This paper examines the effect of surface roughness on the adhesion and morphology of a tenacious calcite scale layer, deposited on mild steel under well-controlled conditions. Metal surface roughness was found to exert a marked effect on the tenacity of the calcite deposit. The tensile stress required to disbond a calcite deposit adhering to a rough metal surface (roughness average Ra = 18-24 μm) was found to be as much as 30 times higher than that required to disbond a deposit adhering to a smooth surface (Ra = 0.1-0.15 μm). Surface roughness also affected significantly deposit structure. Porosity data showed that calcite deposits formed on smooth metal surfaces were three to four times more porous than those formed on rough metal surfaces. SEM photographs suggest that the above phenomena stem from two basic effects induced by surface roughness, viz., enhancement of the surface nucleation density and an orientation of the calcite structure. The end result is a more compact deposit of increased tenacity due to the formation of mechanically interlocking bonds between its crystallites.