Abstract
The Cl−-induced corrosion of metals and alloys is of relevance to a wide range of engineered materials, structures, and systems. Because of the challenges in studying pitting corrosion in a quantitative and statistically significant manner, its kinetics remain poorly understood. Herein, by direct, nano- to micro-scale observations using vertical scanning interferometry (VSI), we examine the temporal evolution of pitting corrosion on AISI 1045 carbon steel over large surface areas in Cl−-free, and Cl−-enriched solutions. Special focus is paid to examine the nucleation and growth of pits, and the associated formation of roughened regions on steel surfaces. By statistical analysis of hundreds of individual pits, three stages of pitting corrosion, namely, induction, propagation, and saturation, are quantitatively distinguished. By quantifying the kinetics of these processes, we contextualize our current understanding of electrochemical corrosion within a framework that considers spatial dynamics and morphology evolutions. In the presence of Cl− ions, corrosion is highly accelerated due to multiple autocatalytic factors including destabilization of protective surface oxide films and preservation of aggressive microenvironments within the pits, both of which promote continued pit nucleation and growth. These findings offer new insights into predicting and modeling steel corrosion processes in mid-pH aqueous environments.
Highlights
Pitting corrosion is a damaging form of localized metallic corrosion that has been a subject of research for several decades[1,2]
electrochemical scanning tunneling microscopy (ECSTM) can only probe a small field of view (FOV) on the order of a few square microns, whereas atomic force microscopy (AFM) is unable to image sharp and deep pits (>10 μm) due to its limited vertical range
At t0, the distribution was characterized by a sharp peak centered at 0 μm. This ‘peak’ persisted even at later times indicating that the majority of the surface featured a largely uniform, unchanging topography across the FOV, and that only localized domains on the surface showed substantial changes that are characteristic of pit formation[8]
Summary
Pitting corrosion is a damaging form of localized metallic corrosion that has been a subject of research for several decades[1,2]. Electrochemical scanning tunneling microscopy (ECSTM) has been used to study oxide layer removal and pitting initiation on metal surfaces[17,18], whereas atomic force microscopy (AFM)[19] has been utilized to observe the morphology of single pits. An electrochemical surface forces apparatus (EC-SFA) has been used to resolve corrosion pits in confined spaces in situ, and in the presence of Cl− ions and applied potential, albeit with restrictions in the nature of geometries that can be examined[20] To overcome these limitations, vertical scanning interferometry (VSI) is used for the first time to study the initiation and development of pitting corrosion. To demonstrate this new approach of investigating pitting corrosion, an AISI 1045 steel was exposed to deionized (DI) water and Cl− containing solutions to gain quantitative insights into surface morphology evolutions as corrosion proceeds and to clarify the effects of aqueous Cl− species on amplifying corrosion activity
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