Some effects of aqueous silica on the corrosion of iron.

Silica is an important natural component of ground and surface waters, and is sometimes added as an inhibitor to control "red water" problems caused by corroding iron pipes. However, the effect of silicates on many aspects of iron corrosion has never been assessed. Experiments with water containing 0.5, 10, 25 or 50mg/L of SiO(2) demonstrated a significant interplay between aqueous silica and iron corrosion. During this 4-month experiment, higher levels of silica caused more iron release to the water and decreased the size of suspended iron particles. The process of iron corrosion also changed aqueous silica concentrations; silica was released into the water from the cast iron during corrosion and was removed from the water by incorporation into the scale layer. Silica also affected the type of scale that formed on the iron coupons. Scale at the lower silica concentrations was fairly uniform and easy to remove from the coupons, while the scale from the high silica reactors was more dense, and was more difficult to remove. Scale from the high concentration silica reactor also developed tall tubercles, and hydrogen gas-containing bubbles were channeled to solution through these tubercles. Iron corrosion occurring via the evolution was significant under all experimental conditions.