Waterwall - Fire-Side Corrosion
As the title of this chapter indicates, the type of corrosion next discussed
affects the fire side of waterwalls in coal-fired boilers. Boilers with low
burner-to-side-wall or burner-to-rear-wall clearances are often affected.
Waterwall-fire-side corrosion is frequently found in the wind-box area or
Waterwall-fire-side corrosion may develop when incomplete fuel com-
bustion occurs (reducing conditions). Incomplete combustion causes re-
lease of volatile sulfur compounds, which may form pyrosulfates. Sodium
and potassium pyrosulfates (Na2S2O7 and K2S2O7) may have melting
points of 80O0F (4270C) or less. Both have a high chemical activity. These
molten slags may flux the protective magnetite on tube surfaces, causing
accelerated metal deterioration along the crown of the tube.
Depending on the amount of SO3 in the salt, the melting point may be
less than 77O0F (3960C) for K2S2O7 and around 754
0F (3870C) for Na2S2O7
(Fig. 11.1). As temperature increases, the amount of SO3 required to form a
liquid phase also increases significantly. Consequently, neither sodium nor
potassium pyrosulfates are likely to be present as liquid except on rela-
tively cool surfaces, such as the waterwalls.
Insufficient oxygen in the burner zone is a primary factor in development
of waterwall corrosion. Poor combustion conditions and steady or inter-
mittent flame contact with the furnace walls, combined with coals that are
capable of forming an ash with a low fusion temperature, produce a hot,
fuel-rich corrosion environment.
Waterwall-fire-side corrosion is frequently characterized by metal loss
along the crown of the tube and may extend uniformly across several tubes
in a particular location (Fig. 11.2). Corroded regions may be covered by
abnormally thick layers of iron oxide and iron sulfide corrosion products.
At times, flow patterns of the liquid pyrosulfates are apparent in regions of