Oxide Scale Approach in Metal Temperature Estimation for Boiler Tubes in Creep Service

Introduction

The typical design life of a boiler and the associated components in thermal service is around twenty to thirty years. Depending on the operating temperature, chemical treatment, and pressure, the boiler and the associated components will encounter various damage mechanisms, such as creep, fatigue, corrosion, erosion, etc. [1,2]. When the equipment has been operating close to its design life span, the plant operators will face the following questions:

• Can the current condition of the boiler and the associated components continue to run beyond the design life span?
• If it is possible, how much longer can the thermal plant be in operation?

The above questions can be addressed through remaining life assessment (RLA). The remaining life and metallurgical condition of the superheater and reheater tubes are key considerations of the key life-determining components of a boiler.

Based on current power plant technology advancement, four material families are used for boilers according to their temperature limits: ferritic steels, ferritic-martensitic steels, austenitic steels, and high-nickel alloys. All these materials can be found in ASTM A213 specification. This article is focused on grades 11/12, 22/23, and 91/92, which are common for superheater and reheater applications.

Superheater/reheater tubes are particularly susceptible to creep stress rupture failures due to typical operating temperatures beyond 482°C (900°F). Although creep failures are expected, significant deviation from the design limits can promote early failures of superheater and reheater tubes [3]. Therefore, the main concern is to ensure that the metal temperature in the superheater and reheater tubes is kept within the design limits. One effective way to monitor the tube metal temperature is through internal oxide scale thickness measurement. Periodic evaluation of the internal oxide formation allows the plant operator to monitor for any signs of overheating of superheater/reheater tubes, estimate remaining life, and plan for tube replacement before the tubes reach critical creep damage limits. The advantages of the oxide-scale method are the simplicity, reproducibility, and possibility of non-destructive oxide scale measurements [1].

Oxidation Under Steam – General Concepts

When carbon steel superheater and reheater tubes are subjected to a high-temperature steam environment, an internal oxide scale layer is formed. The oxide scale grows over time and forms an effective insulating layer, which leads to an increase in the tube wall temperature and eventual overheating damage. Studies performed in the power generation industry over the years have indicated that the overheating effect of the oxide scale is relatively insignificant for oxide thickness lower than 0.3 mm (0.012”) [4]. Once the internal oxide grows beyond 0.3 mm thick, the overheating effect could be significant. It was also reported by EPRI that an increase in the internal oxide thickness in a typical reheater tube will increase the tube temperature by approximately 0.28°C (0.5°F) per 0.025 mm of the oxide thickness [5]. As for a typical superheater tube, an increase in the internal oxide thickness will increase the tube temperature by approximately 1.67°C (3°F) per 0.025 mm of the oxide thickness. Therefore, it is important to comprehend the oxide scale formation and kinetics, and its correlation to tube metal temperature.

Three types of oxides can be formed [5]:

1. Magnetite (Fe₃O₄). It is the predominant form of oxide which exists over a wide range of oxygen partial pressures and temperatures. It grows as a tenacious and coherent film which then impedes the transport and diffusion of ions. The rate of oxide formation is initially high but decreases as the layer thickness increases (i.e. parabolic growth) and becomes self-limiting. It is generally a protective form of oxide.
2. Wustite (FeO). The oxide is only stable at high temperatures (i.e. > 550°C (1022°F)) when low oxygen concentration is present. This oxide generally forms between the metal surface and the dominant magnetite layer. It is a concern as the presence of FeO scale indicates the potential for accelerated oxidation if it forms. It is a non-protective form of oxide.
3. Hematite (Fe₂O₃). The oxide is only stable in an environment with high oxygen concentration. Therefore, it is generally present at the outermost layer of the oxide scales.