Page 218 - Handbook of Adhesion Promoters

10

Adhesion and

Corrosion Protection

It can be inferred, even without any studies, that a well-adhered coating acts as an effec-
tive physical barrier that slows/prevents the arrival of corroding substances to the coating/
1
metal interface. It is true for at least two reasons:
• corrosion requires water and oxygen which have to reach metal surface
• once corrosion begins, the reaction of oxygen with metal leads to the formation
of oxides which are more voluminous than the metal itself and therefore
expanded volume lifts coating from the surface which then loses its protective
value.
For a coating to have preventive features against corrosion, it has to uniformly cover
the surface of the substrate and form a chemical bonds. The formation and retention of the
uniform cover by a coating is purely physical process, which depends on such factors as
• surface preparation (elimination of dust particles; removal of surface contami-
nants which may prevent uniform coating and chemical bonding)
• coating rheology (it permits coating to flow to fill surface irregularities of sub-
strate; the rheology of coating permits formation of uniform thickness of coating
free of surface deformations caused by the coating application; rheological prop-
erties regulate retention of coating on the surface because of its thickening when
the shearing effect of coating process stops)
• curing (too fast curing prevents formation of a full contact with the surface of
substrate; too slow curing may cause formation of thickness variations in coating;
change of rheology during heating my cause flow-off coating especially on verti-
cal or slopped surfaces; rapid evaporation may cause formation of bubbles and
blisters; rapid cooling may cause formation of stress within the coating; sudden
temperature drop may prevent migration of elements in coating which could oth-
erwise form chemical bonds with a substrate; overheating may cause degradation
of coating and loss of its barrier properties; etc.)
• environmental factors (excessive moisture presence may change character and
direction of curing process and cause penetration of moisture to the interface; dry
conditions may prevent formation of bonding with a substrate; temperature
higher than the glass transition temperature of coating may cause deformation of
coating; elevated temperature may cause delamination due to the differences in
thermal expansion of coating and substrate; UV degradation changes chemical