The grain phase has a significant influence on the extent and mechanism of
attack of low-cement castables with similar bond systems in a crucible corr
osion test. The general corrosion mechanism, as determined by post mortem s
canning electron microscopic examination of corroded samples and thermodyna
mic calculations for all four grain types examined (white-fused alumina (WF
A), tabular alumina (TA), brown-fused alumina (BFA), and alumina-rich spine
l (S)), involves initial reaction of the most penetrating slag (enriched in
calcium, manganese, and iron, because these elements diffuse rapidly) with
the fine alumina and calcium aluminates of the matrix, This reaction gives
a CaO-rich local liquid, which can then react with each grain predominantl
y to form calcium hexaluminate (CA(6)) and hercynitic spinel, In the WFA sy
stem, a complete CA(6) laver forms around the grain, whereas in the TA syst
em, this layer is incomplete, In both systems, extensive penetration occurs
, although corrosion is low, In the BFA system, titanates are released from
the grain into the bond, leading to increased densification of the refract
ory, via liquid-phase sintering, and consequent low penetration. Hoc-ever,
the resulting fluid liquid dissolves easily in the slag, so that corrosive
wear is high, even though a Cli, layer forms around the grain. In the S gra
in system, uptake of the rapidly diffusing cations into the spinel crystal
structure leads to silica-rich and viscous local liquid which leads to low
penetration and corrosion.