Combating pipeline corrosion (part 2)
The surface preparation of steel before external coating application is the foundation of any corrosion control programme. Any compromise made in the degree of surface preparation will usually compromise the field joint coating performance, regardless of the technical quality of the field joint coating. Surface preparation is defined as the cleaning and/or pre-treatment of the metal and the adjacent coating surface to ensure the best possible bond between the surface to be coated and the coating to be applied explains Dr JF Doddema, CEO of the Monti-Group. (the first part of the article can be read in the latest issue of PCE-International
During the coating application (installation and curing, if needed), temperatures should be within limits as stated by the coating manufacturer.
Relative humidity can be of interest to the coating application process within eight hours after surface preparation to avoid flash rust and should not exceed the minimum and maximum humidity as specified in the PDS. In cases when the relative humidity exceeds permissible limits, habitats/tents or other containments should be set to allow air treatment inside. When using cleaning materials such as water jetting or wet abrasive blasting, the cleaned surface will rust very rapidly. It is therefore essential that a rust inhibitor can be applied to the surface itself immediately after cleaning, or mixed in the water being used during the actual cleaning process.
Adhesion mechanisms can basically be divided into mechanical interaction, thermodynamic mechanisms, Van der Waal’s forces and chemical bonding. Among these different adhesion mechanisms, a significant contribution is due to mechanical -adhesion. Mechanical adhesion on a loose abrasive-blasted substrate, or a bristle-blasted substrate, relies on the curing (hardening) of the coating inside the surface profile and asperities of the substrate surface and physical anchorage resulting therefrom. Mechanical bond may be assisted by contact friction between substrate and coating in areas where the actual adhesion is inadequate. It is important to note that mechanical adhesion in tension differs significantly from mechanical adhesion in shear. For example, a high interface roughness may improve shear bond strength, whereas tensile mechanical bond strength primarily depends on vertical anchorage in the surface profile.
Surface energy is more important for bonding. Areas on a loose abrasive and/or bristle-blasted surface are not flat but three-dimensional, and roughness of a three-dimensional surface cannot be accurately characterised by using a single roughness parameter in linear length. Parameters that characterise surface profiles Ra, Ry, and Rz, or peak count are two-dimension parameters. Although they are widely utilised in different applications, they are not really able to provide the full information on three-dimensional surfaces. Most importantly, these linear length parameters do not form a linear relationship with surface area or surface energy by themselves. There are 3D roughness profile testers on the market.
Damage and cleanliness
During handling, turning or laying damage to the field joint welds, edges and to the surface by the use of sharp-toothed clamps should be avoided by taking precautionary measures. Touch-up and repairs can easily be handled by the Monti Bristle Blasting method and subsequent coating systems.
Just as surface cleanliness before the first coating layer is fundamental to performance of the complete system, so is cleanliness as important to the coated surface prior to the application of subsequent coating layers within a system.
Cleaning methods without anchor profile generation
Laser and oscillation cleaning methods can be used to remove rust, dust, and contaminants. However, these methods do not create a roughness profile.
Pure strength cleaner/degreaser removes dirt, grease, oil, adhesives, road tar. Agitate for one minute before use. Apply directly on the surface, rub with a brush or cloth, and rinse thoroughly with clean water or wipe with a damp cloth. For large jobs dilute cleaner/degreaser with water. All surfaces should be dry before coating is applied.
Steam cleaning is recommended for removing grease, oil, salt, acid, alkali, and similar chemical residue from large areas. For maximum effectiveness, steam cleaning should be used in combination with alkaline cleaning. The surface should be thoroughly dry and free of residue before coating.
Alkaline cleaning is used for removal of dust, dirt, wax, grease, oil, fat, salt, acid residue, etc. Scrub the surface with a strong commercial detergent solution such as trisodium phosphate (TSP) and then flush thoroughly with fresh water. The surface should be completely dry and free of any residue before it is coated.
Volatile solvent cleaning can be used to remove grease and oil. Make certain the area is well ventilated and apply the solvent to the surface with cloths, sponges, or brushes and scrub to remove grease and oil. Several successive wipings are usually necessary, using clean cloths and solvent each time. For optimum results follow with alkaline cleaning.
Surface preparation of the substrate
Prior to the coating application, the surface should be dry and free of any contamination (such as detritus, dust, non-adhering particles, grease, oil, soluble salts) detrimental to surface preparation or to adhesion of the coating on the steel.
Oil, grease and wax should be removed by solvent cleaning in accordance with SSPC-SP1.
All visible surface imperfections of substrate caused by the girth welding operation, such as welding slag and spatter, sharp edges or burrs that could damage the coating, detected before or during surface preparation, should be removed by an approved grinding method or filing techniques according to the following grades.
Surface preparation of welds
The surface preparation grades of welds, cut edges, and other areas on steel with imperfections are described in P3 of standard ISO 8501-3 for components and fittings to be used in submerged conditions (permanent or intermittent).
P2 of standard ISO 8501-3 for components and fittings to be used in buried conditions
P1 for light preparation is not suitable in this chapter (? Does this mean a section of the ISO?) for field joint coating.
Grinding of steel defects should not reduce the wall thickness below the specified minimum wall thickness of the pipe.
Areas of rust or scaling should be removed by spot abrasive blast-cleaning, bristle blasting, and/or power tool cleaning methods, such as grinding or other agreed method.
Loose abrasive blasting
Selection of a suitable method is necessary to achieve the required standard of surface preparation. In this field joint standard it is not normative (?) to use only the method of loose abrasive blasting to achieve ISO 8501-1 Sa 1-3 grades. In this standard the loose abrasive blast cleaning method ISO 8504-2 is informative. ‘Sa’ is the designation for blasting cleaning by loose abrasives only.
Surface preparation methods by hand- and/or power tools such as needle guns, or wire brushing, are designed by ‘St’. Descriptions are given in ISO 8504-3 for cleaning, including treatment prior to, and after. Preparation grade St is not included, as it would correspond to a surface unsuitable for field joint coating.
Abrasives used in the preparation of field joints should comply with ISO 11124 or ISO 11126.
Compressed air for blast-cleaning should be free of oil, condensed moisture and any other contaminants, and should conform to the requirements of ASTM D4285.
Bristle Blasting, the alternative method
This method can be used for removal of mill scale, rust and old coatings. Creating a profile is mechanical and by hand-held pneumatic, electric power or water-driven tools and semi-automatic or automatic machines. During operation, the bristle tips will create a dense, angular, regular roughness profile of more than 50 micron Rz and a cleanliness equal to the requirements of ISO 8501-1 Sa21/2.
10,000 strikes per second without heating up the surface per 23mm belt
The core feature is the accelerator bar. It suspends each separate bristle during rotation and accelerates it to increase the kinetic energy of the bristle tips impacting the surface. The system combines the ability to produce an abrasive-blasted finish with the high mobility and flexibility of a portable hand-held tool.
The procedures prior to Bristle Blasting and after are similar to loose abrasive blasting.
The applicator should wear glasses. Visual sight during the operation can check cleanliness. The bristles can be reclaimed and dust control vacuum machines can be attached.
Compressed air for bristle blasting by a pneumatic tool should be free of oil, and condensed moisture.
The cleanliness should be checked in accordance with the requirements of ISO 8501-1. Acceptance criteria are given for each system in Clauses 10 to 20. Sa grades are applicable for this method. Loose abrasive blasting is informative only for this standard.
- Sa 3 is approximately equivalent to NACE No.1/SSPC-SP5 (White metal blast cleaning)
- Sa 2½ is approximately equivalent to NACE No.2/SSPC-SP10 (Near-white metal blast cleaning)
- Sa 2 is approximately equivalent to NACE No.3/SSPC-SP6 (Commercial blast cleaning)
Bristle Blasting, depending on the brush and the type, meets the requirements of power tool cleaning and the current normative loose abrasive method to achieve an Sa cleanliness. The general rule of thumb or production is that one operator can prepare 1m2 per hour with one Bristle Blaster of 23mm width. Bristle Blaster Double, Prepper Q4 and Prepper Q10 are available in wider configurations. Subsea bristle blasting is also directly available.
Other power-tool cleaning according to ISO8501-1 grade St2/3
For coating in the field requiring minimal surface preparation, the surface should be prepared according to ISO8501-1 grade St2/3. Tightly adhered mill scale and rust coatings remain provided they cannot be removed with a blunt putty knife. Please note that tightly adhered invisible mill scale can affect the adhesion and performance of the coating system.
Hand tool cleaning can be an ideal method to prepare small areas, areas with difficult access, or areas where the use of blast cleaning is not permitted or is impractical. There are many different tools available to manually prepare surfaces. Some of the most common include:-
- Rotary wire brush – available in various forms to fit specific machines, including cup and radial form, knotted or crimpled tips. Wire brushes have good resistance to wear and tear.
- Reciprocating impact tool (needle gun) – this tool consists of a group of steel needles that are struck by a piston (like a chisel). The needles project out of the gun simultaneously and they will strike the surface individually and hence adapt to irregular surfaces. This is most effective on brittle or loose surface contaminants.
- Grinders or sanders – for example, cutter bundles or stars are hardened steel washers that are grouped together on an axis and rotate individually. These are used in metal and non-metal surface preparation, grinding concrete, and for generation of non-slip surfaces.
- Rotary impact or scarifying tools – generally consist of an abrasive material spinning at high speeds, using centrifugal force to project cutters or hammers against the surface. These tools accomplish most cleaning jobs rapidly and leave surfaces fairly smooth but frequently leave oil or grease on the surface. The surface should be cleaned of oil and grease before using rotary cleaning tools. While it is possible to achieve an acceptable standard of surface cleanliness with power tools, the surface profile is unlikely to be the same as that achieved with abrasive blasting because they do not produce a uniform pattern.
Tools that do not generate a profile but are well-equipped for the removal of polypropylene, polyethylene, glue residues or vinyl without damaging the primer are technologies such as Monti Vinyl Zappers based on butyl or silicone for wear-less properties.
Don’t forget, the first part of this article can be found in the latest issue of PCE-International