Preparing Steel Surfaces for Corrosion Protection Coating
In our previous article in this series discussing ISO 12944, the standard providing guidance for the corrosion protection of steel structures by protective paint systems, we looked at what the standard says about steel structure design and corrosion. In this article, we examine the main points of Part 4 of ISO 12944 which lays out instructions covering surface preparation for different types of surface.
What Is Surface Preparation?
Surface preparation is the first-stage treatment of a steel substrate which must be undertaken before applying a protective coating. Correct surface preparation is essential to the success of the corrosion protection coating being applied.
The objectives of surface preparation are to:
- Remove deleterious matter
- Produce a surface that enables paint to adhere to the steel
- Minimise the contaminants that initiate corrosion
Mill scale must be removed from new hot rolled steel (in accordance with ISO 8501-1) in order to produce a surface profile that is appropriate for the coating system to be applied (which will be defined in the painting specification).
When surface preparation is carried out, health and safety regulations must be observed, and the surfaces that are to be prepared must be accessible and adequately illuminated. Burrs, sharp edges, and weld spatter should normally be removed around weld details and steel edges; this is generally carried out during the fabrication stage of new steel structures. More details on preparation grades can be found in ISO 8501-3, which define acceptance criteria based on a given specification requirement.
Factors that affect surface preparation
There are many factors that affect the surface preparation strategy and method chosen. These include:
- Whether it is an existing or new structure (and age)
- The structure’s location
- The quality of the previous surface prior to preparation
- The performance of the coating system
- The type of corrosive environment
- The coating system that is likely to be used
The preparation grade necessary to provide the required surface cleanliness and the surface profile (roughness) should also be considered.
Types of surfaces covered by ISO 12944-4
The standard covers surfaces of steel structures consisting of carbon or low-alloy steel of the following types:
- Uncoated surfaces, consisting of bare steel (which are assessed in accordance with ISO 8501-1)
- Metal coated surfaces, which include hot-dip-galvanised surfaces, zinc electroplated surfaces, sherardized surfaces, and thermally sprayed surfaces
- Surfaces painted with prefabrication primer (in accordance with EN 10238)
The standard is mainly concerned with the protection of carbon steel new-build structures or extensive maintenance projects to be cleaned back to bare metal.
Surface preparation methods
The method used to prepare the surface for its corrosion protection coating should remove as much oil, grease, dirt, salts, and other contaminants as possible before further surface preparation is performed. Once known contaminants have been removed, testing may be required to detect less visible contaminants such as soluble salts (as per ISO 8502-6).
Any corroded material that exists should be removed by manual or mechanical techniques, though no sound metal should be removed unnecessarily. When preparing surfaces, the two types of surface preparation are:
- Primary – which takes the whole surface to bare steel
- Secondary – which leaves the sound parts of any coatings as they are
Methods that might be used to prepare surfaces for their protective coating include (but are not limited to):
· Water, solvent, and chemical cleaning
A water jet may be used if the contaminants to be removed include water soluble materials and loose rust or paint coatings.
If detergents are used in the cleaning process, then the surface should be rinsed with clean, fresh water after cleaning.
· Steam cleaning
Steam cleaning removes oil and grease, though again the surface should then be rinsed with clean, fresh water if detergent is added to the steam. Further testing may be carried out to determine the presence of oil and grease in accordance with ISO 8502-7.
Other methods to remove oil and grease include emulsion cleaning, alkaline cleaning, and organic solvent cleaning.
· Chemical conversion cleaning
For hot-dip-galvanized surfaces, electroplated-zinc coatings, and sherardized surfaces, cleaning may be carried out by chemical conversion. Such treatments can only be used if the manufacturer of the paint system to be applied approves this type of cleaning.
Solvent-borne pastes or alkaline pastes may be used to strip paint coatings, though this is usually used only on small areas and then followed by other appropriate cleaning.
· Mechanical cleaning
Mechanical cleaning methods include cleaning with power tools, blast cleaning (of various types), and flame cleaning. Blast cleaning may be used to remove contaminants, to smooth or roughen the surface metallic coatings, or to remove a surface layer. ISO 8503-3 details these methods.
What if the surface preparation doesn’t achieve its aim?
The requirements of surface preparation are based on a range of preparation grades that are laid out in ISO 12944-4. If the required preparation grade is not achieved – or if the surface condition has changed before the coating system is applied – then further surface preparation must be carried out.
Who can carry out surface preparation work?
The standard is specific in describing who can carry out surface preparation work, saying that:
‘Personnel carrying out surface preparation work shall have suitable equipment and sufficient technical knowledge of the processes involved to enable them to carry out the work in accordance with the required specification.’
After the surface has been prepared, it must be assessed as fit for the coating system to be used. There are separate standards that regulate testing and how testing must be carried out – ISO 8501 and ISO 8502.
The Institute of Corrosion Coating and Inspection Training Courses – presented by IMechE Argyll Ruane and Corrodere – will help ensure your painters and inspectors work to the latest industry standards and benefit from new technology and innovative approaches. For more information, contact us today.
In our next ISO 12944 article, we look at laboratory performance test methods and the changes that were made to these in the latest revision of the standard.