucosehaltigen Mineralsalzlosung durchstromt wurden.Aufgrund der ermittelten Abtragungsgeschwindigkeiten war fur die homogene Mischelektrode selbst bei freier Korrosion eine unmittelbare Korrosionsgefiihrdung durch SRB nicht abzuleiten. Es trat lediglich ein erhohter Schutzstrombedarf im Vergleich zu sterilen Bedingungen auf. Die Wasserstoffpermeation durch den niedriglegierten Stahl war in Gegenwart SRB-haltiger Mischkulturen gegenuber sterilen und H,S-freien Bedingungen leicht erhoht. Aus den ermittelten Permeationsstromdichten liel3 sich ableiten, da8 bei Potentialen Ucu,cuso, < -0,85 V unter sulfatreduzierenden Bedingungen nicht mit einer wasserstoffinduzierten Werkstoffschadigung zu rechnen ist.Sulfate-reducing bacteria (SRB) are frequently encountered at coating defects of cathodically protected pipes buried in soil. In laboratory experiments the corrosion behaviour of steel St 37 was studied at potentials in the range of -0.5 V > Uc,,cuso, >

Correct diffusion measurements using electrochemical methods must be carried out under careful consideration of the boundary conditions. — It is shown that the diffusion of recombined H‐atoms from the metal surface into the electrolyte does not seem to be the main reason for a loss of hydrogen concentration, but the presence of oxidizing species (generally oxygen), solved in the electrolyte, which lead to an enhanced electrochemical oxidation of hydrogen. In order to avoid this source of systematic errors, deaerated H2SO4 was used for measuring time lag curves of hydrogen diffusion in palladium. Measured time lag curves, obtained by the current pulse method, agree very well with theoretical curves, evaluated from the solution of Fick's second law. — When determining diffusion coefficients by measuring the break‐through time one has to take into account the ratio of the amount of hydrogen, discharged on the entry side of the diffusion foil, to the content of hydrogen within the sample from precharging runs.