Assesment of molten Na₂S· xH₂O/NaOH solutions as a cause of composite tube corrosion in kraft recovery boilers

Cladding/overlay thickness measurements were made on several primary air ports fabricated from alternative composite tubes installed in a kraft recovery boiler to document the fireside corrosion. Laboratory corrosions tests were then conducted to reproduce the relative corrosion rates determined by the field thickness measurments. It was found that all of the major available composite tube systems are suceptible to corrosion. Hydrated sodium sulphide and oxygen in combination with sodium hydroxide are implicated as major components in the liquid environment that causes the corrosion. Prevenative measures discussed include the need for a well-sealed port, and the likely need to avoid having black liquor droplets contacting the port tubes while dehydration is incomplete.