In most chemical plants and refineries, the process piping in the process unit suffers from metal thinning, especially the carbon steel components such as elbow, tee, direct piping, branches, and reducers. Therefore, the inspectors should have accurate knowledge about the deterioration conditions of the piping system in order to maintain the personal safety and the structural integrity of the chemical plants and refineries. Basically, ultrasonic thickness spot measurement is an efficient way to check the thinning condition of the piping spools. However, the variations of the thickness readings always influence the evaluation of the corrosion rate of the piping spools. In this study, in order to waive the influence of the uncertainties of the thickness measurements, a statistical methodology is proposed to calculate the corrosion rate of each piping spool. With the proposed statistical approach, the corrosion rate of the piping spools in the process unit can be easily understood by plotting the histogram and using the upper limit of calculated corrosion rates. By utilizing the trend shown in the histogram, one can get knowledge about the deterioration condition of the piping system in the process unit. Finally, in order to discuss the feasibility of the proposed methodology, two sets of inspection data in a selected process unit from a lubricant plant were adopted as the examples. Also, a site survey result performed in the lubricant rerun unit is presented and shows the effectiveness of this statistical methodology. (C) 2008 Elsevier Ltd. All rights reserved.