Metal Adherend Surfaces

Real joints do not of course consist of simple, separate, elastic materials with a clear mathematical geometry. Metal adherend surfaces are micro-rough, possessing oxide layers, while concrete surfaces are macro-rough comprising aggregates and cement paste, and both surfaces readily adsorb air-bome contamination. The thickness and modulus of primer layers, if employed, is often unknown, and the thickness and properties of the adhesive layer are difficult to regulate and to determine. 

ISS is most useful in the analysis of metal adherend surfaces as a characterization technique prior to bonding, or for analysis of failure surfaces. 

Metallic adherends were cut from. 032" (.08 cm) thick sheets into lx4-inch (2.54 cm x 10.16 cm) specimens. Cleanliness of the surface, is required in order to facilitate good adhesion. Steel, as bonded within the automotive industry, often experiences a variety of surface contaminants which are not removed prior to bonding. 

Since the choice of surface pretreatment prescribed for a metallic adherend has a direct effect on the performance of a joint in humid conditions, four types of commonly utilized automotive surface preparations were examined. The effects upon durability of no cleaning, alkaline cleaning, lubricating or zinc phosphating were examined. Accordingly, adherends were prepared using one of the four methods detailed below. 

The Effect of Adhesive Primers. In practice, adhesive bonds involving metal adherends often use primers as pretreatments of the metal surface prior to bonding. Table IV shows the durability of composite-metal bonds prepared with adhesive C over a series of primers (of varying corrosion resistance) in 240 hour salt spray test. The results indicate that the performance of bonds is directly related to the corrosion resistance of the primer used to prepare the adherend surface. In general, the adhesion of the primer to the steel adherend, rather than the adhesive chemistry. 

Based on the studies of adherend surfaces by Fowkes 12), Parks 13) and Zisman and Shafrin 14), Bolger 15) developed a model (Fig. 1) which depicts the fundamental characteristics of the hydrated oxide surface of any metal, metal oxide or silicate. 

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