Impact of Soil Characteristics and Moisture Content on the Corrosion of Underground Steel Pipelines

Abstract

External corrosion of underground steel structures is a major environmental and economic burden. The severity of such detriment depends on many factors that encompass: soil properties, moisture content, total soluble salts, the presence of microorganism, and method of metal protection. The investigative endeavor put forth bestows prime attention to the effect of soil attributes and moisture loading on the corrosion of submerged steel structures. The exposure to four discrete soil types (saline, clay, calcareous and sand) was explored in this study. Electrochemical linear polarization measurements were deployed as a means of assessing corrosion performance. The results revealed corrosion event severities endured by steel to be strongly underpinned by soil overall characteristics and moisture content in the vicinity. Corrosion potentials were found to possess more negative values with rising soil moisture content. The corrosion rate was rendered consistently peaking in coincidence with 20% (by wt.) moisture contents pertinent to sand, and with a corresponding 25% content in clay, saline and calcareous soils. The corrosivity of soil at the critical soil moisture content increases in the order: calcareous < clay < saline < sand. Scanning electron microscope observations confirmed the existence of corrosion product (loose and porous) with evidence of pitting attack. EDX analysis proved that the corrosion product was mainly composed of iron oxide. Plate counts revealed the absence of bacteria which warrants the exclusion of any microbial detriment as a contributing factor that might underpin corrosion.