Exposure times, high-temperature alloys

Internal or subsurface attack (oxidation). High-temperature corrosion can be identified by simple visual observation of the surface. However, subsurface phenomena within the matrix of the alloy, as well as obscured relations at the interface of the alloy with the surface films formed in many high temperature exposures can be seen in Figure 6.24. Electrochemical corrosion at high temperature at the interface also involves the diffusion of the aggressive gas phase to the vulnerable phase in the subsurface, leading to corrosion most of the time. 

During high-temperature exposure in air, active elements, e.g., Cr, in alloy substrates are preferentially oxidized, forming an oxide scale on the alloy surface. The scale growth is often modeled with a parabolic relationship with time t °... 

The metallographic analysis of the reactors with the optical microscope shows not worth mentioning trace of corrosion in the high temperature section of the reactor The elemental analysis of the exposed surface revealed a layer reach in Ni and Fe oxides and depleted in Cr and Mo. These changes affect only a thin layer. In account to the long exposure time of more than 1000 h and the high (up to 0.2 mol/kg) HCl concentration, corrosion rates of alloy 625 in high temperature - low-density supercritical water solutions are definitely low. For such HCl and Oj solutions Ni-base alloys, similar to alloy 625, can be used. 

Although data reported by suppliers on technical information brochures, low alloy steels, exposed to inhibited solutions, exhibited in laboratory test conditions severe corrosion rates, higher than maximum allowed. Most likely this is primarily due to the high temperature and relatively long exposure time. Of the two corrosion inhibitors tested, that designated B showed a better effectiveness in all experimental conditions. 

The present chapter describes the service conditions at the flame tube of an intermittent operating recirculation burner measured in a burner rig (maximum temperature experienced by the material 1000°C). Three different austenitic high temperature Ni-base alloys are being tested in the rig in an ongoing experiment. The effect of the parameters material temperature, duration of the air ventilation after burner shutdown and sulphur content of fuel on the structure and growth of the oxide scale on the alloys after 50 h exposure time have been investigated. 

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