Alkali Induced Corrosion of 304L, The Influence of Potassium Salts and Water Vapour
This work investigates the influence of alkali salts on the oxidation of
304-type (Fe18Cr10Ni) austenitic stainless steel in laboratory and field
exposures. Stainless steel coupons were exposed isothermally in
laboratory furnaces at 400-600 °C in 5% O2 and 5% O2 + 40% H2O.
Thermobalance exposures were also performed. The samples were coated with
KCl, K2CO3 or K2SO4 prior to exposure. The amount of salt was equivalent
to 0.1 mg/cm2 KCl, based on potassium. The exposure time was 1-168 hours.
Uncoated samples were exposed for reference. In the field exposures,
sample rings mounted on a cooled probe were exposed in the 75MW waste
fired boiler at Händelö, Norrköping, Sweden. The fuel was a mix of
household and industrial waste. The exposure time was set to 10 hr and
the material temperature was 450 °C. The flue gas temperature was 650°C.
Field exposures were performed with and without adding sulphur to the
fuel. The samples exposed in the field and in the laboratory were
analysed by SEM/EDX, XRD, FIB, TEM, IC and AES.
The results from the laboratory exposures show that KCl and K2CO3
strongly accelerate the high temperature corrosion of 304L. Corrosion
starts by the formation of potassium chromate(VI) through the reaction of
potassium with the protective oxide, i.e. the initiation step.
Chromate(VI) formation is a sink for chromium in the oxide and leads to a
loss of its protective properties. This results in rapid oxide growth. In
the propagation stage there is a thick scale consisting of an outer
porous hematite layer with K2CrO4 particles and an inner layer consisting
of spinel oxide, (Fe,Cr,Ni)3O4. The duplex scale is relatively poorly
protective and is permeable to, e.g., chloride ions. In contrast to KCl
and K2CO3, K2SO4 does not react with chromium in the oxide to form
chromate. Therefore, potassium sulphate does not destroy the protective
oxide and is only slightly corrosive towards 304L.
The field exposures focused on the effect of adding sulphur to the fuel
on corrosion. The results showed that adding sulphur to the fuel reduced
the amount of deposits and corrosion products formed. Chromate(VI) was
detected only on samples exposed in the absence of sulphur