Slagging is a major problem in boilers, especially the low-rank coal applied in boilers. In this study, the influence of heat transfer surface on the slagging characteristics of a pilot-scale furnace was investigated. Ni coatings were applied in modifying the deposition surface to control slagging. The growth characteristics of the slag were studied using an online digital image technique. Scanning electron microscopy linked with energy-dispersive X-ray analysis and X-ray diffraction (XRD) were applied to investigate the microstructure, semi-quantitative chemical composition, and mineralogy of slag samples. Ni coating demonstrated a positive effect on the mitigation of slagging. Results revealed that the thicknesses of the slag initially increased with experimental time and then inclined toward stable values for both cases (Case 1: substrate material; Case 2: modified surface). The stable thicknesses for Cases 1 and 2 were 4.91 mm and 3.95 mm, respectively. The heat transfer efficiency was improved by approximately 18.2% with the application of Ni coating for probe surface modification. The mechanism of the mitigation of slagging was investigated in this study. XRD results revealed that the content of alkali reduced when the surface was coated with Ni. The alkali significantly affected the adhesion behavior of the deposition. Hence, Ni coating showed potential in reducing slagging and increasing the efficiency of boilers. The overall study makes a contribution to a deep understanding of the effect of Ni coating on the growth characteristics of the slag. (C) 2018 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved.