In a boiler, various kinds of corrosion damage occurs including high temperature corrosion at the combustion gas side and steam oxidization at the high temperature steam side as described below.
Corrosion is influenced by temperature, environment and material and is mainly led to thickness reduction.
Steam oxidation may cause increase of metal temperature accelerating creep.
Generally, thickness reduction due to corrosion is measured by the thickness measurement method using ultrasonic testing (UT). Based on the results, the maximum thickness reduction is calculated by a statistical analysis using various methods including the extremes method, and its remaining life is evaluated according to the aged deterioration.
Steam oxidation is a direct oxidization reaction between high temperature steam and tube material : it advances according to the parabolic rule since diffusion of oxygen or metal in the oxide layer determines reaction speed.
3Fe + 4H2O ⇒ Fe3O4 + 4H2
The higher temperature leads to the higher oxidization speed and it is controllable by increasing the amount of Cr, Si, or Al in the material, grain refining, or plastic working (shot blast, etc.)
While steam oxidization speed is predicted based on the material and temperature conditions, the temperature is predicted based on the steam oxidation scale thickness and operating period.
