With safety in mind

Cathodic zinc protection for holds, including lower hoppers and tanktops

Coating Integrity

In the protection of highly stressed parts of ships such as bulk cargo holds and including lower hoppers and tanktops, choosing the right product is of utmost importance. According to Steelpaint GmbH, its cathodic zinc protection withstands even the greatest strains.

The 94,000dwt Alfred Oldendorff is a bulk carrier operating on relatively short voyages in the Middle East and Arabian Gulf. The vessel frequently carries cargoes of iron ore pellets that are loaded quickly and often unloaded with assistance from grabs, bulldozers and front loaders. With loading rates of up to 2,000 tonnes of pellets per hour, the ship’s cargo holds and their coatings are regularly subject to major stresses and strains. During the bulk carrier’s construction at the New Yangzijiang Shipyard in China, STEEL-ZINC was applied in the cargo holds in 2015. Despite concerns over whether the Zinc protection would withstand the frequency of cargo handling, the mechanical stresses, and the impact of cargo handling machinery, the SP-ZINC-System is still performing well. Even after nearly nine years of intense operation, it remains largely intact across the hoppers and tanktops and continues to provide reliable protection.

In 2020, it was time for the first survey docking at Oman Dry Dock Company in Duqm on the Arabian Sea. After five years of hard operation, the STEELPAINT-ZINC protecting the holds of the ship were expected to require significant attention. However, the Steelpaint Zincing was found to be remarkably intact with only a few areas requiring remedial attention. The STEELPAINT-ZINC has a number of key properties that make it an excellent protection for this type of high intensity deployment:

It has excellent adhesion properties v that ensure a strong bond between the SP-ZINC protection and the steel substrate in the holds; Its high zinc content – 93% in pigment v mixture – provides the cathodic protection. In the case of damage, the surrounding zinc sacrifices itself to safeguard the steel, thereby preventing structural corrosion; Even at the first five-year survey, v mechanical damage to the zinc was surprisingly low despite the frequent deployment of grabs and bulldozers in the holds. Meanwhile, the STEELPAINT-ZINC could be reworked with another application following the careful removal of all the release agents;

The condition of the hold after five v years of iron ore transport. No corrosion protection treatment was necessary. The upper hoppers, frames, corrugated bulkheads in the upper and middle area were in good condition. No renovation work was necessary.

Despite the STEELPAINT-ZINC’s fine performance, small areas of the Alfred Oldendorff’s holds had suffered damage. These included the lower hoppers, lower stools, tank tops, small areas of hatch covers and under the main deck, and limited parts of coamings. These relatively small areas could be repaired by spotblasting to abrasive grade SA2 followed by reapplying with two applications of STEELPAINT-ZINC. Furthermore, the SP-ZINC-SYSTEM offers several other practical advantages in through-life repairs. It requires comparatively low surface preparation, saving time. For example, manual rust removal St2-St3, water blasting

WJ2-WJ3 or sand blasting Sa2 are perfectly adequate. Further advantages include the STEEL-ZINC’s flexible application properties, the company said. It can be safely and effectively applied in highhumidity conditions up to 98%, while drying effectively at temperatures as low as –5°C. During its lifetime, the STEELPAINTZINC can be managed and applied without the use of additives, thereby preserving its viscoplastic properties over its entire service life, the company added. Meanwhile, high abrasion and impact resistance, the possibility of reworking damaged areas, and the unlikely requirement for steel repairs, form the basis for a strong lifecycle business case. One proviso here that Steelpaint noted: old coatings must be thoroughly inspected, prepared and treated to remove any loose material and interfering release agents such as oils, greases, and chlorides. This also applies to touch-up treatment between dockings. However, a requirement for steel repairs is unusual. In comparison, traditional epoxy coatings show embrittlement, micro-cracking and a tendency to porosity at an earlier stage in their application service life. Equally, the adhesion of these coatings frequently deteriorates over time. This, in turn, can lead to partial rust formation and steady degradation of the material strength of the steel due to the frequency and extent of mechanical loads. Expensive steel repairs are often required and through-live costs can be much higher.

Thus by preventing corrosion damages to lower hoppers and tanktops the structural integrity of the steel can be protected and it can be avoided to have longer subsequent docking intervals due to the welding in of new steel plates in these areas.