In this series of papers, AC induced corrosion of steel pipe under cathodic protection was investigated. In this Part III, the most realistic model was devised, i.e., the cathodic current was composed of the reductions of dissolved oxygen partly controlled by diffusion and of the water following the Tafel law. The cathodic protection potential necessary to mitigate the corrosion for various AC amplitudes were evaluated. In this modeling approach, no change of soil properties due to AC corrosion is considered. Mean DC values of corrosion potential shift and the corrosion current density in presence of AC signal were calculated in reduced scale making possible field applications by introducing an appropriate set of corrosion kinetics parameter collected without AC signal. In a highly corrosive soil, the reduction of dissolved oxygen is slow compared with that of the water reduction, and then the corrosion behavior becomes similar to the case examined in Part II.

AC corrosion enhancement of buried steel pipe is induced by high stray AC electromagnetic field and to the faradaic rectification due to non-linearity of current-potential characteristics. In Part I, this phenomenon was evaluated by neglecting the electrical resistance of soil electrolyte. In this part, we will examine when this resistance is no longer negligible. The mathematical model can be expressed by an ordinary nonlinear differential equation. In this part, the corrosion mechanism is modelled by two reactions obeying Tafel law, one for anodic dissolution of steel and another for cathodic process. This corrosion mechanism is valid when the corrosion current density is much higher than the diffusion limited current of dissolved oxygen. Compared with the system where the electrolyte resistance is negligible, the corrosion enhancement induced by AC signal is markedly attenuated because of the ohmic drop through the electrolyte resistance, and the double layer capacitance constitution a bypass circuit.

Tubulações subterrâneas protegidos por um revestimento grosso e por polarização catódica podem sofrer danos externos sérios por corrosão na presença de tensão de corrente alternada (AC) dispersa induzida por sistemas industriais de transporte elétrico de alta tensão, como linhas de tensão ou ferrovias eletrizadas. A origem do aumento da corrosão vem da não-linearidade das características corrente-potencial da interface metal-solo. Neste trabalho, avaliaremos teoricamente o aumento da densidade de corrente de corrosão e o deslocamento de potencial induzido por um sinal AC de alta amplitude a modelos de sistemas sofrendo corrosão: curvas de polarização anódicas obedecendo uma lei exponencial em relação ao potencial, e processo catódico sob a cinética de ativação-difusão mista. A originalidade do presente trabalho se encontra no uso de um número relativamente pequeno de variáveis sem dimensão para descrever a retificação faradaica no deslocamento do potencial e no aumento da corrente de corrosão. Neste artigo, o efeito da resistência do eletrólito foi negligenciado.

Underground pipelines protected with a thick coating and by cathodic polarisation may suffer a serious external corrosion damage in the presence of stray alternating current (AC) voltage induced by high voltage industrial electric transport system, such as power lines or electrified railroads. The origin of the corrosion enhancement comes from the nonlinearity of the currentpotential characteristics of the metal-soil interface. In this paper, we will theoretically evaluate the increase of the corrosion current density and the potential shift induced by a high amplitude AC signal to models of corroding systems: anodic polarisation curves obeying an exponential law with respect to the potential, and cathodic process under the mixed activation-diffusion kinetics. The originality of the present work lies in the use of a relatively small number of dimensionless variables to describe the faradaic rectification for the corrosion potential shift and the corrosion current enhancement. In this article, the effect of the electrolyte resistance was neglected.