Volume 3 Paper 5
The Protective Action of Organic Coatings on Steel: A Review
David Greenfield and David Scantlebury
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JCSE Volume 3 Paper 5
Submitted 9th August 2000
The Protective Action of Organic Coatings on
Steel: A Review
David Greenfield and David Scantlebury
Corrosion and Protection Centre, UMIST, PO Box 88 Manchester M60 1QD, UK
This review considers the past fifty one years of study in the field of
corrosion prevention by organic coatings. It concentrates mainly on the
mechanisms associated with the paint binder. Areas covered include the
importance of the coating ionic resistance and the concept of D and I areas.
The effect of the environment on the ionic resistance is examined.
Blistering and delamination is considered in the light of movement of ions
through coatings and along interfaces. Specific attention is paid to the
metal paint interface with emphasis on adhesion, underfilm contamination and
surface tolerant coatings.
§2 Keywords: anti-corrosion paints, ionic transport, blistering,
cathodic disbonding, underfilm contamination, surface tolerant coatings
§3 In this review, we are concerned with the effects of the polymeric binder
and the anti-corrosion properties of paints. Aspects of corrosion inhibition
by anti-corrosion pigments have not been emphasized. Also within the field
of underfilm corrosion and blistering, we have not considered the the highly
important and topical area of filiform corrosion.
Inhibition of Corrosion by Barrier Coatings
§4 Any discussion which considers the prevention of aqueous metallic
corrosion, usually takes as its starting point the electrochemical model of
the corrosion of mild steel in a neutral electrolyte with the four important
processes involved linked in series namely; the anodic reaction, the cathodic reaction
and the conductive
pathways for ions and electrons. Inhibition of the anodic or cathodic
reactions or an interruption of the ionic flow will cause the rate of
corrosion to be significantly reduced. Thus, all
corrosion prevention measures are aimed at removing or suppressing one of
the three elements. One of the most convenient, and
certainly the oldest method of protecting a substrate from the detrimental
effects of the environment, is to coat it with a polymeric barrier paint to
isolate it from its surroundings.,
This barrier might be due purely to the properties of the polymer, or to the
inclusion of inert pigments that act to increase the length of the diffusion
path through the coating.
§5 The inclusion of inert pigments into the film can increase the
effectiveness of the coating providing the formulation is not too heavily
loaded. Too high a concentration of pigment is liable to reduce the
effectiveness of the coating. The Critical Pigment Volume
Concentration has been shown to be a transition point where a number
of significant features of the coating change.,
The loading of a coating with pigment particles is made more complex
by the effect of the distribution of pigment particle size.
A wide range of pigment particle sizes favours a high Pigment Volume
� is PVC the same as CPVC � define it!
§6 The nature of this barrier has been the subject of much work over many
years by workers in the field of protective organic coatings. The
original assumption was that organic coatings act as a barrier to water and
oxygen from the environment.
of the subject over the last half century have determined that the limiting factor in the protective
mechanisms of barrier coatings is frequently their resistance to the flow of ionic
Electrolytic Resistance of Organic Coatings
§7 Figure 1 : Schematic of resistance behaviour of coatings in
immersed metals after ref 
§8 The importance of the electrolytic resistance of organic coatings has
long been appreciated. Bacon, Smith and Rugg,
after measuring the resistances of over 300 coating systems, determined a
direct correlation between these resistances and the ability of the coating
to protect the underlying steel from corrosion. Three general
classifications, based on a sustained resistance value, were established
during this investigation: good, fair and poor - see Figure 1. The
classification that a coating was allocated was dependant upon its long term
resistance value upon immersion in solution �Most coatings, if continuous,
have resistances in the neighbourhood of Log R = 9 during the first 5 to 10
minutes of immersion, followed by a decrease in resistance which may vary
considerably in steepness and duration�, the units of R were Ohms for one
sq. cm. Their subsequent behaviour determined which category they
should be placed into. All coatings were found to exhibit an initial
decrease in resistance, which varied in terms of rate and duration.
For a good coating, this initial decrease was followed by an abrupt recovery
to around the original value. The subsequent resistance of the coating
remained either essentially constant or fluctuated within the high
resistance region. A coating designated as fair either levelled off or
displayed a slight increase in resistance. However, a subsequent sharp
drop in resistance was a pre-cursor to coating failure, which occurred
within six months of the exposure period. The resistance of a poor
coating continued to decrease resulting in failure within sixty days.
A coating that maintained a resistance of 108 Ohm cm2
provided good corrosion protection while one whose resistance fell below 106
Ohm cm2 did not.
§9 Although this work identified the relationship between the ionic
resistance of the coatings and their ability to protect, the permeability of
other corrosive agents other than ionic species was not considered.
Therefore, the results were presented as providing a measure of the likely
protection that could be expected from a particular system rather than
proposing a mechanism for the protective action of organic coatings.
The Effect of the Environment on the Performance of Coatings
§10 A further important result from the work of Bacon, Smith and Rugg
identified the effect of the environment on the protective merit of coated
submerged steel. One of the variables taken into consideration was the
level of dissolved salts in the test solution. It was found that the
resistance of the coating fell with increasing salt concentration and from
this result it was concluded that the dissolution of environmental water
into the coating was more important than the uptake of salt from the
solution by the coating. That the level of dissolved salts of the exposure
environment had an effect on the properties of coatings had been reported
previously by Kittelberger and Elm .
This work showed a relationship between the level of dissolved salts in the
solution, expressed as osmotic pressure, and the water uptake of the films
under investigation, expressed as percentage weight gain. It was shown
that the nature of the solute did not affect the results; rather it was the
osmotic pressure that determined the level of imbibition is this a word? of
water into the coating. When the coatings were exposed to a salt
solution, after an initial high rate of water absorption, the curve levelled
off and eventually an equilibrium was attained. Where the films were
exposed to distilled water, however, no equilibrium was reached and the film
continued to gain in weight. It was concluded that the degree of water
absorption was a function of the activity of the water in solution, due to
the differences in osmotic pressure between the exposure solution and that
within the film, see Figure 2.
§11 Figure 2: The relationship between osmotic
pressure and water uptake after ref 
Permeability of Organic Coatings to Corrosive
§12 In order that a paint coating should offer protection from corrosion, it
should interact with one or other of the processes involved in the overall
series of corrosion reactions. This was
considered by Mayne, who
determined that the permeability of organic coatings to both water and
oxygen is so high that the rate at which they arrive at the interface of the
coating and the steel is greater than that required for corrosion to proceed
and so could not be rate determining. Therefore, Mayne reasoned that the
method by which an unpigmented coating protects steel from corrosion was by
the introduction of a path of high electrolytic resistance in between the
metal and the environment; the term resistance inhibition was coined for
Selective Permeability of Organic Coatings
§13 Further work by Mayne 
showed that, upon immersion in water or an aqueous solution of electrolyte,
most organic coatings acquire a negative charge. According to Mayne,
the acquisition of this charge has the effect of creating a selectively
permeable membrane, which is preferentially permeable to cations; that is, a
film that has gained a negative charge due to immersion may be regarded as a
large poly-anion. An alternative view is expressed by Corti et al ,
who claim that a charged film would slow down the passage of counter ions
(those having the opposite charge) whilst having no effect on the movement
of co-ions through the film.
§14 These contradictory views stem from the proposed mode of transport of
ions across the film. According to Corti, the rate of permeation
across the film is affected by the presence of small imperfections or pores,
which extend through the film and have cross sections distinctly larger than
the free areas normally present between the atomic groups in the membrane
matrix. Mayne�s model, on the other hand, infers that the passage of
ions is through the bulk matrix of the film. If the ionic conduction
takes place through pores, it is reasonable to claim that a negative charge
on the polymer would impede the passage of cations due to electrostatic
attraction between the ions and the pore walls. However, if the
movement of ions through the coating is considered to be through the bulk of
the film, the proposal that a negative charge on the film would act as a
repulsive force towards anions seems plausible.
§15 Transport of cations through polymer films has been observed elsewhere
with the passage of iron ions through a range of coatings on steel immersed
in NaCl. These coatings were continuous and free from defects and the
ionic transport was considered to have taken place through the bulk
polymer. Of the coatings tested, a �practical anticorrosive coal tar
epoxy� was found to retard the process. The inference of this work
is that the conduction of ions through the film takes place through the bulk
polymer. The work reviewed in the following section proposed
mechanisms to account for this.
Mechanisms of Through Film Transport of Ions
§16 Ionic conduction through the bulk of the polymer matrix was studied by
Maitland and Mayne.
This work, which mainly considered the properties of an unattached,
unpigmented pentaerythritol alkyd varnish, found that the resistance of the
film was affected by the state of the exposure solution and identified two
distinct processes, which took place in succession of each other: The Fast
Change and The Slow Change. The Fast Change was observed to take place
within a few minutes of immersion where the films attained a steady
resistance value, Ro. This was followed by The Slow Change,
which took place over a number of weeks or months.
§17 The Fast Change was shown to be a reversible process and was independent
of both the nature of the solute in the exposure solution and the state of
the film in respect of the slow change. The controlling factor in the
process was seen to be the osmotic pressure or water activity of the
solution; this process is shown in Figure 3
§18 Figure 3: The Fast Change;
Comparison of the effect of 3.5N KCl with isotonic sucrose solution after
§19 The Slow Change was considered to be controlled by the concentration of
the electrolyte in solution. This was confirmed by experiment, whereby
the reaction was accelerated by an increase in temperature.
§20 Maitland associated this phenomenon with an ion exchange process where
cations from the electrolyte were exchanged with hydrogen ions from carboxyl
groups within the polymer. The ion exchange behaviour was found to be
affected by the pH of the system or the concentration of potassium ions in
solution. An increase in either the pH or the potassium concentration
resulted in a greater rate of ion exchange, causing a fall in the resistance
of the coating.
§21 Work on the effect of the Slow Change on the reversibility of the Fast
Change was carried out by Cherry and Mayne,,
who found that the coatings under investigation altered their mode of
conduction at a given value of pH, the value of which varied according to
the particular coating. Working on the assumption that the slow change
was an ion exchange process, it was postulated that the resistance of the
film should be a function of the ratio of the activities of the metal cation
from the electrolyte (in this case potassium) and that of the hydrogen
ions. The term rK was defined, where:
§22 Films were conditioned in solutions of various rK values and then
the temperature coefficient of resistance for each film was measured.
It was found that there was a small range of rK values, specific to
each polymer, where the mode of conduction ceased to run counter to that of
the solution and commenced to follow it. The point where this change
in the conductive behaviour of the film took place was named the reversal
point and was seen to indicate a change in the mechanism of ion transport
from one of activated diffusion to that of aqueous conduction. This
conduction was considered to take place through virtual pores.
§23 The model whereby the electrolytic conduction takes place through pores
is supported by Miskovic et al
who, as a result of their AC impedance examination of electrodeposited
epoxy, proposed a model whereby electrolytic conduction through a coating is
dependant only on conduction through macro-pores. These pores were
examined microscopically and were determined to have radii between 3 and
Modes of Ionic Conduction Through Polymer Films
§24 The question of the possibility of conduction through pores was addressed
This work identified different modes of ionic conduction through polymer
films. Two distinct forms of conduction were identified. The
first was I, or inverse type conduction, where the resistance of the film
coincided with that of the solution; this type of conduction was in line
with that identified by Bacon, Smith and Rugg .
§25 The second mode of through-film transport that was identified was named
D, or direct type conduction where the film�s resistance ran counter to
that of the solution�s. Initially, the films that exhibited D type
conduction were thought to be due to coating deficiencies such as dust wicks
passing across the film. However, investigation of the temperature
coefficients of resistance for these films showed them to be higher than
that of aqueous conduction. In addition, the films were shown to be
preferentially cation permeable and thus considered to be continuous.
§26 Figure 4: Schematic plot of I
and D type conduction, after ref 
§27 A series of papers were produced as a result of this work, which
addressed various different factors affecting ionic conduction in polymer
films. The first,
examined the influence of the electrolyte on the resistance of films.
It was found that the films investigated followed the general trend
exhibited in Figure 4. However, when an I Type film was exposed to a
solution of very low electrolyte concentration, and therefore a high water
activity, the film displayed D Type characteristics over a small range of
concentrations, at the lower end of the scale. Once the electrolyte
concentration reached a critical level, the film began to exhibit more
typical I Type characteristics. This led to the oft quoted conclusion that the
difference between D and I Type conduction was �more one of degree rather
than of kind�.
§28 Scantlebury examined the distribution of D and I Type areas across a
given film. It was
found that the films investigated were all very inhomogeneous and had a
distribution of both D and I Type areas over the film. Two
conclusions, relating to the mechanisms responsible for the different modes
of conduction were drawn from this investigation. Firstly, that D Type
conduction could not be attributed to the presence of pores unless they were
of molecular dimensions.
§29 The second conclusion that was drawn from this study, was that the
resistance of both D and I Type areas bore a relationship to their hardness
values, as determined via microhardness tests. D Type areas were found
overall to have a lower hardness value than that obtained for I Type.
The hardness of the film was equated to its crosslinking density. This
result was reinforced by swelling experiments, which showed that D Type
areas swelled to a greater degree than I Type. Therefore, it was
concluded that D Type conduction was a function of lower crosslink density
in the film.
§30 This conclusion reinforced Kinsella�s view 
that the initial drying conditions had a marked effect on the final
properties of the film. Other work on crosslink densities of polymer
films has shown that the crosslink density appears only to affect the ionic
conductivity of the film.
The study was concerned with the effect of crosslink density upon water
permeation through films and concluded that the permeation of water was not
noticeably affected by alteration of the crosslink density. The final
two papers in the series by Mayne and co-workers considered the influence of
temperature and finally the
effect on the barrier properties of a coating due to the inclusion of inert
§31 Evidence that D Type areas are detrimental to the protective properties
of organic coatings was produced by Mills, 
who showed a direct relationship between the presence of D Type areas in the
film and the occurrence of underfilm corrosion. With regard to the
term �virtual pores� put forward by Cherry17 to explain the
mode of conduction in an apparently continuous film, Mills suggests the term
�conductive polymer phase� as more appropriate.
This term is intended to describe more accurately the apparent properties of
D Type films.
§32 Mills further observed that the proportion of D Type areas was a function
of the thickness of the film.
It was found that the incidence of D Type areas decreased significantly once
the film�s thickness was increased to around 75μm, this finding was
used to conclude that D Type areas were spherical and approximately 75μm
Breakdown Mechanisms of Coatings
Blistering and Delamination
§33 Amongst the most common forms of failure found in organic coatings are
those of blistering and delamination. Much debate has ensued as a
result of the research into these failure modes and a number of governing
mechanisms have been put forward. The mechanisms involved in the two
modes of failure are similar, but it is unclear whether they are the same
§34 Within each of these failure modes, there are sub-classifications, which
may or may not be important for a particular system. The components of
a system are the substrate, the coating and the exposure environment.
Factors affecting the performance of a system include surface preparation,
coating application, cure regime and film integrity.
Factors Affecting Blister Initiation
§35 The blistering mechanism of paint coatings upon exposure to aqueous
environments was addressed by Gay,
who found that all the systems examined displayed four common features:
The higher the osmotic pressure of the immersion liquid, the smaller
the amount of blistering.
The fluid in blisters formed on seawater immersion is almost
invariably alkaline and its chloride content is lower than that of the
The steel under such blisters is usually bright and free from
Areas of blistering are frequently associated with adjacent areas of
§36 In the majority of cases, before any blistering occurred, some corrosion
was observed at weak points in the coating and no substantial amount of
blistering was found where corrosion was not present. The suggested
sequence of events leading to the formation of blisters is as follows.
The paint film imbibes water from the solution, possibly containing
some dissolved salts.
Eventually, sufficient liquid containing chloride ions passes through
to the underlying metal and activates primary corrosion sites at the
As the corrosion proceeds at the anodic areas under the film, hydroxyl
ions build up at the cathodic sites.
The alkaline environment at the cathodic sites weakens or destroys the
adhesion of the film whilst producing osmotically active substances at
the paint metal interface.
The presence of these substances at the interface is favourable to
either osmotic or endosmotic passage of water through the film from the
§37 The principle of endosmotic transfer of water through the coating was
supported by Mayne  who
concluded that the contribution of osmotic transfer of water was much
smaller; 6% in the case of the coatings studied.
§38 Figure 5: Stages in the
development of an osmotic blister.
§39 Subsequent work on blistering has produced three individual
§40 This form of failure is one consequence of a contaminated substrate.,
Soluble salts at the interface can form a concentrated salt solution that,
due to decreased water activity, acts to draw water through the coating,
which behaves as a semi-permeable membrane, from the exposure
environment. The osmotic pressures exerted can be expected to range
between 2500 and 3000 kPa compared with a mechanical resistance to
deformation of a coating ranging from 6 to 40 kPa
In addition, the pressure within the blister becomes greater than the
atmosphere on the free side.
So, provided the film is not ruptured in the process, the conditions exist
for a continually expanding blister.
§41 Figure 5 schematically represents the stages in the formation of an
osmotic blister. In Figure 5A a layer of interfacial water in contact
with soluble salts is shown, in B the salts at the interface have formed a
concentrated solution and finally C represents the stage when the osmotic
pressure, produced by the concentration gradient across the coating, has
drawn water through from the outside environment and formed the
blister. The foregoing is attractive due to its simplicity;
however, it has been shown29 that electroosmosis plays a predominant role in
this form of blistering with osmosis only contributing to about 6%.
§42 Gowers and Scantlebury
were able to measure currents flowing between different areas of the
substrate under artificial blisters. They found that contaminating the
surface of the substrate with NaCl promoted the corrosion process, the
current flow and the blister formation.
§43 One suggestion that has been made to reduce the tendency for a coating to
suffer osmotic blistering hinges on the formulation of the film. In a
solvent borne coating, it is normal to find that a combination of different
solvents is employed. According to Storfer and Yuhas, hydrophilic
solvents can facilitate and accelerate several of the generally recognized
blister-formation mechanisms, particularly osmotic blistering.
An inverse relationship was determined between the solubility of water in
the last solvent in the formulation to evaporate (the tail solvent) and the
blister resistance of the coating. Hydrophilic solvents are more prone
to cause blistering.
§44 This mode of failure was addressed by Koehler,
who considered liquid filled blisters to be anodic in nature.
Experiments were conducted an on epoxy phenolic resin exposed to 0.01N
calcium chloride. Calcium was chosen as the cation because it does not
produce an alkaline environment. No deliberate faults or holidays were
introduced into the coating. The samples were anodically polarised and
blisters appeared after seven days. The substrate in the centre of the
blister was dished and corroded while the periphery was bright, indicating
cathodic detachment. The explanation of these results was that during
the test, chloride ions passed through the coating at thin points and
created an acid environment under the coating within the blister. The
cathodic reaction was postulated as being the reduction of the ferric oxide
on the steel to the soluble ferrous state. This type of failure
requires a low pH environment to proceed. Such low values may be found
in many canned foods and drinks.
§45 It is to this form of blistering that the majority of the literature is
directed. Cathodic blistering is the result of an alkaline environment
under the coating caused by the cathodic reaction, associated with corrosion
that occurs at a damaged site of the film.,
Early work by Kittelberger 
identified a relationship between bare areas of steel and the occurrence of
blistering on immersed panels; it was shown that an exposed area of of
the coated specimen was enough to exert a significant influence on the
blistering of the coating. Figure 6 shows the proposed failure
mechanisms, which may occur when a film containing a fault is exposed to a
§46 The fault may take the form of mechanical damage to the coating or may be
an inherent fault of the coating i.e. pores or holidays. The primary
pre-requisite for this form of failure is that the substrate should support
a cathodic reaction. In the case of a neutral or alkaline environment the cathodic reaction would be the
reduction of oxygen. Tests with cathodically polarised steel coated
with polybutadiene, exposed to a NaCl solution,
developed blisters containing a solution of high pH after 7 days, which
coalesced in 18 days; no intentional damage was made to the coatings in this
§47 Figure 6: Possible
consequences of a damaged coating.
§48 As no defects were introduced into the coating material, the question of
the route taken by the reactants is raised. Mayne12 showed that the
transmission of water and oxygen through the film was in excess of that
needed for corrosion and attributed the protective mechanism to the film�s
high ionic resistance. If we accept this hypothesis, some pathway must
exist through the film to allow the sodium ions to the interface in order to
produce the alkaline environment. These pathways could be due to
pores. However, an alternative theory is proposed by Leidheiser,
who in a cautionary technical note suggests that above a given
concentration, alkali cations may have a deleterious effect on the coating,
which leads to morphological changes hence introducing conductive pathways
to the interface.
§49 Akin to cathodic blistering, cathodic delamination is also the result of
alkalinity at the interface. Again, this alkalinity is the result of
cathodic activity under the coating. It is associated with faults,
either inherent or induced, in the coating. Cathodic polarisation may be a
consequence of either corrosion at the point of damage or the application of
cathodic protection. Resulting from experiments carried out by Smith
and Dickie on primer failure,
it has been shown that under impressed cathodic conditions, corrosion
inhibitive pigments play no part in the reduction of disbonding. Under
such conditions, the performance of the system reflects the resistance of
the primer resin system to alkali displacement. During salt spray
exposure, it was found that the corrosion inhibitive pigments did exhibit a
degree of control of the rate of failure by inhibiting the anodic reaction.
The Mechanism Of Delamination
§50 A number of explanations have been put forward whereby the alkaline
environment under the film affects the integrity of the metal - polymer
interface, or perhaps more properly the interface between the oxide and the
showed that this form of failure, which he called �halo detachment�,
only occurs when there are alkali metal cations available in the environment
to act as counter ions to the cathodically generated OH-.
§51 One explanation is that dissolution of the air-formed oxide layer is
responsible for the loss of adhesion. Various techniques have been
adopted to confirm this view. Ritter
carried out ellipsometric studies on a range of coatings in tandem with an
investigation of uncoated steel exposed to saturated NaOH. The
coatings studied were collodion, polystyrene and two proprietary acrylics,
all of which were cured at room temperature. Additionally, a
heat-cured acrylic was examined. The argument put forward was that
delamination is a direct result of oxide dissolution and this is supported
by the presence of iron ions in the interfacial region of the polymer in the
heat cured specimens. The oxide layer present on the metal under these
circumstances, is thicker than the normal air-formed layer, as a result of
the curing regime.
§52 Further work carried out using this technique
concluded that the high pH generated under the coating was conducive to
oxide growth and surface roughening of the substrate. This conclusion
was the result of tests carried out on uncoated substrates in a high pH
environment. Taken in conjunction with the findings cited in the
previous paragraph, the inference of this could be that the thicker oxide
layer, developed due to the high pH generated under the film, could cause
the oxide film to become mechanically weaker and result in a cohesive
§53 Castle and Watts, in an
XPS investigation of cathodic delamination, concluded that dissolution of
the oxide layer was not a significant factor in the process. It was
found that the oxide within the disbonded region was reduced only in
localised patches and the disbonded area extended well beyond this.
§54 In a similar vein, Wiggle et al
considered different pre-treatments and the effect upon them of cathodically
produced hydroxide. Both zinc and iron phosphate pre-treatments along
with bare metal was included in the study. Initial tests were carried
out to determine the porosity of the conversion coatings, which yielded
results of 0.2% for the zinc and 23% for the iron phosphate; these porosity
values were linked to the poor resistance of the iron phosphate to anodic
undermining. Experiments were carried out under both anodic and
cathodic polarisation as well as at open circuit. Cathodic
polarisation trials resulted in dissolution of the zinc phosphate and hence
loss of adhesion of the coating. Knaster and Parks 
also identified dissolution of zinc phosphate under paint coatings on
pre-treated steel but considered this to be a secondary reaction. The
contention in this case is that delamination of coatings from phosphated
panels, due to a defect, is caused by oxygen depolarisation along the
phosphate-paint interface and hydrogen evolution at the delamination front
that results in a force, which disrupts the adhesion of the polymer to the
§55 Dissolution of phosphate conversion coatings on steel was also examined
by Sommer and Leidheiser.
The variables they considered were the effect of the pH and also the alkali
cation involved. The pH effects were investigated by exposing test
panels to 0.01, 0.1 and 1.0-M solutions of NaOH for ten minute intervals up
to one hour. The importance of the cation involved was determined with
0.1-M solutions of hydroxides of Na, Cs, K and Li. It was found that
increasing pH values from 11.5 to 13.5 resulted in a higher rate of
dissolution of the phosphate ions from the conversion coating.
§56 With regard to the cation type, the investigators were surprised to find
that Na was the most effective of the cations in the dissolution of the
coating (1.6, 2.6, & 4.1 times K, Cs & Li respectively).
Additionally, the cation type not only affected the rate but also the mode
of dissolution. With Na, Cs and K, the phosphate ions were dissolved
more rapidly than the zinc ions, whereas in the case of Li, the zinc was the
one to be dissolved at a higher rate.
§57 Dissolution of the conversion layer is also suggested by Dickie,
after an investigation of the failure of an epoxy coating. This
mechanism is proposed to account for the slower delamination and longer
delay time observed with alkali resistant coatings.
§58 An alternative view to the dissolution of the substrate theory is that
the alkaline solution under the film attacks the coating itself.
Koehler�s investigation 
of oleoresinous and polybutadiene coatings on steel concluded that
delamination was due to saponification of the polymers. In his
discussion, he states that disbonding may occur at pH 11.7, which is well
below that required to dissolve the oxide layer. That higher pH�s
may form under the coating at cathodic sites is not disputed, however it is
suggested that this may take place subsequent to the loss of coating
adhesion. Furthermore, it is proposed that regardless of any other
phenomena a film of water is required under the coating for disbonding to
§59 Attack on the polymer is also a mechanism supported by Hammond et al 
who examined three different epoxy systems on steel with a coating range of
10 - 30 �m. In their examination of the interfacial composition of
delaminated surfaces, using XPS, predominant traces of polymer were in
evidence on the metal surface relative to iron oxide; this was in
conjunction with little or no traces of iron in the interfacial
polymer. Their conclusion from this was that the failure mechanism was
due to resin degradation and that the locus of failure was within the
polymer, resulting in a failure that was cohesive in nature.
§60 Another XPS investigation was undertaken by Castle and Watts ,
who looked at fusion-bonded epoxy coated mild steel. The results of
this study suggested that the failure was essentially adhesive in
nature. One of the variables in the experiments carried out was
surface texture, ranging from grit blasted (Ra 3.80 �m) to a
mirror finish polish with 1�m diamond paste (Ra 0.15 �m).
It was found that as the Ra values increased, there was a slight
tendency toward cohesive failure of the oxide layer. However, although
some iron was transferred to the polymer, the amount was a trivial component
of the locus of failure and was consistent with cohesive failure of the
oxide covering at protruding asperities.
Transport Pathways for Aggressive Species
§61 The nature of the delamination mechanism is not the only area of debate
within the overall subject of cathodic delamination. Whilst there is
agreement as to the species involved in the process, it is still open to
conjecture how those elements arrive at the delamination front.
Considering a coated steel substrate, immersed in an electrolyte of neutral
or near neutral pH, the half-cell reaction responsible for the delamination
process is generally agreed to be oxygen reduction. This reaction
generates OH- at the cathodic site and is responsible for the
alkaline environment at the delamination front. The elements required
for the process to proceed are water, oxygen and free electrons. The
electrons may be generated by either an anodic reaction or through the
application of cathodic protection. Additionally, in order to maintain
electroneutrality, some form of counter ion is required; the high pH values
recorded at the reaction zone indicate that these counter-ions are alkali
cations rather than protons.
§62 If we assume that cathodic delamination is a result of a damaged coating,
there are two possible routes that the reactants for the cathodic reaction
may take. The two alternatives are either through the coating or along
the polymer-metal interface. An extensive review of the delamination
process was carried out by Leidheiser et al .
The results of the experiments carried out indicated that water was
transported to the reaction zone through the coating. It was suggested
that a certain fraction of this could be in the form of a cation such as H3O+,
as the cathodic nature of the front may favour the transmission of water
associated with an ion possessing a positive charge. The supply of
oxygen to the cathodic site was found to be largely through the coating,
with a small contribution from interfacial transport, in the case of the
epoxy coating studied. Other work on the subject of the supply of
reactants to the delamination front with pigmented coatings,
suggests that supply of oxygen along the interface may be the
§63 The major unresolved factor in the overall picture is the route taken by
the charge-neutralising cations. Leidheiser�s study identified a
linear relationship between the diffusion coefficient in aqueous solutions
of the different cations, used in his experiments, and the delamination
rate. This suggested that the cations were supplied along the
interface. However, as delamination rates showed a strong dependence
on film thickness, through
film migration of cations could not be discounted.
§64 An important feature identified in Leidheiser�s work was an incubation
time needed before delamination commenced. This was dubbed the �delay
time� and the factor controlling this parameter was considered to be
cation ingress into the coating, due to an observed correlation between
ionic mobility and the delay time.
§65 Interfacial migration of cations is supported by the work of Castle and
Watts55. A number of factorsdrew them to this
conclusion. Firstly, EPMA examination of a cross section of the
polymer showed Na ingress into the coating from both the interface and the
bulk solution; however there was no Na detected in the bulk of the polymer,
suggesting that Na+ did not fully penetrate the film. Also,
the disbondment velocity (Dk) was related to the surface profile of the
substrate and the parameter referred to as tortuosity (τ) as
§66 A tortuous interface is one whereby the route that must be taken by a
species in order to diffuse from one point to another, is made more
difficult by roughening the substrate, thereby making the effective path
longer. It follows from this that a rougher surface should result in a
lower lateral degree of delamination, if the controlling mechanism is the
interfacial migration of a species.
§67 Castle and Watts� results for a steel/epoxy system produced a constant
disbondment velocity (Dk*) of 0.4 � 0.08 mm day-1.
This again was felt to point towards interfacial migration as being
important to the process.
§68 The effect of the substrate upon the delamination process was discussed
by Skar and Steinsmo  in an
investigation of the importance of through film ionic migration. Their
experiment, which aimed to clarify the confusion as to the route taken by
the cationic species, showed a strong linear relationship between the dry
film thickness and the disbonding rate. The explanation offered for
this result considered three alternative routes by which the counter-ions
may reach the delamination front:
Along the interface.
Through the detached coating.
Through the attached coating ahead of the delamination front.
§69 These alternative routes are illustrated in Figure 7. The first
option was discounted due to the strong dependence on film thickness.
Of the remaining two alternatives, it was felt that the second was the most
§70 Figure 7: Suggested ionic
migration routes from ref. 
§71 The logic behind this conclusion stems firstly, from work carried out by
Mayne and Mills,25 whose investigation of detached films showed a notable
difference between the ionic resistances of attached, compared to detached,
films. Secondly, it was proposed that the conduction mechanism of the
detached film exhibited �D� type behaviour, as defined by Kinsella,
due to the high pH under the coating. Whilst the second proposition
may well be valid, the first is dubious. The conclusions drawn by
Mayne and Mills, were indeed that detached films showed lower resistances,
as quoted, but not on ordinary mild steel substrates - where the detached
films showed similar resistances to those attached to the steel. It
was when the coatings were applied to inert substrates (platinum and steel
passivated with a zinc chromate pigment) that the quoted difference in
resistance was found.
§72 Working with both pigmented and unpigmented chlorinated rubber coatings,
Sharman  identified a
number of features worthy of note. Referring to the delay time defined
by Leidheiser, he concluded for the system under investigation that it was
essentially independent of cation mobility. Furthermore, it was found
that the iron oxide pigmented systems showed longer delay times than the
clear varnish coated substrates; this was unexpected as the former had
higher ionic permeabilities. This situation was considered by
pre-soaking the pigmented coatings in water prior to testing; this resulted
in a reduction in the delay times from 5-7 hours to 1-2 hours. This
result pointed to ingress of either water or oxygen as the factor
responsible for the duration of the delay time.
§73 With regard to the species responsible for the delamination process and
the route taken, different mechanisms were seen to be responsible for the
two systems. For the unpigmented coating, interfacial cation migration
was considered to be the determining factor as the delamination rate
constant increased in a linear fashion with cation mobility. With the
pigmented coating, which displayed a greater through-film ionic transport
rate, coupled with lower oxygen permeability, interfacial migration of
oxygen was suggested as a likely rate determining mechanism; this was
suggested to be virtually independent of cation mobility. A further
result, which reinforced the hypothesis of interfacial migration of a
species as the rate-determining factor, was that the rate of disbonding
eventually became zero. The explanation for this was that the
interfacial pathways became blocked with corrosion products.
§74 Armstrong and Johnson,
working with unpigmented chlorinated rubber coatings subjected to cathodic
protection, concluded that the corrosion current passed through faults in
the form of cracks or crevices in the film. It is proposed that degradation
of the film due to OH- was essentially mechanical in
nature. This argument is supported by a number of factors. Free
standing films exhibited conductive behaviour only after long exposure to
concentrated OH-; this was lost after the films were dried in a
dessicator. It was felt this result supported the hypothesis of
conduction through cracks or crevices rather than through the bulk
film. FTIR (Fourier Transform Infra Red) spectra of membranes exposed
to OH- displayed no detectable chemical change from the untreated
§75 The underside of the films (that in contact with the metal), which was
subjected to the alkaline conditions, was found to contain holes that were
not visible on the upper face of the coating. This is thought to be
due to pockets of high solvent or plasticiser concentration; it is suggested
that the OH- opens up such defects by a solvent cracking process,
which reinforces the assertion that the breakdown is mechanical rather than
§76 In an ideal world, the coatings that we applied to protect against
corrosion would be a perfectly homogeneous mix and be free of defects of any
kind. Also, the surface to which the coatings were applied would be in
an ideal condition, free of any form of contamination and with an
§77 This utopian situation does not occur in the real world and the
substrates are commonly far from pristine. There are occasions where
adequate preparation of the surface to be coated is either technically
unfeasible or prohibitively expensive.
In such circumstances, there is a need for a coating that will perform well
on a contaminated substrate. Mayne�s12 model of underfilm
corrosion, considered earlier, was applicable to a clean surface. In
the presence of contaminants on the surface, resistance inhibition does not
come into play, as all the required elements for corrosion to proceed are
present at the interface.
Effects of Contamination
§78 Surface contamination is conventionally considered to be due to such
factors as salt deposits or corrosion. Indeed, this is the source of a
great deal of concern and has been the basis of much work,
Additionally, it has been demonstrated that, even on substrates cleaned to
laboratory standards, minor inclusions such as sulphides can have a
detrimental effect on the performance of an organic coating.
§79 An established method for dealing with the problem of painting over steel
that is rusted, contaminated with salts or, as is usually the case, a
combination of the two, has been to coat with an oil-based coating pigmented
with red lead (Pb3O4). Environmental concerns
have limited the use of lead compounds
and forced formulators to look to alternative means of addressing the
problem. In the
pursuit of a viable alternative, the red lead formulation is still used as a
benchmark by which to compare the various contenders ,
The exact mechanisms by which red lead protect a compromised substrate are
unclear, but its efficiency is undisputed.
§80 Rust, in itself, is not seen as a problem;
rather it is the contaminants that the rust contains. Mayne 
identified that sulphates were embedded into adherent rust close to the
metal surface; this rust layer was not easily removed by wire brushing.
These sulphate �nests� are felt, to a large degree, to be responsible
for the formation of blisters at anodic sites .
Such blisters lead to failure, due to local bulging and eventual cracking of
§81 Mayne�s experiments
were carried out in Cambridge, over the period 1945 to 1953. The
contamination of his specimens was determined to be due to the burning of
fossil fuels, this was justified by a seasonal variation in the degree of
contamination giving higher levels in the winter months. One could
possibly argue that with the advent of smokeless zones in Britain that, in
non-industrial urban areas, this problem should be greatly alleviated.
Having said this, industrial areas still provide an adequate level of
contamination to require attention. The problem of sulphates in
industrial areas becomes one of chlorides in marine environments.
Surface Tolerant Coatings
§82 In a search for environmentally acceptable coatings that may be applied
over compromised metal surfaces, attention has been focused on the field of
�surface tolerant coatings�. Frondistou-Yiannas 
surveyed the range of these coatings and their reported modes of
action. The mechanisms considered were: conversion of rust to
magnetite, conversion of iron oxides to other compounds and inertisation of
soluble salts. In addition to these, Thomas 
also considered barrier coatings, the ability of the coating to wet
and penetrate the rust and anti-corrosive pigments. The field trials
of the former investigator, which were carried out in both marine and
industrial areas, concluded that none of the systems tested was expected to
provide long-term protection in the aggressive environments used in the
§83 So called rust converters are mainly formulations based upon tannic
acid. An extensive review of the mechanisms involved in the action of
tannin based coatings,
details the reaction mechanisms by which it is claimed that tannins help to
protect steel. The action of tannin-based treatments is explained by
the inhibition of the formation of magnetite, which if allowed to develop,
would increase the area available for oxygen reduction. An alternative
name of �rust deactivators� is proposed for these formulations, which
may lead one to think that they have some effect upon the contaminants that
are invariably present. However, this statement is followed by the
fact that tannins are unable to neutralise surface contaminants such as
§84 In her review of rust conversion preparations, Thomas 
states that the premise behind their use consists of the stabilisation of
the rust, so that the redox reaction between Fe(II) and Fe(III) is no longer
possible. In this case, in principle, corrosion would be
prevented. It is pointed out, however, that tannic acid reacts with
Fe(III) within regions underlying thick layers of surface rust; therefore,
it would be necessary to wire brush the surface prior to treatment.
§85 A further assessment of the action of tannic acid has been conducted by
Morcillo et al.
Panels that had been exposed outdoors for up to two years were thoroughly
wire brushed and immersed in tannic acid of various concentrations.
Fresh panels were also immersed, to assess the effect on clean steel.
A film of ferric tannate was found to form on the surface of the rusty
panels, this film was found to be extremely porous. The results from
these tests showed that tannic acid promotes rusting on clean steel and does
not significantly reduce that of rusted steel. Furthermore, it was
suggested that due to the nature of the ferric tannate film formed on the
surface, that the tannic acid might attack the iron at the base of the
§86 Perhaps the most poignant point comes from Frondistou-Yiannas� paper
where, all the rust converter systems under examination did not make it to
the field trials, as they failed the initial laboratory screening due to
blistering and intercoat disbonding.
§87 An alternative method of dealing with contaminated surfaces proposed by
Sykes is the use of
water-borne paint. The contention here is that if the requirements of
the coating fall into any of the following criteria:
Removal of soluble salts from the surface.
Conversion of salts into safer species.
Electrochemical alteration of rust involving the underlying steel.
§88 Then an aqueous medium, in the form of a water-borne coating probably
offers the best chance of success; providing a method to either dissolve or
bind the soluble ions while the paint is drying.
§89 Experimental work was carried out on a 120�m thick water-borne vinyl
acrylic latex system applied over pre-rusted steel. The substrates
were coated in the as-received condition, wire brushed or grit blasted prior
to application of the coating. The formulation from which the coating
was taken normally has its pH adjusted to 4.5 - 5.0. During the
investigation, the pH was taken down to 2.7. This lower pH was found
to aid in the removal of light deposits of rust resulting in a decrease in
the degree of blistering of the coating.
Effect of Surface Preparation Techniques
§90 Another feature highlighted in this work was that dry grit blasting could
exacerbate the problem of coating rusty steel by exposing contaminants
buried in the rust layer. This feature has been noted elsewhere.
Research sponsored by Nuclear Electric determined that dry blasting not only
exposes buried salts but also spreads them.
The research was primarily aimed at the determination of an acceptable level
for residual salt contamination. Substrates were pre-contaminated with
two salt compositions as shown in Table 1, prior to being coated with a
three-coat alkyd paint system.
§91 Table 1: Composition of
salt mixtures, after ref. 
(% by weight)
§92 Exposure tests were carried out on panels contaminated with progressively
higher levels of salt contamination. The exposure trial ran for 6 �
years, after which pull-off adhesion tests were carried out. With zero
contaminant, the mode of failure was cohesive within the primer coat.
As the salt levels increased, progressively less primer remained adhered to
the steel surface. At higher contamination levels, there was a change
from mixed to total adhesive failure of the primer. This change was
abrupt and corresponded to the values shown in Table 2
§93 Table 2: Critical
Contamination Levels Applicable to Different Environments.
Critical Contamination Level
2 �g cm-2
6 - 20 �g cm-2
§94 The conclusion drawn by this investigation was that the adhesion, and
therefore the performance, of the system was severely affected by salt
contamination of the substrate, where the levels were in excess of 2 �g cm-2
when the salts were predominantly chlorides.
§95 An epoxy coating used as a tank lining was also tested. In this
case, both the marine and the urban salt contamination resulted in coating
failure at levels above 20�g cm-2. The marine salt,
however, brought about the failure in 6 days compared with 56 days for the
urban salt composition.
§96 The more aggressive nature of the chloride ion is attributed to higher
osmotic pressures exerted. Additionally, chlorides tend to be more
soluble than other salts and of a lower formula weight. Hence, a given
salt burden of a chloride represents a higher molar concentration, which
produces a higher osmotic pressure.
§97 As often, in the field of organic coatings, the literature provides what
appears at first sight, to be contradictory data. The critical
chloride levels determined by Morcillo 
for example was determined to be 50�g cm-2. It is
important that one realises that each different coating/metal system is
likely to have various parameters, including the chloride levels it can
tolerate, that are unique to itself.
§98 Another effect of contaminated surfaces found by Morcillo was the
degradation of a chlorinated rubber primer applied over contaminated rusted
chloride levels found under the films in this investigation were too high to
be accounted for by the initial contamination or the exposure
environment. An XPS analysis was carried out which indicated that the
C-Cl bonds in the polymer underwent a chemical transformation that yielded
Cl- and accounted for the increased chloride concentration in the
rust at the interface. So the, initially present or subsequently
formed, interfacial rust appeared to catalyse the decomposition of the
§99 Morcillo discussed a further aspect of the importance of proper surface
preparation, in a paper that considered the significance of the size and the
type of abrasive used during pre-cleaning operations.
One important point highlighted was that there is a critical surface
profile, the value of which is determined by the environment along with the
type and thickness of the coating. As the coating increases in
thickness, the effect of the surface profile diminishes. The critical
surface profile was found to be a function of the aggressiveness of the
environment � a more aggressive environment resulted in a lower critical
§100 In conclusion, the surface to be coated is as important a part of the
overall system as any other, so the state and condition of the substrate
prior to painting significantly affects the demands that are placed upon the
coating. Traditionally tried and tested methods for coating
compromised steel are no longer environmentally acceptable, therefore new,
safe, ecologically sound alternatives are demanded. Should the
emerging water-borne paint technology provide a solution to the
contamination problem, it would also have the added advantage of addressing
the impending volatile organic content legislation.
§101 Within the debate regarding the important features of the protection
offered by organic coatings, the argument over the importance of adhesion is
amongst the most contentious. It is often claimed (or at least
inferred) that the adhesion is of paramount importance, to the exclusion of
all other features. According to Bullett and Prosser �the ability to
adhere to the substrate throughout the desired life of the coating is one of
the basic requirements of a surface coating, second only to the initial need
to wet the substrate�,
Funke describes adhesion as the most important and decisive property of a
§102 In order to enter into the debate as to the importance of adhesion in the
protective mechanisms of paint systems, it is necessary to consider the
process of adhesion itself. In particular, the principle of adhesive
failure needs to be addressed. For two surfaces to adhere to each
other, be they of the same or of different materials, there is a need for
intimate contact on an atomic scale between them. In the case of a
liquid coating, the degree of intimate contact is governed by the ability of
the coating to wet the substrate. This wetting ability needs to be
present not only in the static sense, where the contact angle between the
substrate and the liquid may be used to determine how efficient the wetting
is; but also in a dynamic sense, where the rate of wetting of the substrate
is not countered by the rate of shear involved in the application of the
§103 According to Bikerman a
true adhesion failure is improbable, a view shared by others.,
 If one
subscribes to this point of view, then it must be the case that all failures
that involve detachment of the coating from the substrate, must include some
element of either cohesive failure of the coating or the oxide layer on the
surface of the steel. This view is reinforced by XPS work which has
been carried out by several workers, who have found that traces of polymeric
material is normally found on the �metal� surface of a metal/polymer
interfacial fracture, which may have appeared to be a purely adhesive
failure from a visual examination.,
Although the explanation of a cohesive polymer or oxide failure may account
for the observed failure modes, this is probably still an
§104 The function of the adhesive joint is to effectively remove the interface
from between the two different materials. In order to do this, a
gradual change must occur in the chemical composition of the coating and the
substrate in the interfacial area. Plato elucidated this concept in
his explanation of how the different elements in the universe could exist as
§105 It is not possible for two things to be fairly united without a third for
they need a bond between them which shall join them both, that as the first
is to the middle, so is the middle to the last, then since the middle
becomes the first and the last, and the last and the first become the
middle, of necessity, all will come to be the same. And being the same with
one another, all will be a unity.
§106 The inference of this is that in a true adhesive joint, there is no
clearly defined interface and that one needs to see the interfacial area as
a gradual transitory phase where the coating and the substrate affect and
are effected by each other. So, in the same way that the atomic
structure of the metal gradually changes from that of the bulk phase to that
of the oxide on the surface � it is proposed that the properties of the
surface of the uncoated metal are changed when a polymeric coating is
applied to it. In the same way, Leidheiser suggests that the structure
of the coating varies as one passes through the bulk of the film to the
interfacial region with the metal. This region, which extends into the
polymer, is known as the interphase. The size of the interphase is not
known, but is thought to be in the nanometer range.
§107 Whilst loss of adhesion may occur under a number of situations, a major
form of this type of failure is loss of adhesion resulting from exposure to
aqueous or humid environments.,
 As a result of work
into loss of adhesion due to water ingress, it has been concluded that
adhesion tests in the dry state are of little use for applications in
immersed or humid environments.,
 Indeed, it has been
proposed that, in isolation, adhesion tests are insufficient to determine
the ability of a coating to control corrosion.
§108 It is true that the adhesion plays an important role in the protective
mechanism of coatings, but one should take care when claiming that it is the
overriding consideration. When one considers the process of cathodic
delamination it is clear that, once the paint has become detached from the
substrate, the underlying metal is exposed to the environment and is no
longer afforded any protection from the coating system. One should,
however, be careful to ensure that the whole process is viewed in
context. Whilst the loss of adhesion, resulting from the delamination
process, effectively negates the protection afforded by the coating, it is
important to consider whether the original adhesion is the deciding factor
in the delamination process. Gowers and Scantlebury suggested that the
beneficial role of the adhesion of a paint coating is due to the impairment
of the formation of a layer of electrolyte at the coating/substrate
interface, preventing ionic current flow and the spread of corrosion over
§109 A case in point is that of the use of silanes as adhesion
promoters. There has been a considerable amount of work carried out in
this field and the results may, at first sight, appear contradictory.
The fundamental reason why the results of various investigations lead to
different conclusions is the starting point and the assumptions made by the
§110 Some work has been carried out which has shown that the use of silane
coupling agents leads to an improvement in wet adhesion.
This result has been used to assert that this treatment will therefore
result in enhanced protection against such phenomena as underfilm corrosion
or cathodic delamination. This claim is based on the assumption that
good adhesion to the substrate will necessarily lead to improvement in the
§111 The work of Marsh et
al has tested this assumption and has shown that although silanisation does
in fact provide better wet adhesion, the treatment afforded no improvement
in resistance to delamination and that underfilm corrosion was significantly
worse on the silane treated specimens. It was pointed out by Marsh
that the use of the cathodic delamination test as a determinant of adhesive
strength is inappropriate, as the work showed that improved adhesion played
no part in the rate of delamination. Furthermore, the cathodic
delamination test is therefore a measure of cationic mobility rather than
adhesive strength. Other work on salt spray exposure of coated panels
has found that no adhesion loss had occurred although severe underfilm
corrosion had taken place.
§112 Good surface preparation is the key to good adhesion but the type, as
well as the condition, of the substrate has been found to have a strong
influence upon the initial dry, and the subsequent wet, adhesion of a
metal/polymer system. Gosselin showed that the metal used as the
substrate affected the strength of the final dry bond and Arslanov found
that the adhesive strength of a coating on aluminium initially decreased
upon exposure to water, but subsequently got stronger.
§113 One method that has been suggested by Funke to increase the adhesive
strength of coated systems is the application of thin layer technology.,
 Here, it is suggested
that the adhesion to the substrate could be enhanced as a result of
co-operative bonding between the intermediate thin polymer layer and the
main bulk of the coating.
§114 The proposed thickness of these coatings is in the nanometer range. 
While this is in conflict with common coating practice whereby the base
primer coat is thicker than the surface profile of the metal, when viewed in
conjunction with Leidheiser�s contention regarding the interfacial region,
and Kumins� model of restricted chain mobility within thin layers;
this may well be an avenue worthy of investigation.
§115 In conclusion, though the adhesion of a coating to the substrate is an
important factor, which must be taken into consideration when addressing the
mechanisms of corrosion protection afforded by the coating, the overall
picture is much more complex and the overall mechanism that must be
considered is a combination of various contributory factors. Karyakina
and Kuzmak have produced an extensive overview of the processes of adhesion
and other contributory factors, together with a review of experimental
techniques for the evaluation of coating properties.
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