ABSTRACT

For many years, it has been state of the art to use zinc dust in heavy duty corrosion protection according to SSPC Paint 20. The recent revision to SSPC Paint 29 allows the use of zinc in a flake-shaped form in heavy duty corrosion protective systems provided the performance meets the specification requirements.

Since zinc flakes have a significantly higher surface area compared to zinc dust, formulations using zinc flakes need to be different from previously used zinc rich primers based on dust to achieve adequate performance.

The presentation outlines fundamental formulation principles using zinc flakes in corrosion protective primers and explains where new options for formulating can lead to improved paint characteristics (storage stability, sprayability and the like) while meeting Paint 29 requirements.

In addition comparative Open Circuit Potential OCP data is presented and a final look onto commercial aspects is made.

INTRODUCTION

For many years, sacrificial corrosion protective paint formulations based on Zinc consisted of zinc dust at approximate 80% loading and about 10% epoxy binder. This composition is necessary for corrosion protection, as direct zinc to zinc dust contact is only possible at high zinc dust loadings. This goes back to the spherical shape of the zinc dust particles with their low surface area.

Following this concept heavy duty corrosion protection on railway bridges, offshore constructions etc. was very successful. The historical specifications for this application have been SSPC Paint 20 in North America and ISO 12944 in Europe.

For thin films - with dry film thicknesses around 15 pm and below - to be applied e.g. on screws, bolts or fittings zinc flakes with greatly reduced particle thicknesses compared to zinc dust have been in use for many years. The thin lamellar zinc pigments align more or less flat and parallel to the substrate surface, and thus low dry film thicknesses are possible to generate.

In 2018, an up-date / revision of ISO 12944 was approved taking the latest trends in corrosion protective technology into account. The revision included a review of the corrosivity categories (with e.g. - introduction of corrosion category C5), the introduction of a cycling ageing test, and the implementation of a modified design of the scratch into the test regime.

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