Influence of silane-based treatment on adherence and wet durability of fusion bonded epoxy/steel joints

Three layers polyolefin coatings are widely used in Europe to protect pipelines against corrosion. Loss of adhesion at a fusion bonded epoxy (FBE)/steel interface has occasionally been observed even on pipelines without external defects. Silane-based surface pre-treatments are developed to improve adhesion with limited impact on the environment unlike usual chromate conversion pre-treatments; however the mechanisms involved in silane action need to be more deeply understood. The application process of silane surface treatment implies a series of key parameters of which optimization is imperative to form a silane layer with good properties at the interface. This paper studies the influence of the cure temperature and the pH of an aminosilane based pre-treatment on adhesion strength and durability via single lap shear tests. SEM/EDX, FT-IR and XPS analyses are used to characterize the silane and silane/epoxy structures. Silane pretreatment improved the FBE joints durability and appears as a relevant solution to replace the usual chromate conversion pretreatments. The joints pretreated at pH 10.6 had better durability than at pH 4.6. This was related to the formation of a bridge across the silane/epoxy interface at pH 10.6, whereas, at pH 4.6, formic acid reacts with amino groups of silanes, limiting the reaction between amino groups and epoxy.