Alternating Current Potential Drop Technique to Detect Pit-to-Crack Transition
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Alternating Current Potential Drop Technique to Detect Pit-to-Crack Transition Xuejun Huang1, Lun Yu1 and Ronald Ballinger1 1 H.H. Uhlig Corrosion Laboratory, MIT, MA 02139, U.S.A ABSTRACT Pitting and localized corrosion can occur under various conditions and accelerate the subsequent cracking and the failure of a component. Pit-to-crack transition is considered a critical transition process and has a significant impact on the total lifetime of a component. In this study, an alternating current potential drop (ACPD) system is built and applied the detection of the pit-to-crack transition. The results show that the ACPD system is capable and sensitive to crack initiation and that the sensitivity increased with increased frequency. Crack initiation sites from pits are characterized and discussed. INTRODUCTION Pit-to-crack transition Pitting corrosion is a form of localized corrosion in which small, separated, areas are penetrated more than other areas [1]. For materials that form a passive film (e.g. stainless steels, aluminum alloys), pitting corrosion initiates due to the breakdown of the passive film. For materials which do not form a passive film under certain environments (e.g. carbon steel in neutral pH seawater), pitting corrosion is not well defined and is usually related to the formation of a corrosion product layer on the metal, which is not fully protective, or only partially protective [2]. Pitting may encourage crack nucleation due to the increased stress/strain intensification owing to the pit size and geometry and the aggressiveness of the local chemistry (e.g., acidic pH, high concentration of aggressive anions) [3], and significantly reduce the lifetime of the component [4]. Thus it is of value to detect pit-to-crack transition and study the underlying mechanism for crack initiation from pits. Kondo [5] attempted to predict the pit-to-crack transition in corrosion fatigue by introducing a critical stress intensity factor range due to the presence of the pit (ΔK)p above which cracks would initiate. In his model, the stress intensity factor for a critical pit was calculated using fracture mechanics by assuming that the pit is a sharp crack. The conclusion of his analysis indicated that when the fatigue crack growth rate exceeds the pit growth rate at a the critical pit-to-crack transition condition (ΔK)p, a crack will initiate from the pit. Kondo’s model provides insights into the pit-to-crack transition process. However, adopting an LEFM parameter, such asο, as the point where the pit-crack transition occurs is not considered appropriate because this parameter is derived from long crack studies [6]. Also, ΔK is an elastic parameter, which may be inappropriate considering the microplasticity generated by the pit.
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