THE PROBLEM
An area of the access gantry steelwork had become heavily corroded through atmospheric and chemical exposure, requiring replacement. The new and sound old steel required a suitable protective coating system.
Access Gantry Protective Coatings Norfolk
THE SOLUTION
We proposed the application of the same protective coating system as used previously by ourselves on the immersed section of the structure, shown in this case study.
Initially all steel surfaces were pressure washed to remove soluble salts and any oil/grease contamination from the fabrication process.
This was followed by preparation to SA2.5 as per ISO8501-1 by method of abrasive blasting. The new steel had a layer of mill scale that must be removed prior to protective coating application. Mill scale is a poorly adhered mineral layer which over time through a differential of movement potential compared to the steel will debond, causing any coating which is adhered to this layer to fail also.
As part of our standard quality assurance the surface roughness/blast profile was tested and recorded using a surface profile needle gauge. Nearly all coating materials prescribe a minimum surface profile which must be achieved in order for the protective coatings to adhere correctly.
All surfaces were vacuumed clean and dust tape tests conducted to ensure that the protective coatings were applied directly to the steel and not to a layer of dust which would prevent proper adhesion.
Because of the steelworks external location and without extensive and costly encapsulation it was not possible to maintain the blast standard by dehumidification. This however was not a problem for the prescribed coating system as Corroless EPF contains a mined pigment that stabilises any corrosion present to stable iron oxides such as magnetite. Corroless EPF can be applied to steel prepared to standards as low as ST2 as per ISO8501-1. This meant that flash rusting post blasting was not an issue.
Prior to any coating application commencing and during climatic conditions were tested and recorded as per our standard quality assurance procedures.
A stripe coat of Corroless EPF was applied to all angles and edges in a buff colour. Stripe coating ensures full coating thickness in areas where coatings pull thin through gravity and surface tension.
A full coat was then applied in black to aid application as per best coating practice. A second stripe coat was then applied in the buff colour, followed by a full coat of buff. The target thickness for each coat was 200 microns, checked by the taking of wet film thickness readings as per protective coating best practice.
As well as an anti corrosive pigment Corroless EPF contains self leafing glass flake which laminate within the coating to provide a more tortuous route for oxygen and moisture to reach the steel substate. All carbon based coatings such as epoxies, polyurethanes, alkyds etc are semi permeable membranes. Each coatings rate of permeability determines its corrosion resistance and the inclusion of glass flake reduces this rate of permeability, thus increasing corrosion protection.
Corroless RF35 glass flake reinforced epoxy was then applied as a stripe and full coat. What is unusual about this specification is that all three coats are glass flake reinforced for maximum corrosion protection.
Dry film thickness readings were taken throughout, with a total system thickness in excess of 600 microns being targeted which was comfortably achieved.
The multi layer specification allows three stages to measure and rectify any areas below specification, simplified by the first two layers consisting of the same material.