2016 Volume 57 Issue 9 Pages 1652-1659
The microstructures of scales adhered to the inner walls of elbow steel pipes, used in the transport of hot spring water, are analyzed. The system examined in this study is from a geothermal plant in Obama town, Unzen city, Nagasaki, Japan, using pipes with 3.5 months of prior use. The adhered substance consists of four layers: amorphous magnesium silicate, aragonite, amorphous magnesium silicate, and iron corrosion products, on the carbon steel from the inside of the pipe to the outside. The corrosion product fully covers the steel surface. The magnesium silicate (1–2 mm thick) is initially generated as an adhesion substance on the corrosion product. The layer thickness of aragonite (orthorhombic calcium carbonate (λ-CaCO3)) is 15–70 mm. Carbon, oxygen and calcium are dissolved in the magnesium silicate, which later precipitates as calcium carbonate with large and/or stratiform features. The chemical contents in the magnesium silicate layers on both the top and bottom sides are nearly identical. Therefore, the precipitation of aragonite and its growth in the magnesium silicate may form the aragonite layer, which shows a columnar structure along the heat flux direction.