Volume 2 Paper 36
Filiform Corrosion of Aluminium Alloy 3003 H14 under Humid and Immersed Conditions
Z. Marsh, J. Marsh and J.D. Scantlebury
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JCSE Volume 2 Paper 36
Submitted 13th September 1999, published for public review 29th October 1999
Filiform Corrosion of Aluminium Alloy 3003 H14 under Humid and Immersed
Z. Marsh, J. Marsh* and J.D. Scantlebury
Corrosion and Protection Centre, UMIST, Manchester 60 1QD, United Kingdom.
*CAPCIS Ltd, CAPCIS House, 1 Echo Street, Manchester M1 7DP, United Kingdom.
Epoxy coated aluminium 3003 H14alloy panels were examined for
susceptibility to filiform corrosion under both humid and immersed conditions.
The panels were not artificially damaged, but were artificially contaminated
with sodium chloride. Significant filiform corrosion was found to occur at 84%
relative humidity (RH). However, the extent of filiform corrosion was greatly
reduced at 100% RH. Extensive filiform corrosion was also noted for samples
immersed in 1% NaCl solution, especially during the early stages of exposure,
and in glycerol:water mixtures. If a cross-cut specimen is examined under
immersion conditions, severe filiform corrosion rapidly occurs, before
steadily converting to general under film corrosion, especially in the
vicinity of the cross-cut. The results clearly show that aluminium 3003 H14
alloy can undergo filiform corrosion under immersion conditions.
§2 Keywords: Filiform Corrosion, Aluminium 3003 H14 Alloy.
§3 One of the most common forms of corrosion occurring on coated aluminium and
aluminium alloys is filiform corrosion. While this form of corrosion rarely,
if ever, leads to a working failure, the consequences can be severe for any
decorative function. In appearance, filiform corrosion on aluminium alloys
resembles long thin thread like filaments, often up to several centimeters
long, but less than 1 milimeter wide. The filaments are generally white, with
a grey head. This head forms the active corrosion site, with corrosion
propagation and filament growth taking place here.
§4 Filiform corrosion is often induced by the presence of chloride salt
contamination on the aluminium surface. As described by Slabaugh et al,
the head of the filament usually contains significant quantities of
chloride ions, with little or no free chloride in the long tail. Any chloride
present in the tail is tied up as an aluminium oxychloride. The chloride
containing electrolyte appears to move forward with the head as the filiform
corrosion progresses across the aluminium substrate.
§5 Two primary filiform corrosion mechanisms are put forward in the
literature[2,3,4,5]. These consist of a cathodic delamination mechanism,
discussed by Funke, and an anodic undermining mechanism, as proposed by
Ruggeri and Beck. Lenderink believes that the first mechanism is
generally thought to be more important for steel, and the second for aluminium.
§6 Aluminium 3000 (Al-Mn) alloys are most often used in the manufacture of
cooking utensils and in the canning industry, although some limited use as
an architectural material also occurs. These are two areas where a retention
of the visual and cosmetic properties of the coated product is extremely
§7 It has been generally accepted that filiform corrosion only occurs under
humid conditions. However, the authors have attended a number of public
presentations where speakers described observing "filament like"
structures under immersion conditions on lacquered aluminium surfaces,
especially with respect to food and drink cans. However, the speakers were
extremely reluctant to describe this as filiform corrosion because "filiform
corrosion does not occur under immersion conditions". This publication
examines the tendency of epoxy coated aluminium alloy 3003 H14 to undergo
filiform corrosion under a variety of humid and immersed conditions.
§8 The composition of the aluminium alloy 3003 H14 (Q-Panel, U.K.) is given in
Table 1. The alloy is one of the 3000 series Al-Mn alloys.
§9 Table 1 Composition of Alloy 3003 H14
§10 The as received sample panels were rinsed with de-ionised water and
degreased with acetone. They were then contaminated at the centre of the panel
with a 5μl droplet of 0.1g/L sodium chloride
solution using a micro pipette. This gives 0.5μg
of contamination. Upon drying the contaminated zone was of approximately 2mm
diameter. The panels were then coated with a 50μm
wet thickness coating of a 2-pack epoxy/polyamide clear lacquer coating (Epilife/H
Marcel Guest, Manchester, U.K.) using the draw bar technique. The curing
regime consisted of 48 hours at room temperature followed by 48 hours at 60oC.
The resultant coating was of 20-25μm thickness.
§11 Coated samples were exposed to relative humidities of 84 and 100% at 28-29oC.
Samples were also immersed in 1% sodium chloride solution, 1% hydrogen
chloride solution or 1:10 glycerol:water mixtures. The samples were generally
undamaged, but some cross cut specimens were also examined in 1% sodium
chloride and 1% hydrogen chloride solutions.
§12 Figure 1 shows examples of epoxy coated aluminium 3003 H14 panels exposed
to an environment of 84%RH. Only a very limited amount of filiform corrosion
can be observed, with only a small number of filaments growing from the
§13 Little or no evidence of filiform corrosion was observed at this humidity.
Some general corrosion and blistering occurred in the vicinity of the
Immersion in 1:10 water:glycerol
§14 Figure 2 shows an example of a sample exposed under immersion to 10:1
water:glycerol. An area of general corrosion surrounding the central
contaminated area is clearly visible after 7 days. However, beyond this
region, filiform corrosion is clearly observed to have occurred at 14 days. An
area of general corrosion has lead to the initiation of filiform corrosion.
Immersion in 1% sodium chloride solution - intact specimen
§15 Figure 3 shows an example of a specimen immersed in 1% sodium chloride
solution. Here, filiform corrosion initiates around the general corrosion in
the contamination zone. However, with time further general corrosion
initiates, spreading across the area affected by filiform corrosion. Filiform
corrosion was far more extensive than that observed on any of the samples
exposed to humidity.
Immersion in 1% sodium chloride solution - cross-cut specimen
§16 Figure 4 shows an example of a cross-cut specimen immersed in 1% sodium
chloride solution. The cross-cut is surrounded by a region of general
corrosion. This region is surrounded by extensive filiform corrosion after 35
days of exposure, covering most of the available exposed area.
Immersion in 1% hydrogen chloride solution – scribed specimen
§17 Samples were partly immersed in 1% hydrogen chloride solution. Within the
immersed region, general corrosion rapidly initiated, with undermining of the
coating occurring within 48 hours. However, filiform formation occurred at the
scribed defect in the region above the immersion line. Figure 5 shows a region
of a specimen immediately above the immersion line after 21 days of exposure.
General corrosion is beginning to advance from the scribe cut into the region
where filiform corrosion has occurred.
§18 A summary of the results for each system is given in Table 2.
84% Relative Humidity
Small number of slow growing filaments
100% Relative Humidity
Little or no evidence of filiform corrosion
Immersion in 10:1 Water:Glycerol – intact
Larger number of faster growing filaments initiating from area of
general corrosion in vicinity of contamination.
Immersion to 1% sodium chloride solution - intact
Initially severe filiform corrosion, changing to general corrosion
after several weeks of exposure.
Immersion in 1% sodium chloride solution - scribed
General corrosion at scribe, surrounded by severe filiform
Part immersion in 1% HCl
Severe general corrosion in immersed zone. Filiform corrosion above
immersion line with general corrosion advancing from the scribe with
§19 Table 2 A summary of the exposure conditions and observations for epoxy
coated aluminium 3003 H14 alloy samples exposed to a variety of conditions.
§20 It is not the purpose of this publication to give a detailed mechanistic
interpretation of filiform corrosion, or to give anything more than a general
indication of corrosion rates in the environments examined. The primary
objective is to establish whether filiform corrosion occurs under immersion,
as opposed to humid conditions. From specimens exposed to 84% humidity, it is
noticeable that there is only a limited level of filiform corrosion, even
after 8 weeks of exposure. When the relative humidity is raised to 100%,
little or no filiform corrosion is present. This would seem to be in agreement
with the theories generally put forward for the initiation and propagation of
filiform corrosion, where a humidity range of 60-95% RH is suggested[2-5]. The
indication would be that filiform corrosion would not occur under immersion
conditions, as these would be representative of 100% RH, when a liquid water
film would form on the sample surface. However, the authors have attended a
number of public presentations where speakers described observing
"filament like" structures under immersion conditions on lacquered
aluminium surfaces, especially with respect to food and drink cans. However,
the speakers were extremely reluctant to describe this as filiform corrosion
because "filiform corrosion does not occur under immersion
§21 When the corrosion of these panels is examined under immersion conditions
it is apparent that the conventional theory has some flaws. For intact panels
immersed in both 1% sodium chloride solution and 10:1 water:glycerol, general
corrosion occurs in the vicinity of the salt contamination. However, beyond
this region extensive filiform corrosion is observed. The level of filiform
corrosion is substantially greater than that noted for any of the specimens
exposed to humidity. When the scribed panels immersed in 1% sodium chloride
solution are examined, general corrosion occurs at the scribe, but severe
filiform corrosion can be noted in the surrounding zone. Filiform corrosion
clearly can occur for this aluminium alloy under immersion
conditions where the water activity is close to unity, and under conditions
where the water activity is rendered less than unity by the use of
§22 Filiform corrosion can occur on epoxy coated aluminium alloy 3003 H14 under
both immersed and humid conditions.
§23 Also, filiform corrosion is far more extensive under the immersion
conditions examined, compared to the humid conditions.
S. Wernick, R. Pinner and P.G. Sheasby, "The Surface Treatment
and Finishing of Aluminium and its Alloys", 5th Ed.,
Vol 1, p14 (1987), ASM International-Metals Park-Ohio/Finishing
W.H. Slabaugh, W. Dejager, S.E. Hoover and L.L. Hutchinson, J.
Paint Technol. 44 (1976) 76.
W. Funke, Prog. Org. Coatings, 9 (1981) 29.
R.T. Ruggeri and T.R. Beck, Corrosion-NACE, 39 (1983)
H.J.W. Lenderink, PhD Thesis, Delft University of Technology (1995).
S. Wernick, R. Pinner and P.G. Sheasby, "The Surface Treatment
and Finishing of Aluminium and its Alloys", 5th Ed.,
Vol 1, p5 (1987), ASM International-Metals Park-Ohio/Finishing
§24 Figure 1 Filiform corrosion of epoxy coated aluminium alloy 3003 H14,
after 8 weeks exposure to 84% RH (Left - 20μm
coating, Right - 40μm coating).
§25 Figure 2 Filiform corrosion of epoxy coated aluminium alloy 3003 H14,
after exposure to 10:1 water:glycerol (Left - 7 days, Right - 14 days).
§26 Figure 3 Filiform corrosion of epoxy coated aluminium alloy 3003 H14,
after exposure to 1% sodium chloride solution (Left-14 days, Right-35 days).
§27 Figure 4 Filiform corrosion of cross-cut epoxy coated aluminium alloy
3003 H14, after exposure to 1% sodium chloride solution for 35 days.
§28 Figure 5 Filiform corrosion of dual circle cut epoxy coated aluminium
alloy 3003 H14, after exposure to 1% hydrogen chloride solution for 21 days.
The sample was partly immersed. The section shown is just above the immersion