Inhibition Properties of Some Aircraft Corrosion Protective Coatings

Chapter 18

Inhibition Properties of Some Aircraft Corrosion Protective Coatings

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A. W. Fangmeier , E. Kock , F. Vohwinkel , and C. P. Brandt

Downloaded by TUFTS UNIV on June 5, 2018 | https://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch018

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Department of Surface Protection and Chemistry, Daimler-Benz Aerospace, Airbus GmbH, 28199 Bremen, Germany Department of Material Science and Processes, Fachhochschule Osnabrück, 49076 Osnabrück, Germany

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Starting from the general Airbus corrosion prevention philosophy requiring a certain leachability of the corrosion protective pigments of the coating, extracts of several chromate containing and chromate-free primers were prepared. The extracts were used to run special alternate immersion/emersion (A.I.E.) tests as well as electrochemical measurements such as equilibrium potential and polarisation resistance measurements. These results enable a classification of the primers into types of different efficiency and mechanism and can be compared to results of standard filiform corrosion tests. In the past, most of the approaches to qualify chromate-free corrosion inhibiting primers for aircraft application failed. These efforts were the starting point for some further general investigations of chromate-free primers without needing too much detail as to the primer formulation. Following the general Airbus corrosion prevention philosophy which requires a certain solubility of the corrosion inhibiting pigments of the coatings, water based extracts of several chromated and chromatefree primers were prepared. Chromated and Chromate-Free Primers Chromated primers have been applied in the aerospace industry for over 30 years. One of their most important properties is the leachability of the corrosion inhibiting pigment, the chromate. In addition to under the primer this promises good protection on areas which are not directly covered by the primer such as drill holes, scratches or other damaged sites. It is known that chromate acts on both cathodic and anodic site inhibiting pit formation as well as corrosion propagation. Daimler-Benz Aerospace Airbus GmbH, Huenefeldstr. 1-5, D-28199 Bremen. Fachhochschule Osnabriick, Albrechtstr. 30, D-49076 Osnabriick.

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©1998 American Chemical Society

Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


239 Due to its long use, the behavior of numerous chromated primers under many of the world wide applied corrosion test conditions is well characterized. In contrast the numerous chromate-free corrosion inhibitive primers which have been developed during the last few years are not so well understood. Their corrosion prevention behavior has been evaluated mainly by standard salt spray tests, filiform corrosion tests, a few alternate immersion/emersion (A.I.E.) and crevice corrosion tests. In Figure 1 the representative leaching behavior of a chromated primer is shown. Although there are some primers which have an initial leaching rate of only ten percent of the shown value, these primers are still active showing the same inhibition behavior. Even if the leaching rate starts at a level comparable to that of the specimen leached for 29 days, there is still a corrosion protective effect. This can be illustrated by observing the leaching rates of old aircraft parts shown in Figure 2. These parts were in service in an aircraft for over 15 years but still do not show any corrosion. Even when scratched after 22 days immersion in distilled water, a significant rise of the leaching rate appears. For the investigation described, the most commonly applied, chromated primers of the european aerospace industry and the most promising chromate-free ones of the qualification attempts of the past years were used (Table I).

C h r o m a t e [mg/l]

1

2

3

4

7

10

16

23

29

Duration [Days]

Fig. 1. Chromate leaching of a primed specimen.


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Chromate [pg/l]

1

2

3

4

14

7

22

23

Duration [Days]

WÊ AC

342

Kl

AC 3 3 3

Fig. 2. Chromate leaching of old aircraft parts.

Table I: Types of Primers under Investigation Polymer base

Number of primers

Designation

Epoxy (EP), chromated Polyurethane (PUR), chromated Epoxy (EP), chromate-free Polyurethane (PUR), chromate-free

2 2 4 1

A,B C,D E,F,G,H I

While both EP-primers (A,B) and one of the PUR-primers (C) are inhibited by 20-25% zinc- or strontium chromate, the PUR-primer (D) is only inhibited by about 5% barium chromate. The inhibiting pigments of the chromate-free primers in most cases are not known at this time. Extract Preparation and À.I.E. tests As a first screening, an attempt was made to prepare extracts with acetone from the different paint base compounds. Although this approach was successfull with some primers for some A.I.E. tests, it was juged to be - An unrealistic procedure compared to in service conditions. - Not applicable to some primers. - Uncertain due to the unknown solubility of the different pigments in acetone.


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Therefore this method was replaced by preparing aqueous extracts from cured paint films which is more realistic and easier to perform. Extract Preparation. Cured and powdered primer was stirred for three days with distilled water and then filtered. Conductivity measurements showed that after three days the conductivity of the extract became constant for all primers under investigation. In order to get a suitable electrolyte for A.I.E. tests the extracts were diluted 1:10 with distilled water and sodium chloride was added to give a 3% solution. To half the solution a part of the insoluble residue of the extraction process was again added to determine the influence of the water insoluble part of the primer on the A.I.E. test results. After preparation of the extracts, A.I.E. tests were performed on the substrates and with the conditions given in Table II. Table II. Test Conditions Characteristic

Property

Temperature Duration Cycle Substrate

20-25°C 1000 hrs. 2 hrs. wet / 2 hrs dry Al alloy 2024 (bare) Al alloy 2024 (Al clad) Chromic acid anodizing (CAA) Chrome sulfuric acid pickling (CSA)

Tretreatment

Results. Without discussing any specimen individually, the results of the A.I.E. test can be summarized as shown in Table III. While the chromated primers do not show any significant difference on pure aluminium clad (Al-clad) and unclad (bare) material or CAA and CSA pretreated substrates, the chromate free ones in general show a better protection behavior on clad than on bare material and on CAA than on CSA surfaces. Table III. A.I.E. test Results Primer

Test series with residue

+ A,B,D 0 C Ε + 0 F,G,I Η 4- = very good corrosion protection 0 = poor corrosion protection - = very poor corrosion protection

Test series without residue

+ + 0 -


242 The reason for this behavior is assumed to be the lower protection performance of these primers such that the anodic protection by the cladding and the passivation by the CAA layer become more important for the test results. Although well re cognizable, these effects are not strong enough to change the classification given in Table III. It is remarkable that most of the chromate-free primers, especially primer (E) give a better corrosion protection if some insoluble primer residue is present which means that there must be still some interaction. This was not observed with the chromated primers.


Electrochemical Measurements For the electrochemical measurements (determination of the equilibrium potential and the polarisation resistance), primer extracts were used which were prepared as previously described. Different dilution ratios were prepared from these extracts. They included from full concentration to 1:1000 extract/water. Sodium chloride was added to give a 0.1M NaCl electrolyte solution. The measuring equipment used was a VersaStat 250-1. As shown in Table IV, the chromated primers in general give an initial high polarisation resistance at a low concentration. There is a drop when going to the chromate-free primers which even at a high concentration give a significantly lower polarisation resistance. Thus these results corroborate those found in the A.I.E. tests. Table IV. Results of the Polarisation Resistance Measurements Primer

Dilution ratio [extract/water]

Polarisation resistance [kQ]

A Β C D Ε F G H I

1:10 1:100

780 660 800 660 690 390 350 400 200

1:15 1.5:0.5 1:10

-

Second Test Cycle Following this first test cycle, more detailed investigations were started (in order to get a better correleation to in service conditions) to take into account the real situa tion in scratches, well defined scratches were prepared and the chromate leaching out of these artificial damage sites was measured. For this purpose, 80x150mm specimen with primer and top coat, were prepared and scratched twelve times. Measuring the leached chromate gave 8.5 mg/1. This



243 value was taken to define the chromate concentration of at least one of the solutions used in the following electrochemical and A.I.E. tests. As there was nothing known about the leachability of inhibitors from the chromate-free primers, the extracts prepared from them were diluted to the same extent as the extracts of the chromated primers. So all concentrations given in the following for chromate-free systems are based on these dilution ratios and do not represent real concentrations. A second question to be answered by the following tests was that of the importance of the relative chloride to inhibitor concentration ratio. For steel it was reported (1) that log [chloride/chromate] should not exceed two, or the inhibitor loses its effectivness. This was the second point considered in diluting extracts used in the following tests: 1. Modified A.I.E. test 2. Polarisation resistance measurement - with oxygen aeration - with nitrogen aeration 3. Equilibrium potential measurements The composition of the electrolytes prepared from an extract of primer A is given in Table V. The concentration is based on a measurement which gave 560 mg chromate/1 » 4.8 χ 10" mol/1. All ο ;r extracts were diluted in the same way without measuring the real inhibitor concentration which was not possible without knowing the exact inhibitor composition. 3

Table V: Composition of Electrolytes Prepared from Primer A Dilution

NaCI [mol/1]

log [CI"]/ [inhibitor]

Abs. chromate cone.

Al A2 A3 Al A2 A3

0.05 0.05 0.05 0.10 0.10 0.10

1.00 1.50 2.00 1.30 2.15 2.50

4.80 1.60 5.00 4.80 7.10 3.16

χ χ χ χ χ χ

3

10 10"' 10^ ΙΟ" 3

W

1

10^

Results. The results of some of the electrochemical measurements are given in Tables VI-VIII (different test conditions) and Figures 3 and 4 (different primers). The results given in Tables VI-VIII show that chromate inhibits the anodic process as well as the cathodic process while the chromate-free systems preferably influence the cathodic reaction. A summary of the test results of the A.I.E.-tests of this second cycle are given in Table IX and Figures 5-7. The corrosion categories which are essential for the understanding of the above mentioned figures, as they are not yet standarized, are defined in Table X. All results from the previously described corrosion tests corroborate the electro chemical measurements and confirm the somewhat poorer corrosion inhibition properties of the chromate free primer systems.


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P o l a r i s a t i o n R e s i s t a n c e [k Ohm]

0,06

0,6

6

60

600

Extract / 600ml Electrolyte Primer A

Hi

[mi]

i ^ l Primer Β

I Primer C

ϋ ϋ

Primer D

Fig. 3. Polarisation resistance of chromated primers (0.1 M NaCl).

P o l a r i s a t i o n R e s i s t a n c e [k Ohm]

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60

180

420

E x t r a c t / 6 0 0 ml E l e c t r o l y t e Hi

Primer Ε

ϋ ϋ

Primer G

BS

Primer H

lui

Primer I

600

r

[ml]

Fig. 4. Polarisation resistance of chromate-free primers (0.1 M NaCl).


245 Table VI: Polarisation Resistance and Equilibrium Potential of a Chromated Primer 0 aeration 2

Dilution

0.05M NaCl


Al A2 A3

O.lMNaCl


Equilibrium potential [volt]

Polarisation resistance [kO]


400 300 900

-0.645 -0.681 -0.872

530 450 400

-0.569 -0.686 -0.668

600 800 1000

-0.705 -0.787 -0.867

630 830 510

-0.688 -0.700 -0.680

N aeration 2

Al A2 A3

Table VII. Polarisation Resistance and Equilibrium Potential of a Chromate-Free Primer 0 aeration 2

Dilution

O.lMNaCl

0.05M NaCl

Al A2 A3

Polarisation resistance [kO]




5.8 100 400

-0.526 -0.507 -0.563

6.5 7.3 100

-0.529 -0.544 -0.532

1200 1300 1400

-0.748 -0.789 -0.712

590 900 670

-0.605 -0.633 -0.714

N aeration 2

Al A2 A3

Filiform Corrosion Test Results One of the standard qualification test procedure run on the primers evaluated in this study was filiform corrosion. The results on CSA pretreated Al-specimen are given in Figure 8 as they reflect the tendencies of all other corrosion test results run during these qualification test attempts. They also corroborate the other reported investigation results.


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Table VIII. Polarisation Resistance and Equilibrium Potential of Reference Electrolyte O.lMNaCl

0.05M NaCl


Polarisation resi stance [kQ]

Equilibrium po tential [volt]

-0.540 / -0.540 4.3 / 15.2 0 aeration / N aeration 2



4.2 / 90

-0.554 / -0.560

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Table IX. A.I.E. Test Results (Summary) Primer

0.05M NaCl

O.lMNaCl

0.5MNaCl

+ A + + 0 Ε F H +/0/- = good/low/very poor corrosion inhibition

+ 0

Category]

L o g [ C h l o r i d e / Inhibitor] H i Primer F

H U Primer H

I I

Primer 1

Fig. 5. A.I.E. test results with 0.05M NaCl.


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[Category]

Fig. 6. A. I. Ε. test results with 0.1 M NaCl.

[Category]

Fig. 7. A.I.E. test results with 0.5M NaCl.


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Table X. Corrosion Categories Categories 1 2 3 4 5 6

= = = = = =

complete area corroded, covered with tightly adhering corrosion products complete area corroded, nearly no tightly adhering corrosion products < 50 % of area corroded < 20 % of area corroded corrosion visible by eye no visible corrosion


Filament Length [mm]

A

F H

AI 2024 (ΑΙ-clad)

I

Ε

H I Al 2024 (bare)

Fig. 8. Filiform corrosion results on CSA pretreated Al-specimen.

Conclusion Chromated and chromate-free primers exhibit a completely different behavior. A significant influence of chloride / inhibitor ratio on the effectivness of the corrosion tests could not be proven. The A.I.E. tests and the electrochemical mea surements show the same trends and also corroborate the results of filiform corrosion testing. The extraction method, together with the described elec trochemical measurements, demonstrated the capability of a good screening test for corrosion inhibiting primers. These methods may be used prior to qualification testing to preclude running costly corrosion tests on poor corrosion inhibiting primer systems and to choose inhibitor systems for further primer development.

Literature Cited [1] Mc Lafferty, E . , "A Competitive adsorption Model for the Inhibition of Crevice Corrosion and Pitting", J. Electrochem. Soc., Vol. 137. No. 12, 3731 (1990)


Inhibition Properties of Some Aircraft Corrosion Protective Coatings

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