Previous work has shown that mixed oxidant corrosion below 600 degrees
C is characterized by the formation of outward growing Fe and Ni rich
sulfide scales and inward growing Cr oxide and sulfide precipitation
zones and/or scales. In syngas without HCl a somewhat protective FeCr2
S4 scale is formed below the outward growing sulfide scale. When the o
xygen pressure of the gas is increased this layer becomes gradually en
riched in oxygen, but corrosion losses only drop significantly,when th
e scale consists of FeCr2O4 only. In this paper we will review the eff
ect of HCl on the corrosion process. In addition, the effect of deposi
ts containing 5-15% chlorides and exposure to high humidity conditions
during downtime will be discussed. The data indicate that the presenc
e of HCl or chlorides will cause accelerated corrosion, once a thresho
ld chloride level is reached. For many alloys the threshold level is v
ery high and is only reached in the presence of chloride rich deposits
. High humidity conditions during downtime can cause the formation of
chloride rich liquids in deposits, which may concentrate at the scale-
metal interface, thus causing accelerated corrosion during subsequent
exposures. In the worst case, aqueous corrosion products can form belo
w the high temperature protective scale, thus causing scale spallation
, which will further increase corrosion losses. Fitting during downtim
e corrosion is also frequently observed.