Aqueous electrolyte traditionally used for electrochemical characterization of soil‐related corrosion in laboratories fails to represent the soil physical features, such as pore structure, soil heterogeneity, soil compaction, and saturation levels, in the diffusion‐controlled corrosion process. This article introduces a semi‐solid agar system to reproduce the physical structure of soil for corrosion study. For feasibility validation of the agar system, direct comparison regarding electrochemical activity, diffusion characteristics, and corrosion mechanisms has been performed on pipeline steel in aqueous sodium chloride (NaCl) solution (5 g L−1), 5 g L−1 NaCl‐containing agar, and 5 g L−1 NaCl in sand, respectively. The results indicates that oxygen diffusion in agar and sand media is similar, which significantly weakens the cathodic activity of steel specimens, but leads to distinct corrosion characteristics from those identified in aqueous NaCl solution counterparts. The high diffusion rate of chloride ions in aqueous solution also accelerates corrosion of pipeline steel in NaCl solution through extensive attack at defect sites, but the limited chloride ion movement and the diminished driving force for anodic corrosion activity reduce such attack in their agar and sand equivalents. The solid nature of agar outperforms aqueous electrolytes as soil replicate to explore soil‐related corrosion responses at laboratory scale.