Protection of pearlitic steel from high-temperature gaseous corrosion

Conclusions

• 1. The structure of the coating cannot be developed prior to heat treatment.

• 2. The laminated appearance of the coating after heat treatment is explained by its consecutive deposition.

• 3. After tests at 650°C in steam and air, the layer's structure consists of a solid solution of phosphorus in nickel with inclusions of the excess phase Ni₃P which becomes coarser with increasing temperature or heating time.

• 4. After prolonged holding (1000 hrs) at 650°C, the hardness of the deposit beneath the oxide film retains a high level (Hμ=728) which gives these coatings a good potential in applications involving friction at normal and elevated temperatures.

• 5. A layer of the nickel-phosphorus coating 30–50μ thick, gives stable protection to low-alloy, pearlitic steels from gas corrosion in elevated temperature service in air or superheated steam at up to 650°C.

• 6. During oxidation of specimens chemically coated with nickel, a very hard oxide film forms on their surface, expecially in contact with superheated steam; this makes them suited for covering parts exposed to wear, particularly for steam turbine components.