A new method of corrosion protection via self-assembled monolayers of functionalized conducting polymers

Aims

In this project a new primer for the corrosion protection of carbon or low alloy steels should be developed. This new type of primer is environmentally friendly because of the very low need of chemicals and the possibility to use water based solutions for the SAM formation.

For this purpose for the first time the application of self-assembled monolayers shall be combined with the use of conducting polymers, for example polythiophenes, in corrosion protection. In this way a new type of corrosion protective coating is to be developed and characterized.     

Working plan

The new primer should be developed in four steps:

1. Thiophene derivatives ( alkylphosphonic acids ) will be solved in a water-based solution in order to form self-assembled monolayers ( SAMs ) on the steel surface.

2. The head groups ( thiophene derivatives ) of this SAMs will be electropolymerized anodically, so that a cross-linked monolayer of polythiophene is formed, which is bound to the
    metal surface via alkyl chains.

3.On top of the anodically oxidised polythiophene layer a top-coat should be applied.

4. Finally thus systems with different commercial top coats will be electrochemically characterised. Also some corrosion tests and measurements of the adhesion of the top coats
    will be carried out to evaluate the effectiveness of this primer system in comparison with recent primers.

Results

After synthesis of suitable monomers, the most important steps in the development of the new type of coating were first of all the successful SAM - formation on passivated carbon steel as well as the subsequent electropolymerization of the head groups. Different passivation methods for the steel surface and also several conditions for the subsequent SAM formation with phosphonic acids were already successfully tested. Cyclovoltammetric experiments to electropolymerise bithiophene- and terthiophene-head groups showed clearly the formation of polythiophene chains and indicated that also this step in the development of the primer system should be feasible.

The adhesion of different top coats on the steel surface after SAM formation showed to be in the same range as for top coats on conventional phosphated steel and will be more than satisfactory for all estimated technical applications.

Recent corrosion tests in a climate chamber indicate, that SAM's with different phosphonic acids can increase the corrosion inhibition properties of stable and compact passive layers on steel surfaces.

First tests with electropolymerised terthiophene head groups so far showed no improvement in corrosion inhibition compared to the non-cross-linked SAMs.

Further developments of this primer system by using oligomeric phosphonic acids or alkyl-phosphonic acids with reactive head groups led to a significant simplification of the described process ( to build up this primer system ) and also to substantial improvements for technical applications.

For a more detailed description of this more recent developments you can download our   latest publication ( paper on CD - ROM, which was available at the EUROCORR 2007 conference in Freiburg / Germany    //    Download link see below )  

Macromolecular Symposia 187 (2002) 65-76

Proc. on CD-ROM EUROCORR 2003, Budapest (2003)

Proc. on CD-ROM EUROCORR 2007, Freiburg (2007)