Page 633 - Corrosion Engineering Principles and Practice

596 C h a p t e r 1 4 P r o t e c t i v e C o a t i n g s 597

exposed to atmospheric humid environments with one important
difference—that is, how the initial charge imbalance is subsequently
satisfied by the movement of ions attracted to their opposite charges.
In the case of uncoated steel, the route for the counter-cations and
counter-anions to respectively the cathodic negative site or the
anodic positive site is straightforward. However, when the steel is
coated the situation becomes more complicated. The path from the
exposure environment to these sites may be either restricted or
blocked completely when the coating adheres well to the substrate.
In such cases the corrosion process would be relatively stifled after
the initial attack.
For less adherent coatings, available pathways (micronic dust,
coating porosity, and holidays) for counter ions can be much less
restrictive and the corrosion reaction would be allowed to proceed at
a much faster pace. When this happens a second cathodic reaction
can be triggered by the accumulation of protons at the anodic site to
produce gaseous hydrogen [Eq. (14.4)]. Molecular hydrogen (H ) is a
2
highly active gas that can simply pry the coating loose.
+
−
2H + 2e → H (g) (14.4)
2
Another aggravating factor is that organic coatings are generally
poorly resistant to alkaline conditions and may be attacked by the
hydroxyl ion causing a serious loss of surface adhesion. The reason
this alkalinity causes such failure has been variously ascribed to
saponification of the coating, dissolution of the oxide layer at the
interface, and alteration of the ionic resistance of the film [6]. External
cathodic currents provided by cathodic protection or internal currents
produced by inorganic zinc additives during immersion service, for
example, would increase the possibility of failure by the hydrogen
and hydroxyl formation because much greater quantities of cathodic
reaction products would be created.
In addition, external or internal cathodic currents on the steel can
force more water through the coating than would be the case without
such currents. The resulting forced diffusion of water through the
coating is termed electro-osmosis (not to be confused with osmosis
which occurs when water is drawn at a higher than normal rate
through the film by a soluble salt lying beneath the coating). Thus,
cathodic currents of sufficient magnitude can strip coatings from a
steel surface. It has been demonstrated half a century ago that organic
coatings were so permeable to water and oxygen that their rate of
permeation to the cathodic region was greater than the amount
required for corrosion to proceed [7].
One particular feature that has been identified by a number of
workers is a delay time or initiation period between a coated substrate
first being exposed to a corrosive environment and the start of the