CORROSIVITY AT ONDO STATE INDUSTRIAL ESTATE, AKURE, SOUTHWESTERN NIGERIA

Abstract

This study was carried out at Ondo State Industrial Layout, Akure, Southwestern Nigeria. The aim of this study is to map the corrosivity of the shallow subsurface soil material within the study area. This study involves fifty two (52) vertical electrical sounding (VES) data and six (6) subsurface layers soil samples collected at depth of 2 m for geoelectric and geotechnical investigations of subsurface layers. The VES results delineated 3 to 5 geoelectric layers across the area which corresponds to five subsurface layers; topsoil, lateritic layer, weathered layer, fractured basement and the presumed fresh bedrock. Layer resistivity values range from 22 to 602 ohm-m, 423 to 2468 ohm-m, 7 to 602 ohm-m, 17 - 436 ohm-m and 308 to 10023 ohm-m in the topsoil, lateritic layer, weathered layer, fractured basement and the presumed fresh bedrock respectively. The iso-resistivity maps at depth slices of 1 and 2 m shows that the northeastern, southeastern and the central part of the area are non-corrosive (above 350 ohm-m) to slightly corrosive (250 - 350 ohm-m), while the central, northwestern and southwestern parts of the area are moderately (150 - 250 ohm-m) to strongly corrosive (60 - 150 ohm-m). Soil sample analysis shows liquid limit results that vary from 37.6 to 59.7 %, while the plasticity limit results vary from 29.3 to 42.5 % and all the plasticity index plots were below the A line indicating presence of non-plastic clay which suggests high rate of ionic movement. The natural moisture content values vary from 21.4 to 35.5 %. The 2 m depth slice isoresistivity map and clay plasticity factor were synthesized using additive model to generate subsurface corrosivity model map which indicates that the northwestern, northeastern and southern parts of the area are moderately (0.4 - 0.6) to strongly corrosive (0.6 - 0.8), while the upper central area and the flanks are slightly corrosive. The moderately and strongly corrosive zones correspond to the low elevation and water logged zones of the study area. The corrosivity model map was validated by the pH and conductivity data