Corrosion under insulation (CUI) refers to degradation of metals under thermal insulations, which reportedly manifests as localized corrosion and pitting. The ASTM standard for the laboratory simulation of CUI (i.e., ASTM G189-07, Standard Guide for Laboratory Simulation of Corrosion under Insulation) proposes an annular gap between insulation and steel coupons to accommodate the counter electrode (CE) to allow for electrochemical measurements. On the other hand, annular gap deprives the proposed arrangement in ASTM G189-07 from oxygen concentration cells that otherwise exist in real-world applications from the direct contact between insulation and steel. Also, there is a literature gap in understanding the long-term implications of direct contact between insulation and steel and consequential oxygen concentration cells, which are crucial for pitting under thermal insulations. This research investigates CUI simulation behavior via direct contact between insulation and steel (i.e., no annular gap) while allowing for electrochemical measurements by incorporating a novel method deploying carbon fiber mesh as CE. Generally, the presence of pitting can greatly affect the rates of CUI in a nonlinear manner. The standard duration for CUI simulation tests has been 72–96 hours (3–4 days), which may not trigger noticeable localized corrosion and pits, especially when insulation grades with corrosion inhibitors are involved. Moreover, short-duration tests may not trigger insulation aging and consequential corrosive leaching that occur otherwise in real-world applications. This study also investigates CUI rates and corrosion modes over longer durations, specifically 15 and 30 days. Following the CUI simulation tests, corroded surfaces were characterized using confocal laser scanning, 3-D topography, and scanning electron microscopy to understand the impact of direct contact versus annular gap as well as corrosion behavior over longer durations from a CUI rate perspective. Finally, the research suggests some improvements to ASTM G189-07 in terms of test setup and characterizations.