Corrosion of carbon steel in soil: differential aeration cells and heterogeneity of the corrosion product layer

摘要

The surface of a carbon steel electrode buried in an unsaturated soil is in contact with wet and dry zones of soil, which is revealed by variations of the active area of the electrode and give rise to differential aeration cells likely to induce localized corrosion phenomena. The corrosion product layer forming on the steel surface may hinder, via a barrier effect, the reduction of oxygen in the cathodic region and thus progressively reduce the impact of the differential aeration. In contrast, if electronic conductors such as magnetite (Fe_3O_4) are favored in the cathodic (aerated) zones, facilitating O_2 reduction on the outer part of the corrosion product layer, the galvanic effect due to differential aeration could persist for longer times. In order to study the possible link between the presence of wet and dry soil areas and the heterogeneity of the corrosion product layer, a multi-coupon electrode (MCE) was used. The MCE consisted of 16 square steel coupons (5 × 5 mm) with an individual electric connection so that electrochemical measurements could be performed on each coupon. A large active area of 25 mm~2 was considered for the coupons so that the corrosion product layer covering each of them could be characterized by X-ray diffraction. The MCE was buried at room temperature during 125 days in an artificial silt loam soil, initially set at 70% of the saturation with a 0.01 M NaCl + 0.01 M NaHCO_3 solution. The experimental cell was hermetically closed for 32 days before to be opened so that the effects induced by the progressive drying of the soil could be studied. Measurements of the open circuit potential of each coupon revealed significant differences that could be associated with galvanic currents via experiments performed with a potentiostat in zero-resistance ammeter mode. XRD analysis of the corrosion product layers revealed significant variations of the composition of the corrosion product layer, associated with heterogeneity of soil composition and/or aeration/wetness.