  We consider the problem of finding pairs $(\lambda; u)$, with $\lambda > 0$
and $u$ a harmonic function in a three dimensional torus-like domain,
satisfying the nonlinear boundary condition $\partial_{\nu} u = \lambda \sinh
u$ on the boundary. This type of boundary condition arises in corrosion
modelling (Butler Volmer condition). We prove existence of solutions which
concentrate along some geodesics of the boundary as the parameter $\lambda$
goes to zero.
