The internal surfaces of thermal power plants become covered withg deposits from the combusted material (fuel) as well as a gradual build-up of various corrosion products. The formed layers are highly complex in composition and structrue, varying depending on the alloy, temperature, type of fuel, etc. The elements present in the deposited layers affect the corrosion processes and thereby the service lifetime of the boilers. In order to better understand these corrosion processes, compositional depth profiling (CDP) of the layers is very valuable analytical information. A radio frequency (RF) Glow Discharge Optical Emission Spectroscopy (GD-OES) method has been developed for quantitative depth profile analysis of these surface deposits.
Some of the major elements of interest are CI, S, Ca, Na and K, and there are no commercially available solid reference materials (RM) with mass fractions in the range found in these materials. It was therefore necessary to produce dedicated RM's for calibration. The method devised within the project was to spray salt solutions onto steel substrates and subsequently dry these, producing coating of well-known composition for calibration. The average thicknesses (or rather coating weigh/are) of the coatings were determined by weighing the samples before and after deposition. This method was shown to work satisfactory.
A comparison with SEM cross section images have also shown that the in-depth structure of the depth profiles qualitatively agree well with the observed structure in the images. However, the analytical wok has also shown that some of these types of materials are "difficult" to analyse by GD-OES, in the sense that the plasma discharge tends to be unstable. It should therefore be noted that not all materials of this type will be amenable to GD-OES analysis.
Comparing some field exposures of 16Mo3 and 310S it has been shown that differences in corrosion mechanisms can be deteced with the GD-OES method. By observing the positions in the depth profile of primarily K and CI, the ability of a material to withstand corrosion can be deduced.
The GD-OES method developed is a valuable analytical tool in continued research on the corrosion properties of thermal power plants, as wlll as th chemical processes responsible for this corrosion.