Volume 2 Paper 15
Short Term Testing and Real Time Exposure
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JCSE Volume 2 Paper 15
ifnotmode(3,'Submitted 13th September 1999')
Short Term Testing and Real Time Exposure
Akzo Nobel Nippon Paint , North Woolwich Road , Silvertown , London . E16 2AP
Three case studies are used to illustrate the difficulties in determining
long term corrosion performance and in-service early failure of pre-coated,
galvanised steel by the use of laboratory accelerated techniques. Variants of
cyclic exposure tests tend to provide better realism in terms of the mode of
corrosion, especially at the cut edge of the coated sheet. However, even these
do not replicate the exact mode of failure, particularly with regard to the
later stages of cut edge corrosion, namely the corrosion of the steel itself.
The examples chosen also illustrate the importance of accelerating factors on
the initiation and propagation stages of cut edge corrosion, arising from
elements such as building design, orientation, and macro / micro-climates. An
integrated, corrosion prediction methodology is suggested, combining the
response data from fundamental tests, from a broad spectrum of accelerated
tests and from a number of outdoor exposure series. These are then linked
together in a central model to provide a reliable, predictive guide of
§2 Keywords: Cut edge corrosion, Coil Coating, Accelerated testing,
Cyclic corrosion tests, UV exposure, Hot dipped galvanised steel, Chromate
free primers, Outdoor exposure, Service life prediction.
§3 Organically pre-coated metal sheeting is a well established product for the
construction industry, comprising about 20% of all roof and wall cladding
areas in the UK and probably more so in mainland Europe . The corrosion
performance of these coated metals is controlled by the combined effects of a
galvanic coating, a pretreatment of a controlled metal oxide/ chromic acid
rinse, and finally by the application of organic coatings. These organic
coatings themselves perform a number of different functions from the point of
view of corrosion protection. The primer layer has to be formulated so as to
maximise adhesion to both the pretreated substrate and the overlying topcoat.
At the same time this coating also has to function as the vehicle in which to
carry the inhibitive pigments. Finally, in combination with the topcoat, the
coating system functions as an ionic barrier coating (Figure 1).
§4 A recent survey of UK installations [2,3] suggested that failures of such
materials within their product lifetimes probably amount to no more than 0.25–0.5%
of the total area usage of such precoated, materials in the UK. That said,
more than 75% of these failures were linked to corrosion problems and
specifically those related to critical areas such as the exposed cut edges of
sheets in roofing and side cladding situations shown in Figure 2.
§5 The issue that is of most concern to the coating’s formulator and user is
that of trying to understand and model such corrosion processes. In many cases
the type of failure will not have been predicted in the development and
quality checks that the same organic coatings were subjected to in the
§6 Another important concern to be addressed is that of predicting the service
life of coatings, particularly in environments where there is no exposure
history of the system. The coatings and painted sheets are often supplied to
the end user with a guaranty of the time to first maintenance . Similarly,
international performance standards are also calling for a minimum level of
performance  and so some means of determining the coated system’s
performance over the broadest range of environmental stresses has to be
established. Again neither the traditional test methods nor a limited range of
severe, natural exposures can be anything other than an approximation of
§7 A further issue to face the coating’s formulator is that of choosing
suitable techniques to use in product development. New demands in the coil
industry mean the development of more environmentally compliant coatings for
the future . This in turn means a move away from traditional pretreatments
based on chromic acid, away from primers containing hexavalent chromate
and from thick film, barrier coatings based on polyvinylchloride, all of which
have been the mainstay for corrosion protection for so long. To avoid
erroneous and untimely conclusions from unreliable tests being made, a new
approach to performance assessment has had to be developed alongside the
development of the formulations themselves.
§8 Several examples are used in this paper to illustrate these issues. The
focus is placed upon the performance of zinc galvanised steel at the critical
area of sheet cut edge, concentrating on the mode, rather than mechanism of
corrosion in the field compared to that generated by accelerated tests.
Case Study 1. Agricultural Building Roof, North West England.
Observation of Corrosion
§9 This building was erected in 1988 on the north west coastal area of the UK
and by 1993 cut edge corrosion was observed at the gutter drip edge of the
roof sheeting on the westerly facing pitch of the roof (Figure 3). This side
of the roof faced towards the Morecambe Bay estuary, about 20 metres away. The
other side of the roof showed no such signs of deterioration (Figure 4).
§10 The edge defect comprised peel back of the organic coating to a maximum of
7mm up the sheet. On the surface of the exposed substrate both zinc and steel
corrosion could be observed. The zinc corrosion product was seen as a light
deposition in a tidemark fashion. At the very edge and, in some cases
extending up to 4 mm, the zinc anode had been totally consumed and steel
corrosion was occurring.
§11 The first response was to review the results of the same coated product on
other exposure sites and accelerated tests. In short could this type of
failure have been predicted?
§12 Saltspray performance, conducted according to ASTM B117 – 90 (Figure 5a),
showed only slight edge corrosion after 1000 hours of continuous testing. At
the exposed cut edge there was no visible peel back of the coating, rather
only very slight blistering (<2/2 according to ISO 4628-2 82) The only
other evidence of corrosion was that of dense, compact, white zinc corrosion
product accumulated on top of, rather than underneath the coating. Where
blisters could be removed, a slightly yellow liquid was released and the
substrate underneath found to be a dull metallic colour with little or no
visible, white zinc corrosion products. This is similar to that reported by
§13 Like saltspray testing, Prohesion� cyclic testing (conducted
according to ASTM G85-94), resulted in little corrosion at 1000 hours of total
testing (Figure 5b). However the cyclic testing conditions resulted in primer
edge peel and lift rather than blistering at the exposed cut edge. There was
also some build-up of solid corrosion product under the coating. This was
beginning to form a wedge and so lift the coating further. However the degree
of delamination was still slight at the end of test (<2mm from the edge).
There was no evidence of anything other than zinc corrosion products, albeit
they had a different, more open and voluminous morphology to that produced in
the saltspray test. Importantly there was no evidence of red rust and
associated steel corrosion products.
§14 After 3� years of Scab Corrosion testing  (Figure 5c), considerably
more corrosion was seen on the panels, particularly at the exposed cut edges.
This comprises a heavy deposition of zinc corrosion products underneath the
coating and (at the extreme edge) on top it. Red rust was also visible at the
edge and below the zinc corrosion layer. In places the organic coating had
become embrittled and had peeled back or flaked off. On a macro scale this
test shows all the components of corrosion seen on the roof of the building.
However the pattern is still not quite the same, particularly in the
morphology and quantity of solid, zinc corrosion product formed, which
differed considerably from the ‘tide mark effect’ seen in the field.
§15 5 years Hook of Holland Exposure performance is shown in Figure 5d. These
panels do show the same type of mechanism of failure albeit on reduced scale
over a similar time period. Although the pitch was similar at 5�
from the horizontal, the test panel didn’t face the prevailing wind
direction during exposure and only the overlap rather then the drip edge was
corroded. That said the corrosion was characterised by organic coating peel
back and flaking, tidemark lines of the zinc and some red rust at the extreme
edge. Unlike the corrosion products generated by the Prohesion�
test, on this exposure they were not voluminous and did not appear to form a
wedge. This infers that other processes were also responsible for the initial
loss of adhesion.
§16 Figure 5e shows the corroded edge of the product after 1500 hours testing
on a combined Cyclic Fog /UV Exposure test. This new test follows the
procedure outlined for testing of other exterior coatings [10,11,12] and which
has recently been embodied in the ASTM D5894-96 standard . At the end of
test the coating had peeled back and flaked from the edge as seen on natural
exposure in Holland and on the building. The zinc corrosion product was not as
voluminous as that generated by the Prohesion� test on its own and
there was evidence of red rust developing at the extreme panel edge.
§17 It is clear that the older, established tests were not able to predict and
or replicate the type of field failure observed. This example evidences the
well documented shortcomings of the continuously wet, saltspray test  In
this test, the conditions used (95-98% relative humidity, 5 % by weight NaCl
at pH of 6.5 – 7.5 and 35�C) result in a pattern
of corrosion and morphology of corrosion products rarely seen in natural
exposures. In particular, the very high concentration of sodium and chloride
ions, in conjunction with the constant high relative humidity, result in
clearly defined anodic and cathodic sites being set up on the principle of the
oxygen concentration cell. The result is that as the pH in the cathodic
regions rises, in some instances to as much as 12 or 13 , the zinc surface
becomes passivated. The high pH would also mean that any corrosion product
would likely remain in solution rather than precipitate as solid corrosion
product to form a wedge. Furthermore this high cathodic pH can also result in
base catalysed hydrolysis and saponification of the organic binder at the
interface leading to wet adhesion loss. The overlying zinc corrosion product
is predominantly made up of dense plate-like structures of hydrated, basic
zinc chloride, Zn5(OH)8Cl2.2H2O
(pseudo-hexagonal crystals), rather than the zinc hydroxy carbonates seen on
natural exposure . Thus both the anodic products and cathodic mechanisms
fall somewhat short of reality and, not surprisingly, do not reproduce or
predict the pattern of corrosion seen on the roof.
§18 By contrast the wet/dry cycling conditions of the Prohesion�
test result in quite a different pattern of corrosion to that of the saltspray.
The inclusion of wet and dry cycling periods is now well recognised  as
significantly influencing the corrosion process. The increase in ionic
concentrations during the drying periods results in some solid species being
formed as precipitates or evaporites, accumulating as a wedge between the
anode and cathode regions. The example here demonstrates further the effect of
cyclic conditions in the formation of solid corrosion products that can
mechanically damage the film . This process, allowing free access of
oxygen and CO2, together with the use of a more dilute electrolyte
fog (0.35wt% (NH4)2SO4 and 0.05 wt % NaCl)
will favour the build-up of different corrosion products from the salt spray;
predominantly the complex salt of zinc hydroxy sulphato chloro hydrate; Zn12(OH)15(SO4)3Cl3
(H2O) 5. . Equally the cyclic test conditions favour
the formation of compact rust layers consisting of �-FeOOH
and g-FeOOH similar to that found on outdoor
exposure testing [19,20]. That said, the standard test conditions used were
not able to produce the less voluminous corrosion products or the degree of
steel corrosion as seen on the cut edge of the roof. It is likely that the
salts used in the test still overwhelm the buffering effect of surface
moisture containing bicarbonate ions which would otherwise lead to the
formation of basic zinc carbonates such as Zn5(OH)6(CO3)2.
§19 Even though the test represents a more realistic one than the continuous
salt fog , the overall performance on this test would still not predict the
extent of cut edge corrosion and particularly anode depletion leading to steel
corrosion, seen in the field.
§20 Both the Scab corrosion test and the outdoor tests at the Hook of Holland
exposure site demonstrate the same type of corrosion performance as seen on
the roof. The former test is the more aggressive. The Dutch exposure was the
closest in terms of the amount of solid corrosion products and the mode of
coating delamination and degradation although this occurred to a smaller
extent and in the area of panel overlap rather than at the free drip edge. The
mode of failure appeared to be the same although it is significantly less in
degree (2 mm total creep against 7 mm for the roof in the same time period).
This suggests that the corrosion process be accelerated by a number of
different factors. In the case of the west facing roof pitch of the building
it may be the combination of more frequent and rapid rates of wetting and
drying and the build up of debris and dirt at the edge that lie behind the
cause of the failure.
§21 For those panels exposed at the Hook of Holland the features and processes
involved in corrosion of overlap regions may also provide a clue to the drip
edge problem. The accelerating influences in the former instance could be
longer wet times (because of entrapped water), crevice corrosion and even
differential aeration cells. It is possible that the same factors may be
operating on the very exposed and wet conditions on the gutter edge of the
§22 Of the laboratory tests the combined cyclic fog/ UV exposure test was
the only one which began to demonstrate the same pattern of performance as
seen on roof. In many ways the edge corrosion appeared similar in basic
morphology and, in particular, less voluminous than that produced by the
Prohesion� test. It was also the only
accelerated technique, which caused the topcoat to loose gloss and become matt
in a similar fashion to that seen on natural exposure. Loss of gloss is a
common feature related to photolysis and/or photo-oxidation of the surface
layers of polyester coatings. The reduction in gloss renders the surface more
hydrophilic, lowering the critical RH for the onset of corrosion. The same
processes can also produce microcracking and micropores, further facilitating
the uptake of moisture and thereby aggressive ions . That said, the test
still did not give the same degree of steel corrosion (visible as red rust) as
seen on the roof, even after completing 1500 hours of testing. It is possible
that a modified wet/dry regime, to encourage more rapid drying and a different
concentration and combination of electrolytes may better replicate the
§23 Thus it is clear that this type of edge corrosion failure could not be
anticipated by the accelerated tests employed. However, the example does
demonstrate the importance of introducing combined stresses into the
accelerated corrosion protocols, rather than just using those of a
continuously wet nature. It would seem that wetting and drying of the panel
together with photodegradation are some of the more important ones to utilise.
Case Study 2. Packaging Factory, South West England.
Observation of Corrosion
§24 This building was erected in 1987 in the Bristol area and by 1991 showed
the type of overlap edge corrosion shown in Figure 6a and 6b. The following
points were noted regarding the occurrence of this phenomenon and were felt to
be significant with respect to the nature of this corrosion:
§25 a) Corrosion was limited to the roof only and
occurred in the trough valleys of the profile.
§26 b) The sheet gauge used on the roof was
heavier (1.1mm) than the side cladding (0.7mm)
§27 c) Corrosion was limited to the south west
facing pitch main roof. Neither the north east pitch nor roofs of smaller
outbuildings showed a similar problem. See Figure 6b.
§28 d) Corrosion was limited to strip overlap
areas. The drip edges of the same sheets at the gutter were corrosion free.
§29 e) In the troughs there were significant
deposits of extract from the factory ovens. When wet this material formed a
sludge or poultice over the coated steel in places up to 0.5 – 0.75 mm
§30 A detailed investigation of the defect was undertaken both in-situ and back
in the laboratory. This lead to the identification of a number of factors
which differed between material that had failed prematurely and that which was
in a good condition. These findings are summarised in Table 1.
§31 Again, none of the traditional laboratory testing techniques predicted that
this type of failure would occur. Salt spray  and high humidity 
testing showed the coated sheets to be giving the level of performance
anticipated for the product. Even the severe, accelerated, outdoor scab
corrosion test  indicated that the pre-coated coil performance was in line
with expectations. Therefore this example demonstrates further the dangers of
relying on short term testing to predict real time performance. It also
illustrates other variables and influences to consider when relating short
term, accelerated testing results to premature in-service failure. Some act as
confirmation of those influences already discussed, but there are others that
§32 a) The importance of mixed electrolytes and
their respective concentrations are a significant influence on the mode of
corrosion. This has been argued for a long time  and the results from
Table 1 clearly support the use of sulphate rich electrolyte solutions. It is
interesting to compare the measured ppm levels of sulphates, and chlorides
with those commonly used in the saltspray and Prohesion�
tests. The levels of chloride ion present at the overlap are two orders of
magnitude less than that employed in the saltspray (50,000 ppm of NaCl).
However the Prohesion� test deposits quantities of NaCl and (NH4)2SO4
(500 ppm and 3500 ppm respectively) almost identical to that measured in
the uncorroded overlaps and 2 –3 times lower than in the problem areas.
§33 b) The effect of the high cathode: anode ratio
significantly effected the rate of anode depletion. Our own work with
accelerated tests supports this, although the picture is less clear cut with
the thinnest gauges. The general trend for the tests was similar at gauges
between 0.5 – 1.3 mm, even though the mechanisms of corrosion clearly were
not. On the roof this ratio was ~27:1 compared with ~10:1 for the previous
example. Whatever the specific process it is clear that the larger the cathode
area the harder the anode is driven at the cut edge. The PUCAT test devised by
Walters can provide a means of assessing the coating’s response as this
factor is varied by increasing the effective cathodic area.
§34 c) The comparatively low Tg (+15�C) of the
topcoat exacerbated the degree of permanent deposition of the factory extract.
This resulted in a rapid and significant build-up of debris at the cut edge
and immediately below it forming a poultice. This, in turn acted rather like a
sponge, to trap and accelerate the concentration of aggressive ions. Current
accelerated tests have no means of determining the effects of either coating
Tg or poultices on the overall corrosion performance.
§35 d) Both the low pitch of the roof (<10�)
and the design of the sheet overlap and fixings combined to cause ponding and
lengthen the wet times at the edge. Additionally no sealant had been used at
the overlap, so allowing moisture to be trapped underneath. This will promote
the set up of differential aeration conditions, setting up conditions which
favour crevice corrosion. These may also have contributed to the premature
corrosion at the cut edge. Whilst recognised as significant influences on the
corrosion process , these influences are not routinely exerted into tests
in the laboratory
§36 In addition to incorporating combined or co-acting stresses during the
accelerated testing, this example argues for careful selection of electrolytes
and concentrations. It also underlines the significant influence of building
design and construction which can result in a coating failing prematurely.
Case Study 3. Comparison of Exposure Results at Two Different European
Observation of Corrosion
§37 A final example from the field illustrates the influence of location and
elevation of the exposure on the corrosion resistance of coil coated systems.
The effect is quite an obvious one to discuss but it is important to
demonstrate just what sort of differences can be seen even with the same
coating system. In this example the coating was applied under commercial
coating conditions on hot dipped galvanised steel (0.5 mm gauge), galvanised
with 250 g/m2 of zinc. The organic coating comprised a primer and
topcoat .The former was an epoxy melamine based primer with strontium chromate
as the inhibitive pigment. The latter was a polyvinylidene flouride coating;
considered to be the most durable generic type used commonly for coil
coatings. Panels of this system were also exposed in the accelerated
environments detailed in Table 2.
§38 The overall patterns for the edge corrosion of each of these tests is shown
in Figures 7a – 7h inclusive. The principal issue is again, not in the
details of the corrosion mechanism, but rather the mode of corrosion generated
by each exposure and to what extent the short term tests replicate this.
§39 From the photographs a number of features are worth commenting on:
§40 a) There are quite distinct differences
between the natural exposure panels on the basis of both geographic location
and orientation. The exposures on the 90 �, north-facing orientation at
Hendaye gave the greatest degree of degradation. However the degradation here
is characterised by significant face blistering and, at the cut edge, the
build up of voluminous zinc corrosion product overlying as well as
undermining the organic coating. In places the coating has embrittled and
flaked but there is surprisingly little evidence of steel corrosion /red rust
considering the severity of the exposure.
§41 b) At the Hook of Holland site on 90�, north
facing exposure the same material shows quite a different corrosion mechanism
and seems to follow the more usual edge disbondment, peel and flake off of the
organic coating. There is considerably less zinc corrosion product and some
bare, presumably passivated zinc exposed. There are traces of red rust at the
extreme cut edge.
§42 c) The exposures on the 45�, south
facing orientation at Hendaye correspond closely with those at the Hook of
Holland at 45�, both showing the beginnings of edge peel after 5 years.
§43 d) The pattern of corrosion of the system
exposed at Hendaye on 90�, north facing elevation is best replicated by the
scab corrosion test and to a much lesser extent the saltspray. As before the
other natural exposures seem better replicated by the laboratory cyclic tests
and in particular the Prohesion� and Cyclic salt fog/ UV exposure
§44 The results show clearly that the outdoor environment, even in the two
locations chosen here, is neither uniform in the mode or, quite probably, the
mechanism of corrosion. Equally as interesting is that all the various
accelerated tests seem to have relevance depending on location, with the
possible exception of the salt fog test.
§45 Table 3 shows the principal climatological differences between the two
sites . The severe marine site at Hendaye is obviously the more aggressive
of the two, evidenced by the corrosion rates of zinc and steel. The reason for
this difference is most likely found in the combination of high Cl-
levels, higher average annual temperatures and radiation levels.
§46 Being sheltered and north facing, the 90� exposed panels at Hendaye
are likely to experience the longest wet times of any natural exposure,
especially at the horizontal cut edge. It may be postulated that at this edge,
conditions above critical RH will persist and ionic concentrations build most
rapidly. Corrosion products will be more formed more rapidly, will be denser
(possibly containing more of the hydrated zinc hydroxy chloride (Zn5(OH)8Cl2.
2H2O)), and more strongly isolating.
§47 A likely consequence will be that the zinc is less able to function
sacrificially, hence anodic depletion will stall, evidenced by the lack of
further coating peel back and red rust.
§48 Being outdoors and sheltered 90� and facing north, the scab
corrosion test conditions are clearly very similar to this, albeit that the
greater Cl- content in the spray appears to accelerate the
degradation. Saltspray testing takes this a step further still, but fails to
produce the same, more open corrosion product morphology.
§49 At other angles of exposure on the same site the effects of more rapid,
wet/dry cycling, direct sunlight and greater temperature cycling would explain
why such exposures are better replicated by the cyclic tests with lower
chloride concentrations and periods of UV irradiation. The resultant, thinner,
less dense and thus more permeable corrosion products are more likely to
contain more hydrated zinc hydroxy carbonate salts. The mode of edge corrosion
in this case and replication of it is shown in Figures 8 and 9 respectively.
§50 This case demonstrates the dangers of assuming that a more severe marine
environment like Hendaye just serves to accelerate the rate of
corrosion. Clearly the mode, rate and mechanism can differ profoundly
between geographic location and elevation. This underlines the importance of
understanding not only the macro and micro climates of chosen test sites, but
also the specific climate of any building before offering advice or guaranties
An Integrated Model for Corrosion Performance Prediction.
§51 The case studies used here illustrate and reinforce the fact that the
nature of corrosion in the environment where pre-coated steels are used is
complex and variable. They also serve to emphasise the influence of product
and application design and the accelerating effects of atmospheric
contaminants and pollutants. Clearly these all contribute towards the overall
corrosion performance of the product in a combined rather than isolated
§52 This being the case it is unlikely any single test will be capable of
reproducing the corrosion process in its many forms and simultaneously take
into account the various design and orientation influences. The demand for a
generally applicable corrosion test is therefore, as Funke put it ‘rather
like the demand for a medicine, which cures all ills’. 
§53 This dilemma can be resolved by two possible approaches. The first is to
use laboratory proving tests which do not necessarily accurately model
atmospheric attack mechanisms but which are consistently reproducible. They
must obviously also produce an adequate acceleration of corrosion. Such
an approach is ideal for quality control and ranking evaluations. Considerable
work has been undertaken by both the SSPC and CSCT in this area, utilising
rank correlation statistics to determine the reliability of an such approach.
[29,30,31]. The key to this approach is to preserve the outdoor, ranked
performance of known systems in a reliable range of accelerated tests. Having
done so, the performance of an untried, new product can then be assessed and
ranked with confidence within this matrix.
§54 The other approach supported here is based on one suggested for the
automotive industry . This entails the integration of information relating
to the corrosion process derived from three sources; firstly, that from
standard, natural exposure sites, secondly from a broad spectrum of laboratory
accelerated performance tests and finally, from measuring system’s response
to the fundamental processes involved in corrosion.
§55 Implicit in this methodology is the recognition that any particular
accelerated or outdoor test will often overstress particular elements of the
corrosion process. The advantage of this approach is that it both recognises
and exploits these emphasised stresses. By integrating the responses from all
three sources into a central model, it then becomes possible to both predict
service life in untested environments and to understand and rectify early
§56 Information from the field is predominantly a question of careful
observation of any failure mode, in particular how it is initiated and how it
proceeds. This should be supported by characterisation of the process
using in-situ, scanning techniques such as SRET, potential mapping or Kelvin
probe analyses. Such techniques and the information about the real time
processes are then utilised to validate the other two approaches in the
methodology. A broad range of exposure sites with detailed information on
their climatological characteristics helps to put the corrosion modes
recognised into the context of the environment of exposure.
§57 The accelerated tests chosen, are done so as to provide a means of
assessing the sensitivity of materials to one or more of the specific
degradation influences outlined below. Ideally these should be applied
at a number of different levels in order to generate a response surface for
§58 a) Mixed electrolytes , pH and concentration,
§59 b) Influence of wetting and drying rates and
§60 c) Influence of photodegradation of the
§61 d) Influence of design –overlaps, drip ends,
scratches and bends, composite panels.
§62 e) Influence of orientation and exposure angle
(90�, 45�, 5� from the horizontal).
§63 f) Influence of temperature
fluctuations (-25�C� + 70�C)
§64 g) Influence of panel moisture/wet time
§65 h) Influence of substrate thickness
§66 Clearly this list is not exhaustive but in an industrial development
laboratory some balance between time, resource and depth of study has to be
struck. A number of these influences are already covered in existing
assessment standards. What is necessary is to utilise a broad range of testing
techniques, each one emphasising one or more of the particular parameters
listed above. Like the jigsaw puzzle it is the combination of the
responses of the system to each individual influence that enables the
formulator to gain the overall picture. Each piece on its own is not only
uninformative but has the potential to be misleading.
§67 The purpose of the fundamental studies are to link up the coated system’s
macro corrosion performance characteristics with its response to
electrochemical and physical stresses, which have been identified and
characterised on examples in the field. These will be the essential chemical
and physical mechanisms lying at the heart of the corrosion processes. The
techniques should be employed to characterise the basic adhesion,
barrier and inhibitive properties of the coating under a variety of different
conditions. The best tools to use here are likely to be those that evaluate
the coating in-situ rather than as a free-film . Electro chemical
impedance spectroscopy (EIS) has been successfully used to measure ionic
barrier properties, water uptake rates through the coating and wet adhesion at
the substrate interface. The protective action of the coating and the
efficiency of the inhibition process at the cut edge has also been be studied
with EIS , atomic absorption spectroscopy and scanning reference electrode
techniques . Measurements of microhardness and adhesion at temperatures
around and exceeding the Tg of the coating will also be of value.
§68 These fundamental, mechanistic responses then need to be integrated with
the responses of the system to the accelerated stresses/artificial
environments of laboratory tests and the information relating to real life
failure modes and service conditions. This is done through the central
predictive model as shown in Figure 10.
Case Study 4: Development of Chromate Free Primer Systems.
Description of the Programme
§69 The integrated testing methodology suggested is particularly suited to the
determination of service life performance of new, chromate free systems.
Typically traditional, accelerated techniques discriminate against such
products and this last example is an illustration of how the proposed
methodology can be used to avoid falsely condemning promising materials. In
this project, fundamental and exterior evaluations are still in progress, but
the accelerated testing protocols and the coating’s responses are worth
§70 All the coated materials in this example were factory applied under
standard commercial conditions. Their general compositions are shown in Table
§71 Panels were against a selection of accelerated tests, the specifications of
which are shown in Table 4b.
Results and Discussion
§72 On accelerated testing the major point of degradation, as expected, is the
exposed cut edge. Edge creep performance, summarised in terms of degree and
mode for the various accelerated methods, is shown in Table 5. The key to
interpretation of the results is now not the performance against a particular
specified, standard test such as ASTM B117, but rather performance of the
material on application of a particular stress or set of conditions. More work
needs to be done in the resolving each procedure into its component stresses.
However, by way of illustration, some tentative conclusions on edge creep
performance of the chromate and chromate-free primers can be deduced.
§73 a) Wet Adhesion and effect of Base
Catalysed Hydrolysis – System A shows the better resistance, especially
in terms of degree of edge delamination.
§74 b) Wet Adhesion and Influence of High RH
under condensing and immersed conditions – Good osmotic resistance and
wet adhesion is demonstrated by both System A and B.
§75 c) Influence of cycling conditions –
These produce edge peel and corrosion product wedges in Prohesion�
and Cyclic Fog/UV Exposure. The cyclic PUCAT test does not seem sufficiently
progressed and the cycling conditions of CCT-1 are masked by the high NaCl
concentrations. At this stage System A demonstrates more resistance to the
stresses built up by the cycling conditions than System B.
§76 d) Influence of increased cathode and anode
ratio. At 750 hours the Cyclic PUCAT test did not produce a high degree of
anodic depletion and red rust was not yet observed. This suggests that both
primers provide good protection for the galvanised substrate and that both
exhibit good wet adhesion and inhibitive properties. System A shows some edge
peel and System B blistering as a response to this applied stress.
§77 e) Influence of UV Degradation. Whilst
the topcoat and its degradation remain the same for both systems the increased
effects of wetting and ionic transport through the film are exerting a
slightly more delamination effect on System A than System B. Both show the
start of steel corrosion at the cut edge.
§78 f) Influence of electrolytes-
System A and System B respond similarly to the NaCl and (NH4)2SO4
electrolyte mix used in Prohesion�, Cyclic PUCAT and Cyclic
Fog/UV exposure tests.
§79 Using this response data coupled with an understanding of the specific,
applied stresses and influences involved, a combined picture of accelerated
performance can be built up. With further evaluations and the required
integration of responses from the field and fundamental studies it will be
possible to confidently construct a comparative model of predictive
performance of this chromate free system.
Summary and Conclusions
§80 The objective of this paper has not been to promote any single, new test to
replace salt fog or other existing laboratory based accelerated techniques. It
is clear that whilst the adoption of cyclic tests such as ASTM D5894-96
represent a move in the right direction they are unlikely to be able to
reproduce all aspects of real time corrosion performance.
§81 Rather the purpose has been to support, on the evidence of real time, field
exposures, the adoption of a more coherent approach to predicting both early
failure and long term service life. This methodology entails the integration
of response data from three different, but complimentary approaches, into a
central predictive model. No one of them stands on its own as an indicator of
performance, contrary to the traditional method of corrosion assessment and
§82 In all but sheltered marine environments, the edge corrosion process for
zinc galvanised, organically pre-coated steel appears to follow a common mode.
It is characterised by peel back and embrittlement of the organic coating , by
relatively thin layers of zinc corrosion product and by an apparently
high rate of anodic reaction and consequently corrosion of the steel to form
red rust. The process is initiated by adhesion loss (electrochemical or
mechanical disbondment) of the organic coating and is accelerated by a number
of factors not necessarily related to the coating itself. These may include
the substrate gauge, the nature of the electrolytes and any contaminant, the
rate and duration of wet and dry and temperature cycles. Propagation can be
similarly accelerated by external influences such as the orientation and
attitude of the coated sheet on the structure (hence wet and dry
characteristics and resultant stresses).To date most, common accelerated tests
do not reproduce this phenomenon.
§83 The author wishes to thank the management of Akzo Nobel Nippon Paint Ltd,
for their permission to use their data and Dr Scantlebury, Dr R Howard and Dr
A Darwin of the corrosion protection centre at UMIST for their help and
co-operation in certain aspects of corrosion protection assessment.
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§85 Figure 1 : Section Through Edge of Pre- Coated Steel Sheet (Not to
§86 Figure 2 : Failure modes on Precoated Steel .Source 
§87 Figure 3 : Drip Edge, West Pitch
§88 Figure 4 : Drip Edge, East Pitch
§89 Figure 5a : Saltspray 1000 hours
§90 Figure 5b : Prohesion� 1000 hours
§91 Figure 5c : 3 � Years Scab Corrosion Test
§92 Figure 5d : Hook of Holland 5 Years
§93 Figure 5e : Cyclic Fog/UV Exposure Test, 1500 hours
§94 Figure 5 : Accelerated Testing : Edge Corrosion Patterns (x 8
§95 Figure 6a Roof South West Pitch, Overlap Edge
§96 Figure 6b Roof North East, Overlap Edge
§97 Table 1 : Characteristics of Different Roof Pitches
South West Pitch
North East Pitch
Organic Coating Thickness
Galvanised Layer Thickness
Extent of Red Rust (from edge)
Extent of Coating Peelback (from edge)
Cross Hatch Adhesion
Pencil Hardness (Faber Castell)
Solvent Resistance(methylethyl ketone)
40-45 double rubs
29-45 double rubs
pH at the overlap edge
Cl - ppm(w/w)
Top of Overlap
Bottom of Overlap
Top of Overlap
Bottom of Overlap
Bottom of Overlap
§98 Figure 7a. Saltspray Test - 1000 hours Continuous Fog
§99 Figure 7b. Prohesion� Test 1000 hours wet/dry cycling
§100 Figure 7c. Cyclic Wet Dry/ UV Exposure Test 1500 hours
§101 Figure 7d. SCAB Corrosion Test 5 years Exposure
§102 Figure 7 : Case Study 3 : Accelerated Exposure Tests (x 0.54)
§103 Figure 7e Hook of Holland, 90� North Facing 5 Years
§104 Figure 7f Hook of Holland, 5� South Facing 5 Years
§105 Figure 7g Hendaye, France, 90� North Facing 5 Years
§106 Figure 7h Hendaye, France, 5� South Facing 5 Years
§107 Figure 7 : Case Study 3 : Natural Exposure Tests . (x 0.54)
§108 Table 2 : Case Study 3 : Corrosion Test Details
1) Saltspray according to ASTM B117 –97
according to ASTM B117 –97
3) Cyclic Fog / uv Exposure according to ASTM
D5894 – 96
4) Scab Corrosion according to SS 11 72 11
5) Hook of Holland 90�, North Facing Natural
Exposure (ECCA T19.5.2)
6) Hook of Holland 5�, South Facing Natural
Exposure (ECCA T19.5.3)
7) Hendaye, France. 90�, North Facing
Natural Exposure (ECCA T19.5.2)
8) Hendaye, France 5�, South Facing Natural
Exposure (ECCA T19.5.3)
Table 3 : Climatological Data Comparison 1995-1996 (Source Reference 27)
Hook of Holland
51� 59' N
43� 28' N
Average Annual Temperature (�C)
Average Annual Relative Humidity (%)
Global Radiation (hJcm-2 )
Average Annual rain mm
Average annual Wind Speed
Predominant Wind Direction
Average annual Deposition Rate
Steel Corrosion Rate
Zinc Corrosion Rate
§109 Figure 8 : Cut Edge Corrosion Mode (x 8 magnification)
b) Edge peel /Zn Corrosion
c) Fe Corrosion Exposure
d) Coating detachment
§110 Figure 9 : Reproducing Cut Edge Corrosion (x 8 magnification)
Scab Corrosion Test 3 Yrs
Hook of Holland 6 Yrs
Scab Corrosion Test Exposure
Prohesion� Test 1000 hrs
Saltspray Test 1000 hours
§111 Figure 10 : Corrosion Prediction Methodology
§112 Table 4a Case Study 4 : Systems Evaluated
System A: Chromate-free Polyester primer on HDG:
Substrate: HDG, 0.5 mm gauge, Zn coating thickness 17-24 microns
Pre-treatment : Bonderite 1303 /Parcolene 62.(Chromic acid based)
Primer : No chromate pigments, 6-8 microns. Polyester/amino binder
Topcoat : Brown polyester/amino topcoat of 18-20 microns dry film thickness
System B: Chromated Polyester primer on HDG :
Substrate : HDG, 0.5 mm gauge , Zn coating thickness 20 microns
Pre-treatment : Bonderite 1303 /Parcolene 62. (Chromic acid based)
Primer : Strontium chromate containing, 6-8 microns. Polyester/amino binder
Topcoat : Brown polyester/amino topcoat of 18-20 microns dry film thickness
Table 4b Case Study 4 : Accelerated Corrosion Test Detail
1) Saltspray according to ASTM B117 –97
2) Prohesion� according to ASTM B117
3) Condensing Humidity according to BS 3900 : Part F2
4) Total Water Immersion according to ASTM D870 –92
5) Cyclic Fog / UV Exposure according to ASTM D5894 – 96
6) Scab Corrosion according to SS 11 72 11, exposed as ECCA T19.5.2
6) Cyclic PUCAT
1 hr immersion in
3500ppm (NH4)2SO4 ,500ppm NaCl
1 hr drying at
coupled SS Cathode coupled to 30mm x 0.5mm Coated Edge
7) CCT – 1 Cyclic Corrosion Cabinet (Automotive Test CCT - A)
Cycle 1 : 4
hours 5% NaCl Fog @ 35�
Cycle 2 : 2
hours drying @ 60� C
Cycle 3 : 30 mins.
drying @ 40�C
Cycle 4 : 2 hours
condensing humidity @ 50� C, >95%RH
§113 Table 5 : Case Study 4 Evaluations – Edge Corrosion Results
Edge Corrosion (mm from edge)
Coating Peel Back (mm from edge)
Edge Blistering (ISO 4628-2 82) Quantity/Size
4 - 6
2 / 4
4 - 9
Prohesion 1000 hrs
1 - 1.5
2 - 2.5
2 - 2.5
Water Soak 1000 hrs
0 - 0.5
0 - 0.5
Cyclic Fog / UV 1320 hrs
3 - 6
3 – 6
Some on edge
3 - 5
3 – 5
Some on edge
Cyclic PUCAT 750 hrs
1 - 2
CCT- 1 937hrs
6 - 10
Very dense Overlying
9 - 12
Very dense Overlying
Scab Corrosion 6700 hrs
1 - 1.5