High durability, quality control and prediction of deterioration are key issues for the thermal spray coatings applied to fossil fuel and waste burning boilers. Small electrical resistance inspection (SERI) has been developed and applied for boilers to quantitatively evaluate the relationship between the deterioration mechanisms and the coating defects as well as life-times of coatings. Changes in the electrical resistance (ER) of coatings showed a good correlation with the deterioration causes and also with the bonding strength of the coatings. On the other hand, new processes such as thermal spraying and automatic fusing (SFU) process by high frequency induction heating have been developed to manufacture highly durable coatings. These SFU coatings, which are 0.5~3 mm in thickness and contain chemically bonded and dense NiCrSiB alloy layers have been found to have excellent durability which is almost equal to that of 625 weld overlays used in many WTE plants. Applications of these technologies are in progress to realize coatings with high cost performance, high efficiency and easy maintenance.