Graphitic Corrosion-Induced Errors in Cast Iron Coating Thickness Measurements

This article presents a non-destructive testing anomaly encountered during the maintenance of a cast iron vertical axial flow pump at a water treatment plant. During coating inspection, a magnetic induction dry film thickness gauge recorded a coating thickness of approximately 300 μm, significantly exceeding the specified primer thickness of 100 μm. A detailed investigation revealed that the elevated readings were not caused by excessive coating application, but rather by the presence of a graphitic corrosion layer. This case study examines the mechanisms behind this measurement error and highlights the limitations of conventional magnetic induction gauges.