Organic metals combat corrosion

Dr. Bernhard Weßling

The original intention was to develop conductive polymers for use in lightweight ordinary and rechargeable batteries with a high storage capacity – an idea that was doomed to failure in practice for various fundamental reasons. The engineers nevertheless succeeded in synthesising a large number of polymers which – once they had been converted into plastics – were conductive on account of their internal structure (intrinsic), rather than as a result of additives such as graphite or metal powder. All the compounds that were evolved shared three crucial drawbacks, however: they were insoluble, infusible and more or less instable.

The insolubility and infusibility of this family is closely linked to its conductivity, and hence it is impossible to produce soluble, conductive polymers. Dispersion is an alternative method of making these substances processable.

The organic metal polyphenylene amine (OM) is an organic polymer consisting of the elements C, H, N, O and S. It is a metal nonetheless, i.e. it has free electrons in a metallic conduction band. Like many other similar compounds, polyphenylene amine is absolutely infusible and insoluble, but can be made processable very effectively by means of dispersion. The saline compound is largely temperature-stable. The maximum temperature in continuous use is 80 °C, with peaks of up to 230 °C a possibility. Polyphenylene amine can exist in three different oxidation states, one of which resembles metal. Two out of the three oxidation states are stable under normal ambient conditions. Oxidation and reduction have no effect on the form of poylphenylene amine. It takes on different colours, however, depending on the oxidation state, in other words it is electrochromic. The stable metallic form is green, while the stable oxidised form is blue and the reduced form that can be oxidised back in air is colourless. In thin layers, metallic polyphenylene amine is transparent with a light green hue.

OM coatings on metals have an anti-corro-sive effect. The organic metal acts like a precious metal with a redox potential approaching that of silver. In addition to refining the protected metal, the polyphenylene amine transforms its surface into a tight, very thin, metal oxide layer. In combination with iron and steel, Fe2O3 is produced in a complex reaction sequence.

The corrosion potential of steel can be shifted by up to 800 mV by coating it with organic precious metal, and galvanised steels too can be protected extremely effectively. The rate of corrosion after refinement is usually only a fraction of that before it. The passive Fe2O3 layer counteracts rusting with an additional chemical and physical barrier. Even copper is passivated by the organic metal according to this same principle. Unlike conventional, purely defensive barrier coatings, the polyphenylene amine-based alternatives (trade name: Corrpassiv(r)) thus offer active protection.

The organic metal is only able to act directly on the metal surface. For this reason it is contained in all Corrpassiv primers that are applied to bare metal surfaces in a 20 µm thick coating. In order to guarantee permanently high efficiency, OM primers at present still have to be sealed with a top coat. The choice of coating is dependent on the specific requirements and intended application of the protected metals. 2-K epoxy resin and 2-K polyurethane systems are ideal under extreme corrosion conditions.

Mutual compatibility of the various paints in a coating system is crucial for optimum corro-sion protection. Above all, excellent adhesion between the different products is essential. Although OM primers are themselves universally suitable, there are certain other types of coating with which they cannot be combined.

The prospects for success of this coating technology are outstanding for almost every kind of corrosion protection task. In practice, therefore, all corrosion problems that occur as a result of corrosive stresses can be overcome with OM. The layer structure of these systems has been specially designed for use in environments where heavy loads prevail, such as steel or hydraulic structures, dockyards, refineries, bridges, pipelines and vessels.

Organic metal-based coatings have also proved expedient in waste water engineering. A medium-sized company has developed innovative, hydraulic weir and filter systems as part of a cost-effective solution for modernising sewage systems. The OM coating has enabled stainless steel components to be replaced by ordinary steel. Internal laboratory tests and external field tests conducted over a period of more than two years have shown that the required properties can be achieved by applying an OM primer and a wear-resistant, 2-K synthetic resin top coat, specified by the manufacturer. Not only does the corrosion behaviour of the parts treated in this way compare in every respect with that of stainless steel parts – OM-coated parts are also significantly cheaper.

One of the consequences of the aggressive atmosphere in the ammonia and urea production line of a north-German chemical firm was severe corrosion damage to the steel girders of pipe bridges. Maintenance, entailing the removal of the rust and the application of an anti-corrosion coating system, was necessary roughly every five years. The Corrpassiv 4903 coating system was used for one of the pipe bridges with the aim of extending these intervals (Fig. 2). The surface was prepared by sand-blasting – a condition laid down by the company. As a further instance proves, however, prior sand-blasting is not absolutely essential. There, the loose rust was simply removed by mechanical means and a surface-tolerant version of the coating system applied.

As a result of the maritime climate at the port of Brunsbüttel on the River Elbe, corrosion phenomena were discovered time and time again in the vicinity of the weld seams of refinery pipelines used for crude oil. In order to protect their surfaces against mineral oils and seawater, the pipelines were examined at regular intervals. Any points found to have suffered damage were sand-blasted within the framework of revamping measures, and then repaired with a coating system consisting of an anti-corrosive primer, an intermediate coat and a top coat. In an attempt to prevent the occurrence of rust spots on a more permanent basis, a Corrpassiv system was then introduced for the pipeline repair work.

Ready-coated pipes, which are bent according to requirements on the actual building site, frequently exhibit micro-cracks follow-ing the bending process. This applies particularly to zinc-dust primers, which some-times flake over a large area. The Corrpassiv primers and top coats, on the other hand, are completely insensitive to all such deforma-tions. The corrosion that is otherwise observed after only a short time at the elongation and compression points is eradicated.

A chemical plant engineering firm decided recently to replace the stainless steel pipes used in sewage treatment plants, which in this case are laid in the system in direct contact with the corrosive medium, with coated ordinary steel pipes.

Preliminary tests and an application at DASA in Finkenwerder, near Hamburg, have shown that this is absolutely unproblematic.

Ormecon

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Further information cpp-259