The flux-creep rate associated with twin boundary pinning is calculate
d in the framework of a Brownian motion model. Systematic elimination
of the velocity degrees of freedom exposes the relevant small paramete
r and validates the strong-friction limit for a ''typical'' high-tempe
rature superconductor. The corresponding expression for the vortex-esc
ape probability is obtained in a closed form. For a piecewise linear p
inning potential approximation this expression yields an analytic resu
lt. The ensuing critical current strongly depends not only on the pinn
ing-potential maximum depth, but also on the existence of deep, steep
features in its shape. For a ''realistic'' twin boundary pinning poten
tial, the calculated vortex-escape probability is compared to the Ande
rson-Kim theory. The comparison shows that the Anderson-Kim expression
greatly overestimates the vortex-escape probability. Inter-defect dif
fusion is found to be particularly important in the ''weak'' current,
low-temperature regime.