Chromium diffusion coatings are one common type of coating applied to increase the corrosion resistance of coal-fired boiler tubes. Chromizing, the process of applying a chromium diffusion coating, is desirable for several reasons. First, the process is relatively cheap and simple. Also, chromizing is successful on a wide range of part size and shape. Another benefit of diffusion coatings over other protective coatings, is that they are more resistant to spalling due to the nature of the bond formed with the substrate.

A microstructural analysis was performed on low alloy boiler tubes to assess the protective ability of the chromizing. In this case, the boiler tubes were removed from a boiler after about 6 years of service. The fire side of the boiler tube panel was covered by a thick but brittle corrosion layer but no failures were detected by visual inspection on the macroscopic scale. The corrosion lay was complex, consisting of at least 3 phases. See Figure 1. EDS analysis indicated high amounts sulfur suggesting that sulfidation is a major corrosion mechanism. See Figure 2.

The microstructure of the fire side showed obvious signs of corrosion. Initially, corrosion of the coating was localized to sensitized regions adjacent to the grain boundaries (see Figure 3).

With time, corrosion eventually consumed whole grains of the diffusion zone and penetrated deeper into the diffusion zone. From microstructural analysis, it appears that failure of the chromized boiler tubes is unavoidable. While chromized boiler tubes offer some protection against corrosion, it would be an unacceptable option for an extended time in a boiler tube environment.

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