Microstructure and Chemical Composition of Natural Scale Layers Available to Purchase

Basic physico-chemical data required to understand the mechanism of corrosion protection by natural scale layers in water pipes were investigated. These included the morphology, distribution of constituent elements, and crystalline phase composition of 15 natural scale layers, formed in potable water supply mains carrying water of various chemical compositions. Using scanning electron microscopy, the morphological studies reveal the following structure: the outer zone (adjacent to the scale/water interface) is relatively compact and consists of contiguous crystals, while the inner zone is considerably more porous. This morphological heterogeneity is explained by differences in crystallographic and chemical composition of the two zones. Crystalline phase analysis of the scales was performed by the Debye–Scherrer method on powdered samples. It was found that calcite–CaCO₃, which favors a compact arrangement, appears in the outer zone, while the inner zone is comprised of geothite [α FeO(OH)], siderite (FeCO₃), and magnetite (Fe₃O₄), that favor both needle–like and granular porous structures. Independent microanalysis studies showed the occurrence of a steep gradient in Fe and Ca concentrations inside the bulk of the scale. The location of this gradient in terms of scale depth varies from scale to scale and plays a significant role in corrosion protection. The Fe/Ca ratio as determined by overall chemical analysis varied over the range 0.2 < /Fe/Ca < 182.