By Carlos Molina (Neiva, Colombia)
The ISO 12944 Standard, “Paints and varnishes — Corrosion protection of steel structures by protective paint systems”, is quickly becoming the gold standard of corrosion control on a global scale, and swiftly replacing regional standards of corrosion control. It is useful for owners of assets such as pipes and tanks who have a maintenance schedule.
ISO 12944 grants you…
• … confidence in the specified corrosion control measures taken.
• … the possibility to have a standardized approach for the selection of coating systems.
• … the chance to verify that a coating system proposed is fit for purpose.
ISO 12944 has 8 parts, which are the following:
Part 1: General introduction
Part 2: Classification of environments
Part 3: Design considerations
Part 4: Types of surface and surface preparation
Part 5: Protective paint systems
Part 6: Laboratory performance test methods and associated assessment criteria
Part 7: Execution and supervision of paintwork
Part 8: Development of specifications for new work and maintenance
ISO 12944 enlists several different examples of coating systems according to the corrosivity of the environment and the intended durability. This information can be found in tables A.1 to A.8 in Annex A of part 5. These systems have a proven track record. The standard, however, doesn´t intend to be a comprehensive list of all the systems available.
To understand it, it is necessary to have a certain background in coatings.
How to select a coating system for your facility?
(the steps are taken directly from ISO 12944)
Step 1. Determine the corrosivity category of the environment where the structure will be located most of the time.
ISO 12944 (part 2) lists the corrosivity of the environment according to the following table:
On the other part, for submerged or buried structures, ISO 12944 provides the following classification:
Im1 – fresh water (examples: Instalaciones rivereñas, plantas hidroeléctricas)
Im2 – seawater or brackish water (examples: compuertas, muelles, estructuras offshore, pasos de agua)
Im3 – buried (examples: underground tanks, buried pipes, stilts)
Step 2. Establish if there are any special conditions that result in a higher corrosivity (known as “microclimate” in ISO 12944 part 2).
Step 3. Select the desired durability of the coatings system.
- a) low (L): 2 to 5 years;
- b) medium (M): 5 to 15 years;
- c) high (H): more than 15 years.
Step 4. Look in Annex A for the relevant table. Tables A.2 to A.5 give proposals for different generic types of paint system for corrosivity categories C2 to C5, while Table A.1 gives an overview of the contents of Tables A.2 to A.5.
Step 5. Identify in the table the coating system with the required durability.
Step 6. Seleccionar el sistema óptimo, tomando en consideración el método de preparación de superficie.
Step 7. According to ISO12944, “Consult the paint manufacturer in order to confirm the choice and to determine what commercially available paint system(s) correspond to the paint system selected”
Each manufacturer produces one or more coating systems compliant with ISO 12944. That means that every manufacturer has several brands of primer, intermediate and finish paints.
Some manufacturers use the same system for similar applications so many times that these systems have started to have a name of their own. For example, almost all of the manufacturers have a “tanklining” system.
In practice, some systems have much more than the specified 15 years of durability. ISO 12944 states: “In general terms, increasing the total dry film thickness and the number of coats will extend the durability of a paint system. In addition, the choice of a system designed for a corrosivity category “higher” than the one envisaged will provide higher durability when such a system is used in a lower-corrosivity environment.”
Let´s verify the coating systems proposed by Hempel, PPG Industries and International Paint, for a steel structure situated next to the sea.
Corrosivity category: C5M
Let´s check against ISO 12944
Let´s look in table A5 of ISO 12944 for the C5M category (have in mind that this table is for Low-alloy carbon steel prepared to Sa 2½, from rust grade A, B or C). The proposed systems are equivalent to system A5M.05 included in the ISO standard.