Effect of temperature gradient and sulfur dioxide addition on erosion – corrosion of iron- and nickelbased alloys

Abstract

In this study, high temperature erosion – corrosion with trace amounts of added SO2 and the role of isothermal or thermal gradient conditions are studied. The studies are focused on 304L as the model material, but results for other alloys (Alloy 625 and 9Cr1Mo – steel) are also included to support the results and trends observed. The experiments include 550°C exposures in the erosion – corrosion rig under isothermal and thermal gradient conditions. The internal cooling used to create the thermal gradient meant that the 550°C material temperature could be maintained, despite of the higher bed temperature of 750°C. Laboratory oxidation experiments in a similar environment are also included as reference tests. The evaluation of material degradation was done by using a combination of different techniques such as scanning electron microscopy (SEM) including X-ray microanalyses, Auger and X-ray electron spectroscopy. The results demonstrate three important aspects related to erosion – corrosion attack in fluidized bed combustion. First, provided oxide growth is rapid enough leading to thickness in the micrometer range, low material wastage is obtained as demonstrated for the 9Cr1Mo– steel and also for the 304L and Alloy 625 materials when exposed to air plus 50 ppm added SO2 in isothermal conditions. Because of the large catalytic surface in the test rig for the thermodynamically favored oxidation of SO2 in air, significant presence of SO3 is expected. The material wastage is then also associated with a shift in erosion pattern with maximum material wastage at the impact of erodent (called type B). Detailed microstructure investigation of oxide in question on the 304L showed that the inner part of the oxide then is dense and possibly isolated from the environment and also Cr-rich. Adding a temperature gradient then returns the wastage pattern to type A for both 304L and Alloy 625 and leads to increased wastage for the former material. The resulting oxide thickness is also smaller and always less than 0.2 µm. The laboratory reference tests showed a similar range of average oxide thickness for air and air plus SO2. However, in air plus 50 ppm SO2 the tendency for break away corrosion on 304L was less than for air exposed samples.