Prevention of metal dusting on Ni-based alloys by MCrAlY coatings

The flame tubes from the domestic heating devices made of different Ni-based alloys (alloy 601/NiCr23Fe and even alloy 602/NiCr25FeAlY) showed after a short operation time severe attack by metal dusting. This phenomenon occurred only during combustion of the fuel oil with a new chemical composition. The german market offers such fuel oils with a reduced sulphur content due to more severe environment regulations. Sulphur is able to stabilize the cementite suppressing or retarding the appearance of metal dusting. Metallographical studies revealed that under this operation conditions the oxide scale on the Ni-based alloy forms only partially.In order to prevent the appearance of metal dusting, the flame tubes were coated by a high performance layer (MCrAlY-alloy) which shows a good stability under these corrosive conditions. Uncoated and MCrAlY-coated samples made of alloy 601 and alloy 602 were used for metal-dusting experiments. The specimens were tested using a computer-controlled special equipment which was developed and built in order to simulate the parameters in a flame tube of an oil heating device.Metallographical investigations of the tested samples revealed that the oxide scale present on the uncoated based alloy was formed only partially, whereas in the case of the MCrAlY coatings the oxide scale has an uniform distribution and a good stability due to the presence of the β-NiAl phase which is considered the Al-reservoir for its formation.