Volume 2 Paper 26
Blistering and Delamination Processes on Coated Steel
D. Greenfield and J. D. Scantlebury
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JCSE Volume 2 Paper 26
Submitted 18th July 2000, published for public review 22nd August 2000
Blistering and Delamination Processes on Coated Steel
D. Greenfield & J. D. Scantlebury
Corrosion and Protection Centre, UMIST, PO Box 88, Manchester M60 1QD
The alkaline nature of the environment at the paint/metal interface has
been addressed. A pH indicator has been incorporated into an alkyd
coating and the alkalinity of the environment at the interface monitored as a
function of time using time lapse photography.
§2 Keywords: Blistering, Delamination, Alkyd, Marine Environment.
§3 When a coating is exposed to an aggressive medium, such as that found in a
marine environment, the associated failure mechanisms of blistering and
delamination are amongst the most important to be considered. These two
modes of failure are often treated separately, but have so many common
features that it could be argued that the differences are a matter of degree
rather than type. There is a general consensus over the integrity of a
typical coating and it is fair to assume that one will find defects, due to
porosity or damage in service. Therefore the following considers the
situation of an organically coated steel structure containing a defect, which
is subjected to aggressive, immersed conditions.
§4 Due to the presence of imperfections in the coating, the steel substrate is
directly exposed to its surroundings. This initiates a corrosion
process, with the anodic reaction occurring at the defect, this reaction
follows that shown in equation (1)
Fe → Fe2+ + 2e
§5 In order to maintain electroneutrality within the system, this reaction is
balanced by a cathodic reaction. In most naturally occurring situations,
this reaction will be oxygen reduction, as illustrated in equation (2).
These two reactions initially take place adjacent to each other but separate
as the process continues with the cathode moving under the coating.
2 H2O + O2 + 4e → 4OH−
§6 The ferrous ions produced in equation (1) go into solution and react to
produce electrically neutral compounds through combination with cations in the
medium. This leaves us with a charge imbalance. The environment
has a surfeit of positive charge, in the form of whatever cations are present
and the environment at the cathode is producing hydroxyl ions, resulting in an
excess of negative charge. In the case of uncoated steel, the route for
the counter-cations is straightforward. However, when the steel is
coated the situation becomes more complicated. The path from the
exposure environment to the cathodic site is either restricted or blocked
§7 It was shown by Mayne  that coatings were so permeable to water and
oxygen that their rate of arrival at the cathodic region was greater than that
required for corrosion to proceed. Figure 1 shows a schematic
representation of the results of the corrosion reactions, along the lines
proposed by Schwenk . It can be seen that the cathodically produced
hydroxyl is present at both the blistering and the delamination sites.
The alkaline nature of this resultant solution at these sites is considered to
be a major contributory factor in the failure of the coating.
§8 Figure 1: An illustration of the blistering and delamination processes
§9 The reason this alkalinity causes such failure has been variously ascribed
to saponification of the coating , dissolution of the oxide layer at the
interface , alteration of the ionic resistance of the film [5, 6].
§10 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 blistering or
delamination process taking place. Leidheiser  ascribes this delay as
due to the time required to set up a steady state diffusion in the film;
whether this diffusion was of water, oxygen or ions was unclear. Nguyen
, on the other hand, suggests that the important route for diffusion is
that of the cations along the paint/metal interface. Both
interpretations provide explanations for the observed phenomena.
§11 The work described here is concerned with blistering and delamination of an
alkyd coating. Part of the essential quality of this research is to use,
wherever possible, non-destructive test methods. To this end, some work
has been carried out to determine the feasibility of incorporating pH
indicators into the organic coating under investigation.
§12 The initial tests, described below, were carried out on an alkyd
resin. The hypothesis of the tests was that cathodic areas under a clear
resin coating may be identified by the high pH generated due to the presence
of OH- produced by the oxygen reduction reaction; it should
therefore be possible to track the delamination front as it proceeds along the
§13 The substrates used for all the experiments were cold rolled, mild steel Q
Panels. The panels were solvent de-greased and dried with paper towels
before being coated in a dust free, ventilated, coating cabinet. The
coatings were cured at room temperature for 48 hours and then stoved at 65-70oC
for a further 48 hours.
§14 In order to provide an indication of the degree of protection offered by
this coating, a control experiment was carried out to demonstrate the effect
of immersing an unprotected substrate in the test solution of 0.5M NaCl.
This was recorded with time lapse photography and the result is available as
video1 in MPEG format in either medium (2.6 Mb) or small
(0.9 Mb) size. [Editor's note: there is always a compromise between
file size and image quality for video clips - these are the
best we can do at present - note that it will take about 1 hour to retrieve 10
Mb using a V90 modem].
§15 In order to track the development of alkaline and therefore cathodic
regions under the film, phenolphthalein pH indicator was incorporated into the
wet coating, as detailed below.
§16 After testing various preparations of the indicator for incorporation into
the coating, the optimum solution for the alkyd coating was determined to be
1g phenolphthalein in 100ml EtOH + 100 ml water. Three panels were
selected at random to test if the indicator would leach out of the
coating. A drop of pH 13.5 buffer solution was placed on the surface of
the coating. Within a minute, the droplet had turned pink as seen in
comment(17)Figure 2 Effect of a phenolphthalein loaded coating upon a droplet of pH
13.5 buffer solution
§18 The average DFT of the final coatings was 22mm. A 2mm hole was
introduced into the cured coatings and the panels were fitted with a 30mm
diameter perspex cylinder, 50mm high, which was filled with a 0.5M NaCl
solution. The panels were left at ambient temperature for 10 days and
were photographed at regular intervals. On the second run of the
experiment, time-lapse photography was employed using a digital camera linked
to a PC. Computerised videos were produced showing the progress of the
breakdown of alkyd coatings, stills from these videos are included in the
§19 Figure 3 shows the progression of the alkalinity of a typical panel.
Note that after 10 days the alkaline front has progressed under the cylinder
and along the panel for a short way.
comment(20)Figure 3: Stages in blistering process as indicated by phenolphthalein
§21 In all cases, the movement of the pink areas followed the lines of the
rolling marks on the surface of the panel. Although this method provided
a picture of the process as it occurred, it was felt that the use of
time-lapse photography would produce data that would illustrate the picture
more clearly, especially in terns of whether the blistering was in deed
affected by the topography of the substrate.
§22 In the first experimental trial, the coating was applied across the
striations of the panels and it was felt that this might have had an effect
upon the ability of the coating to adequately wet the surface.
Therefore, when the experiment was repeated, the coating was applied along the
rolling marks with all other conditions remaining the same.
§23 10mm, single coat alkyd:
This coating failed completely within 30 hours, the progression of its
deterioration is shown in figure 4 and as time-lapse video as video2 in MPEG
format in medium (9 Mb) and small
(3.3 Mb) size.
comment(24) Figure 4: Breakdown of thin alkyd coating (click the image for
a larger view)
§25 Once the exposure had finished, the coating was scraped off the substrate
to reveal the state of the metal beneath, shown in figure 5.
§26 Figure 5: 10mm coated panel at the end of the exposure
trial before and after the removal of the coating
§27 The steel underneath the coating, although corroded, had no corrosion
products attached to it; these were on the outside of the film. This
result is in line with the proposition by Mayne  that coatings acquire a
negative charge when immersed and therefore allow the passage of positive
§28 30mm two coat alkyd.
This coating fared better, the period to breakdown was greater and the mode
of failure was different. Rather than the substrate suffering underfilm
corrosion, the coating failed due to delamination and for the period of the
experiment, the underlying steel was protected by the alkaline conditions
present at the interface.
comment(29)Figure 6: The progress of the blistering and delamination process
(click the image for a larger view).
§30 The progress of the breakdown of the coating is illustrated in figure 6 and
as two sections of time-lapse video as video3a and 3b in MPEG format in medium
(3a, 19 Mb and 3b, 26 Mb) or small (3a,
6.8 Mb and 3b, 9.4 Mb) size. Note that there is
magnification change on going from 3a to 3b. There is an initiation period where there is no sign of any
activity under the coating, the progression as a function of time is as
81 hours: The first sign of alkalinity near the fault.
50 hours: Alkaline regions are initiated some way from the fault.
52 hours: Alkaline regions established and corrosion products are
clearly visible at the fault.
71 hours: Blisters coalesce and substrate at the fault is covered in
81 hours: Delamination front begins to move away from the fault along
the striations on the substrate.
100 hours: Delamination front firmly established either side of the
fault, still following the lines on the substrate. Initiation of new
blisters at the periphery.
125 hours: Original delamination zone lighter in colour.
Peripheral blisters established.
§31 At the end of the exposure period, the photographs were examined and a plot
constructed of the percentage area of the exposed surface that was indicated
as being alkaline. The results of this plot are given in figure 7,
§32 Figure 7: Percentage area of the exposed panel covered by
blistering and delamination.
§33 The results presented here show quite clearly that, for the coating
considered, not only do blistering and delamination processes share many
common features but that they may be considered as constituent parts of one
§34 The process starts with the initiation of blisters around the region of the
point of damage. These blisters become larger as time progresses until
they eventually coalesce. Once this has occurred, the delamination
process takes over and the front progresses rapidly. As the coating
delaminates from the fault, the pH at the interface behind it falls as the
§35 Figure 7 indicates that the processes occurring under the film are cyclic
in nature, starting with an initiation time where there is no apparent
activity. Of course, the indicator used only functions at higher pHs so
there would be a delay in its response. Notwithstanding this, the
initial delay period coupled with the apparent inactivity indicated by the
plateau at about 100 hours does provide evidence of a cyclic process in line
with that proposed by Scantlebury .
§36 Blistering and delamination act in conjunction with each other in the
failure of organic coatings.
§37 As blisters develop and grow, they coalesce to form a large disbonded
region in the vicinity of a fault in the coating. This disbonded area
provides the site for the delamination process to initiate and propagate.
§38 The initial delay time is repeated in a cyclic nature once the delamination
reaches a critical point. At this point, the pH of the solution under
the film at the disbonded area falls as the oxygen reduction reaction moves
away from the fault. At this point, the delamination process gives way
to further blistering under the intact coating. One could view the
delamination front needing the blisters to act as stepping-stones in order to
§39 1. Mayne J.E.O. “The Mechanism of the
Protective Action of an Unpigmented Film of Polystyrene”. J.O.C.C.A.
Vol. 32, No. 352, Oct 1949, pp 481-487.
2. Schwenk W. “Adhesion Loss for Organic
Coatings, Causes and Consequences for Corrosion Protection” Corrosion
Control by Organic Coatings. Ed H. Leidheiser Jr. 1981, pp 103-110
3. Castle J.E. & Watts J.F. “Cathodic
Disbondment of Well Characterised Steel/Coating Interfaces”. Corrosion
Control by Organic Coatings. Ed H. Leidheiser Jr. 1981, pp 78-86
4. Ritter J.J. “Ellipsometric Studies on
the Cathodic Delamination of Organic Coatings on Iron and Steel”. J.
Coat. Tech. Vol. 54, No. 695, 1982, pp 51-57
5. Mayne J.E.O. & Mills D.J. “The
Effect of the Substrate on the Electrical Resistance of Polymer Films”
J.O.C.C.A. Vol. 58, 1975, pp 155-159
6. Skar J.I. & Steinsmo U. “Cathodic
Disbonding of Paint Films — Transport of Charge”. Corrosion Science
Vol. 35, Nos. 5-8, 1993, pp 1385-1389.
7. Leidheiser H. Jr. Wang W & Igtoft L
“The Mechanism for the Cathodic Delamination of Organic Coatings from a
Metal Surface” Prog. Org. Coat. Vol. 11, 1983, pp 19-40
8. Nguyen T. Hubbard J.B. & McFadden G.B.
“A Mathematical Model for the Cathodic Blistering of Organic Coatings on
Steel Immersed in Electrolytes” J. Coat. Tech. Vol. 63, No. 794, 1991, pp
9. Mayne J.E.O. “The Mechanism of the
Inhibition of the Corrosion of Iron and Steel by Means of Paint” Official
Digest, Feb 1952, pp 127-136.
10. Scantlebury J.D. “The Dynamic Nature of Underfilm
Corrosion” Corrosion Science Vol. 35, Nos. 5-8, 1993, pp 1363-1366.