Cargo Tank Coatings

In this article Alan Walker, Marine Market Sector Manager at coatings experts Safinah explores the measures that may be taken by Members to minimise problems related to cargo tank coatings.

One of the many activities that Safinah becomes involved with is surveying specific areas on marine vessels.  Typically, around one third of our work relates to cargo coating disputes.

There are a number of common factors running through these disputes which should be considered when investing in a new build or purchasing second hand.

Factors That Influence the Life of Cargo Tank Coatings

The factors that have the most influence on the life of cargo coatings are tabulated. The table shows that most of these factors can be influenced by ship owners.

Design

Members can influence vessel design and should consider the following:

• Using stainless steel ladders.
• Fitting a stainless steel plate under the suction strum; an area that tends to be prone to damage, abrasion and cavitation corrosion.
• Fitting stainless steel lugs for scaffold attachment at new building to avoid contact damage, masking damage and provide the optimum scaffold height for painting.
• Consider deck heaters rather than stainless steel heating coils:
• Reduced damage during fitting
• Easier maintenance
• Reduced electrochemical potential in the tanks (stainless steel act as a cathode to mild steel).
• Paint all stainless steel in the tanks (heating coils, ladders etc.) to reduce the electrochemical reactivity.

Coating Technology

Using the appropriate coating technology is critical for cargo tanks as incorrect product types will almost inevitably lead to premature coating breakdown. Members should be aware of the available technologies or consult with third party experts as to which is the most suitable for the intended cargo(s).

There are a number of available technologies with each having advantages and disadvantages:

• Pure epoxy: generally best suited for CPP cargos. Generally, have more restrictions on aggressive cargos such as methanol.
• Zinc silicate: recommended for neutral cargoes such as dedicated methanol carriage. Cannot tolerate acidic or alkaline cargos.
• Phenolic epoxy: suitable for carriage of a wide range of cargos including methanol.
• Bimodal epoxy: the most recent technology introduced by some of the paint suppliers offering reduced cargo absorption and ability to carry a wide range of cargoes including methanol. This coating does however require post curing.

Paint suppliers and the Shipyards have their own best interest at heart not that of the Ship Owner. Profit in the case of paint suppliers and ease to apply for the Shipyards.

These may not match the technology best suited to the operational needs of the Member. Guidelines and training are available

Product Selection

The larger marine paint companies generally all have a product offer for each technology. This leads to the situation where too often Ship Owners and Shipyards decide the product mainly on its price.

This can be a short term saving; product selection should be based on:

• Ship owners previous experience of what works.
• Product track record
• Independent expert advice
• Warranties

Project Management

Once the correct technology and product(s) are selected what can go wrong?

Eighty percent of cargo tank coating failures are due to poor surface preparation and/or application.

Experienced project management is very important to ensure the selected coating technology and products are applied correctly and in accordance with the paint makers’ application guidelines:

• Quality of materials such as grit
• Surface preparation standards
• Scaffold installation and removal
• Inter-coat preparation of surfaces
• Correct dry film thicknesses
• Correct over coating intervals
• Curing times
• Heated post cure if applicable
• Etc.

The Paint Makers technical service representative will advise on much of this, but under- pressure from both the Paint Maker and the Shipyard may not always act in the best interests of the Ship Owner. An experienced Project Manager employed by the Owners is the safest way of ensuring the project is successfully completed.

Remember: the cost of a Project Manager will be $30,000 to $50,000 for the entire coating application versus the cost of a tank lining failure that can be >$1 million to fix.

Example / case study: two 8 years old sister ships.

Operation

Ship owners and operators should know the restrictions applicable to the cargo tank linings on their vessel.

• Unrestricted cargoes
• Restrictions on certain cargo types e.g. a maximum moisture content of carriage time.
• Cargo sequencing
• Permitted tank cleaning processes

Cargo resistance guidance and training should be provided by the paint companies as part of the “sales package”.

Independent experts can also provide additional advice, support and training

All deviations from approved cargos, e.g. cargo type, carriage duration the paint suppliers’ guidelines should be fully documented to avoid later disputes between Parties (e.g. between Owner and Charterer).

The Expected Lifetime for a Cargo Tank Coating

* Depending on cargo type and sequencing

It all depends on the influencing factors mentioned and Getting Them Right.

Coating breakdown and corrosion starts on Day 1 although it may not be apparent in early years.

Maintenance and Repairs

By the crew:

This is often done for cosmetic reasons, prior to the visit of cargo vetting agents.

Good lasting repair is difficult due to:

• Inadequate surface preparation
• Insufficient time to apply the correct coating specification and allow the coating to cure
• Limited access

Breakdown can be expected within 12 to 18 months.

Typical crew repairs to a tank top after 6 months service

Anything more than minor touch ups by the crew in cargo tanks are generally a waste of time and serve to hide defects rather than effectively repair and stop corrosion.

Partial Repairs at Dry Dock e.g. Re-blast and Recoat Tank Tops

Limited value:

• The Shipyard will not treat a repair in the same way as a full re-blast and recoat
• There will be no guarantee from paint company
• Less quality control from the Owner, the Shipyard and the paint company

Summary

• Members can influence most of the factors that influence the performance of cargo tank coatings.
• At new building; ensure the correct technology and coating is selected
• Buying a second hand chemical tanker; invest in a professional cargo coating survey. An experienced cargo tank coating surveyor can detect early signs of trouble ahead.
• Poor surface preparation and coating application is the major cause of premature coating breakdown and corrosion.
• Invest in good project management during coating application.
• Cargo tanks that have significant coating breakdown and corrosion almost always require major refurbishment in order to safely carry higher value, pure cargoes.