Underground Corrosion of 1040 Medium Carbon Steel in Andisol, Oxisol, and Molisol Type Hawaii Tropical Soils

Abstract

This research investigates the effect of tropical Hawaii soils on the corrosion behavior of 1040, medium carbon steel. The primary objectives of this research were to

1. measure the rate of corrosion in sterile and unsterile soils from Andisol, Oxisol, and Molisol type tropical soils, and ascertain whether corrosion rates in unsterile soils, under aerobic and/or anaerobic soil conditions, will be greater than those in sterile soils, under aerobic and/or anaerobic conditions, 2. examine the influence of aerobic and anaerobic soil conditions from Andisol, Oxisol, and Molisol type tropical soils on the primary type of corrosion product formed on 1040, medium carbon steel; 3. determine if there was an effect from the presence of microbial activity as a mean for underground corrosion on 1040, medium carbon steel; and 4. interpret the working electrode (WE) and oxidation-reduction (redox) potentials from Andisol, Oxisol, and Molisol type tropical soils at the experimental field sites by correlating the results with soil moisture conditions and against the controlled conditions in the laboratory.

The research was conducted on 1040, medium carbon steel in an Andisol using laboratory bench-scale systems, and between all three soil types using field-scale systems set up in an Andisol on the Island of Hawaii, and an Oxisol, and a Molisol on the Island of Oahu. Andisol soils in the laboratory were compared against sterilized soils of the same type and held under either aerobic or anaerobic environmental conditions.

Based on the laboratory results, the highest corrosion rate for 1040, medium carbon steel was produced in an Andisol held under aerated conditions, regardless of whether or not the soil was sterilized. Therefore, the presence of oxygen played a larger role in increasing the corrosion rate than effects from the presence of microbial activity. However, measured oxygen reduction (redox) potentials indicated anaerobic pockets developed extensively in the aerated soils and correspondingly led to the development of microsites on the surface of the steel. This inconsistency made the measured soil environment results appear anoxic in systems that were aerated.

The influence of aerobic or anaerobic soil conditions had an unascertainable effect on the type of corrosion product formed on the steel, as nitrogen sparged (anaerobic) environments only yielded a trace presence of elemental sulfur, as analyzed by energy dispersive x-ray spectroscopy (EDX), in the surficial corrosion product versus the pure iron (Fe)-oxide product formed on the steel in the aerobic environments.

It was assumed that unsterilized soils in anaerobic (nitrogen-sparged) conditions would produce the highest rate of corrosion on steel. However, from this experiment, in the Island of Hawaii Andisols, both unsterile and sterile conditions resulted in similar degrees of corrosion. This may indicate that microbial activity in the unsterile soil may have been suppressed by the low nutrient levels.

In the laboratory chambers, the water saturated soils became anaerobic despite constant aeration which was likely caused by the nature of the saturated Andisols which prevented air from flowing through the soils. Hence the redox potentials in the aerated laboratory chambers showed that the wet soils were anaerobic. In contrast, the redox potentials in the field experiments showed that significantly more oxygen was present in these soils during rain events, or when wet. Redox in the field experiments was relatively constant even during rain periods, likely due to the soils being relatively well-drained. This indicates that oxygen content in the soil, at a depth of approximately 1 foot, was relatively constant, even as the moisture content fluctuated with the rain events. In other words, the decrease in working electrode potentials as moisture increased (during periods of rain) indicated the anodic dissolution kinetics of Fe increased with soil moisture in the field.

Further, the corrosion rates at the experimental field sites were presumed to be affected by moisture content in the soil and its effect on the anodic dissolution of Fe. In other words, higher rainfall in an area should have increased the rate of corrosion and the amount of visible corrosion product on the surface of the sample coupons. However, from the experiment we learned drier soil conditions in the field raised the corrosion potential, but produced less corrosion product on the surface of the coupons. Wet soil conditions, on the other hand, lowered the corrosion potential, but raised the anodic current and produced more corrosion product.

Despite a constant infusion of air into the aerated laboratory chambers, the redox potential continually produced negative readings, indicating extreme anoxic conditions. This output was likely the result of the saturated Andisol becoming sticky and plastic-like which prevented diffusion of the air evenly throughout the chamber. Therefore, it is advisable to improve the experimental laboratory system to provide better correlation with field results.