The process of osmosis is a natural phenomenon involving the movement of SINGLE small molecules through a " porous or simi-permible " film. The skin of tomatoes will crack after swelling from moisture ingress. Pickles will shrink as moisture is lost from a cucumber soaking in brine. ALL polymeric organic films are subject to small molecule migration within the film. This WILL occur to varying degrees dependent on factors such as the polarity of the migrating specie, degree of cross-linking of the polymer matrix, temperature, amount of formulated inert mineral fillers, and a myriad of other factors. Osmotic activity in polymeric paint films is a common coating failure mode. Corrosion of the metal substrate, typically, will follow if not protected by an active cathodic electrical potential, chemical additives, or other means.
Osmosis involves the transmission of single molecules through the matrix. As such, since the definition of a gas is " a collection of single molecules ", osmosis is a gas phase transmission and follows the General Gas Laws. An understanding of the process of osmotic activity in paint films will assist the practitioner in determining the source that drives the osmosis and in analyzing the paint failure.
Osmotic activity in paint films is explained and the analytical procedures and instruments used to identify its driving source(s) are discussed. Osmotic activity requires a solvent and solute. Not all solutes are introduced during application or manufacturer of the coating. While some are present at or during application, some are generated insitu from chemical reactions during service. The source and chemistry of some of these are discussed. Cathodic protection (CP) generates hydroxyl ions that cause additional osmotic activity. Fermentation of sugars generates acetic acid that can lead to osmotic activity in tank linings.