Forschungsinstitut für Pigmente und Lacke, Stuttgart, FPL

Assignment: Supplementary investigation of protection afforded by selected coating systems based on a polyaniline primer against filiform corrosion of aluminium alloys

Client: Zipperling Kessler & Co(GmbH & Co).

Authors: Dr. T. Schauer, Dipl. Chem. A. Joos, E. Praschak

Stuttgart, 24.07.1996

1. Questions to be investigated and object of study

Filiform corrosion of aluminium alloys is a hitherto unsolved problem that causes considerable damage, for example in aluminium facades of buildings.

The preceding study [1] showed that efficient protection against filiform corrosion of aluminium alloys can be achieved with the polyaniline primer. It was found, however, that the efficiency of this protection may be influenced by the individual top coat used. Possible factors of importance here are both inter-layer adhesion and the properties of the top coat itself. The measurements performed are based on the dry and wet adhesion tests and the filiform HCl test (after DIN 65 472).

To supplement this study, other top coats were subjected to comparative investigation and also exposed to an outdoor weathering test.

2. Experimental Details

2.1 Substrate

The tests were made on AlMg1 alloy panels (200x100x1.5 mm) which were blasted with īElektrokorund NK1 No. 100” (Messrs. Würth) and then degreased in a mixture of acetone, ethylacetate and xylene (1:1:1). The surface roughness was Ra= 2.3 µm.

2.2 Coatings

Polyaniline primer 900226/19 was applied by pneumatic spray. Layer thickness after drying was 17.7 ± 2.5 µm. The top coat was sprayed on after the primer had dried (24 h). Formulations were as such, with thinner added to improve sprayability.

  1. Two-component epoxy top coat, amine hardener - referred to below as 'top coat 1'
    top coat 100,0 parts by wt.
    hardener 17,0 parts by wt.
    thinner 12,5 parts by wt.

    2. The total layer thickness averaged 200,7 ± 23,3 µm.

  2. Two-component epoxy top coat, amide hardener - referred to below as 'top coat 2'
    Component A (paint) 88 parts by wt.
    Component B (hardener) 12 parts by wt.
    Thinner 60 parts by wt.

    The total layer thickness averaged 145,6 ± 23,3 µm.

  3. Two-component polyurethane top coat - referred to below as 'top coat 3'
    Base paint 100 parts by wt.
    Hardener 25 parts by wt.
    Thinner 12,5 parts by wt.

    The total layer thickness averaged 146,2 ± 12,5 µm.

2.3 Conditioning

To ensure complete drying and curing, the coated panels were placed in a climate chamber at a constant temperature of 23°C and a relative humidity of 55%. The state of the coatings was assessed by measuring pendulum hardness using the König method (DIN 53 157, 1/87).

2.4 Measurement of wet and dry adhesion

To measure dry adhesion, stamps (diam. 7mm) were fixed to the coatings with a two-component epoxy adhesive and subsequently removed with the aid of a puller (Messrs. Instron). The pull-off forces were registered with a measuring sensor and averaged from five independent readings.

To measure wet adhesion the test pieces were stored in distilled water. At certain times the test pieces were removed and measured in the same way as for dry adhesion. Since the adhesion of the top coat falls off with storage in water, it was possible for this measurement to fix the stamps to the coating with a commercially available superglue.

2.5 Measurement of filiform corrosion

Filiform corrosion was investigated using the HCl method after DIN 65 472 [2]. The coated panels, three of them for each coating, were scratched with a Sikkens scratching tool and inoculated with hydrochloric acid fumes for one hour. After removal from the chamber the panels were left to stand in room air for a further hour. The panels were then stored for 6 weeks in a climate chamber at 40°C and 82% relative humidity. Assessment took into account the picture analysis and the ratings described in [2]. There are five ratings, which indicate a specific ratio of the surface affected by filiform corrosion to the length of the scratch. Rating I represents the most efficient corrosion control (surface value 0.0 0.5 mm2/cm) and rating V the poorest corrosion control (surface value > 25 mm2/cm).

2.6 Outdoor weathering

Two panels of each top coat system were exposed to the local climate and other environmental influences at Hook of Holland on 03.05.96. The coatings were scratched, the back of the panels covered with an adhesive film, and the edges also treated with a protective varnish.

The outdoor weathering test is scheduled to last at least two years. The first inspection of the panels will be in October 1996.

3. Measurement results

3.1 König pendulum hardness

The results of the König pendulum hardness measurements are shown in Fig. 1.


Fig. 1 König pendulum hardness for the coatings investigated

It is immediately evident that the pendulum hardness of the 'top coat 1' coating is much greater than that of the other two coatings. One the other hand the 'top coat 1' coating is relatively slow to attain its final hardness and does not reach a relatively stable figure until after about 30 days; the 'top coat 1' coating uses an amine hardener. With the other coatings the greatest change in hardness took place within the first 12 days after application.

The pendulum hardness figures recorded agree well with those found in the earlier studies [2].

3.2 Dry and wet adhesion

The results of the dry and wet adhesion measurements are summarised in Fig. 2.


Abb. 2 Fig. 2 Dry and wet adhesion of the coatings investigated

The dry adhesion values at time 0 indicate good adhesion of all three coating systems investigated, the highest figure being achieved by 'top coat 2'.

On exposure to water the adhesion of all coating systems falls off. Here the 'top coat 3' top coat displays the biggest drop in adhesion: after 320 h of storage in water the pull-off strength measured was close to total delamination (2 to 3 MPa). A subsequent increase in adhesion is connected with regeneration processes.

The 'top coat 1' coating was generally found to have the most stable wet adhesion.






3.3 Filiform corrosion

The results of the filiform corrosion test using the HCl method are summarised in Table 1.

Table 1

Results of the filiform test using the HCl method

Coating system

Rating

'top coat 1'

I

'top coat 2'

III

'top coat 3'

IV

The measurement results were evaluated by photographing the sites affected by filiform corrosion and analysing the pictures. Fig. 3, for example, shows the appearance of the panel most strongly affected by filiform corrosion.

'top coat 1' 'top coat 2' 'top coat 3'



Fig. 3 Graphic reproduction of panel surfaces after the HCl test

It is clear that the 'top coat 1' coating possesses very good protective properties against filiform corrosion. Of the other coatings, 'top coat 2' displays markedly better protection efficiency.

4. Discussion

Experience shows that efficient protection against filiform corrosion depends on properties of the entire coating system. To date it has yet to be established which parameters of this system are of decisive importance. The parameters considered, however, include adhesion, porosity and anticorrosive properties of the primer, inter-layer adhesion, and the porosity and barrier properties of the top coat.

In an earlier study, efficient protection against filiform corrosion was found to exist with a polyaniline primer and an epoxy top coat. The present study was intended to clarify whether the same corrosion protection effects could also be achieved with other top coats and what parameters of the coatings were significant here.

The top coats tested were two epoxy coatings with different hardeners: 'top coat 1', with amine hardener, and 'top coat 2', with amide hardener, and one polyurethane coating: 'top coat 3'.

The pendulum hardness tests revealed that the 'top coat 1' top coat has a much greater hardness than the other coatings. As a rule, increased hardness is an indication of good cross-linking of the binder and good barrier properties and low permeability of the coating.

The other tests, namely the adhesion measurements and the HCl test, show a good correlation between efficient protection against filiform corrosion (top ranking) and relatively stable wet adhesion for the 'top coat 1' top coat. Both 'top coat 2' and 'top coat 3' displayed poorer wet adhesion, and these top coats were found to provide markedly inferior protection against filiform corrosion.

5. Conclusions

  1. Very good protection against filiform corrosion was found to be provided by a polyaniline primer and the amine-hardened 'top coat 1' top coat.
  2. Good protective properties of this coating system go hand in hand with the high pendulum hardness and good wet adhesion.
  3. Another epoxy top coat, the amide-hardened 'top coat 2', was found to have a much lower pendulum hardness and poorer wet adhesion and to provide inferior protection against corrosion.
  4. A polyurethane top coat, 'top coat 3', was also found to provide inferior protection against filiform corrosion. This correlates with low pendulum hardness values and poorer wet adhesion for this coating.
  5. The present study confirmed the findings of the previous studies with regard to good correlation between efficient protection against filiform corrosion of aluminium alloys and a high hardness and good wet adhesion of the polyaniline primer/'top coat 1' coating system.
  6. Further verification of these findings will be sought in the outdoor weathering tests.

6. Literature

  1. T. Schauer, A. Joos, E. Praschak, Überprüfung der Schutzeigenschaften von zwei Lacksystemen auf der Basis einer Polyanilingrundierung gegenüber der Filiformkorrosion von Al-Legierungen, Stuttgart 1995
  2. K. Gaszner, M. Heinrich, T. Schuler, Aluminium 71(1995)562

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