This work evaluated the rate at which metal walls of the tubes of the furnace internal tubes of the refinery boiler were corroded due to the harsh environment in which it is operated. The rate of corrosion was investigated in the absence and presence of corrosion inhibitors with the use of a material balance equation of the boiler system where a mathematical model was derived using first order differential equation. The results obtained during the four years period under review were between 0.1425-0.5550mm, 0.1250-0.5000mm, 0.1125-0.4650mm, 0.2250-0.8000mm and 0.1700-0.6000mm in the absence of corrosion inhibitors in the FWT, EWT, IWT, ITURB AND ETURB respectively. The results also revealed that in the presence of corrosion inhibitors under the same period were 0.0945-0.2345mm, 0.0860-0.2135mm, 0.0775-0.1920mm, 0.1375-0.3415mm and 0.1035-0.2560mm in the FWT, EWT, IWT, ITURB AND ETURB respectively. The metals gained during the period under evaluation ranged between 0.0480-0.3205mm, 0.0490-0.2865mm, 0.0350-0.2730mm, 0.0875-0.4585mm and 0.0665-0.3440mm in the FWT, EWT, IWT, ITURB AND ETURB respectively. The resultant percentage efficiencies recorded during the period under review were in the range of 33.68-57.75%, 36.30-57.30%, 31.11-58.71% 38.89-57.31% and 39.12-57.33% in the FWT, EWT, IWT, ITURB AND ETURB respectively. The corrosion evaluation in the furnace internal wall tubes showed that there was a remarkable decrease in corrosion rate due to the application of corrosion inhibitors.