Rust-Layer Characteristics and Pit Morphology on the Inner and Outer Surfaces of a Retired Q355C Hydropower Penstock

A hydropower penstock corrodes on two faces that share one steel wall but almost nothing else, because the inner surface stays permanently wetted by flowing reservoir water while the outer surface is wetted only intermittently by the plant atmosphere. The two surfaces of a service-retired Q355C penstock were compared using cross-sectional electron microscopy, potentiodynamic polarisation, impedance spectroscopy, confocal profilometry of the descaled surfaces and 720 h of continuous galvanic-current monitoring sampled once per minute. The outer rust layer was thicker and exhibited cracks, delamination, and pores, while its fitted rust-layer resistance was lower than that of the inner surface. The laboratory electrochemical measurements did not establish a reliable comparison of service corrosion rates between the two surfaces. One of the 31 outer-surface pits exceeded a depth-to-equivalent-mouth-diameter ratio of 0.5, whereas none of the 59 inner-surface pits did; however, the proportion difference was not statistically significant. The available 720 h galvanic-current records showed overall decreases on both surfaces, with temporal fluctuations.