Impact of General and Local Metal Loss on the API 579-1 Creep Residual Life

Creep and corrosion are typical damage mechanisms of pressure components operated at high temperatures and they are frequently evaluated using the relevant sections of the API 579-1 [1] standard. When a general or local metal loss is expected or assessed via Non-Destructive-Examination (NDE) on a pressure equipment in the creep regime, the time-dependent corrosive phenomenon has a direct impact on the time and stress-dependent creep residual life and the two damage mechanisms are competing and reinforcing one another. For these cases, an API 579-1 Level 3 creep assessment shall necessarily take into account the metal loss induced by the corrosive environment to be effective in predicting the component residual life. In this work, the impact of corrosion on the creep life evaluated according to API 579-1 and the Larson-Miller theory was evaluated for the special case of a low-alloy piping component exposed to both general and local metal loss with variable corrosion and creep rates. A finite-element model was set-up incorporating the inelastic steady-state creep law, the API 579-1 time-fraction creep damage and the progressive metal loss due to uniform and localized corrosion. A parametric analysis was finally carried out to investigate the role of the corrosion rate, and the related, augmented stress field on the overall API 579-1 creep damage.