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The framework that is presented here was developed by Graham Gedge in response to the Corrosion Doctors request to the UMIST Corrosion Discussion Group for help in designing interesting and significant problems on the topic of protective coatings.
It should be possible to take each component of a paint specification and assess it in comparison to other specifications by providing a ranking system. Such a system will not give an answer to a specific question or the suitability of a particular specification for a particular application. However, as an educational tool to demonstrate the important parameters in a paint specification it could be useful. The following is a brief outline of such a ranking system; it is based on only a few resin and pigment types but shows how the ranking system could work. As an educational tool the tables below could be developed into something more interactive such as a spreadsheet, database or web based application and this could also include guidance on the use of various specifications. In such a form it would also be easy to add more resins, pigment etc.
The tables follow a logical progression dealing with each coat in a specification (primer, barrier(s) and finishes). For each coat an assessment is made on the following key parameters:
Resin Type
Pigment Type (although pigment content is not yet included)
Thickness (in the tables thickness is given in microns as the Minimum Dry Film Thickness)
Each variable within each coat is assigned a value and the sum of these values for individual coats and the whole specification gives an overall ranking of the coat or specification. The tables have been compiled such that the higher the final total value the "better" that specification is for a given service condition. But note it is only a ranking system. It is possible that one could compare a primer only specification with a high build Glass Flake Epoxy (GFE) and conclude that the GFE is better. This may well be the case but GFE would be inappropriate for use in an internal environment whereas a primer only scheme would be appropriate. Similarly, a primer only scheme would be totally inappropriate for long-term immersion service in seawater.
In the current form the ranking system is only really appropriate for conventional multi-coat specifications given in things like the SSPC manual and ISO 12944 Part 5. The current form it does not really deal with modern specifications based on high film thickness specifications such elastomeric urethanes nor does it really do justice to things like polysiloxane coatings. However, the system could be developed to include these things.
As it stands, the framework is intended to assess and rank specifications that are commonly used to protect structural steel against corrosion in applications such as building, bridges etc. Other Tables could be developed using a similar ranking procedure to cover other types of coatings or substrates.
The ranking system is also based on conventional and high solids solvent borne coatings and does not include water borne coatings. The ranking system for materials such as water borne epoxies may be different. Similarly, the typical specifications do not take account of water borne coatings and it is possible that to achieve the same performance with a water borne system additional coats would be required to achieve the same total film thickness.
The tables are really a matrix, for example under primers one could have an Epoxy Zinc Phosphate Primer with a thickness of >100 microns.
These tables should not be used to say that the Epoxy based specification is 2 times better than the Alkyd based specification!!!
As a teaching tool for students this system would allow uses to get a feel for the important parameters in paint specifications by changing individual variables and seeing the effect that this has on overall rankings. This would help bring understanding to what is important in coating specifications in the real world of engineering. No doubt someone would come up with a wonderfully bizarre and durable specification that could not be produced!!!
Final comment, all the tables assume that steel has been blast cleaned to Sa 2½ of ISO 8501-1. This document was prepared on 10 November 2000 by Graham Gedge
