From the corrosion protection mechanisms of paint coatings, good adhesion to the underlying material is obviously highly desirable. Adhesion is thought to be a combination of several mechanical, chemical and physical effects (the relative importance attached to each of these has varied somewhat over the years).

These bonding effects all rely on intimate contact between the underlying material and the coating, down to the scale of the molecular level. Effective wetting of the material to be coated is therefore important. Note that, with all other being things equal, a higher surface area provides more mechanical and/or chemical and/or physical anchoring (bonding) sites.

1. Mechanical interlocking: "Lock and key" effect, mechanical anchoring on porous and roughened surfaces. Surface roughening increases the effective surface contact area. Absorption of liquid coating into the substrate is relevant (analogy of a liquid being absorbed by a porous sponge).
   
   
2. Adsorption: "Chemical" bonding effects ranging from strong chemical bonds (chemisorption) across the bond interface to weaker van der Waals forces* (physisorption) that are based on temporary of permanent dipole interactions.
   
   
3. Electrostatic: Double layer of opposite charges created at the bond interface (a simple analogy is that of a charged capacitor).
   
   
4. Diffusion: Applicable to polymeric coatings on polymeric substrates and with the substrate being permeable to the coating; diffusion across the bond interface to produce entanglement of polymer chain molecules.

* Such van der Waals forces are now believed to play a major role in the remarkable adhesion feats of geckos, with billions of microscopic keratin hairs on the surfaces of their feet facilitating such dry bonding. (Editor's note: there goes grandma's suction cup theory ...)

[Source: "Gecko tape - research to  drive you up the wall", Materials World, December 2003 p.25-26].

Chemisorption and Physisorption:

Chemisorption: chemical adsorption - by valence forces of the same kind as found in the formation of chemical compounds.

Physisorption: physical adsorption - by intermolecular forces (van der Waals forces) which do not involve a significant change in the electron orbital patterns.

It has been noted that a sharp distinction between chemisorption and physisorption is not always possible, as is the case for chemical and physical interactions in general. (Reference: IUPAC, Division of Physical Chemistry, Manual of Symbols and Terminology for Physicochemical Quantities by D.H. Everett.)

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