  Typical Weyl semimetals host chiral surface states and hence show an
anomalous Hall response. Although a Weyl semimetal phase is known to be robust
against weak disorder, the influence of disorder on chiral states is not fully
clarified so far. We study the behavior of such chiral states in the presence
of disorder and its consequences on an anomalous Hall response, focusing on a
thin slab of Weyl semimetal with chiral surface states along its edge. It is
shown that weak disorder does not disrupt chiral edge states but crucially
affects them owing to the renormalization of a mass parameter: the number of
chiral edge states changes depending on the strength of disorder. It is also
shown that the Hall conductance is quantized when the Fermi level is located
near Weyl nodes within a finite-size gap. This quantization of the Hall
conductance collapses once the strength of disorder exceeds a critical value,
suggesting that it serves as a probe to distinguish a Weyl semimetal phase from
a diffusive anomalous Hall metal phase.
