Keywords: Poly-aniline, Powder epoxy coatings, Strontium chromate, Corrosion prevention, Cathodic disbonding, Accelerated testing
All materials including epoxy resin, hardener, flowing agent and pigment (different states of polyaniline, strontium chromate) were premixed in 2%w/w concentration, 5 min 1800 rpm, in order to prepare for the next stage of extrusion which is a homogenous blend of ingredients.
Mixed materials were extruded so that the pigment blends and disperses more efficiently. The extruded polymer was chipped using a Cumberland Granulater and granulated and ground to a fine powder by a mini-kek pin disk mill.
The final epoxy powder paint was sieved obtain small particles less than 75 microns in size.
The powder coatings were applied to a thickness of 22± 2 microns using an electrostatic spray gun. The curing schedule used for all powder coated panels was 15 minutes at 180° C.
Types and formulation of the powder epoxy paint are as follows:
|Epikote 3003||855 g||835.9g||835.9g||835.9g||835.9g|
|0% Protonated PANI||20g|
|42% Protonated PANI||20g|
|50% Protonated PANI||20g|
Table 1 is the corrosion performance of the scratched samples after a 1000h continuous hot salt spray test
|Sample||Corrosion at edge||Corrosion at scribe||Size/extent of blistering||Isolated black spots|
|Sample||Corrosion at edge||Size/extent of blistering||General corrosion|
|Sample||Corrosion at edge||Corrosion at scribe||Isolated black spots|
Examination of the salt spray data produces the following interesting observations.To achieve a similar degree of degradation, the test period for the cyclic test had to be twice as long as the hot salt spray test. Even so, the degree of discrimination is somewhat less in the cyclic test. The two protonated PANI samples behave significantly worse than the rest of the samples including the blank in both the cyclic and continuous tests. Behaviour of the exposed steel adjacent to the coating is an important feature of an anticorrosion system, i.e. at an edge or a scribe. With the systems tested, the 0% protonated PANI provides a significant improvement compared with the blank and the two protonated forms and in certain situations is comparable or even better than the conventional strontium chromate pigment. However, the cathodic disbonding experiments provide completely contrary data. The 0% protonated PANI is now the system with the highest rate of cathodic disbonding; i.e. the lowest performing system. It is necessary explain why under free corrosion this system is good and at a relatively negative potential this system is the worst. It is likely that the poor performance is associated with the oxygen reduction reaction which is accelerated at negative potentials. One suggestion is that the 0% protonated PANI is in contact with the electroactive steel surface and it catalyses the peroxy intermediates that are a feature of the oxygen reduction reaction. A second suggestion is that this particular PANI is unstable at the alkali pHs generated by the cathodic reduction reaction. A third suggestion is that this PANI accelerates the oxygen reduction reaction in general. At this stage it is not possible to be more definitive about which mechanism is most likely.
The coating containing 0% protonated PANI showed the good protective properties against a corrosive environment including a conventional hot salt spray test and a more unconventional wet dry cyclic test Performance was assessed visually and seemed as good as a more traditional anti-corrosion pigment containing strontium chromate.
In a cathodic disbonding test, the 0% protonated PANI came out worst. Suggestions are made to explain why this might be so.
2. M.M. Attar, J.D. Scantlebury, J.Marsh, Proceeding of the symposium on Advances in Corrosion Protection by Organic Coating I I I , Proceeding, 97-41, 1 (1998).
3. S.B. Lyon, J.B. Johnson, J.D. Scantlebury, Realism in cyclic cabinet corrosion tests - the use of artificial acid rain solution and objective materials assessment", in "Cyclic cabinet corrosion testing", ASTM STP 1238, ed. G.S. Haynes (Proceedings, ASTM Fall Meeting, Fort Worth, TX, USA, 15-16 November 1993), American Society for Testing and Materials, Philadelphia, PA, USA, 3-17 (1995).
4. A.G.MacDiarmid, J.C.Chiang, A.F.Richter, N.L.D.Somasiri, Conducting polymers, Alcacer, L., Ed., Reidel Publications Co: Holland, 105 (1987).
5. Y. Cao, A. Andreatta, A.J. Heeger, P. Smith, Polymer, 30, 2305, (1989)
6. A.T. Evans, J.D.Scantlebury, L.M.Callow, These Cambridge Conferences, (1994).
7. E.L. Koehler, Localized Corrosion, eds Staehle, R. et al. NACE (1974).
8. D.J. Mills, PhD Dissertation, University of Cambridge (1973).
9. J. Marsh, Private Communication (1999).
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