Diffusion of Water in Steel‐to‐Steel Bonds

Abstract

The long‐term durability of steel‐to‐steel bonds is affected by the ingress of water in a bondline. Sorption of water by an adhesive changes its physical properties and degrades the adhesion between the adhesive and the steel. Diffusion coefficients and equilibrium water concentrations are reported for a specific poly‐methyl methacrylate (PMMA) based adhesive. Changes in modulus with water concentration are reported. The effects of water immersion on the rapid tensile strength of circular butt bonds are determined; the ingress of water produces annuli of adhesion failure. The scale of the bond affects the fraction of the initial strength that is retained after a period of immersion. Fick's diffusion equation is used to compute evolutionary water concentration profiles in bondlines subject to cyclical boundary concentrations. The diffusion coefficient of the particular adhesive is such that daily boundary concentration fluctuations are filtered from all the bond area except an annular region extending about 0.07 in. in from the boundary; yearly boundary concentration fluctuations are also filtered except for an annular region extending about 0.6 in. in from the boundary.