October 15th, 2021 by . Posted in In Focus, In Focus Marine

Corrosion protection of cargo holds on board iron ore carriers

Using Single-pack Moisture Cured PU Zinc


The market share of moisture cured urethanes for paint projects is gradually increasing due to their long term corrosion protection, resistance to chemicals and graffiti, superior gloss retention and colour appearance. On top of that moisture cured urethanes offer cost and time savings based on their surface tolerance, quick drying capability, applicator friendliness, and the fact that they can be applied under adverse weather conditions.

The technology was originally invented by Bayer Material Science (now Covestro) from Germany. The most striking difference with epoxy coatings is that, instead of adding two components together, the curing process is initiated by the reaction of the polyurethane binder system with atmospheric moisture. This is the reason why moisture cured urethanes can be applied up to 98% relative humidity. Surface moisture actually completes the chemical reaction, which is why these coatings show excellent adhesion to visibly damp surfaces. In the pores and capillaries of the steel, where moisture is usually present, the coating penetrates deeply to form strong chemical bonds. Since the moisture is consumed in this process there is a strongly reduced risk of blisters caused by water entrapped under the paint film.

Productivity and Performance advantages

In a time where the life cycle costs of a structure are more and more calculated over an extended period, sometimes even as long as 50 years, it becomes increasingly important to offer a corrosion protection solution which allows a long interval until first maintenance.

With the asset management tools available today it is possible to set the actual maintenance costs off against the projected life cycle maintenance costs. Deviations caused by e.g. premature paint failure can be easily monitored and the financial consequences accurately quantified. This trend may be one of the most important reasons why companies are moving away from short term thinking during the construction- or major maintenance phase of a project. More and more it is preferred to opt for long term corrosion protection, even if this results in an increase of the overall project costs. Paint related costs in general however present only a small percentage of the total project costs. If the running time of a new building- or M&R project can be shortened, and the time to first maintenance is extended by selecting a suitable coating system, the financial advantages outweigh the extra paint related costs by far.

Moisture cured polyurethanes offer long term corrosion protection for steel structures in the most challenging environments.

On top of that they also offer a number of practical advantages during new construction as well as maintenance activities. One of the major advantages is that these coatings can be applied to steel substrates with minimal/reduced surface preparation such as power tool cleaning ST3, UHP Waterjetting WJ 2 – 3 L, gritblasting SA1.5 – SA2. Other advantages of moisture cure polyurethanes are that they can be applied up to 98% relative humidity, cure down to minus 5 degrees Celsius without any aditives, are permanently flexible, are highly abrasion- and impact resistant, can be applied over most existing types of paint.

Steelpaint GmbH

Steelpaint is a German coatings manufacturer specialized in Moisture Cured Polyurethanes, active for over 30 years.  

Our main markets are bridge, hydraulic steel structures (tubular- and sheet piles, jetties, locks, storm defence structures etc), marine (cargo holds of bulk carriers, ballast tanks), offshore (platforms, rigs, barges), Industrial (pipelines, storage tanks).

Corrosion protection in Cargo holds on board of bulk carriers

The very nature of the cargoes carried by bulkers makes bulk carrier inspection in critical areas mandatory; especially where high tensile steel plates have been used in the hull. The hulls are subjected to longitudinal bending stress and shear forces during loading and unloading of the cargo

Bulk carriers have become larger, carrying ever increasing tonnage of various cargoes. They are required by law to have a hull inspection carried out yearly.

Structural damage to the hull can be sustained during cargo loading and discharge.

Nowadays modern bulkers are designed to carry iron ore, bauxite, phosphates, coal and grain. Bulkers making up almost 40% of world shipping fleet.

There are number of methods of loading:

Modern cranes can load 1000-2000 T/hour. Conveyor systems can load between 100 and 700T/hour with the modern systems capable of 16000T/ hour. Imagine the stresses imposed on the hull structure at these rates!


Depending on the cargo, cranes are normally used and the rate of discharge is dependent on the size of the bucket/grab and capacity of the crane averaging around 800T/hour.

Once most of the cargo has been discharged, front loaders and bulldozers are lowered into the holds to direct the residue into sizeable mounds for collection by the crane buckets, then manual labour is used to brush out the holds.

The most critical areas in the cargo holds are the lower hopper- and bottom plates. Here the impact of the bulldozer and grab is the most extreme. The bottom plates normally are not painted because there are no other coating systems which can withstand the extreme impact.


In 2008 Steelpaint was invited to participate in a full scale test on board of  M/V Sophie Oldendorff, a Panamax bulk carrier carrying highly abrasive cargoes such as iron ore, coal etc. owned by Oldendorff Carriers GmbH.

The purpose of the test was to find the most abrasion- and impact resistant coating system for the cargo holds. A different coating system was applied in each hold, in order to be able to compare the performance of each coating system under exactly the same circumstances. 

Steelpaint was appointed hold No. 3, which on this type of vessel is also used as ballast space during empty runs. The exposure to abrasion and mechanical impact in combination with seawater makes this hold the biggest challenge for the performance of a coating system.

The coating system applied was 2 x 100 microns of PU-Zinc. The other holds were coated with reinforced epoxy coatings at dry film thicknesses varying from 600 to 1200 microns.

Advantages PU-Zinc

The PU-Zinc has a number of principle characteristics which make it a superior coating for this particular purpose:

  1. It has outstanding adhesion to the steel substrate
  2. There is 93% zinc in the dry film which provides cathodic protection. If there is a damage the surrounding zinc will sacrifice itself instead of the steel. In this way corrosion is prevented
  3. It remains flexible and therefore bends with the steel when it is damaged by the grab or bulldozer
  4. It can be applied in multiple coats so a thick layer of zinc can be applied tot he steel
  5. It is surface tolerant and can be maintained by the crew until the next drydocking

Test results

After 6 months all holds were inspected and the performance of the various coating systems was evaluated. It appeared that only hold No. 3 was in excellent condition. The coating systems in all other holds showed the usual signs of breakdown and corrosion.

During the next maintenance drydocking, 2.5 years after the trial application, a final evaluation was made. As it turned out hold No.3 showed minor spot breakdown of less than 5 percent oft he total surface. The corroded spots were repaired by powertool cleaning ST3 and touched-up with two coats of PU-Zinc. The repairs were carried out from a basket, no scaffolding needed.

The coatings in the other holds were broken down to an extent of 50-70% and required scaffolding, almost a full blast, and total repainting  job.  There was an immense difference in repair costs compared to hold No. 3 and from then on the owner gradually changed the cargo holds of its owned fleet over tot he PU-Zinc.


When in 2013 Oldendorff Carriers launched an extensive newbuilding program the PU-Zinc system was considered as a possibility for the cargo holds of the new vessels.  In order to justify this decision an inspection was conducted on board M/V  “E. Oldendorff“ a Panamax self unloader.

Here the PU-Zinc had been in service for 1.5 year. This vessel was in use as an overload station in the Persian Gulf. The loading- and unloading operations were practically going on continuously, which also meant that the PU-Zinc was exposed to maximum impact and abrasion.

 M/V “E. Oldendorff” pictured in the middle


Overall view cargo hold after 1.5 year overloading iron ore

The damaged areas on the lower hopper plates at first sight appeared to be corroding. However, the rust stains  were just iron ore residue which had settled on top of the PU-Zinc. After removal by wirebrush or scraper the metallic action of the PU-Zinc appeared immediately. Measurements with a dry film thickness gauge showed that in all damaged areas sufficient zinc remained to provide adequate corrosion protection.

In fact the inspection proved that after 1.5 years service there was zero percent corrosion in the hold which was inspected.

Following this successful inspection Steelpaint was awarded the supply of PU-Zinc for all cargo holds of 11 newbuildings in China.

9 x 39000 DWT at Nanjing Jinling

2 x 94000 DWT at New Yangzijiang

Trials on bottom plates

Test area:

Based on the excellent results on the lower hopper plates we launched a test program on board of M/V “Berge Bureya” a VLOC owned by Bergebulk. The purpose of this trial was to find out if the PU-Zinc would show satisfactory performance on the bottom plates as well.

Two areas of in total 100 m2 were applied. One in the centre of the hold, were the impact of the grab and bulldozer is worst, and one partly on the tanktop and lower hopper.

The surface preparation was slurryblast SA2 and the coating system 2 x 100 micron PU-Zinc and in one third of the area 3 x 100 microns of PU-Zinc was applied. All work was done by a riding crew during an empty run of the vessel to Tubaro in Brazil. The vessel was on a steady trading pattern between Tubaro (loading) and Rotterdam (unloading). This made it easier to monitor the results of the test.

The results after the first two runs looked very promising. We were seeing that even in the heaviest damages there was still enough zinc on the surface to provide adequate corrosion protection. In the few spots where the PU-Zinc was almost totally gone the surrounding zinc prevented the steel from corroding.

Full scale test:

Following the satisfactory test results on board M/V “Berge Bureya” it was decided to do a full application, as a large scale test in all 9 holds on board the Capesize bulker M/V “Berge Matterhorn”. During drydock in Shenzen China the PU-Zinc was applied on the bottom plates and lower hoppers up to approximately 1.5 mtr. Surface preparation gritblasting SA2, coating system 2 x 100 microns PU-Zinc.


After 15 months the cargo holds were inspected in Krishnapatnam India.

The results were very good. No corrosion was found. In 80% of the surface area the thickness of the PU-Zinc was fully intact, no maintenance required. In 7% of the area the dry film thickness of the PU-Zinc was diminished to 100 micron, no maintenance required. In 3% of the area the dry film thickness was reduced to 50 micron, no maintenance required at this point. On top of the double bottom frames the coating is abraded more than in other areas because these areas are higher and therefore more exposed to the impact of the bulldozer. In total this area represents 10% of the total surface. In a lot of places on top of the frames the thickness was reduced below 50 micron.  Here it is recommended to bring the thickness of the zinc back up to the originally specified thickness.


For these areas a maintenance plan was developed for the crew. The maintenance activities can be carried out with simple tools during empty runs. The only surface preparation required is high pressure water washing to remove all cargo residues properly and light abrading of the frame areas by power wire brush or disc. After which the PU-Zinc can be applied by spray or roller making sure to apply 2 – 3 coats.

In this way the coating system can be maintained until the next drydocking, corrosion is prevented and loss of steel thickness is kept under control in a very cost effective way.