Assessment of Chemical Conversion Coatings for the - ESMAT

ESA STM-276
February 2008
Assessment of Chemical
Conversion Coatings for the
Protection of Aluminium Alloys
A Comparison of Alodine 1200
with Chromium-Free Conversion
Coatings
A.M. Pereira, G. Pimenta
Instituto de Soldadura e Oualidade (ISO), Oeiras, Portugal
B.D. Dunn
Product Assurance and Safety Department, ESA/ESTEC,
The Netherlands
'uropean Spate Agenty
Agenle spatiale europeenne
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ESA STM-276
Publication:
Assessment of Chemical Conversion Coatings for the
Protection of Aluminium Alloys (ESA STM-276,
February 2008)
Editor:
K. Fletcher
Published and distributed by:
ESA Communication Production Office
ESTEC, Noordwijk, The Netherlands
Tel: +31 71 565-3408
Fax: +31 71 5655433
Printed in:
The Netherlands
Price:
€20
ISBN:
978-92-9221-897-2
ISSN:
0379-4067
Copyright:
(Q 2008 European
Space Agency
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ESA STM-276
Contents
Introduction
1.1 Project Background and Objectives
1.2 State of the art
1.2.1 Commercially Available Chemical Conversion
Coatings
...
1.2.2 Review of Published Papers
2
ExperimentalPhase
2.1
2.2
3
Materials, preparation of specimens, and treatments
Evaluation of the aluminium alloys samples
2.2.1 Surface roughness
2.2.2 Surface electrical resistivity
2.2.3 Thermal cycling tests
2.2.4 Salt spray tests ...
...
2.2.5 Scanning electron microscopy and energy dispersive
X-ray spectrometry (SEM/EDS)
2.2.6 Chemical conversion coatings procedure
5
5
6
6
9
...
17
17
17
17
18
18
18
19
19
Results / Discussion...
3.1 Visual inspection
3.1.1 'As received'
3.1.2 'As produced'
3.1.3 After thermal cycling tests
3.1.4 After salt spray tests
3.2
3.3
3.4
21
21
... 21
21
21
21
3.1.5 After thermal cycling + salt spray tests
,
21
Surface roughness.
29
Surface electrical resistivity measurements
30
Surface morphology analysis - Scanning electron microscopy.. 31
3.4.1 After cleaning with Novaclean AI86
31
3.4.2 After chemical conversion coating treatments
32
3.4.3 After thermal cycling tests
39
3.4.4 After salt spray tests
43
3.4.5 After thermal cycling plus salt spray tests
47
3.4.6 Comparison and correlation between Aluminium
alloys and techniques performed
51
4
Conclusions
57
5
References
59
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ESA STM-276
Abstract
The most common conversion coating (CCC) currently used for improving
the corrosion resistance of aluminium and its alloys that are used in the
construction of ESA spacecraft is Alodine 1200 manufactured by Henkel.
This commercial product is a chromium (VI)-based chemical conversion
coating. However, due to the environmental impact of hexavalent chromate,
and recent EU regulations enforcing prohibition of the use of certain
hazardous substances in electrical and electronic equipment, there is a great
need for chrome (VI)-free anti-corrosion coatings.
The aim ofthis work was the study of commercially-available
alternatives to
chromium (V I)-based treatments. They should assure the same level of
performance
in terms of corrosion resistance, thermal and electrical
properties. The objective of this study was to perform an assessment of
commercially existing chemical conversion coatings for the protection of
aluminium alloys that are to be used in the hardware of ESA space vehicles.
In this study, the aluminium alloys A17075, Al22l9 and Al5083 were used
as substrates for evaluation of the chemical conversion coatings.
Among various chemical conversion coating systems, three alternative
conversion coating pretreatments were selected for evaluation: Alodine
5700, Nabutan STI/310 and Iridite NCP. The chromium (VI) conversion
coating Alodine 1200 was used as the reference.
The quality assessment of conversion coatings was evaluated by optical and
scanning electron microscopy. Coated samples were characterised by salt
spray exposure in order to simulate ground storage conditions; thermal
cycling under vacuum, which simulates an in-orbit space environment; and
electrical resistivity, as spacecraft sub-systems are electrically grounded to
one another.
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ESA STM-276
1 Introduction
1.1 Project Background and Objectives
Aluminium and its alloys are largely used on board space vehicles due to a
good combination of specific weight and mechanical properties. Although
space hardware for spacecraft use is usually confined to controlled
environments,
in emergency conditions, the pre-launching
phase, or
prolonged storage for reusable spacecraft, it may be exposed to harsh
environments, such as the marine atmosphere that exists at coastal launch
sites. This requires that all aluminium alloys must be protected by a proper
anti-corrosion treatment. It is usual for all aluminium alloys to receive a pretreatment and a coating scheme. The most successful pre-treatment for Al
and Al alloys used so far are Chromium (VI) based chemical conversion
coatings. A notable representative of this is a product commercially available
under the brand name Alodine 1200.
Recent EU legislative restrictions enforced strong limitations to the use of
chromate conversion coatings. Member states should ensure, from 1 July
2006, new electrical and electronic equipment put on the market shall not
contain hexavalent chromium [1]. Exempted are materials and components
in which design changes are technically, or scientifically, impracticable, or
where the negative environmental, health and/or consumer safety impacts
caused by the substitution are likely to outweigh the environmental, health
and/or consumer safety benefits.
Although the aerospace and aeronautic industries have been exempted from
compliance with the EU regulations, ESA is preparing itself to comply with
the ROSH directive [1] and simultaneously to keep the lead in the search for
an environmentally cleaner, socially responsible industry.
With this goal in mind, ESTEC contracted the Instituto de Soldadura e
Qualidade, ISQ, to produce a benchmarking study on the viable alternatives
for Cr (VI) based coatings. The main objective of this applied research work
is to assess existing commercially-available
chemical conversion coatings to
be used as pre-treatments coating on aluminium alloys. Candidate pretreatments for this study must:
be industrially available for ready application;
ensure at least the performance level achieved by current Cr (VI) pretreatments
include one coating having a European origin.
Further, technical performance requirements
conversion coatings in this study, are:
for viable alternative chemical
good corrosion resistance to tropical environments: the European
spacecraft
launching base is located in the tropics, a region
characterised by high humidity, high temperature and salty atmosphere;
Good electrical properties: many Al alloys are used in the spacecraft as
structural
components.
However,
besides
adequate
mechanical
properties, they must ensure proper electrical resistance in order to
obtain an equipotential surface;
Thermal properties: extreme temperatures can be achieved either in
space (::I::
100°C) or in prolonged storage condition, on Earth, often in
tropical climates (+50°C and very high humidity).
6
ESA STM-276
In this study, Alodine 1200 (Henkel) will be used as the state-of-the-art
chemical conversion coating.
Table I illustrates the aluminium alloys compositions
used in this study.
Table 1 - Aluminium alloys, chemical composition [2].
Aluminium alloy
:
Composition
AI7075
5.5% Zn; 2.5% Mg; 1.5% Cu; 0.3% Cr
AI2219
6.3% Cu; 0.3% Mn
AI5083
4.5% Mg; 0.7% Mn
It should be noted that the low strength aluminium alloys (5000 and 6000
series) are permitted to have a final finish of chromate conversion coating.
However, ESA prescribes in ECSS-Q-70-71 ('Data for the selection of space
materials and processes'), that the high strength alloys (2000 and 7000
series) shall have a further painted finish applied to the chromate pretreatment, or alternatively be anodised.
1.2 State of the art
In this section a literature survey of recently published studies related to
chromate-free coatings has been performed.
1.2.1
Commercially
Available Chemical Conversion Coatings
A study has been conducted in the USA under contract by the Deputy
Undersecretary of Defense for Environmental Security. Its overall objective
was to validate and implement multiple CHROMATE-free aluminium pretreatment alternatives for a broad range of applications [4]. Alternatives
studied were all aqueous solutions designed to obtain a thin coating on the
substrate that would enhance paint adhesion and corrosion protection. Some
of the alternatives provide unpainted corrosion protection as well as
electrical conductivity in corrosive environments, especially in the 2xxx
series alloys. The best results have been obtained with a Cr (III) based
composition. The second best alternative, and the best of the chrome free
alternatives, was Alodine 5200/5700, although it did not perfonn as well in
unpainted applications as Navair Trivalent Chromium Pretreatment (TPC)
did. Alodine 5200/5700 did not perform well against filiform corrosion, but
was found to be process flexible and could be applied similarly to chromate
conversion coatings.
NASA tested three alternative pre-treatments: Alodine 5700 (Henkel Corp.),
Okemcoat 4500 (Oakite Products, Inc.) and Chemidize 727ND (MacDermid
Inc.). Okemcoat 4500 was dropped from qualification
due to poor
performance during Bend Strength - Flexibility and Water Resistance
qualification tests. The other two were found to be acceptable for flight.
Alodine 5700 was classified as having flexible processing parameters and
was recommended for implementation as a pre-treatment alternative. NASA
changed to Alodine 5700, as part of their new Hentzen system
(05510WEPX10551I CEH-X
primer
and 4636 WUX-3/4600CHA-SG
topcoat) from Hentzen Coatings Inc. 1 Alodine 5700 in June 2002, and the
first hardware flew in late 2002.
At the ASST 2006 Conference [5] a session was dedicated to Al
pretreatments. This session started with an overview from Alcan R&D
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ESA STM-276
concerning new alloys for aircraft application, namely AI-Cu-Li alloys.
Some emphasis was also given to joining techniques with potential for
application, namely laser beam and friction stir welding of panels.
Furthermore, some studies with sol-gel coatings were presented [6-8].
Sepulveda et al. [5] implemented a sol-gel conversion coating process to
produce a zirconium oxide coating on A12024- T3 alloy, that was
characterised
morphologically
and by electrochemical
techniques. The
coatings produced presented a uniform film, with no evidence of cracking or
delamination. Polarisation studies revealed no breakdown potential up to
+0.5 V (SCE), indicating an ennoblement of the pitting potential of over
+ 1.0 V, at a low scan rate, and the passive region exhibited variations in the
current density that suggest a self-healing capacity ofthe coating. This study
allowed establishing conditions for optimisation of zirconia sol-gel coatings
for structural
aluminium
alloy,
using
electrochemical
impedance
spectroscopy. Hughes et. al. [7] studied the performance of a cerium
tridibutylphosphate-based
inhibitor for corrosion of aluminium alloys.
Raman spectroscopy and SEM/EDS were used to characterise the reactions
of A12024- T3 with Ce( dbp)3 solutions in the presence ofNaCI.
Raps et al. [8] tested alternative systems based on permanganate, cerates,
zirconate and trivalent chromium, which form a chemically-growing
oxide
film in a traditional tank process. Also investigated was a different approach
based on organic-inorganic hybrid sol-gel coatings and silane films, which
can develop an oxide layer on the natural aluminium oxide by formation of
covalent AI-O-Si bonds. Thus, those coatings offer the possibility to link the
metal oxide and the organic coating (primer). The film morphology and
corrosion protection properties were studied by SEM, salt spray test, filiform
corrosion test and electrochemical test methods. They concluded that the
trivalent chromium-based conversion coating and the sol-gel coatings are
able to close the gap between barrier functionality and adhesion promotion.
The chromium (III) process was reported to have the most promising
performance of all the chemical conversion coatings tested, being the best
alternative and thus the most effective substitute for Cr (VI) based
conversion systems.
Under this present ISQ-ESA research contract, contacts with different
suppliers (HENKEL, ATOTECH, CHEMO-PHOS, SURTEC, MacDermid,
Kluthe and Enthone, Alcan) were undertaken for an update on alternative
industrial chemical conversion pretreatments. However, only some of these
companies sent pre-treatment solution samples and product information,
namely Henkel (by Henkel Iberica Surface Technologies), CHEMO-PHOS,
SURTEC and MacDermid (through its sales representative in Portugal,
Artegalva Lda.).
An overview ofthe alternative chemical conversion pretreatments,
the conventional chromium based coatings is described.
a) Alodine 12005 (Reference)
as well of
[9J
Alodine 1200S (Henkel Surface Technologies) is a hexavalent chromium
based chemical conversion coating. In this study it will be the reference for
this pretreatment evaluation. Alodine 1200S treated surfaces have an
excellent corrosion resistance, and an increase in the paint adherence. The
immersion bath has a long term stability and does not often need to be
replenished. The solution can be used by immersion, spray or wipe
processes. The immersion bath has a long term stability and does not often
need to be replenished. The pretreatment by immersion is performed at
8
ESA STM-276
temperatures in the range of 25-35°C for 15 seconds to 3 minutes, adjusting
the pH 1.3 to 1.8.
Furthermore, ALODINE 1200S treated surfaces have colours ranging from a
light iridescent golden to tan, depending on alloy and coating thickness.
b) Henkel A Iodine 5200/5700 [4, IOJ
Alodine 5700 (Henkel Surface Technologies) is a chromium-free conversion
coating, formulated as a ready-to-use material for spray or immersion
application, and is a diluted form of the Alodine 5200. The solution is used
in a similar fashion to conventional chromate pretreaments and is performed
at a temperature of 25-35°C for 1-5 minutes. Alodine 5200/5700 is an
organometallic zirconate complex based pretreatment. Deposited coatings
have a light colour ranging from blue to tan, depending on the alloy [4].
c) Sanchem
Safeguard
7000 with seal [4}
It is based on potassium permanganate, with a complex sealant composed of
polyacrylic acid polypropylene glycol and fatty acid esters. It is processed as
a two-component product (one solution potassium permanganate, and one
seal). The pretreatment is performed at room temperature; however, it
requires high temperatures for seal cure (about 92°C, 200°F). Application is
performed by immersion, spray, and wiping. It induces a bronze gold colour
coating.
d) Surtec Chromital TPC [11/
On a benchmarking performed by Surtec, using Alodine 1200S as reference,
Surtec 650 pretreatment presented very similar performance characteristics
to the control product. Surface preparation is dependent on the silicon
content of the alloy: for Al alloys containing less than 1% Si, surface
preparation includes alkaline degreasing, alkaline etching and deoxidation;
for Al alloys containing more than 1% Si, surface preparation does not
require the alkaline etching step.
In application by immersion, Surtec 650 is to be applied in a 20%
concentration in deionised water. The solution pH must be adjusted to 3.9
(3.7 - 3.95), with sulphuricacid or sodiumhydroxide.The pretreatmentis to
be performed in the range 30-40°C. This temperature will influence the
bathing time. Pre-treatment times range from 0.5 to 4 minutes.
e) Nabutan STl/3IO [12J
Nabutan STI/310 is a chrome-free, no-rinse, mixture of inorganic and
organic acids composed fundamentally of fluorotitanic acid and can be
applied to the surface treatment of aluminum alloys. The corrosion resistance
and the adherence of subsequent paint coats to the treated surface will be
improved. This coating is applied preferentially by dipping and spraying.
J) lridite NCP [13}
The Iridite NCP is a chromium-free
passivation treatment designed
specifically for aluminium and its alloys; it provides both high adhesion
values for paints and powder coatings, and excellent corrosion resistance.
Iridite NCP also has the ability to withstand high temperatures, so it is
suitable for use on items that require high operating temperatures. This
unique property, not found on chromates, also allows applicators to increase
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ESA STM-276
the temperature of their dry-off ovens prior to painting. Unlike traditional
chromate treatments that require 24 h to cure to a hard film, lridite provides
a hard amorphous crystalline coating as formed. The IRIDITE NCP coating
is normally clear at low coating weights to a light blue colour at highest
coating weights. There are times when the coating is iridescent due to
refraction of light by the conversion coating. The colour is very much
dependent on the chemical processing conditions, substrate alloy and surface
condition being processed. Because of this, coating quality cannot be
assessed by judging the colour of the coating [14].
Based on the properties of each of the commercially-available
chemical
conversion coatings mentioned above, ESA and ISQ selected three of them
for evaluation.
The chromium (VI) conversion coating Alodine 1200 will be used as
reference. The characteristics of these chemical conversion coatings are
summarised in Table 2 [9,10,12 and 13].
Table 2 - Summary of chromate and chromium-free conversion coatings.
i Chemistry
Conversion
coating
(from MSDSs)
(Henkel, Germany)
Alodine 5700
(Henkel, Germany)
Nabutan STI/31 0
(Chemo- Phos,
Germany)
Iridite NCP
1.2.2
Application
~Methods
Disadvantages
. Advantages
I
I
Alodine l200S
Reference
(MacDermid,
I
USA)
Chromic acid,
complex
fluorides
Immersion,
spray, wIpe
Easy to use;
standard
Contains hexavalent
chromium
Organometallic
zirconate
complex
Immersion,
spray, wIpe
Easy to use;
drop-in
replacement for
chromates
Minimal corrosion
inhibition;
unperceivable colour
change
Fluorotitanic
acid
Immersion,
spray
Easy to use;
chromium- free
Probably
fluorotitanate
based
Immersion,
spray
Easy to use;
chromium- free
Review of Published Papers
Chromate conversion coatings (CCC) have been intensively studied over the
past decades due to the effective corrosion protection they confer on Al
alloys and the desire to develop an effective and environment-friendly
replacement. The composition and structure of CCCs formed on pure Al and
Al alloys have been investigated using a variety of analytical tools [15].
The influence of surface preparation on performance of chromate conversion
coatings on Alclad 2024 aluminium alloy was investigated by Campestrini et
al. [16, 17]. Since the parameters of each pretreatment step, performed
before the chromating process, give rise to important modifications of the
morphology and microstructure of the aluminium surface, they strongly
influence the nucleation and growth of the chromate layers, and therefore,
their final properties. Researchers studied the influence of both the
immersion in a nitric-hydrofluoric acid solution, carried out between the acid
pickling step (immersion in sulfuric-phosphoric acid solutiQn) and the
chromating step, and the duration of the acid pickling step on the surface
Minimal corrosion
inhibition;
Colourless
Minimal corrosion
inhibition;
Colour depends on the
chemical processing
conditions and alloy
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ESA STM-276
properties of Alclad 2024- T3 aluminium alloy was studied. Scanning
Electron Microscopy
(SEM), Atomic Force Microscopy
(AFM) and
electrochemical investigations highlighted the fact that this intermediate step
in the pretreatment route gave rise to a more etched but less reactive surface,
due to the removal of a large number of cathodic sites, mainly related to AIFe-Si phases present in the clad layer. A similar effect, but on a smaller
scale, was caused also by the increase of immersion time in sulphuricphosphoric acid solution. The decrease of galvanic coupling gives rise to a
more homogeneous nucleation of the chromate conversion coating on the
surface of the Alclad 2024- T3. Moreover, the chromate film tends to be
denser and with fewer defects. In fact, removal of galvanic coupling from
the aluminium surface appears to be a requirement for the formation of a
chromate layer with good corrosion resistance [17]. Besides, the surface
properties of the aluminium substrate largely influence the morphology and
composition, not only of the chromate film but also of the corrosion products
formed at weak spots, which seem to play an important role in the level of
protection provided by the chromate conversion coating.
Waldrop and Kendig [18] studied the formation of CCC on AA2024- T3
using AFM. The CCC on the Al matrix was found to nucleate and grow very
fast in the form of nodules. Nucleation and growth of the coating were faster
on AI-Fe-Cu-Mn
particles than on AI-Cu-Mg. A similar AFM study of
nucleation and growth of CCC formed on AA2024- T3 in combination with
FE-SEM and TEM, was conducted by Brown and Kobayashi [19]. They
concluded that CCC formation and growth on intermetallics
strongly
depended on the size, shape and composition of intermetallics.
Sun et at. [20] used SEM, transmission election microscopy (TEM), and Xray photoelectron Spectroscopy (XPS) to examine chromate coatings formed
on 2024- T3 samples, to which different pre-treatments had been given.
These samples have also been evaluated for their corrosion behaviour.
Although a previous work has shown that different pre-treatments can
directly influence the compositions at surface regions of Al alloys, this work
highlights the fact that this initial enrichment can affect the composition and
properties of a subsequent conversion coating. The present study for
chromate coating provides a good reference for comparison with parallel
work being done on alternative non-chromated coatings.
Treacy et al. [21] carried out several surface analysis studies on chromate
conversion coated 2014- T6 aluminium alloy prior to, and after exposure to a
neutral sodium chloride salt fog environment. The development of dark
stains on surfaces after 48 h of salt spray fog exposure coincided with the
loss of chromium, indicative of the coating failure. This fact was observed
by surface analytical techniques: XPS, AES (Auger electron spectroscopy)
and LIMA (Laser induced mass analysis. From LIMA analysis it would
appear
that these regions
are associated
with copper-containing
intermetallics, suggesting that chromate coating deterioration during salt fog
exposure occurs at highly active copper-containing intermetallic sites.
Lunder et al. [22] investigated the role of microstructure in the formation of
a chromate conversion coating on extruded AA6060. The surfaces were
given a conventional surface treatment, involving alkaline etching and
deoxidation prior to treatment in a commercial chromate solution (Alodine
C6100). The coated surfaces were examined by surface analytical techniques
and electrochemical techniques. A non-uniform growth of the chromate
conversion coating (CCC) on AA6060- T6 resulted in a porous morphology,
with cracks extending down to the base metal. Poor coverage was
particularly observed at the grain boundaries.
ESA STM-276
It is generally accepted that CCCs form via a redox reaction between Al in
the alloy and Cr (VI) species in the chromating solution. They are
amorphous and mainly composed of hydrated mixed Cr (III) /Cr (VI) oxide.
The Cr (VI) species can be stored in CCC and released as soluble chromate
species when CCC is exposed to solution. This characteristic of CCC is
believed to lead to the important 'self-healing' ability of CCC-treated alloys.
Chromate conversion coatings (Alodine 1200S) on AA7075-T6 were
characterised by Meng et at. [23] using SEM, focused ion beam sectioning
and s-TEM with nanoelectron dispersive spectroscopy line profiling. The
CCC formed on AA 7075 is heterogeneous and the coating formed on the Al
matrix was much thicker than that formed on the coarse AI-Cu-Mg, AI-FeCu and Mg-Si intermetallic particles. The coating formation on the
intermetallics has been shown to be dependent on the electrochemical
reactivity of the intermetallics, the local pH change and the interaction of the
intermetallics with HF and ferricyanide.
A review by Kending et at. [15] of studies carried out on aluminium and
aluminium alloys, showed that the oxoanions of hexavalent chromium
uniquely inhibit the corrosion of many metals and alloys. They are
particularly useful for protecting the high strength aluminium alloys against
corrosion. The inhibition of Al corrosion is derived from both inhibition of
oxygen reduction and inhibition of metal dissolution reactions, mainly due to
a delay in the onset of pitting. Inhibition of corrosion by chromates appears
to be closely linked to their ability to irreversibly adsorb on to metal and
oxide surfaces.
In order to characterise filiform corrosion on a commercial AA2024- T351
aluminium alloy, a detailed microscopic study using SEM and EDX was
performed by Mol et al. [24]. Some of the aluminium alloy samples were
alkaline-cleaned,
deoxidised, and coated with a chromate conversion and
others were only alkaline-cleaned
before coating. Both samples were
similarly spray-coated with a 42 /lm clear polyurethane topcoat. Cross
section and top views examined by SEM revealed varying degrees of attack
ranging from generalised etching without local attack, to severe local attack
in the form of pitting, resulting in grain etch out, grain boundary attack and
subsurface etchout. EDS revealed the presence of chloride inside the pits,
and subsurface etch out.
Characterisation
of the metal-coating
interface
is crucial to the
understanding,
and prediction
of the corrosion
protective-coatings
performance. Bierwagen et at. [25] reported the use of different methods to
examine the interface between steel and marine coatings and to image the
surface of a) untreated and b) chromate/phosphate treated aircraft aluminium
alloys. Electrochemical
noise (EN), EIS and Prohesion TM testing were
performed in parallel with AFM/SEM measurements. They proved to be
beneficial in the characterisation of surface morphology, corrosion resistance
properties of coatings and possibly for determining
mechanisms of
corrosion. The e-coat systems displayed resistance values higher than the
spray coat systems and their delamination areas were lower, indicating they
are better for corrosion control for undamaged systems.
Fedrizzi et al. [26] studied the effect of chemical cleaning (alkaline, acid,
combination of the two) and environmentally-friendly
and chromate pretreatment baths on the filiform corrosion behaviour of aluminium AA6060
alloys. They concluded that the poor behaviour of samples cleaned in an
alkaline bath can be related to a remarkable chemical surface modification
induced by the chemical etching. In fact, after this alkaline etching, the
aluminium surface was found to be highly enriched in elements like copper,
zinc or magnesium, which has been shown to affect the filiform corrosion
11
12
ESA STM-276
resistance to varying extents. The use of acid cleaning after the alkaline
etching allowed a significant improvement of the aluminium surface
chemistry, and consequently the filiform corrosion resistance. The best
behaviour was obtained by chromate or fluorotitanate conversion coatings
after an alkaline plus acid chemical cleaning.
Zhao et at. [27] studied the effects of chromate conversion coatings (CCC)
and chromate in solution on the corrosion behaviour of AA2024-T3, and
pure AI. The influence of the chromate on pit growth was studied by the thin
film pit-growth technique. The release and migration of chromate from
CCCs and the comparison of the CCCs chemistry with various synthetic
mixed oxides composed of Al (III), Cr (III) and Cr (VI) species, was
investigated by Raman spectroscopy. The effect of released chromate was
also studied by electrochemical methods. Using an artificial scratch cell,
chromate released from CCCs was seen to migrate to, and protect, a nearby
uncoated area. However, experiments investigating the effect of chromate in
solution on the anodic dissolution kinetics under potentiostatic control
indicated that large chromate concentrations were needed in order that an
effect can be seen.
John Bibber [28] published an overview of non-hexavalent
chromium
conversion coatings that could provide excellent corrosion resistance and
paint adhesion characteristics when used with aluminium and its alloys,
compared with the chromium-based conversion treatment. The four main
types of conversion coatings in common use are based upon:
I) The formation of a chromium hydroxide and/or chromium oxide film;
2) The production of a precipitated heavy metal phosphate or oxide film;
3) The use of various synthetic
phosphates or oxides; and
4) The formation
permanganates.
of a manganese
polymers,
with or without
oxide/aluminium
heavy metal
oxide film by use of
The author concluded that the manganese oxides produced by the
heptavalent-manganese-based
system are by far the most closely related to
the chromium oxides and hydroxides in terms of their respective chemistries.
Thus, the aluminium oxide/manganese
oxide film, as produced by the
heptavalent-manganese
conversion coating system, is the alternative pretreatment most closely matching the actual CCC in terms of actual chemistry
and performance.
NCAP
('Non-Chromate
Aluminum
Pretreatment
Project')
[4] has
undertaken a project to validate, and implement, multiple chromate-free
aluminium pretreatment alternatives. The researchers concluded that all
alternatives tested showed acceptable performance. From the evaluated
chemical coatings, the only pretreatments that came close to matching the
technical performance, cost and flexibility of chromates are those based on
trivalent chromium (Navair TCP). Furthermore, the Alodine 5200/5700
pretreatment performed similarly to TCP in most painted systems, but not in
unpainted applications. In general, although none of the other selected pretreatments can compete with the overall performance
of these two
pretreatments,
many could be selected for specific coating system
applications, and achieve the desired performance.
Cerium-based Al and AI-alloy pre-treatments have been proven as efficient,
environmentally-clean
alternatives
to the environmentally-undesirable
Cr (VI) compounds. Thus, full immersion in cerium salt solutions allows an
identical corrosion protection level to that of chromate-based treatments. A
ESA STM-276
major drawback is that treatment time is too long for industrial applications
[29].
Bethencourt et al. [30] proposed accelerated methods for obtaining ceriumrich conversion coatings on aluminium-magnesium
alloys (AI5083). The
films developed have been characterised by SEM and EDS. These studies
revealed that the coatings have a mixed or heterogeneous nature, being
composed of a layer of alumina covering the matrix, together with islands of
cerium formed over the cathodic intermetallics present on the alloy surface.
Furthermore, electrochemical
studies indicate that the protection level
provided by these coatings is several orders of magnitude higher than
achieved with other treatments, such as chromate-based treatment.
Work by Arnott et at. [3 I] has shown that cerium-based coating deposition
relies on electrochemical interactions between the aluminium matrix and
intermetallic inclusions that make up structural aluminium alloys.
Fahrenholtz et al. [32] studied the performance of a cerium-based conversion
coating formed by spontaneous reaction between a water-based solution
containing CeCh and aluminium alloy 7075-T6 substrates. This study has
shown that the cerium conversion coating morphology and salt fog
performance were affected by the type of pre-treatment of the panel prior to
coating. The best pre-treatment consisted of desmutting, degreasing, and
acid activation.
Rivera et al. [33] developed an improved process for the spontaneous
deposition of cerium oxide conversion coatings for corrosion protection of
aluminium alloy 7075- T6, by a spontaneous dip-immersion process that
resulted in improved salt fog corrosion performance. The role of substrate
surface preparation and post-treatment was discussed in terms of the surface
morphology, thickness and performance of coatings on aluminium alloy
7075-T6.
Decroly et al. [34] investigated a chromate conversion coating alternative
treatment
(cerium salt-based
conversion)
that could meet all the
requirements for industrial application. The corrosion protection properties
of this Ce conversion coating were investigated by cathodic and anodic
polarisation measurements and by electrochemical impedance spectroscopy
(EIS). However, although the coatings formed presented interesting
properties, their performances did not match those of chromate conversion
coatings.
The cerium-based chemical conversion coating is an effective method to
improve the resistance to localised corrosion of the aluminium borate
whisker reinforced AA6061 composite (AI1sB4033w/606IAI). Hu et al. [35,
36] reported a cerium conversion coating formed by an immersion process
that resulted in improved salt spray corrosion performance. The CeCh
concentration in the test solution has been shown to have a significant
influence on the corrosion behaviour, and surface morphologies of the
coated samples.
The electrochemical behaviour of a Ce conversion coating formed on Al
alloy 2024- T3 covered with a Cu-rich smut has been studied by Palomino et
al. [37]. The microstructural characterisation has shown that the precipitation
of the Ce conversion layer seems to occur homogeneously in the presence of
the Cu-rich smut originating a uniform coating. Moreover, it has been
confirmed that the dry-mud structure of this kind of conversion layer is just a
superficial phenomenon, with only a few cracks attaining the metallic
substrate) most likely due to stress relieving. Electrochemical results and
SEM/EDS characterisation have evidenced the self-healing properties of the
13
14
ESA STM-276
Ce conversion layers, which allow ions released from the coating to
precipitate above active sites during the immersion of the coated metal in an
aggressive electrolyte.
The same researchers investigated the corrosion behaviour of a bi-Iayer
cerium-silane
pretreatment
on Al 2024- T3 in 0.1 M NaCI [38], by
electrochemical techniques (anodic polarisation curves and electrochemical
impedance spectroscopy (EIS)). EIS has shown a continuous increase of the
impedance response during the whole test period, which was interpreted on
the basis of a pore-blocking mechanism. This theory was supported by SEM
observation and equivalent circuit fitting. On the other hand, mechanical
tests have evidenced the good adhesion of the silane layer to the Ce
conversion layer, which has been interpreted as a better linking between the
silane molecules and the cerium bottom layer.
Hughes et at. [39] studied filiform attack on a cerated AA2024- T351
aluminium
alloy with a polyurethane
topcoat by SEM/EDS.
The
observations revealed that filiform corrosion results were similar to those
obtained on chromated and coated surfaces. This conclusion led to
development of a filiform corrosion model, attributing the main cause of
delamination to the volume expansion of the corrosion product.
Guan et at. [40] published an overview describing the formation, chemistry,
morphology, and corrosion protection of a new type of inorganic conversion
coating. This coating, referred to as a vanadate conversion coating (YCC),
forms on aluminium alloy substrates in a matter of minutes by deeping the
substrate in aqueous vanadate-based
solutions at ambient temperatures.
Electrochemical testing was used to confirm earlier reports of Al corrosion
inhibition by soluble vanadates, which showed that in near-neutral solutions,
vanadate increases the pitting potential, and decreases the oxygen reduction
reaction rate on AA2024-T3. Impedance experiments suggest that slow film
formation may be occurring on Al in contact with vanadate-bearing
solutions.
Khoabaib et at. [4 I] evaluated the corrosion resistance behaviour of a
proprietary sol-gel based coating system, as replacement for Alodine 1200,
on AI2024 alloys. The sol-gel method is also being applied to obtain a thin
protective coating of cerate, molybdate and other rare-earth compounds on
the aluminium substrate through environmentally-friendly
processes. The
performance
of this coating evaluated by electrochemical
techniques
concluded that the sol-gel treated E-coat provided an acceptable corrosion
protection level, similar to that of the conventional chromate system.
Yang et at. [42] investigated the behaviour of a Si02 and Zr02 sol-gel
conversion coating, both alone and in combination with a polyurethane
unicoat (TT-P-2756, self-priming topcoat) on Al 2024-T3 alloy. The
evolution of the coating system under immersion was followed by AFM,
SEM, EIS, and XPS. Although after two days of immersion, pitting
corrosion and degradation products on the sol-gel single coating surface
were observed, further pitting corrosion ceased after a few days of
immersion. Undercoating blisters in the sol-gel plus polyurethane topcoat
system were found at 4 weeks of immersion, after which no further increase
in blister size was observed. Results are explained based on a theory that
after initial corrosion, aluminium and silicon oxides may form a stable
mixed-oxide barrier layer at the interface hindering further pitting corrosion.
Parkhill et at. [43] demonstrated that an epoxide modified silicate sol-gel
film spray coated on aluminium alloy 2024-T3 panels provides exceptional
barrier and corrosion protection when compared to chromate-based surface
treatments (Alodine 1200). The ormosil film was characterised using
ESA STM-276
profilometry,
optical spectroscopy,
and electrochemical
analysis. The
authors concluded that a spray-coated
water-based
environmentallycompliant sol-gel coating exhibits great potential for the replacement of the
chromate-based
conversion coatings presently used in the corrosionprevention scheme of aircraft aluminium.
CCC on Magnesium Alloys
Zhao et at. [44] reported on the chromium-free
conversion coating
performance for application on magnesium alloys. In one of these works the
researchers studied the coating performance of phosphate-permanganate
treatment that adopted phosphate, incorporated with di-potassium hydrogen
phosphate and permanganate, on magnesium alloys. They concluded that
this treatment is capable of producing uniform coatings and with reasonable
adhesion to the substrate. Furthermore, the results of the corrosion testing
showed that the coating corrosion resistance was equivalent to the protection
afforded by the traditional chromate treatment.
The same authors [45] developed a chromium-free multi-element complex
coating (MEEC), that was obtained using a new kind of chemical conversion
process
and an environmentally-friendly
treatment
technique.
The
composition and microstructure of the coating layer formed on magnesium
alloys were characterised and showed good corrosion resistance.
The growth process of the same multi-element complex coating (MECC) on
magnesium alloy was also studied [46]. The morphological and structural
composition ofthe coatings studied by SEM, EDX, XRD and AFM, allowed
the conclusion that coating formation is a three-stage process: initial stage
with formation of a compact, amorphous layer; metaphase stage where the
intermediate layer consists of a mixture of amorphous and crystalline
material, and a final stage, in which the crystallinity increases and the
coating surface becomes smooth and compact.
The degradation process of a chromium-free conversion coating on Mg
alloys was investigated in NaCI solutions [47], by EIS, and a mathematical
model for the degradation mechanism was proposed.
The development of a potassium permanganate bath chemical conversion
coating for magnesium alloys was investigated by Umehara et at. [48]. The
coating formed consisted of an amorphous film composed mainly of
manganese oxides, magnesium oxide or hydroxide.
The corrosion protection supplied by an aluminium arc-spray coating, posttreated by hot pressing and anodising on an AZ31 magnesium alloy, has
been studied by Chiu et al. [49] by using electrochemical polarisation and
EIS. In this study, the Al arc-spray coating as well as a post-hot pressing and
anodising treatment were carried out on an AZ31 Mg alloy, and the effect of
the processing on the corrosion resistance was evaluated. The corrosion in
the AI-sprayed AZ31 specimen was enhanced by a galvanic cell occurring at
the interface of the Al coating and the AZ31 substrate, under the
circumstances when electrolyte was supplied through the porous Al coating
to the interface region.
Montemor et at. [50] investigated the composition and corrosion resistance
of cerium conversion films on the AZ31 magnesium alloy and its relation to
the salt anion. For that purpose, pretreatments based on different cerium (III)
salts were performed by immersion of Mg alloy in a solution of cerium
chloride, cerium nitrate, cerium sulphate and cerium phosphate. The
chemical composition of the treated surfaces was investigated by X-ray
photoelectron spectroscopy and Auger electron. spectroscopy, whereas the
corrosion behaviour of the pretreated AZ31 substrates was investigated in
15
16
ESA STM-276
0.005 M NaCI solutions, using potentiodynamic
polarisation and opencircuit potential monitoring.
The results showed that the chemical
composition and the corrosion protection ability are dependent on the cerium
salt used for the deposition of the conversion layers. Conversion layers
obtained by immersion in CeCh are the thickest and the most protective. The
electrochemical results show that all the conversion pre-treatments reduced
the corrosion activity of the AZ31 Mg alloy substrates in the presence of
chloride ions. The corrosion protection efficiency is related to the anion
present in the cerium salt.
In previous work performed by the same authors [50] it was demonstrated
that conversion pre-treatments based on cerium nitrate or lanthanum nitrate
could protect the AZ31 Mg alloy, the effectiveness of the conversion layer
being dependent on the treatment time. Electrochemical
and analytical
techniques were combined to study the rare-earth conversion films formed
on the AZ31 Mg alloys and their ability to provide corrosion protection.
Cerium and lanthanum conversion layers provide corrosion protection on the
AZ31 Mg alloy. The corrosion protection seems to be independent of the
chemical composition of the conversion layer, however it does depend on
the thickness.
Magnesium-rich
coatings present a new and challenging
field of
development for the corrosion protection of aluminium structures [52].
These coatings are capable of sacrificial protection, but assessment of their
efficiency and durability can be strongly affected by the testing environment.
The corrosion behaviour of two aluminium alloys (A12024 and A17075)
coated with a magnesium-rich coating, also of pure magnesium and of the
bare aluminium substrates, was assessed in the two solutions (0.1 % NaCI
and dilute Harrison solution, DHS) using electrochemical techniques (OCP
and EIS). The corrosion rate of pure magnesium was higher in DHS,
although the dissolution rate of the magnesium embedded in the polymer
matrix was not significantly affected. The impedance spectra of the scribed
samples resembled that of the bare substrates in NaCI solution but not in
DHS.
17
ESA STM-276
2 Experimental
Phase
2.1 Materials, preparation
of specimens, and treatments
Specimens of three aluminium alloys (Table 3) were mechanically cut in
4 x 4 cm dimensions and used as received for coating with the chemical
conversion coating treatments selected for evaluation. For each pair,
chemical conversion coating / aluminium alloy triplicates were prepared.
Table 3 - Chemical composition of aluminium alloys selected (wt %).
Alloys
AI
Cu
Mg
Mn
Fe
Zn
Cr
Si
Ti
AI7075
Bal.
1.2-2.0
2.1-2.9
0-0.3
0-0.5
5.1-6.1
0.2-0.4
0-0.4
0-0.2
AI2219
Bal.
5.8-6.8
0-0.02
0.2-0.4
0-0.3
0-0.1
0-0.2
0.02-0.1
AI5083
Bal.
0-0.1
4.0-4.9
0.4-1.0
0-0.4
0-0.25
0-0.4
0-0.15
0.05-0.25
To assure that the metal surface was free from oil, grease and other foreign
matter before the treatment, an appropriate cleaner was used. Because of its
availability, Novaclean A186 was used. This is a cold degreaser for
aluminium
used in immersion processes, combining degreasing and
deoxidising characteristics.
2.2 Evaluation of the aluminium alloys samples
The following tests were performed on the samples.
Table 4 - Tests performed on aluminium alloys samples.
'As
received'
'Cleaned'
'As
produced'
(coated)
After thermal
cycling
After 55
exposure
Surface roughness
X
X
X
X
-
Surface electrical resistivity
-
-
X
X
-
Thermal cycling*
-
-
X
Salt spray (ASTM B 117/07)
X
-
X
X
-
Scanning electron microscopy
(SEM)
X
X
X
X
X
-
* The thermal profile corresponds to a standardised profile of -50°C and +1 OO°C,with dwell times of 15 min,
at cooling/heating rates of 10°C/min;
2.2.1
Surface roughness
The surface roughness of the test panels was measured using a Mitutoyo
Surftest 201. A stylus attached to the detector unit of the SJ-20 1P is
displaced at constant speed across the sample's surface, tracing surface
irregularities. The vertical stylus displacement produced during tracing of
the sample's surface is converted into electrical signals. The parameter used
for expressing roughness was Ra, which is the arithmetic mean of the
absolute values of the profile deviation from the centre line within the
evaluation length [53].
18
ESA STM-276
2.2.2
Surface electrical resistivity
Surface resistivity could be defined as the material's inherent surface
resistance to current flow multiplied by the ratio of the specimen's surface
dimensions (width of electrodes divided by the distance between electrodes),
which transforms the measured resistance to that obtained if the electrodes
had formed the opposite sides of a square [54]. Surface resistivity does not
depend on the physical dimensions of the material.
The surface electrical resistivity of the test panels was carried out with a
Microohmimeter Schuetz Messtechnik, model MR 300C, based on the 4
point method. The resistance measured is then calculated through the
equipment by applying Ohm's Law. The measured electrical resistance is
proportional to the length of the sample and to the surface resistivity, and
inversely proportional to the cross-sectional area of the sample:
p = (R x S) /l
where:
p = surface electrical resistivity (Q m);
R = electrical resistance of a uniform specimen of the material (Q);
S = cross-sectional
area of the specimen (m2);
I = length of the specimen (m).
2.2.3
Thermal cycling tests
Thermal cycling is a test consisting of cycling a sample between two
extreme temperatures which are held for a certain amount of time. The
temperature change is performed at a constant rate. This is done for a
specified number of cycles. As samples heat up and cool down they expand
and contract, possibly causing fatigue or adhesion failure of the coating over
time [55].
The thermal cycling tests were performed in air using a thermal chamber
ARALAB, model Fitoterm 300. The thermal profile corresponds to a
standardised profile of -50°C and + 1OO°C, with dwell times of 15 min., at
cooling/heating rates of 10°C/min. The 500 cycles test was interrupted at the
200th cycle, for resistivity measurements.
2.2.4
Salt spray tests
Neutral salt spray testing was carried out according to ASTM B 117 [56]
with a 5% NaCI solution, at 35°C. The panels were placed in the cabinet
between 15 - 300 from the vertical, for 7 days. Photographic recording was
performed each 24 h. The test panels were not allowed to contact other
surfaces in the chamber and condensed or corrosion products on their
surfaces were not permitted to cross-contaminate each other. After exposure,
test panels were rinsed in deionised water to remove salt deposits from their
surface and then immediately dried.
The salt spray exposure was carried out in a salt spray chamber ERICHSEN,
model 606/400 L.
The samples were subsequently
stored in a desiccator.
19
ESA STM-276
2.2.5
Scanning electron microscopy and energy dispersive X-ray
spectrometry (SEM/EDS)
Surface morphology and chemical composition of the samples was
performed on a scanning electron microscope JEOL-JSM model 6700,
equipped with an Oxford energy-dispersive X-ray spectrometer (EDS),
operated at 20 keY.
2.2.6
Chemical conversion coatings procedure
Figure 1 summarises treatment steps to which samples were submitted. The
chemical conversion coatings were treated according to the respective
technical data supplied (see Annexes).
Ultrasonic cleaning in acetone (10 min.)
Drying
Treating with
AlodJne 12005
conversion coating,
3 min. (350C)
Treating with
AlodJne 5700
conversion coating,
3 min. (35°C)
Treating with
Iridite NCP
conversion coating,
5 min. (35°C)
Figure 1 - Cleaning and treatment processes scheme.
Treating with
Nabutan 5TI/310
conversion coating,
2 min. (35°C)
21
ESA STM-276
3 Results / Discussion
3.1 Visual inspection
3.1.1
'As received'
The Al2219 test specimen' As received' presented a visually rougher surface
compared to the Al7075 and Al5083 aluminium alloys, as shown in Figure
2.
3.1.2
'As produced'
Figure 3 shows the test panels surfaces after application
conversion coatings:
of the chemical
Alodine 1200S tested specimen surface presented an iridescent golden
colour for all the alloys;
.
Alodine 5700 provided a light golden appearance brighter on Al 2219;
Iridite NCP gave colours ranging from light blue (AI 2219 and AI 7075)
to yellowish (AI 5083) over the samples' surfaces;
Nabutan coated samples kept the initial metallic aspect, and appeared
colourless.
All pre-treatment coatings applied on Al2219 kept a mirror-like aspect ofthe
surface. This can perhaps be interpreted as the deposited films in this alloy
not being as thick as the other substrates.
3.1.3
After thermal cycling tests
The thermal cycling test did not produce visible changes in the samples
(Figure 4). No cracks, delamination, or other form of degradation after
thermal cycling exposure were observed.
The only visible effect of thermal cycling on the treated test panels was to
darken the coating colours, probably as a result of dehydration caused by the
combined effects of thermal contraction and expansion due to t~e
temperature changes applied during the tests.
3.1.4
After salt spray tests
Figure 6 illustrates the coated test panel surfaces after the exposure to salt
spray test:
There is no evidence of corrosion on the three types of aluminium test
panel coated with the Alodine 1200S, after 7 days of exposure;
Alodine 5700 and Nabutan coatings showed surface corrosion within 24
h on AI7075 and A12219, with formation of pits;
Iridite NCP presents some corrosion on AI7075 and Al2219 after 48 h
of exposure;
There is no evidence of corrosion on the Al5083 coated samples after 7
days of exposure, for each of the treaments studied.
3.1.5
After thermal cycling + salt spray tests
Figure 7 shows the coated test panel surfaces subjected to thermal cycling
measurements after the exposure to salt spray test. The results are very
similar to those obtained for the samples after salt spray test:
22
ESA STM-276
There is no evidence of corrosion on A17075 and A15083 test panels
coated with the Alodine l200S, after 7 days of exposure;
A122l9 samples coated with Alodine l200S present some evidence of
corrosion after 7 days of exposure;
Alodine 5700 and Nabutan coatings presented surface corrosion on
A17075 and A122l9 alloys, within 24 h of exposure; formation of pits;
Iridite NCP shows some corrosion on A17075 and A12219 after 48 h of
exposure with formation of pits;
There is no evidence of corrosion on the A15083 coated samples after 7
days of exposure, for each of the treaments studied.
23
ESA STM-276
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3.2
29
Surface roughness
Test specimen roughness was measured on samples at each step of the
process: 'As received', cleaned with the N ovaclean Al86 agent, coated, after
thermal cycling tests, after salt spray tests, and after thermal cycling plus salt
spray tests. For each test specimen, three measurements were carried out in
two perpendicular directions (longitudinal and transverse) and the average
results are shown in Figures 8 and 9.
In general, roughness measurements
a)
b)
c)
can be summarised as follows:
'as received' roughness is higher for A12219;
Alodine 1200 and lridite NCP provide a higher average surface
roughness for Alloys 2xxx and 5xxx. On the contrary, for Al 7xxx
higher roughness was obtained for Alodine 5700 and Nabutan;
A decrease in roughness was seen for all substrates after thermal
cycling, as compared to the 'as coated' condition measurements. This
evidence was probably related to the fact that after thermal cycling
measurements
the coated specimens exhibit a more compact and
uniform surface.
Effect on Ra (LD)
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30
ESA STM-276
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Figure 9 - Roughness measurements on the transversal direction.
For the Al7075 alloy, there is a decrease in roughness for all treatments;
there is a decrease in roughness with the coating application as well as with
the thermal cycling.
For the Al 5083 alloy the influence of the different surface conditions is the
smallest, but it follows the same tendency as the AL 7075 alloy: a decrease
tendency in roughness with coating application and thermal cycling.
The Al 2219 alloy presents a decrease of roughness with the chemical
cleaning but the coating deposition strongly increases the surface roughness,
which decreases again with the thermal cycling.
In summary, thermal cycling always decreases surface roughness. This
feature can be related to a dehydration of the surface layer. Treatment
application also decreases roughness, except for A12219, for which this
treatment makes roughness values higher than those obtained in the 'as
received' condition.
The behaviour
directions.
described is essentially
the same for the two perpendicular
In general, the Al2219 test panels show a
other two aluminium alloys. It should be
treatments Alodine 1200 and Iridite NCP
roughness, except for the Al7075 samples,
5700 and Nabutan showed higher values.
higher surface roughness
noted that the conversion
provide a higher average
where those coated with
than the
coating
surface
Alodine
The surface roughness measurements after the salt spray tests were only
performed on the test panels treated with the Alodine 1200S and on samples
of A15083, since that in general the surface of samples presented visible
evidence of corrosion to allow the measurement of the roughness. In those
samples measured it can be seen that there is no significant differences
between the roughness values obtained after salt spray tests.
3.3
Surface electrical resistivity measurements
The surface resistivity of the test specimens was measured for the samples
'as produced', at 200 cycles of the thermal cycling test. Once again, these
measurements were not performed on test panels after salt spray exposure,
since in general the surface was attacked. For each test specimen /
31
ESA STM-276
conversion coating treatment, 10 measurements were carried out. Figure 10
summarises the average results obtained.
Surface
0,12
0 as produced
Resistivity
I
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Figure 10 - Surface electrical resistivity measurements of test panels.
In general, the surface electrical resistivity of all test panels 'as produced' is
very similar for all the chemical conversion treatments under evaluation and
did not suffer significant change after the thermal cycling tests, if the
experimental standard deviation is considered.
3.4 Surface morphology analysis
-
Scanning
electron
microscopy
3.4.1
After cleaning with Novaclean Al86
Figure 11 illustrates the SEM micrographs of Al2219 and Al7075
specimens, after cleaning with Novaclean A186. Samples presented higher
roughness before the cleaning step, and some dirt was eliminated in this step.
The remaining particles on the surface were secondary phases from the
corresponding aluminium alloy. This was confirmed by chemical analysis by
SEM/EDX. Some holes can also be seen on the surface; this is attributed to
the mechanical removal of the secondary phases, either during material
processing or selective leaching of secondary phases by the cleaning agent.
No cleaning test was performed on the Al5083 samples, due to insufficient
material quantities.
32
ESA STM-276
(a) Al2219 before cleaning (200x)
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(a) Al2219 after cleaning (200x)
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cleaning with Novaclean A186.
3.4.2
After chemical conversion
before and after
coating treatments
Figures 12 to 14 show the secondary
coated test panels. The EDS spectra
conversion coating were stored.
electron photomicrographs
of the
correspondent
for each chemical
Apart from the Alodine 5700, SEM observation showed relatively uniform,
closed coatings for all the treatments tested. Discontinuities found on the
surface can be related to previous surface defects or microstructural
heterogeneities induced by the secondary phases. In fact, it is expected that
varying substrate composition and reactivity presented by the three alloys
(AI7075, AI2219 and A15083) will influence the formation of the surface
films.
EDS analysis revealed that these morphological heterogeneities are also
coated, making the coating a continuous film over the complete alloys
surfaces.
The Alodine 5700 coating peeled off in significant areas of the A17075, a
stronger attachment being observed on AI2219 surfaces. This peeling off can
be interpreted by a poor adhesion to the substrate or internal stresses of the
coating or a combination of both of these effects. In the Al5083 alloy, on the
contrary, this treatment formed a compact film (Figure 13). EDS spectra
revealed the presence of F, Ti, Zr, 0 and C, along with the alloying elements
characteristic of the alloys under study (all results and images have been
stored electronically for further reference).
ESA STM-276
In an attempt to overcome the poor adhesion of Alodine 5700 on AI7075 and
AI2219 alloys, coating tests were performed with varying immersion times
(30 seconds, 1, 3 and 5 minutes), with all other parameters kept constant.
The corresponding morphologies are illustrated in Figures 15 to 16. For the
AI2219 alloy, the best result was obtained for 3 minutes immersion, while
for the 7075 alloy the best result was obtained for I minute immersion.
Nevertheless, the surfaces were very similar to those obtained previously, in
terms of morphology and composition. Furthermore, it was possible to
observe that the surface was probably composed of two coating layers: one
more superficial and the other beneath.
SEM observation shows that films formed on the test panels are probably
thin. In fact, it must be emphasised that some defects were present in
coatings, probably related with the secondary phase distribution over the
surface.
SEM images of the test panels coated with the Alodine 1200S treatment
show a typical mud-cracked surface morphology and are very similar for all
the aluminium alloys. It was possible to identify some black spots over the
surface, which indicates that the coating did not cover the whole surface of
the sample. These sites are supposed to be related to intermetallic particles
presented on the surface of these alloys, suggesting that the thickness of the
coating is less on top of these particles. The same mud-crack surface
morphology has been observed by other authors [23], in a study performed
on chromium conversion coating AI7075 alloys.
The semi-quantitative analysis of the chemical elements reveals the presence
of some constituents of the aluminium alloys and elements that proceed from
the conversion chemical coating, mainly: Cr, F, Zr and O. However, due to
the limitations of this technique, namely the inability to analyse light
elements, the percentage of 0 cannot be consider in a reliable quantitative
way.
The surface morphology of test panels coated with the Nabutan and lridite
NCP showed a uniform surface, nevertheless some black holes existed on
the surface, suggesting that the coating did not cover the whole surface of
the samples. The EDS analysis identified that the treated layers are formed
mainly by F, Ti, 0 for the Nabutan treatment. F, P, Ti and 0 plus the
elements of each alloy were associated with the Iridite NCP treatment. Once
again, the percentage of 0 cannot be considered in a quantitative form.
33
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3.4.3
39
After thermal cycling tests
SEM observation after the thermal cycling tests revealed minor changes in
the morphology of all coated surfaces except for the Al7075 and Al2219
(Figures 17 to 19). This was more evident in test panels coated with Alodine
1200 and Alodine 5700. In effect, the comparison of SEM images before and
after thermal cycling shows clearly that, after thermal cycling, the coated
specimens exhibit a more compact and uniform surface. For Al7075 and
A12219, the non (poor)-adherent bits of the coatings observed in the 'as
deposited' condition were absent after thermal cycling. This is coherent with
lack of adherence to the substrate, and the thermal stress induced by the
temperature cycling promoted the complete peel-off of the non-adherent
films parts.
All these results support surface roughness measurements,
decrease in Ra after the thermal cycling test.
The representative
which revealed a
EDS spectra have been retained at ISQ.
The dark areas observed on the coated samples surfaces are related with the
coating layer formed, while the white area corresponds to the surface where
the coating treatment had poor adhesion to the substrate, which is confirmed
by energy dispersive spectroscopy (EDS) analysis. This fact was more
evident on samples A17075 and Al2219 submitted to the Alodine 5700
treatment. The test panels treated with Nabutan and lridite NCP were
relatively unaffected by the thermal cycling tests.
ESA STM-276
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ESA STM-276
3.4.4
After salt spray tests
The evaluation of samples after salt spray exposure by SEM revealed that
Alodine 1200S coated samples had a similar surface morphology to the 'As
produced' samples. Nevertheless, the mud-cracked surface appearance was
not so evident, except for samples from A12219. The semi-quantitative
analysis of the chemical elements identified the constituents of the
aluminium alloys, as well as elements that proceed from the conversion
chemical coating and a small percentage ofNa.
In general, SEM observation of the test panels AI7075 and AI2219 treated
with the three chemical conversion coatings under study, showed that the
surface morphology is formed by a 'white' corrosion product, mainly
composed of AI, 0, Na and the elements of each alloy, indicating Al
substrate degradation and sodium chloride residues.
The AI5083 test panels exposed to salt spray did not present evidence of
corrosion, even after 7 days of exposure. SEM images of these test panels
showed an unaffected surface morphology, although a small amount of white
corrosion product could be observed. The EDS analysis identified the
elements revealed before the exposure, with an increase in 0 percentage and
the presence of a small percentage ofNa.
43
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ESA STM-276
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ESA STM-276
3.4.5
After thermal cycling plus salt spray tests
On the whole, SEM observation of the test panels that were submitted to
thermal cycling followed by salt spray exposure brought out changes in the
morphology of all coated surfaces quite similar to those obtained after the
salt spray exposure.
Once again, AI5083 samples showed no signs of corrosion for all treatments
studied.
The surface morphology of test panels from the other two treated aluminium
alloys revealed an equivalent morphology to those obtained before. The
surfaces were covered with a layer of white corrosion products, formed
mainly of AI, 0, Na and other constituents of the aluminium substrates.
In summary, sample exposure to thermal cycling did not introduce
significant changes on the corrosion behaviour.
47
48
ESA STM-276
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ESA STM-276
3.4.6
Comparison and correlation
and techniques performed
51
between
Aluminium
alloys
Tables 5 to 7 summarise the results obtained for each aluminium alloy and
characterisation performed.
Al7075
Generally, the surface roughness of test panels treated with the four
conversion chemical coatings was very similar. However, for Alodine 5700
and Nabutan coated samples, the values were slightly higher. It should be
noted that there was a decrease in roughness values for all treatments after
coating application, as well as after the thermal cycling. This feature can be
related, on one side to the chemical etching performed by the cleaning agent
and coating deposition and on the other side, to the fact that after thermal
cycling the coated specimens exhibited a more compact and uniform surface,
probably due to surface layer dehydration and spalling of poor-adherent
coated areas.
The surface electrical resistivity of all test panels 'as produced' was very
similar for all the chemical conversion treatments under evaluation, and did
not suffer significant changes after thermal cycling.
Alodine 1200S treated samples showed a typical mud-cracked surface
morphology,
and semi-quantitative
analysis of the chemical elements
revealed the constituents of the aluminium alloy, as well as elements that
proceed from the conversion chemical coating. The Alodine 5700 coating
morphology developed on the surface test panel exhibited a surface where
the coated layer is not continuous, indicating a poor substrate adhesion. The
EDS spectra on the coated layer revealed the presence of elements
characteristic of the chemical coating and Al7075 alloy. The surface
morphologies of the Nabutan and Iridite NCP coated test panels showed
uniform surfaces.
After thermal cycling, the coated specimens display a compacter and more
uniform surface, and the morphology in terms of the elements constituents of
the coating layer appears to be quite similar. This was more evident in
Alodine 5700 coated test panels since the thermal stress induced by the
temperature cycling promoted the complete peel-off of the non-adherent
films areas.
After the salt spray tests, except for Alodine 1200S treated samples, the
surface morphologies are covered by a layer of 'white' corrosion products.
Al2219
Al2219 test panels present higher initial surface roughness, a roughness
decrease could be observed after chemical cleaning. However, the coating
deposition strongly increases the surface roughness, which decreases again
after thermal cycling, probably related to surface layer dehydration. The
Alodine 1200 and lridite NCP conversion coating treatments provide higher
average surface roughness.
The electrical surface resistivity of all 'as produced' test panels were very
similar for all the chemical conversion treatments under evaluation, and did
not suffer significant changes after the thermal cycling.
Alodine 1200S coated samples present a typical mud-cracked surface
morphology, and the surface is mainly composed of elements from the alloy
and from the chemical conversion treatment. The Alodine 5700 coating
peeled off in significant areas of the Al2219 surface, which can be
52
ESA STM-276
interpreted as a poor adhesion to the substrate or internal stresses of the
coating, or a combination of both of these effects. On the other hand, the
surface morphology of test panels coated with the Nabutan and Iridite NCP
showed a uniform surface.
After thermal cycling, the coated samples showed a compacter and more
uniform surface, and the morphology of the coating layer in terms of the
elements constituents was very similar. This was more clearly seen in
Alodine 5700 coated test panels since the thermal stress induced by the
temperature cycling promoted the complete peel-off ofthe non-adherent film
areas. In Nabutan and lridite NCP treated test panels, it could be seen that
they were roughly unaffected by the thermal cycling.
After the salt spray tests, except for Alodine
surfaces exhibit a white corrosion product layer.
12008 coated test panels,
AI5083
A15083 alloy showed the same influence of different surface conditions, but
it followed the same tendency as the Al7075 alloy: roughness decrease with
coating application and thermal cycling.
In general, surface roughness and surface electrical resistivity of all test
panels 'as produced' were very similar for all the chemical conversion
treatments under evaluation, and did not suffer significant changes after
thermal cycling.
Coated specimen surface morphologies displayed a compacter and more
uniform coating. Furthermore, Alodine 5700 treated samples did not present
the lack of substrate adhesion observed on samples of Al7075 and AI2219
alloys.
Thermal cycling did not significantly affect sample surface morphologies;
however they exhibited a more compact surface after cycling. The coating
layers were composed essentially of the same elements: element constituents
of the aluminium alloy and those that provided for the chemical conversion
treatment.
After the salt spray tests, there was no evidence of corrosion on the AI5083
coated samples after 7 days of exposure, for each of the treatments studied.
ESA STM-276
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ESA STM-276
4 Conclusions
Chemical cleaning of the as-received sheet samples in Novaclean Al86
has been seen to decrease the surface roughness of all alloys (AI7075,
Al2219 and AI5083).
The coating supplier recommended
processes were applied. Only
Alodine 1200S was observed to be defect-free after its application. The
other coatings contained some defects. For Alodine 5700, even
immersion time variations did not exclude defects (except for the 30second immersion).
Only the Alodine 1200S samples (for all alloys) survived thermal
cycling; the other conversion coatings were seen to partially spall-off.
The electrical resistivities of all samples were very similar, even after
thermal cycling.
Only the Alodine 1200S samples (for all alloys) can be considered to
have survived the salt spray test. The other coatings (A Iodine 5700,
lridite NCP and Nabutan STi/310) did not provide adequate corrosion
protection.
For A15083, chromium free coatings seem to have promising corrosion
behaviour.
Only Alodine 1200S provided a compact finish. The other coatings
tended to spall-off and are considered to be a severe source for
contamination in any given spacecraft application.
It is recommended
that corrosion protection schemes for ESA
spacecraft continue to utilise Alodine 1200S, as the other (hexavalent
chromium-free) coatings did not provide suitable protection.
57
ESA STM-276
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