Angle-Resolved Scattered Light Measurements for the Assessment of Visual Preparation Grades of Blast-Cleaned Low-Carbon Steel Substrates

Abstract Visual substrate cleanliness is an important parameter for the assessment of steel substrates prior to coating or adhesive applications. The standard assessment procedure for the evaluation of blast-cleaned substrates categorizes various preparation grades (Sa). It is subjective, inaccurate and dependent on the environmental site conditions. This contribution investigates for the first time the application of an angle-resolved scattered light sensor to the visual preparation grade evaluation of blast-cleaned low-carbon steel. Twelve different surface conditions were generated, and a full factorial design was built. Factors were abrasive type (three levels), surface preparation grade (two levels), and average maximum roughness (two levels). Target parameter was the mean back-scattered light intensity. Results of design of experiments (DoE) and analysis of variance (ANOVA) revealed that all factors were extremely significant. The surface preparation grade had by far the largest contribution (66.7%), followed by roughness (19.4%) and abrasive type (0.3%). The mean light intensity was consistently higher for the Sa 3 preparation grade (2272 to 3079 W/m2) compared to the Sa 2½ preparation grade (1443 to 2112 W/m2). A preliminary threshold of ∼2100 W/m2 is proposed, that separates Sa 2½ from Sa 3. False-color patterns, indicating four light intensity levels, were generated, which were consistent the results from the mean light intensity measurements. The findings indicated the suitability of angle-resolved scattered light sensors for the objective in-situ evaluation of blast-cleaned carbon steel substrates.