Theoretical Development of a Corrosion Rate Prediction Model for Sweet Gas Pipelines in Tropical Lateritic Soil Environments

External corrosion of buried gas pipelines in tropical lateritic soils exhibits fundamentally different electrochemical characteristics from the temperate soil environments for which existing corrosion rate prediction models were developed, creating a critical theoretical gap that undermines pipeline integrity management in sub-Saharan Africa. This study develops and validates an original theoretical model for predicting external corrosion rates of buried gas pipelines in tropical lateritic soil environments, grounded in electrochemical theory and calibrated through an extensive experimental programme. The theoretical framework is built on a modified Butler-Volmer kinetic model that incorporates the distinctive properties of tropical lateritic soils: elevated iron and aluminium oxide content, variable seasonal moisture, high organic acid concentrations from plant litter decomposition, and pronounced wet-dry seasonal cycling. Laboratory corrosion testing is conducted using simulated soil electrolytes representing lateritic soils from Ogun, Benue, and Adamawa states, using linear polarisation resistance (LPR) and electrochemical impedance spectroscopy (EIS) techniques on API 5L Grade X52 pipeline steel coupons. Field validation is performed against 5 years of corrosion coupon data from cathodic protection test stations on gas pipelines in the studied states. The study's original contributions include: a seasonal soil moisture correction factor for tropical laterite corrosion rates, a temperature-dependent soil resistivity model for the laterite-steel system, and a corrosion rate prediction algorithm suitable for implementation in pipeline integrity management software. Recommendations include adoption of the model as a national standard for soil corrosivity classification of Nigerian gas pipeline routes and mandatory application in pipeline design life calculations. Keywords: corrosion prediction, lateritic soil, gas pipeline, electrochemistry, tropical environment.