OFFSHORE & MARINE TECHNOLOGY | CORROSION PROTECTION
A light, durable and
economical material for
the offshore industry
ALUMINIUM Long experience with numerous aluminium alloys in marine climates and seawater has shown aluminium, especially when suitably designed, to be highly corrosion-resistant.
In recent years it has also been increasingly used in the offshore wind energy sector: as a
structural material in the construction of supply and maintenance boats, as a lightweight
material for helicopter hoist platforms as well as platforms in the towers of wind energy installations, and for cables, transformers and housings for switch cabinets. As Dr Thomas Hentschel
from Hydro Aluminium Rolled Products GmbH in Bonn, Germany, points out in the following
article, even more marine applications are possible for durable, economical aluminium.
T
here are a number of reasons to use aluminium
as a construction material for offshore installations:
u Light weight: The density
of aluminium is only a third
that of steel, resulting in considerable cost savings and
easier transport of construction parts to – and assembly
at – the offshore site. Expensive, complex crane work is
not required since many parts
can be moved and fitted manually. What is more, supporting structures can be slimmer
when lighter components are
used.
u Strength: In spite of their
lower densities, aluminium
materials suitable for seawater use have a specific strength
(strength-to-weight ratio) three
times higher than that of steel.
Consequently, it is possible to
make aluminium components
up to 60% lighter than equivalent steel ones.
u Formability and versatility: Aluminium sheets can
be easily formed and joined.
Aluminium profiles with complex geometries and integrated
functionalities can be manufactured – so joints can be optimised, making assembly sim-
Corrosion resistance
Figure 1: The Oseberg platform with helipad
CORROSIVITY
CATEGORY
DEFINITION
C1
very low
C2
CORROSION RATES rcorr in g/(m2·a)
Carbon steel
Zinc
Copper
Aluminium
r corr ≤ 10
rcorr ≤ 0,7
r corr ≤ 0,9
negligible
low
10 < r corr ≤ 200
0,7 < rcorr ≤ 5
0,9 < r corr ≤ 5
r corr ≤ 0,6
C3
medium
200 < r corr ≤ 400
5 < rcorr ≤ 15
5 < r corr ≤ 12
0.6 < r corr ≤ 2
C4
high
400 < r corr ≤ 650
15 < rcorr ≤ 30
12 < r corr ≤ 25
2 < r corr ≤ 5
C5
very high
650 < r corr ≤ 1500
30 < rcorr ≤ 60
25 < r corr ≤ 50
5 < r corr ≤ 10
CX
extreme
1500 < r corr ≤ 5500
60 < rcorr ≤ 180
50 < r corr ≤ 90
r corr > 10
Table 1: Corrosion rates rcorr for the first year of exposure for different corrosivity categories,
according to [3]
72
Ship & Offshore | 2013 | No 6
pler and assuring the structure
a long service life.
u Recycling: At the end of
their service lives, aluminium
materials can be 100% recycled,
making aluminium a sustainable resource. The high value of
aluminium scrap also makes it
a valuable capital asset.
u Corrosion resistance: This
is a crucial property in unmanned offshore wind energy
converters. Since such installations are never brought back
to shore during their lifetime,
maintenance is very expensive
and complex. Aluminium assemblies, if constructed and
fitted properly, will not corrode. Periodic applications of
corrosion-inhibiting coatings
are therefore unnecessary.
To be more specific, aluminium
displays excellent corrosion resistance under various environmental conditions. Its natural
protective oxide layer prevents
general surface corrosion,
which, if it occurs at all, is of a
localised nature, especially in
the form of pitting. After a few
years of outdoor exposure, the
depth of corrosion normally
does not increase substantially,
thereby ensuring a lifetime of
decades.
A large number of aluminium
alloys are recommended for use
in a marine environment [1; 2].
The 5000 series (AlMg) alloys
are preferred for aluminium
sheets and plates. Aluminium
alloys most commonly used
for profiles are from the 6000
series (AlMgSi).
The excellent corrosion resistance of aluminium is documented in ISO 9223 (2012),
which lists the corrosion rates
for various metals during the
first year of exposure (Table 1).
Corrosion rates for aluminium
are about 100 times less in all
categories than those for steel,
and almost ten times less than
those for zinc and copper. Marine climates are categorised
as C5 (coastal) and CX (offshore).
Because of its high corrosion
resistance in marine environments – even without protective coatings – aluminium has
been successfully employed in
offshore applications and structures for decades. One example
is the Oseberg platform in the
Norwegian North Sea, whose
crew quarters and helicopter
landing pad are made of uncoated aluminium sheets and
profiles (Figure 1). Aluminium
is also being used now in offshore wind energy installations
as a construction material for
stairs and railings, platforms
for maintenance crews as well
as for various other internal
components of the tower.
Figure 2: The North Sea Buoy II
protected by sacrificial anodes
made of zinc that are typically
affixed 2m apart (Figure 3).
The body of the buoy is regularly brought to shore and cleaned
with high-pressure water jets to
remove fouling. Other periodic
maintenance has not yet been
carried out. According to the
latest evaluation, NSB II can
continue operating for another
ten years. The buoy’s excellent
condition includes:
u No appreciable loss in wall
thickness;
u No cracks in the base material or weld seams;
u Weldability equal to that of
new material;
u All screws without problems
(screw material: 1.4571
stainless steel).
A boat striking the buoy in
2010 necessitated unscheduled
maintenance. Samples were
taken from replaced parts for
laboratory analysis: a plate
of aluminium alloy 5083
(AlMg4,5 Mn0,7), which was
located about 2m above water,
and a pipe of aluminium alloy
6082 (AlSi1MgMn), which was
located about 2m below water.
The plate exhibited a corrosion
depth of 0.2 to 0.5mm, and the
pipe a corrosion depth of about
0.1mm. The exceedingly low
corrosion in the submerged
part showed how effective protection with zinc anodes is.
This favourable corrosion behaviour has been confirmed
and statistically supported by
data from the experimental test
station at the North Sea island
of Helgoland. They show that
for AlMg(Mn) sheet alloys located within the spray zone, the
maximum corrosion depth is
reached within a few years, after
which corrosion progresses very
slowly. This evidently holds true
for more than 30 years, since
the corrosion depths measured
on the buoy samples from the
spray zone were only slightly
higher than those measured on
the Helgoland test samples.
Weathered aluminium materials generally do not corrode
uniformly, but experience local pitting corrosion. Therefore
the average depth of attack is
much less than the maximum
depth. Even after ten years in
the splash zone, only a few
micrometres of material degraded on average, equivalent
to several g/m2, as shown in
Table 1.
Telescopic gangways
Four aluminium telescopic
gangways that have been used
on North Sea platforms for up
to 17 years were also inspected.
Samples could not be taken,
however, because the gangways
were free of damage and meant
to be used further.
Figure 4 shows one of them in
its working environment on the
oil platform Safe Scandinavia in
the Scottish North Sea. Like the
NSB II, the gangways were built
with uncoated aluminium extrusions and plates using the
aluminium alloys 5083 and
6082.
North Sea Buoy II
Owing to the very long service
lives and complete recyclability
of aluminium constructions,
there is an acute shortage of
samples available for material analysis. The North Sea
Buoy II (NSB II) of Germany’s
Federal Maritime and Hydrographic Agency is an exception. Located 120km west of
the island of Sylt, it has been
part of a network of meteorological measurement stations
for over 30 years. The body of
the 15m-high buoy is made
of uncoated aluminium sheets
and profiles, and the mounted
measuring container is painted
bright yellow (Figure 2). When
the buoy is on site, most of it is
under water. While the part of
the buoy above water does not
require any corrosion protection, the parts below water are
Figure 3: Zinc anodes protect under water
Figure 4: Telescopic gangway
Figure 5: Condition of weld seams
Figure 6: Condition of a steel flange
Ship & Offshore | 2013 | No 6
73
OFFSHORE & MARINE TECHNOLOGY | CORROSION PROTECTION
With the help of replicas, local corrosion attacks typically
0.1mm to 0.15mm deep were
determined. All welds were in
excellent condition; there were
no indications of increased
corrosion attacks in the heataffected zones of the welds
(Figure 5). Figure 6 shows
the condition of a steel flange
that was installed on the same
gangway and exposed to the
same environmental conditions as the aluminium components.
Deeper pits in the aluminium
were found where salts could
accumulate without being
washed away by rainwater,
and where there were gaps between aluminium and steel in
direct contact with one another. In no case was the function
of the components affected.
Galvanic corrosion
As already stated, aluminium
materials are highly corrosionresistant thanks to the natural
oxide layer on their surface.
Aluminium is a base metal,
however, and defects in the
oxide layer lead to pitting corrosion. As described above,
these local corrosion attacks
are mostly not very deep because the points of attack are
deactivated by a new formation
of the oxide layer. This is called
“repassivation”.
But when aluminium materials are assembled with noble
metals, repassivation can be
suppressed in the vicinity of
the joint, intensifying the corrosion attack. This is known as
bimetallic or galvanic corrosion. So when connecting aluminium with a nobler material
such as steel, some rules should
be observed.
Aluminium sheets of the alloy 5083 were connected with
various steel screws and exposed to the splash zone of
Helgoland for ten years. Unalloyed steel screws caused a
“corrosion moat” a few millimetres wide and up to 3mm
deep around the joint. It is
therefore essential that direct
contact of aluminium materials with unalloyed or low-alloy steels be avoided. Galvanic separation of the materials
still allows joints.
74
Ship & Offshore | 2013 | No 6
Figure 7: No galvanic corrosion between aluminium and
galvanised steel
Figure 8: Seals prevent direct
contact with stainless steel
screws
MATERIAL
POTENTIAL [V]
Stainless steel type 316
from +0.00 to -0.15
Carbon steel
from -0.60 to -0.70
Aluminium alloys for marine applications
from -0.70 to -0.90
Table 2: Practical potentials of different metals in seawater
(values versus saturated calomel electrode), following [5]
In the case of stainless steel
screws (1.4571), corrosion attacks about 0.5mm deep were
measured in the aluminium
next to the screw connection.
Aluminium in contact with
seawater-resistant
CrNiMo
steels (type 316), may be used
without galvanic separation if
the steel surface is small compared with that of the aluminium and moderate corrosion
can be tolerated [4].
This has been confirmed by
field observations. An aluminium ladder was inspected
after assembly ten years earlier
at a height of about 20m on
ZONE
NO
PROTECTION
a wind turbine tower at the
Kentish Flats wind farm, off
the southern English coast.
The aluminium used, 5083
and 6060 (AIMgSi), had the
expected superficial corrosion
as described above. Of particular interest was the area around
the stainless steel screws in direct contact with the aluminium. There was only moderate
galvanic corrosion around the
point of contact, similar to the
attack observed on the panels
in the Helgoland test. The area
of contact between the aluminium and washer was free
of corrosion.
PASSIVE
PROTECTION
It is noteworthy that aluminium is essentially compatible
with stainless steels but incompatible with unalloyed steels.
Considering the electrochemical potentials in seawater for
the materials in question, different behaviour would be expected. The potential difference
between aluminium and stainless steels is in fact much larger
than that between aluminium
and carbon steels (Table 2).
However, both stainless steel
and aluminium have a passivating oxide layer, so galvanic
corrosion is inhibited.
There are numerous other examples of successful structures
mixing aluminium with steel
components and fasteners.
The stainless steel screws on
the NSB II have already been
mentioned. The four aluminium telescopic gangways show
the effectiveness of targeted
anti-corrosion measures. At
the galvanised and painted
steel joints, the different metals were mostly isolated with
neoprene seals, so no galvanic
corrosion could occur (Figure 7). Such seals also prevented direct contact with stainless
steel screws (Figure 8).
Coated aluminium
In contrast to steel, the organic
coating of aluminium materials
in the marine sector is less suitable for corrosion protection
than for decorative appearance
[4] [7].
ACTIVE (CATHODIC) PROTECTION
coating
avoid galvanic corrosion,
assure air ventilation,
allow for natural cleaning
by rain where possible
uncoated surface
above
sea level
anodizing
TO BE
CONSIDERED
not applicable
for decorative surfaces
anodes
insufficient
below
sea level
avoid galvanic
corrosion
insufficient
coating + anodes
Table 3: Corrosion protection measures for aluminium in the marine environment
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OFFSHORE & MARINE TECHNOLOGY | CORROSION PROTECTION
Sometimes a signalling effect
may be the purpose of a coating,
as in the NSB II’s bright yellow
measuring container (Figure 2).
A well-executed protective coating, though rarely necessary,
provides effective corrosion
protection. It is included with
other possible corrosion-protection measures in Table 3.
When painting aluminium
structures, pre-treating the surface is essential. In addition to
degreasing, the oxide layer has
to be removed either chemically (pickling) or mechanically
(sanding, sweeping or grinding) prior to coating [6; 7]. This
is the only way to permanently
prevent paint delamination in
a marine environment. Good
adhesion to a pre-treated metal
surface is ensured by an epoxybased primer. The total thickness of the paint system is typically 200-300 microns.
An alternative coating concept
for the marine sector is recommended by GSB International,
a German quality-assurance
association for batch paint-
Cleaning
NORSOK
STANDARD
M-501
GSB
GUIDELINE
AL631
Sweep blasting
=> anchor profile 25-45μm
Degreasing with ~1g/m2
material removal
Conversion
layer < 1g/m2
(Cr-VI, Cr-III or Cr-free) or
pre-anodisation 3-8μm
Passivation
epoxy primer 50μm
one coat topcoat 50μm
Total
thickness
225μm
Table 4: Coating concepts for aluminium in the marine environment
ing [8]. Especially careful pretreatment of the aluminium
surface is required. The concept involves chemically etching the surface and subsequent
passivation by a conversion
layer (Cr VI, Cr III or Cr-free)
solutions
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Shanghai
one coat 75μm
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Coating
or pre-anodisation. A coating just 50-100 microns thick
is then applied to the treated
surface. This coating concept
has proven itself in the field of
architecture and could bring
significant savings compared
with thick coatings.
The two coating concepts are
compared in Table 4. Experience
has shown the concept recommended by GSB to make better
use of aluminium’s potential
since the corrosion resistance
of coated surfaces is promoted
more by the surface pre-treatment than by the coating. However, the necessary chemical
conversion requires a bath treatment, which must be integrated
into the production process of
an aluminium construction.
Delamination of coated aluminium is more of a cosmetic
problem. The typical filiform
corrosion – starting from cut
edges or paint defects – is a
near-surface phenomenon and
does not affect the load-bearing
capacity of a part.
To protect coated surfaces in
a submerged zone, galvanic
anodes must be used just like
for uncoated aluminium.
This effectively suppresses the
progress of corrosion on damaged parts.
Another interesting coating
method is the application of
corrosion protection films. The
Halunder Jet is a 52m-long, allaluminium catamaran that has
been shuttling daily between
Hamburg and Helgoland during the summer season for almost ten years. The skin of its
hull is not painted, but covered
with a surface protection film
to give the ship a distinctive design and keep it running.
Outlook
Aluminium can often be used
in the marine sector without
surface coating. In a splash
zone, corrosion depth increases
very little after three years. Even
after many years, it does not exceed 0.5mm in most cases. To
preserve aluminium’s natural
resistance to corrosion, the following guidelines apply:
u Natural cleaning of the surface by rain is advantageous;
u Permanent moisture in crevices is to be avoided;
u Galvanic contact with carbon steel is to be avoided;
u Galvanic isolation from
stainless or galvanised steel
is advantageous but not always necessary;
u In a submerged zone, galvanic anodes provide effective protection.
Aluminium has enormous potential in the entire range of
offshore applications. Ideally,
it can be used in auxiliary structures such as railings, ladders,
platforms, stairs, etc. But even
large-scale structures are a possible field of application. Given
the example of the all-aluminium crew quarters of the major
oil and gas production platforms, aluminium could also
be a sustainable, cost-effective
alternative in transformer and
work/living platforms in the
offshore wind energy sector.
References
[1] BS EN 13195:2010-02, Aluminium and
aluminium alloys. Specifications for wrought
and cast products for marine applications
(shipbuilding, marine and offshore).
[2] DIN 81249-2:2013-05, Corrosion of metals
in sea water and sea atmosphere - Part 2:
Free corrosion in sea water.
[3] ISO 9223:2012-02, Corrosion of metals and
alloys – Corrosivity of atmospheres – Classification, determination and estimation.
[4] Germanischer Lloyd, Rules for Classification and Construction, Chapter VI-10-2, Section 3: Materials.
[5] ASM Handbook, Vol. 13: Corrosion.
[6] Germanischer Lloyd, Rules for Classification and Construction, Chapter VI-10-2, Section 4: Coatings.
[7] NORSOK Standard M-501, Edition 6 (2012):
Surface preparation and protective coating,
Annex A.6..
[8] GSB AL 631, International Quality Regulations for the Coating of Aluminium Building
Components, Edition May 2013.