Introduction

Protective coatings are the most widespread solution for controlling steel (mild and carbon) corrosion, due to their low cost and high versatility. Anti-corrosion primer is generally applied immediately after steel is manufactured in order to prevent rusting during storage and transportation, and before further processing (cutting, welding, drilling, etc).
To avoid removal of the primer before further processing, weldable or weld-through primers were introduced in the 1990’s. Most conventional primers last a maximum of 6 months, due to their formulation or to subsequent mechanical damage, leading to corrosion.
Anti-corrosion primers are often too thick (~50μm) for welding, giving rise to poor weld quality or the need for time consuming localised grinding to remove the primer. Where primers are thin enough (~25μm) to allow welding, their anti-corrosion properties are limited that it does not reduce the corrosion. Above all, weldable primer formulations contain high zinc content that results in health hazards during fabrication, such as zinc fever and discomfort due to highly unpleasant odours.
An alternative is to not use primers at all, but this needs expensive and hazardous blast cleaning to be carried out before fabrication, which may sometimes be too late to prevent irreparable pitting damage, as well as incurring additional material waste and cost.

Baseline for the WeldaPrime project:

The vast majority of primers currently used on steel have far poor adhesion, either by simple electrostatic attraction or more frequently simple mechanical attachment based on a conformal interface between the coating and the substrate. Since most corrosion of coated steel structures occurs at voids or cracks within the coating, improving adhesion is key to the success of the WELDAPRIME project. The exposed steel can then corrode and the corrosion products then undercut. The coating then blisters and is readily detached, exposing more of the steel surface and allowing further corrosion to occur.
The WELDAPRIME project will focus on the development of thin coatings which minimise the amount of material on the surface of the steel structure and so reduce the impact of the coating on the welding characteristics of the protected substrate. These coatings will be developed to have excellent adhesion to the substrate due to the development of formal covalent chemical bonds to the surface of the steel. Formal covalent bonding between the coating and the steel substrate however, does not allow the undercut mechanism to occur and so the progression of the corrosion will be limited to areas where the steel is exposed due to the removal of the coating. This superior corrosion protection therefore means that the WELDAPRIME project will be able to deposit thinner coatings, which will reduce the impact of the coating on the welding conditions.





Principle of corrosion

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