Assessment of Corrosion of Mild Steel Buried In Soils of the Niger Delta, Nigeria

This study determines the corrosion rate and percent weight loss of mild steel buried in soils of the Niger Delta Area of Nigeria. Six geologic zones representing the upland and wetland soils were used for the assessment. The corrosion rates of mild steel in these soils were monitored to assess the extent of corrosion. However, the upland soils (Odagwa, Ogoni, Ahoada and Omoku) were more resistant to corrosion than the wetland (Kaiama and Elebele) soils. The corrosion rate of mild steel at the 24 month was in the following order of corrosivity: Elebele > Kaiama > Omoku > Ahoada > Ogoni > Odagwa. The percent weight loss was higher at the 24 month, with the highest values found at the Meander Belt Deposits of Elebele. Alternately, the Coastal Plain Sands were found to have the least percent weight loss with the lowest value recorded at Odagwa site at the 24 month. The percent weight loss at the 24 month is in the following order: Elebele > Omoku > Kaiama > Ahoada > Ogoni > Odagwa. The variation in corrosion rate and weight loss of mild steel buried in the different soil types is caused by the aquic moisture regime of the soils, anthropogenic activities carried out, microorganisms present in the soils, and also, the physico-chemical properties of the soils.

concentration cells (Davies, 2000). In addition, soils with high moisture content, high electrical conductivity, high acidity and high dissolved salts are usually the most corrosive (National Association of Corrosion Engineers, NACE, 2000). The process involving gradual chemical or electrochemical reactions between a metal and its environment, leading to the destruction or denaturing of the metal surface (coloradogeologicalsurvey.org/geologic-hazards/corrosive-soils/) is known as corrosion. The most commonly observed corrosion process is the rusting of iron exposed to air or moisture (Douglas, 1982;Raymond, 2010).
All metals are unstable, and their instability is due to their tendency to revert to their original native state when in any reactive environment. Thus, in acidic, alkaline or neutral environment metals will reactthis is the foundation of corrosion. Steel quickly corrodes in acidic environments and gradually or not at all when alkaline is added (Davies, 2000;Industrial Galvanizers Corporation, 2003). The reaction of metals in the environment is related to the movement of electrons and is classified as an electrochemical reaction. The ability to lose electrons varies from metal to metal and the greater the readiness the more reactive or corrosive is the metal. Formed corrosion products, or oxides, are usually insoluble and form a protective skin on the metal surface. In general, corrosion is an oxidation-reduction reaction involving an electron gain or loss (Ferreira et al., 2007;Terence, 2019). In the presence of various metals conducting fluid, known as the electrolyte, the electric potential is generated which causes the current to flow when there is a suitable path. Such electrical potential can also be developed between two regions of a single component made of metal due to small variations in structure or differences in metal surface exposure conditions (Davies, 2000).
Some of the major harmful effects of soil corrosion can be summarized as follows:  Reduction of agricultural productivity through metal contamination of soils (predominantly by iron and aluminium).  Ecological damage to aquatic and riparian ecosystems through precipitation of iron, fish kills, increased fish disease outbreaks, dominance of acid-tolerance species, etc. (Erker and Yuksel, 2005)  Contamination of groundwater with iron and other heavy metals, etc.
The specific objective of this study is to determine the corrosion rate of mild steel in different soil types, mild steel being used as a predictive tool for monitoring corrosion in soils using weight loss technique.

Materials and Methods:
Six sampling locations were selected within the Niger Delta. The area has a surface area of 19,420sq.km, out of which 15,570sq.km of the land is on submerged or saturated conditions for most part of the year. However, while the submerged or saturated part of the land forms the wetland soils, the remaining part of the land (3,850sq.km) forms the upland soils (Mordi, 1986). The sites are located within latitudes 4 0 15`and 5 0 47'N and longitudes 5 0 22' and 7 0 37'E.
(1) Odagwa site is situated at Adaobi at the back of the Assemblies of God Church 100m away from the crude oil pipeline in Etche Local Government Area; (2) Ogoni site situates at Wiyakara (200m away from the river) in Khana local Government Area; (3) Ahoada pedon was sampled 6km away from Ahoada town along the East-West road, 150m off the road in Ahoada West Local Government Area, while

Preparation of Specimen:
Mild steel pieces with dimensions 2.5 x 2.5 x 0.4cm were cut out with a lathe machine in the Science and Engineering Workshop of the University of Port Harcourt. A hole of 2.5mm in diameter was bored at one end of each of the mild steel coupons (Plate 1). Each of the mild steel coupons was then fastened to long copper wires through the hole with the aid of araldite. The coupons were polished with silicon carbide paper, degreased with acetone and stored in the dessicator prior to commencement of experimental procedure. The mild steel coupons under study were taken from the dessicator and labeled. Each labeled specimen was carefully weighed using mettler H35 AR analytical balance. This weight was recorded as initial weight. The soils were excavated by digging a mini-pit in each location up to 80cm covering the A and B horizons. Ten weighted mild steel coupons were buried in each soil type, five at the depths of 0-30cm and the other five at the depths of 30-80cm, respectively. The investigation was carried out under natural environment. The metal coupons were periodically monitored every four months for two years. After each four months, the buried mild steel coupons were dug out from the ground and taken to the laboratory for weight measurement. Before weighing, they were cleaned with a pickling solution to remove corrosion products from the surface, washed with distilled water using a clean brush, and with ethanol to remove grease or oil and finally rinsed in a fast drying solvent, acetone (National Association of Corrosion Engineers {NACE}, 2000). The coupons were finally weighed and the average weights obtained were recorded as the final weights for each of the four months.

Plate 1: Showing some mild steel coupons buried in the soils
The Corrosion rate and % Weight loss of the mild steel buried in the different soil types were then calculated using the first order equation below: Loss in weight = Wo -Wf Time t  Elebele. The range of the percent weight loss at the 12 th month was between 0.2% and 11.2%, with the lowest value found at the surface and sub-surface soils of Ahoada and the highest found at the surface soils of Elebele, while the percent weight loss in the 16 th month was recorded from 0.3% at Ahoada surface and sub-surface soils to 12.0% at Elebele surface soils. At the 20 th month, the percent weight loss of mild steel ranged between 3.8% at Ahoada sub-surface soils and 14.6% in the surface soils of Elebele, while at the 24 th month, it ranged from 5.8% at the sub-surface soils of Odagwa to 20.8% in Elebele surface soils, respectively. These results imply that Odagwa soils are less corrosive and therefore, the mild steels buried in them will have higher life expectancy than in the other locations, while Elebele in Meander Belt Deposits are most corrosive among the soils studied because of its aquic moisture regime. Generally, the corrosion rates tended to be higher in the surface than in the subsurface soils, which signified a higher activity zone, resulting from both human and microbial activities. While the effects of corrosive soil can cause structural failure and financial burden, it also has attendant effect on the pollution of the environment, for example, oil pipeline rupture and leakages (Nduuku, 2015) leading to low fertility of the soil and low crop yield. Because of the changes in soil properties as a result of oil spill and waste deposition, and their effects on plant growth, the problem of the quality of pipes (iron) to be laid becomes a great challenge to the agronomists in other to reduce corrosion rate and avoid oil leakages and high iron and heavy metals accumulation in soils.