Selection of processing parameters for conversion coatings on high-strength aluminium alloys by cyclic voltammetry
A method of recording copper enrichment on AA2000 series alloys by means of cyclic voltammetry in borate buffer preceded by immersion of the samples in 0.6 M NaCl solution is extended to the assessment of various surface treatments like alkaline cleaning, desmutting, phosphating and also inorganic a...
Gespeichert in:
| Datum: | 2014 |
|---|---|
| Hauptverfasser: | , , |
| Format: | Artikel |
| Sprache: | English |
| Veröffentlicht: |
Фізико-механічний інститут ім. Г.В. Карпенка НАН України
2014
|
| Schriftenreihe: | Фізико-хімічна механіка матеріалів |
| Online Zugang: | http://dspace.nbuv.gov.ua/handle/123456789/135879 |
| Tags: |
Tag hinzufügen
Keine Tags, Fügen Sie den ersten Tag hinzu!
|
| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Zitieren: | Selection of processing parameters for conversion coatings on high-strength aluminium alloys by cyclic voltammetry / L. Kwiatkowski, M. Grobelny, P. Konarski // Фізико-хімічна механіка матеріалів. — 2014. — Т. 50, № 5. — С. 13-22. — Бібліогр.: 10 назв. — англ. |
Institution
Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
irk-123456789-135879 |
|---|---|
| record_format |
dspace |
| spelling |
irk-123456789-1358792018-06-16T03:03:15Z Selection of processing parameters for conversion coatings on high-strength aluminium alloys by cyclic voltammetry Kwiatkowski, L. Grobelny, M. Konarski, P. A method of recording copper enrichment on AA2000 series alloys by means of cyclic voltammetry in borate buffer preceded by immersion of the samples in 0.6 M NaCl solution is extended to the assessment of various surface treatments like alkaline cleaning, desmutting, phosphating and also inorganic and organic corrosion inhibitors. Since the height of the peak determined from voltammograms results from Cu/Cu+ charge transfer taking place on Al–Cu surface a selection of adequate substances or technological parameters inhibiting this reaction may find an application in elaboration or modifying of surface technology. In this work the particular technological steps of the complex electrophosphating process of AA2017 were elaborated using results obtained from voltammetric transients. It was shown that a partial removal of Cu from the surface without affecting bulk content was the most effective de-smutting stage while inhibitors of Al/Cu corrosion were effective in the post-treatment stage. Розширено застосування методу дослідження збагачення міддю сплавів серії АА2000 за допомогою циклічної вольтамперометрії у боратному буфері після попереднього витримування зразків у 0,6 M розчині NaCl для оцінки різних поверхневих обробок, зокрема лужного очищення, освітлення, фосфатування, а також неорганічних та органічних інгібіторів корозії. Оскільки висота піка, визначена з вольтамперограм, є результатом перенесення заряду Cu/Cu+ на поверхні Al–Cu, вибір відповідних речовин чи технологічних параметрів, що інгібують цю реакцію, може знайти застосування у розробці або модифікації технологій поверхневої обробки. Розроблено конкретні технологічні кроки для комплексного електрофосфатування сплаву АА2017 з використанням результатів вивчення вольтамперометричних перехідних процесів. Показано, що часткове усунення Cu з поверхні, без впливу на її об’ємний вміст у сплаві, найефективніше на стадії освітлення, тоді як інгібітори корозії сплаву Al/Cu найкраще діють на стадії фінальної обробки. Расширено применение метода исследования обогащения медью сплавов серии АА2000 с помощью циклической вольтамперометрии в боратном буфере после предварительной выдержки образцов в 0,6 M растворе NaCl для оценки разных поверхностных обработок, в частности щелочной очистки, освещения, фосфатирования, а также неорганических и органических ингибиторов коррозии. Поскольку высота пика, определенная из вольтамперограмм, является результатом перенесения заряда Cu/Cu+ на поверхности Al–Cu, выбор соответствующих веществ или технологических параметров, ингибирующих эту реакцию, может найти применение в разработке или модификации технологий поверхностной обработки. Разработаны конкретные технологические шаги для комплексного электрофосфатирования сплава АА2017 с использованием результатов изучения вольтамперометрических переходных процессов. Показано, что частичное устранение Cu из поверхности, без влияния на ее объемное содержание в сплаве, наиболее эффективно на стадии освещения, тогда как ингибиторы коррозии сплава Al/Cu лучше всего действуют на стадии финальной обработки. 2014 Article Selection of processing parameters for conversion coatings on high-strength aluminium alloys by cyclic voltammetry / L. Kwiatkowski, M. Grobelny, P. Konarski // Фізико-хімічна механіка матеріалів. — 2014. — Т. 50, № 5. — С. 13-22. — Бібліогр.: 10 назв. — англ. 0430-6252 http://dspace.nbuv.gov.ua/handle/123456789/135879 en Фізико-хімічна механіка матеріалів Фізико-механічний інститут ім. Г.В. Карпенка НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| description |
A method of recording copper enrichment on AA2000 series alloys by means of cyclic voltammetry in borate buffer preceded by immersion of the samples in 0.6 M NaCl solution is extended to the assessment of various surface treatments like alkaline cleaning, desmutting, phosphating and also inorganic and organic corrosion inhibitors. Since the height of the peak determined from voltammograms results from Cu/Cu+ charge transfer taking place on Al–Cu surface a selection of adequate substances or technological parameters inhibiting this reaction may find an application in elaboration or modifying of surface technology. In this work the particular technological steps of the complex electrophosphating process of AA2017 were elaborated using results obtained from voltammetric transients. It was shown that a partial removal of Cu from the surface without affecting bulk content was the most effective de-smutting stage while inhibitors of Al/Cu corrosion were effective in the post-treatment stage. |
| format |
Article |
| author |
Kwiatkowski, L. Grobelny, M. Konarski, P. |
| spellingShingle |
Kwiatkowski, L. Grobelny, M. Konarski, P. Selection of processing parameters for conversion coatings on high-strength aluminium alloys by cyclic voltammetry Фізико-хімічна механіка матеріалів |
| author_facet |
Kwiatkowski, L. Grobelny, M. Konarski, P. |
| author_sort |
Kwiatkowski, L. |
| title |
Selection of processing parameters for conversion coatings on high-strength aluminium alloys by cyclic voltammetry |
| title_short |
Selection of processing parameters for conversion coatings on high-strength aluminium alloys by cyclic voltammetry |
| title_full |
Selection of processing parameters for conversion coatings on high-strength aluminium alloys by cyclic voltammetry |
| title_fullStr |
Selection of processing parameters for conversion coatings on high-strength aluminium alloys by cyclic voltammetry |
| title_full_unstemmed |
Selection of processing parameters for conversion coatings on high-strength aluminium alloys by cyclic voltammetry |
| title_sort |
selection of processing parameters for conversion coatings on high-strength aluminium alloys by cyclic voltammetry |
| publisher |
Фізико-механічний інститут ім. Г.В. Карпенка НАН України |
| publishDate |
2014 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/135879 |
| citation_txt |
Selection of processing parameters for conversion coatings on high-strength aluminium alloys by cyclic voltammetry / L. Kwiatkowski, M. Grobelny, P. Konarski // Фізико-хімічна механіка матеріалів. — 2014. — Т. 50, № 5. — С. 13-22. — Бібліогр.: 10 назв. — англ. |
| series |
Фізико-хімічна механіка матеріалів |
| work_keys_str_mv |
AT kwiatkowskil selectionofprocessingparametersforconversioncoatingsonhighstrengthaluminiumalloysbycyclicvoltammetry AT grobelnym selectionofprocessingparametersforconversioncoatingsonhighstrengthaluminiumalloysbycyclicvoltammetry AT konarskip selectionofprocessingparametersforconversioncoatingsonhighstrengthaluminiumalloysbycyclicvoltammetry |
| first_indexed |
2025-07-10T00:17:50Z |
| last_indexed |
2025-07-10T00:17:50Z |
| _version_ |
1837217020561588224 |
| fulltext |
13
Ô³çèêî-õ³ì³÷íà ìåõàí³êà ìàòåð³àë³â. – 2014. – ¹ 5. – Physicochemical Mechanics of Materials
SELECTION OF PROCESSING PARAMETERS FOR CONVERSION
COATINGS ON HIGH-STRENGTH ALUMINIUM ALLOYS
BY CYCLIC VOLTAMMETRY
L. KWIATKOWSKI 1, M. GROBELNY 1, 2, P. KONARSKI 3
1 Institute of Precision Mechanics, Warsaw, Poland;
2 Motor Transport Institute, Warsaw, Poland;
3 Tele&Radio Research Institute, Warsaw, Poland
A method of recording copper enrichment on AA2000 series alloys by means of cyclic
voltammetry in borate buffer preceded by immersion of the samples in 0.6 M NaCl solu-
tion is extended to the assessment of various surface treatments like alkaline cleaning,
desmutting, phosphating and also inorganic and organic corrosion inhibitors. Since the
height of the peak determined from voltammograms results from Cu/Cu+ charge transfer
taking place on Al–Cu surface a selection of adequate substances or technological para-
meters inhibiting this reaction may find an application in elaboration or modifying of
surface technology. In this work the particular technological steps of the complex electro-
phosphating process of AA2017 were elaborated using results obtained from voltammetric
transients. It was shown that a partial removal of Cu from the surface without affecting
bulk content was the most effective de-smutting stage while inhibitors of Al/Cu corrosion
were effective in the post-treatment stage.
Keywords: Al alloys, 2000 series, surface inhomogeneity, corrosion, Cu enrichment,
cyclic voltammetry, phosphate coatings, impedance
Pure aluminium is resistant against corrosion due to passivity. The oxide film is
adherent, transparent, stable in pH 4–9, self-healing and also is an electrical insulator.
However, for increased strength the metal is alloyed with several mass.% of Mg, Zn,
Cu and other elements producing a large number of alloys, usually inferior in corrosion
resistance in comparison with the pure metal. One of the most common representative
alloy used in an aircraft industry is AA2024-T3. Even newer and improved variants of
precipitation age hardened Al alloys which offer combinations of strength, toughness,
and weight reduction often exhibit susceptibility to localised corrosion, as a result of
surface heterogeneity. Numerous constituent particles formed in these alloys can be
divided into three major types based on composition [1]:
1. Al–Cu–Fe–Mn particles, which are more noble than Al matrix and act as
cathodes;
2. Al–Cu–Mg particles, which are less noble than Al matrix and act as anodes;
3. Al2Cu particles, which are more noble than Al matrix.
According to [2], corrosion leads to preferential dissolution of Al and Mg leaving
Cu-rich remnants which act as cathodes. Pitting is often induced by local galvanic cells
formed by Cu-intermetallics or replated Cu on AA2024-T3 surface. According to [3]
the deleterious effect of intermetallic compounds in Al–Cu alloys on their corrosion
resistance especially in the presence of conversion coating is considered to be the result
of copper dissolution and re-deposition creating Cu-rich cathodic areas on which the
oxygen reduction reaction occurs at a faster rate than on the Al alloy. The re-deposition
Corresponding author: L. KWIATKOWSKI, e-mail: lech.kwiatkowski@imp.edu.pl
14
of Cu takes place during formation of majority of conversion coatings. The early stage
of this process is pickling of a surface layer leading to Al = Al3+ + 3e. At the same time
cleaned and reduced, thus very active un-noble surface area is an excellent place to re-
deposit more noble Cu.
While in the case of chromate coatings on AA2024-T3 surface the effect is not so
pronounced, an application of non chromate conversion coatings on Cu-containing Al
alloys may lead to a dramatic decrease in corrosion resistance. Conversion coating
(CC) process, whether it is chromating or non- chromium treatment, consists of the
following steps: cleaning, rinsing, deoxidising, rinsing, conversion coating, post-trea-
tment rinsing or decorative colour rinsing and drying. At least three of them: cleaning,
deoxidising and main CC process create the risk of copper re-plating due to the high
or/and low pH of solutions used. Therefore protection against corrosion of AA2000 se-
ries is a challenging task. Some problems are also related to the methods of assessment
of corrosion resistance of protecting substance, coating or system. The common
method of determination of pitting potentials is doubtful for many high strength alloys
since pitting occurs in the vicinity of the open circuit potential, impedance spectra are
difficult to interpret, chamber tests are too simplistic and field exposition is time con-
suming. Therefore a robust and precise set of methods for the evaluation of protective
systems, especially on high strength Al alloys would also be useful. The results pre-
sented in this paper are taken from the extensive research on chemical and electroche-
mical surface treatments of high strength aluminium alloys [4]. The aim of this paper is
to show, how simple electrochemical characterisation of the surface of aluminium alloy
originally described in [5] may be helpful in establishing the anticorrosion protective
system.
Experimental. Materials. Discs of 30 mm diameter cut from a rod of AA2017
alloy were used in most of the experiments. A number of tests were also carried out on
AA2024-T3 samples cut from sheet.
The dimensions of the samples were 2.57.5 cm but the measured area was re-
stricted to 1 cm2 for both kinds of the samples. The following solutions for surface
treatments were used:
– for alkaline degreasing: 0.5 M Na3PO4 adjusted to pH 12, temperature 50C;
– for de-smutting: 3% HNO3, ambient temperature and so called chromosulphuric
acid treatment; a procedure contains hexavalent chromium;
– for electrochemical de-smutting: a solution and procedure indicated below.
In terms of electrochemical treatment a process of Cu removal is based on US
Patent 5635084:1997 [6]. The method relies upon a the sample polarization in
HNO3/NaNO3 solution at –55 mV vs. SCE during 30 min. This procedure has got an
abbreviation here as US patent. A number of substances which belong to inhibitors of
Cu corrosion was tested usually in 0.1 M concentration and they are mentioned along
with description of the results.
Methods. Electrochemical corrosion tests were carried out by means of polarisa-
tion curves and by means of electrochemical impedance spectroscopy. Both measure-
ments were recorded mainly in 0.5 M Na2SO4 + 0.01 M NaCl of pH 6.0. Some of tests
were also carried out in 0.15 M NaCl. Potential scan rate was fixed to 1 mV/s. Impe-
dance spectra were recorded at the open circuit potential using AC amplitude 10 mV in
the frequency range of 106…10–3 Hz. A cyclic voltammetry method based on a work
published in [5] is applied for the assessment of Cu enrichment in corrosive medium
(0.6 M NaCl) in the presence of inhibitors / complexing agents. The first step of the
procedure is 24-hour immersion of the samples in 0.6 M NaCl containing inhibitor.
Afterwards a sample is transferred to electrochemical cell containing borate buffer of
pH 8.4. Cyclic voltammetry is conducted at a rate 1 mV/s from –700 mV to + 300 mV
and back to –1200 mV (all potentials quoted vs. SCE). These voltages are in the range
15
of copper oxidation / reduction but not for the corrosion of the base metal. Three scans
are performed and the result of the 3rd scan is taken into consideration. Electrochemical
methods were carried out alternatively by the following systems: Autolab Eco Chemie,
Solartron 1287 and ACM Instruments along with relevant software for experiment con-
trol, data acquisition and data analysis. Surface examination was carried out by means
of SEM/EDXS analysis using scanning electron microscope LEO model VP435 equip-
ped with EDXS produced by Roentec. “Preliminary attempts were also undertaken to
analyse quantitatively surface components by means of glow discharge mass spectro-
metry along with a depth profile analysis. In the depth profile analysis the sputtering
process in a glow discharge erodes the sample surface rather evenly, i.e. “atomic layer
by layer”. The methodology was elaborated using SMWJ-01 glow discharge prototype
spectrometer [7]”.
Results. Assessment of Cu enrich-
ment as a result of corrosion of alumi-
nium alloy. The first step of the testing
procedure is the exposition of a sample in
0.6 M NaCl to initiate Cu enrichment.
Detection of this Cu enrichment is recor-
ded in the second step in borate buffer.
Cyclic voltammograms of two represen-
tatives of AA2000 series recorded after
18 h of samples immersion in 0.6 M NaCl
are shown in Fig. 1. Two anodic peaks at
E = –0.09 V and E = 0.03 V vs. SCE are
recorded in borate buffer. The second
peak is more pronounced for AA2017.
The two anodic current peaks cor-
respond to the following reactions:
2Cu + H2O → Cu2O + 2H+ + 2e; (1)
Cu2O + H2O → 2CuO + 2H+ + 2e. (2)
In order to check whether Cu enrichment is a dominating factor of the electroche-
mical behaviour a surface analysis was carried out by means of Glow Discharge Mass
Spectrometry (GDMS). The results are shown in Fig. 2.
Fig. 2. Concentration profiles of Al, Cu and Mg in surface layer of AA2017 before (a)
and after (b) 18 h of the samples exposition in 0.6 M NaCl.
It is seen from the results presented above that significant Cu surface enrichment
takes place during 18 h of samples immersion in NaCl solution. Therefore cyclic volt-
ammograms show two well defined anodic current peaks which is consistent with the
results published in [5]. The results of surface analysis presented in this work proved
Fig. 1. Cyclic voltammetric curves
for AA2024 (1) and AA2017 (2) alloys
recorded in borate buffer (pH 8.4)
after 18 h immersion in 0.6 M NaCl.
16
that cyclic voltammograms can be used as direct indication of Cu active presence on
the Al surface and indirectly may be related to initiation of corrosion process. The
current peaks are related to Cu/Cu+ and Cu+/Cu2+, however, we cannot exclude that the
second peak may also contain additional contribution due to the direct oxidation of
Cu/Cu2+. Hence for further analysis the height of both peaks was measured, but the
height of the 1st peak was usually considered as a measure of Cu activity on the surface
of aluminium alloy.
Applicability of the voltammetric method to the assessment of conversion coa-
ting. A common process used as a pre-treatment prior to organic coating on AA2000
series was known as Alodine process which was carried out in the solution containing
chromates and phosphates with some minor additives [8]. The “pure” phosphating was
never applied because of technical difficulties related probably also to Cu enrichment
of the surface. The best result which was obtained in our trials for chemical phospha-
ting of AA2017 is shown in Fig. 3a. As it is seen from this result and data presented in
a caption of Fig. 3a, coverage of the surface is incomplete and Cu rich areas are deter-
mined between phosphate crystals. Such results were independent of the chemical con-
tent of a phosphating solution, as well as on the process parameters. From this reason
the efforts were focused on the elaboration of modified phosphating process which was
supported by electrochemical aid of cathodic polarization (Fig. 3b). The details of such
process were presented in [9]. Comparison of surface coverage obtained in each pro-
cess leaves no doubt that the latter process is much more effective. These variants are
signed here as un-modified and modified, respectively.
Fig. 3. The effect of the type of phosphating process un-modified (a), modified (b)
on the topography and substrate coverage of zinc phosphate coating. Elemental concentration
of Cu measured by EDAX between crystals: a – 13.24; b – 1.02 mass.%.
Since crystalline phosphate coa-
tings exhibit a certain degree of porosity
it is also interesting whether a substrate
at the bottom of the pores and voids of
the coating contains Cu deposit. Basi-
cally, phosphate nuclei should crystallize
on cathodic areas of the surface, hence
Cu remnants have to be covered by the
phosphate coating, but due to the mobi-
lity of Cu during dissolution process of
intermetallics [2] a nucleation of phos-
phate crystals may proceed in different
way. Therefore, the next example served
as a verification of applicability of the
monitoring of Cu release from the pho-
sphate samples by means of cyclic volt-
Fig. 4. Cyclic voltammetric curves
for AA2017 phosphated in un-modified (1)
and modified (2) process recorded
in phosphate buffer after 18 h exposition
in 0.6 M NaCl.
17
ammetry. The results of the tests are shown in Fig. 4.
It can be concluded that a coverage of Cu-rich surface remnants by the layer of
hardly soluble phosphates is much more efficient for modified version of the process.
The question is how this result corresponds to the assessment of anticorrosion proper-
ties evaluated by common electrochemical methods. Therefore, a set of the samples
coated in phosphating solutions described above was tested by means of electrochemi-
cal impedance spectroscopy (EIS). The measurements were carried out in 0.15 M NaCl
after 1 and 24 h of immersion (Fig. 5).
Fig. 5. Impedance spectra for AA2017 coated with phosphate coating of different quality:
a – un-modified; b – modified process recorded in 0.15 M NaCl
after 1 (1), 3 (2), 6 (3), 24 h (4) of immersion.
Fig. 6. Electrical equivalent circuits used
for the analysis of impedance spectra. Rs – solution
resistance, Rp – polarisation resistance, Cdl – capacity
of the metal–solution interface (double layer),
Rc – electrolytic resistance of the solution
in the coating pores, Cc – capacity of the coating.
In most results a constant phase element (CPE)
was used instead of capacitance.
The results of EIS measurements confirm
the results obtained from the cyclic voltammetry
in borate buffer (Fig. 4). The values of charge
transfer resistance and interfacial capacitance
calculated using equivalent circuit (Fig. 6) were
estimated after 1 h of immersion as Rt =
= 103 ·cm2, Cdl = 10–5 F/cm2 and Rt = 104
·cm2, Cdl = 10–8 F/cm2, for un-modified and modified process respectively. It can be
assumed that surface Cu enrichment, in this particular case, which takes place at the
bottom of pores and voids of the coating can be a direct measure of the initiation/onset
of substrate corrosion. Therefore a modification of pre-treatment and post-treatment of
the conversion coating process in order to decrease undesirable activity of
intermetallics was undertaken. Two possible routes were taken into consideration. The
first one is to remove surface copper without affecting a bulk material. The second
relied upon a use of inhibitors that may adsorb on Cu rich areas during pre- or post-
treatment. For both approaches a voltammetric method served as a screening tool.
Surface pre-treatment. It is common that a surface pre-treatment affects corrosion
behaviour of a surface. Prior to every surface treatment a clean and free of foreign
mater surface is required. Aluminium alloys frequently undergo alkaline degreasing
18
followed by deoxidising (de-smutting). The latter is carried out in acidic solutions. A
traditional solution was called a chromosulphuric treatment. Alternative preparations
are based on HNO3 or mixtures of inorganic/organic acids.
The treatment tested in this work is based on already cited US patent [6], known
as Cu removal method. The comparison of the effect of particular surface pre-treat-
ments on the electrochemical state of the surface characterized by cyclic voltammetry
is presented in Fig. 7.
For acetone degreasing and alkaline degreasing followed by HNO3 de-smutting
both peaks of Cu oxidation are relatively high which means that copper enrichment
was taking place during 18 h of sample immersion in 0.6 M NaCl. On the other side,
two other treatments changed the state of the alloy surface resulted in significantly
lower 1st current peak and also lower but to a lesser extent 2nd peak. The lowest value
of peak current densities corresponding to Cu oxidation process for electrochemical
treatment decreasing the Cu content on the surface was obtained which seems to be
logic. However, in the case of regular alkaline degreasing without de-smutting an
accumulation of impurities containing also Cu oxides takes place covering the surface
with black smut. This deposit loosely adheres to the surface but may block some areas
against action of NaCl during 18 h exposition. This feature as well as inactivity of
already formed CuO leads to recording of a current signal from the smaller area. These
are probable reasons of lower heights of current density peaks for alkaline etched
AA2017 surface.
The method based on cyclic voltammetry in borate buffer was previously applied
to assess mixtures of inhibitors used for the protection of high strength Al alloys [5]. In
our studies two types of substances were tested. The first one belongs to the group of
inhibitors of Cu corrosion. An application of substances of this type is due to the fact
that re-plated Cu is detected on investigated Al alloy surfaces. The second group is related
to corrosion inhibition of both Cu and Al–Cu substrate. The results are shown in Fig. 8.
Fig. 7. Fig. 8.
Fig. 7. CV voltammograms for AA2017 in borate buffer at pH 8.4 after 18 h exposition
in 0.6 M NaCl. The effect of surface preparation: 1 – acetone; 2 – alkaline degreasing;
3 – alkaline degreasing, HNO3 deoxidising; 4 – alkaline degreasing, potentiostatic treatment [6].
Fig. 8. The effect of inhibitors: 1 – without inhibitor; 2 – 1-allyl-2-thiourea; 3 – sodium
gluconate + citric acid; 4 – chromates; 5 – benzotriazole, added to 0.6 M NaCl and exposed
for 18 h on the course of CV voltammograms recorded in borate buffer after exposition.
Backward cathodic transients are eliminated for the clarity of the figure.
The AA2017 sample exposed to 0.6 M NaCl without any additional substance
shows the highest cd peaks related to Cu two-step oxidation. On the other hand, sam-
ples immersed in Cr(VI) containing chloride solution show almost negligible current
signal. This means that chromates effectively inhibit reactions of Cu species. Similar
properties were obtained for citric acid-sodium gluconate mixture. There is however an
increase of current density observed at +300 mV vs. SCE but it appears beyond the po-
19
tential range of two peaks determined for
the AA2017 reference sample. Therefore
this mixture can also be considered as
effective. A lower ability to inhibit Cu
enrichment for 1-allyl-2-thiourea is
observed, however a certain positive
effect is still recorded. The values of
current densities at relevant potentials are
shown in Table 1.
From comparison with the results
obtained in the previous chapter one may
conclude that both attempts decreasing
the possibility of surface reactions
involving Cu ions are effective in terms of significant decrease of current densities.
Again, the question is how these results correspond to corrosion resistance of a phos-
phate coating formed in electrochemical process on AA2017 aluminium alloy.
The effect of pre-treatment on corrosion resistance of the phosphate coating.
Electrochemical impedance spectroscopy was employed to study corrosion resistance of
pre-treated phosphate coatings (Fig. 9). For all the treated samples, the impedance spectra
exhibit two time constants (see relevant Bode plots) which comply with the so-called
“coating model” (Fig. 6) [10]. The results of EIS data analysis are shown in Table 2.
Fig. 9. Impedance spectra of phosphated AA2017 depending on surface pre-treatment:
1 – HNO3; 2 – chromosulphuric; 3 – US patent; 4 – benzotriazole + butindiol; 5 – pre-treatment
with HNO3 (no coating) after 1 h of immersion in 0.5 M Na2SO4 + 0.01 M NaCl, pH 6.
Table 2. Electrochemical parameters for phosphated AA2017 resulted
from the analysis of EIS spectra (Fig. 9)
Electrochemical
parameters
HNO3
Benzotriazole +
+ butindiol
chromo-
sulphuric
US patent
HNO3 pre-
treatment
(no coating)
Rs, cm2 3.94 3.079 3.31 4.03 4.47
CPEcoat, F/cm2 4.2810–8 1.1110–7 1.1110–7 3.1610–6 –
n1 0.83 0.78 0.74 0.75 –
Rcoat, cm2 5.73102 2.88102 5.19102 7.12102 –
Rp, cm2 1.56105 4.17104 6.15105 9.69105 4.36104
CPEdl, F/cm2 1.0810–7 1.4810–7 1.7010–7 3.3310–7 3.5410–6
n2 0.83 0.78 0.74 0.85 0.89
Table 1. Peak current densities
and potentials shown in Fig. 7
1st current peak
Treatment with
E, V i, A/cm2
HNO3 –0.092 24
Benzotriazole –0.092 0.67
Chromates –0.261 2.25
Citrates/gluconates –0.092 3.2
1-allyl-2-thiourea –0.112 10.7
20
The results indicate that surface pre-treatment directly influences the protective
properties of the phosphate coatings which are represented here by polarization resis-
tance. This parameter is related to the physicochemical state of the substrate in the
pores of the coating and reflects corrosion resistance. Another matter is whether all pre-
treatments applied here affect positively crystals structure. From our experience [4]
chromosulphuric acid and mixture of inhibitors favor the formation of coarse crystals
which is not acceptable prior to organic coatings while de-smutting by nitric acid or
electrochemical Cu removal stage provide formation of the coating with much more
finer crystals. Unless it is stated, in all cases described above an activation step using
Ti colloid containing solution was used after de-smutting. As a final conclusion from
this part of the work it is concluded that some kind of Cu removal as exampled here by
cited treatment is more beneficial for the formation of the phosphate coating than a
direct inhibition of copper relating reactions by inhibitors. The latter however may be
applicable in the final stage of the complex process which is post-treatment.
The effect of post-treatment. Every
crystalline phosphate coating is porous. Its
corrosion resistance depends on the degree
of porosity and physicochemical state of the
substrate at the bottom of the pores. Con-
cerning properties of the coatings obtained
in this work, an additional element is in-
cluded. This is Cu content in pores or/and
voids of the coating. Therefore just for
highlighting the problem two kinds of
sealing were applied. One, taken from the
former investigations related to sealing by
Cr(VI) added as K2Cr2O7 and another rela-
ted to actual work on Cu inhibitors which is
a solution containing benzotriazole. The
main constituent of sealing solution was
0.01% H3PO4. The pH of both solutions was equal to 3.3.
The results are shown in Fig. 10 and Table 3. All spectra exhibit two time con-
stants and can be represented by already described “coating model”. The results of
calculations of particular elements of the equivalent circuit are given in Table 3. The
results indicate that an improvement of protective properties can be seen, especially
taking into account the values of polarisation resistance (Rp). It should be underlined
that sealing solutions as well as parameters were not optimised in this experiment.
Table 3. The results of fitting the data to the “coating model” (Fig. 10)
Electrochemical
parameters
without sealing sealing: benzotriazole + butindiol sealing: Cr(VI)
Rs, cm2 3.94 5.66 4.41
CPE1, F/cm2 4.2810–8 3.7610–7 2.4810–7
n1 0.834 0.936 0.774
Rcoat, cm2 5.73102 2.18102 5.15102
Rp, cm2 1.56105 3.15105 5.94105
CPEdl, F/cm2 1.0810–7 1.4110–7 1.7510–7
n2 0.826 0.793 0.854
Fig. 10. Bode plots for phosphated/sealed
AA2017 and exposed to 0.5 M Na2SO4 +
+ 0.01 M NaCl without sealing (1),
benzotriazole + butindiol (2), Cr(VI) (3).
21
It is obvious that the best sealing and hence protective properties for Cr(VI) are
obtained. It is also necessary to conclude that the improvement in this case is not as
significant as for commercial products but in the present work simplified solutions
were used just for evaluation of the effects and trends. However, it is proved that other
compounds like Cu inhibitors can be employed for sealing rinse of the phosphate
coatings as potential candidates. As a result of the conclusion stated above, one may
assume, that the procedure comprising sealing rinse with solution containing inhibitor
of Cu corrosion which adsorbs on the substrate containing Cu remnants shall be opti-
mised in terms of the chemical content and technological parameters.
CONCLUSIONS
The method of recording surface copper enrichment of AA2000 series alloys by
means of cyclic voltammetry in borate buffer preceded by immersion in 0.6 M NaCl
solution which was originally used as an assay for screening of inorganic inhibitors
performance was extended in this work to the assessment of various surface treatments
related to Cu surface enrichment like alkaline cleaning, de-smutting, phosphating and
also inorganic and organic inhibitors. The applicability of the method to this wider
range of subjects was basically proved positively but some situations need an additio-
nal knowledge on the system in order to withdraw proper conclusions. It means that the
results obtained by this method can not be accepted without prior knowledge analysis
of an investigated system.
It was proved that surface enrichment of Cu can be assumed as a direct indicator
of corrosion of Al–Cu alloys, especially at the beginning of degradation process. For
this kind of a substrate the corrosion process is always preceded by the activity of inter-
metallics. It can be also useful for Al–Cu alloys coated by crystalline phosphate coa-
tings which are porous and their corrosion resistance depends mainly on the state of the
substrate in pores and voids of the coating. It is also applicable for other protective
coatings that locally expose bare substrate as well as for selection of components and
content of various surface treatments and their partial operations (pre- and/or post-
treatments).
The extension of the applicability of the method to the evaluation of the effect of
surface treatment performed in this work shows that a peak height is not only propor-
tional to the amount of Cu on the surface as was assumed earlier but may also be de-
pendent on the form of Cu compound formed on the surface and the electrochemical
activity of this substance under corrosion load that means depend not only on the
amount but also on kinetics of Cu reactions on AA2000 series surface.
Acknowledgements. The authors gratefully acknowledge the financial support of
Technology Partners Foundation and Airbus Co. under the FAST-AA project.
РЕЗЮМЕ. Розширено застосування методу дослідження збагачення міддю сплавів
серії АА2000 за допомогою циклічної вольтамперометрії у боратному буфері після попе-
реднього витримування зразків у 0,6 M розчині NaCl для оцінки різних поверхневих обро-
бок, зокрема лужного очищення, освітлення, фосфатування, а також неорганічних та орга-
нічних інгібіторів корозії. Оскільки висота піка, визначена з вольтамперограм, є результа-
том перенесення заряду Cu/Cu+ на поверхні Al–Cu, вибір відповідних речовин чи техно-
логічних параметрів, що інгібують цю реакцію, може знайти застосування у розробці або
модифікації технологій поверхневої обробки. Розроблено конкретні технологічні кроки
для комплексного електрофосфатування сплаву АА2017 з використанням результатів ви-
вчення вольтамперометричних перехідних процесів. Показано, що часткове усунення Cu
з поверхні, без впливу на її об’ємний вміст у сплаві, найефективніше на стадії освітлення,
тоді як інгібітори корозії сплаву Al/Cu найкраще діють на стадії фінальної обробки.
РЕЗЮМЕ. Расширено применение метода исследования обогащения медью спла-
вов серии АА2000 с помощью циклической вольтамперометрии в боратном буфере после
предварительной выдержки образцов в 0,6 M растворе NaCl для оценки разных поверх-
22
ностных обработок, в частности щелочной очистки, освещения, фосфатирования, а также
неорганических и органических ингибиторов коррозии. Поскольку высота пика, опреде-
ленная из вольтамперограмм, является результатом перенесения заряда Cu/Cu+ на поверх-
ности Al–Cu, выбор соответствующих веществ или технологических параметров, ингиби-
рующих эту реакцию, может найти применение в разработке или модификации техноло-
гий поверхностной обработки. Разработаны конкретные технологические шаги для комп-
лексного электрофосфатирования сплава АА2017 с использованием результатов изучения
вольтамперометрических переходных процессов. Показано, что частичное устранение Cu
из поверхности, без влияния на ее объемное содержание в сплаве, наиболее эффективно
на стадии освещения, тогда как ингибиторы коррозии сплава Al/Cu лучше всего действу-
ют на стадии финальной обработки.
1. Suter T. and Alkire R. Microelectrochemical Studies of Pit Initiation at Single Inclusions in
Al 2024-T3. // J. Electrochem. Soc. – 2001. – 148, № 1. – P. B36–B342.
2. Local dissolution phenomena associated with S phase (Al 2CuMg) particles in aluminum
alloy 2024-T3 / R. G. Buchheit, R. P. Grant, P. F. Hlava et al. // Ibid. – 1997. – 144.
– P. 2621.
3. Ilevbare G. O. and Scully J. R. Oxygen Reduction Reaction Kinetics on Chromate Conver-
sion Coated Al–Cu, Al–Cu–Mg, and Al–Cu–Mn–Fe Intermetallic Compounds // Ibid.
– 2001. – 148, № 5. – P. B196–B207.
4. Functional and advanced surface treatments of 2xxx aluminium alloys / L. Kwiatkowski,
M. Grobelny, A. Zych et al. – Technology Partners Report, Warsaw, 2009, unpublished, and
IMP Report 2007 unpublished.
5. Chambers B. D. and Taylor S. R. High-throughput assessment of inhibitor synergies on alu-
minum alloy 2024-T3 through measurement of surface enrichment // Corrosion. – 2007.
– 63, № 3. – P. 268.
6. US Patent 5635084 “Method for creating a corrosion resistant surface on an aluminium cop-
per alloy”, 1997.
7. Quadrupole-based glow discharge mass spectrometer: Design and results compared to
secondary ion mass spectrometry analyses / P. Konarski, K. Kaczorek, M. Ćwil, J. Marks
// Vacuum. – 2007. – 81, № 10. – P. 1323.
8. Biestek T. and Weber J. Electrolytic and Chemical Conversion Coatings. – Warszawa: Port-
cullis Press Ltd-Redhill, 1976. – P. 203.
9. Kwiatkowski L. and Zych A. On the phosphatibility of high strength aluminium alloys
// Proc. 18th Int. Corrosion Congress. – 2011. – 2. – P. 1529–1540.
10. Mansfeld F. Electrochemical Methods of Corrosion Testing in ASM Handbook // Corrosion:
Fundamentals, Testing and Protection”. – 2003. – 13A. – P. 446–462.
Received 07.07.2014
|