Fractal behavior of surface oxide crack patterns on AISI 4140 high-strength low-alloy steel exposed to the simulated offshore environment

Abstract

Wear and corrosion are surface-dependent degradation phenomena pertinent to the hostile environment existing in deep-sea oil and gas wells containing aggressive chemical species such as chloride/bromide salts, organic acids, carbon dioxide, and hydrogen sulfide,etc. Apart from the identification of the potential failures of alloys that can occur on a large and devastating scale by its interaction with these deleterious species, it is also essential to comprehend the nature and character, particularly the cracking pattern of the surface oxides formed on alloys undergoing corrosion in a particular environment. This article uses fractal geometry to study the branching of the surface oxide crack patterns in AISI 4140 high-strength low-alloy steel after exposure in a simulated offshore corrosive environment containing aqueous 3.5 wt.% sodium chloride solution. The fractal geometry was evaluated using the box-count technique in three different electrolytic test conditions: Electrochemical Anodic Polarization, Electrochemical Cathodic Polarization, and Immersion Corrosion for 24 h. Our results showed that the magnitude of fractal dimension is highest during immersion corrosion for 24 h and lowest during anodic polarization. Fractals can provide complementary quantifiable information about the surface geometries and fracture crack patterns from macrostructural to microstructural length scales by image analysis. This approach can be leveraged by emerging technologies like Artificial Intelligence (AI), Machine Learning (ML) and Computer Vision (CV), in structural health monitoring of alloys used in exotic and challenging conditions.

Keywords

• Corrosion
• Low alloy steel
• Fractal
• Polarization
• Raman spectroscopy
• Surface degradation