© 2006 by Taylor & Francis Group, LLC

Introduction 5
1.2.2 ELECTROLYTIC RESISTANCE
Perhaps the single most important corrosion-protection mechanism of organic coatings
is to create a path of extremely high electrical resistance between anodes and cathodes.
This electrical resistance reduces the flow of current available for anode-cathode
corrosion reactions. In other words, water — but not ions — may readily permeate
most coatings. Therefore, the water that reaches a metal substrate is relatively ionfree
[12]. Steel corrodes very slowly in pure water, because the ferrous ions and
hydroxyl ions form ferrous hydroxide (Fe(OH) 2). Fe(OH) 2 has low solubility in
water (0.0067 g/L at 20° C), precipitates at the site of corrosion, and then inhibits
the diffusion necessary to continue corrosion. On the other hand, if chloride or
sulphate ions are present, they react with steel to form ferrous chloride and sulphate
complexes. These are soluble and can diffuse away from the site of corrosion. After
diffusing away, they can be oxidized, hydrolyzed, and precipitated as rust some
distance away from the corrosion site. The stimulating Cl – or SO 4 2– anion is liberated
and can re-enter the corrosion cycle until it becomes physically locked up in insoluble
corrosion products [16-21]. This mechanism of blocking ions has several names,
including electrolytic resistance, resistance inhibition, and ionic resistance. The
terms electrolytic resistance and ionic resistance are used more-or-less interchangeably,
because Kittleberger and Elm showed a linear relationship between the diffusion
of ions and the reciprocal of the film resistance [22].
Overall, the electrolytic resistance of an immersed coating can be said to depend
on at least two factors: the activity of the water in which the coating is immersed
and the nature of the counter ion inside the polymer [1]. Bacon and colleagues have
performed extensive work establishing the correlation between electrolytic (ionic)
resistance of the coating and its ability to protect the steel substrate from corrosion.
In a study involving more than 300 coating systems, they observed good corrosion
protection in coatings that could maintain a resistance of 108 Ω/cm 2 over an exposure
period of several months; they did not observe the same results in coatings whose
resistance fell below this [23].
Mayne deduced the importance of electrolytic resistance as a protection mechanism
from the high rates of water and oxygen transport through coatings. Specifically,
Mayne and coworkers [7, 24-27] found that the resistance of immersed
coatings could change over time. From their studies, they concluded that at least
two processes control the ionic resistance of immersed coatings:
• A fast change, which takes place within minutes of immersion
• A slow change, which takes weeks or months [26]
The fast change is related to the amount of water in the film. Its controlling factor is
osmotic pressure. The slow change is controlled by the concentration of electrolytes in
the immersion solution. An exchange of cations in the electrolyte for hydrogen ions in
the coating may lie behind this steady fall, over months, in the coating resistance. This
theory has received some support from the work of Khullar and Ulfvarson, who found
an inverse relationship between the ion exchange capacity and the corrosion protection
efficiency of paint films [13, 28]. The structural changes brought about by this ion
exchange might slowly destroy the protective properties of the film [29].
© 2006 by Taylor & Francis Group, LLC