This article concerns the performance of painting systems applied in water ballast tanks as required by International Maritime Organization Resolution MSC.215(82), keeping in mind that use of ultra high-pressure waterjetting (UHP WJ) will increase following the requirements for reducing the environmental fingerprint.
The ultra high-pressure water-jetting (UHP WJ) method could be integrated as a secondary surface preparation technique in a future revision of the performance standard for protective coatings.
This would involve adding a checkpoint in the inspection plan to verify the blasting profile of the primary surface preparation. Doing so would enable the prequalification of painting systems compatible with UHP WJ. Painting is considered a “special process,” as the performance of the painting job can be assessed months after the vessel was delivered.
Performance of a painting system depends heavily on surface preparation. The surface must be clean and free of any contamination, such as oil and grease, dust, moisture, or soluble salts, and a surface profile should provide sufficient conditions for good adhesion. Such is the case with water ballast tanks, as it is not easy to inspect the area and maintenance costs are high. Painting system failure implies corrosion protection failure, which poses a great risk to assets.
Adopted on August 12, 2006, International Maritime Organization Resolution MSC.215(82) is a performance standard for protective coatings (PSPC) for dedicated seawater ballast tanks in all types of ships and double-side skin spaces of bulk carriers.
This standard is based on specifications and requirements that intend to provide a target useful coating life of 15 years, which is considered to be the time period, from initial application, over which the coating system is expected to remain in “good” condition.1
Nevertheless, the necessity of an increased coating life remains valid for any type of structure.
Primary surface preparation aims to remove mill scale and provide conditions for application of shop primer, which has to provide temporary protection to the steel for periods up to six months without interfering with steel cutting and welding.
Regarding primary surface preparation, PSPC requires full blasting to ISO Sa2,5, followed by application of a (preferably prequalified) shop primer.1 In a shop priming plant, the steel plates are blasted with spherical steel or cast-iron shots and then shop primed, as seen in Figure 1 (top).
Secondary surface preparation provides the foundation of good corrosion protection and is done after steel parts have been cut and welded/assembled.
Regarding secondary surface preparation, PSPC states: “If the complete coating system comprising epoxy-based main coating and shop primer has passed a prequalification, certified by test procedures in 1.3, intact shop primer may be retained provided the same epoxy coating system is used. The retained shop primer shall be cleaned by sweep blasting, high-pressure water washing or equivalent method.”1
General Considerations
Today, new parameters are taken into consideration for characterization of blasted surfaces. Two additional attributes can be equally important to helping ensure long-term coating system performance: angularity and frequency (density) of the peaks of the generated surface profile.2
These attributes are not often taught in training courses and may not be well recognized in the industry but, depending on the coating system and service environment, may be critical and may be invoked by specification.
According to the Society for Protective Coatings (SSPC) SPCOM2017,3 if the valleys of the surface profile are too deep and narrow, the applied coating may not be able to penetrate to the bottom of the valleys, due to viscosity or reduced wet-out time, leaving a void at the bottom of the “valley.” So, maximizing peak count/density may not be advantageous for all coatings.
Several studies4-5 show that the main factors that affect the roughness of the blasted surface are:
- abrasive shape and particle size
- air blast pressure
- abrasive hardness
- abrasive particle size distribution
The primary surface preparation is usually done in automated lines (i.e., at shop priming plants) where steel plates are preheated, blasted with spherical/round shots on both sides, and then coated with one thin layer of shop primer at dry film thickness between 15 and 25 µm, as seen in Figure 1. Studies show that topography of the shot-blasted surface is different from the topography of a surface blasted with angular abrasive.4, 5
When using round particles of blasting abrasive (i.e., shots), relatively uniform deformation of the surface is achieved. The surface consists of intersecting spherical dimples that are not producing the angularity and the peaks density required by good adhesion of the painting system. Sharp angular particles of abrasive (i.e., grit) cause notches in the substrate whose orientation on the surface is stochastic.
During their random movement within the abrasive stream, particles can impact the surface with their edges, flat surfaces, or tips, depending on blasting process parameters such as abrasive velocity, blasting distance, impact angle, and the hardness of both the abrasive and the substrate.
The profile created by angular abrasives is considered superior to the shot-peened profile, offering bigger areas and, thus, better conditions for paint adhesion. Figure 2 shows the difference between the topography of a shot-peened surface and a surface blasted with angular abrasives.
By using angular abrasives, in addition to a better profile, any traces of mill scale are removed — that is, if they were somehow missed during the primary surface preparation.
Zinc salts of the approved shop primer are also removed, and the excessive thickness of the remaining shop primer, traces of welding slag, and some of the weld spatters are removed, exposing the welding seams’ pores. Figure 3 shows a blasted water ballast tank, where traces of the approved (gray) shop primer can be seen.
The area of water ballast tanks represents a big percentage of vessel area, as seen in Figure 4. The costs associated with surface preparation and painting are significant—therefore good surface preparation is necessary.
Impact of New Technologies: Ultra High-Pressure Water-Jetting
Considering new environment, health, and safety regulations, along with general trends to reduce the environmental footprint, the use of UHP WJ performed at pressures above 210 MPa (30,000 psi)6 becomes increasingly popular.
In line with the new trends and efforts of reducing the environmental footprint, there are some attempts to prequalify painting systems that can be applied in water ballast tanks where surface preparation is done by water-jetting.7
Technical literature recommends centrifugal blasting equipment that is commonly used in an automated line for abrasive blasting a mixture of grit and shot.8
The shot is best for breaking and removing hard mill scale common on new steel, while the grit is best for removing rust and imparting the required surface profile, including these new attributes: angularity and peaks density.
Besides the advantages of water-jetting, there is one unavoidable drawback that may affect the painting system performance: water-jetting does not create roughness. Figure 5 shows a keel block mark from the new building stage that was treated by power tooling to ISO St3. After UHP WJ, the power-tooled area surrounded by the initial blasting profile can be seen.
Assuming that a surface-tolerant painting system may be prequalified, which will allow the use of UHP WJ, it may be necessary to ensure good ventilation in order to get a dry surface as soon as possible and minimize flash rust. Another important consideration may be to increase the number of technological openings, which allow a faster evacuation of water.
A proper substrate profile following primary surface preparation is crucial, regardless of the method used for secondary surface preparation. One critical aspect remains the substrate preparation; this can be scheduled, as usual, prior to secondary surface preparation.
For small areas that may be polished too much during substrate preparation, the minimum roughness, as mentioned in the primer’s paint data sheet, can be obtained by using Bristle Blaster† or small grinders.
Conclusions
Given that a better profile (i.e., meeting PSPC requirements) can be achieved by using a mixture of grit and shot for primary surface preparation, there is potential to incorporate UHP WJ as a secondary surface preparation method in a future version of the PSPC standard.
This can be achieved by introducing a holding point in the inspection plan for checking the blasting profile of primary surface preparation and by allowing the prequalification of new painting systems based on surface-tolerant paints compatible with UHP WJ.
Incorporating these prequalified systems will reduce the environmental impact, improve the removal of soluble salts, and increase productivity.
References and About the Author
† Trade name.
Editor’s note: This article first appeared in the November 2024 print issue of Materials Performance (MP) Magazine. Reprinted with permission.