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Published: October 2017      

Two issues for reefer box owners

The COA has issued guidelines for the carriage of NOR cargo in reefers, and identified a growing problem of corrosion, caused by modified atmosphere when carrying certain reefer cargoes.

Refrigerated cargo carried in containers has seen steady growth over many years, but reefer trades are often characterised by a heavy imbalance, in some cases with no cargo at all on the return leg, combined with freight rates that are low and make it difficult to justify an empty positioning move.

In advance of the fruit season in southern hemisphere producing countries such as Chile or South Africa, shipping lines move in many thousands of empty reefers that are stored waiting for fruit to become ready to harvest. To mitigate the substantial cost of doing this, lines have increasingly looked to carry non-refrigerated cargo in empty reefers to earn revenue, or at least offset some costs.

NOR basics

Non-reefer cargoes are referred to as Non Operating Reefer (NOR) cargo. While this has been happening for many years on a small scale, low revenues from carrying reefer cargo has put more focus on cost, so carriage of NOR cargo has been increasing, and is likely to remain an important tactic in the years ahead.

It’s important to remember that reefer containers were designed predominantly for the carriage of chilled and frozen food. They are heavily insulated, have relatively thin outer and inner linings to control tare weight, use lightweight materials such as aluminium for the T section floor, and much of the interior metal surfaces remain unpainted. A reefer container typically costs seven times more than a dry container to buy new – and repair costs can be at least seven times higher.

Reefers predominantly use stainless steel lining panels, which are good at resisting corrosion, as are FRP lining panels sometimes used in lightweight reefers. While aluminium T floors, scuff liners, evaporator coils and copper refrigeration tubing have been in use for decades, they are not as resilient as stainless steel or FRP.

Many NOR cargoes are fairly benign. However, if a cargo is corrosive or emits any corrosive vapour, the aluminium and copper components inside the reefer can corrode. Likewise, cargoes that are sharp or heavy may damage the interior of a reefer by bending or puncturing, though they may be just fine in a dry container with its thicker Corten steel walls and marine plywood floors.

New guidelines

For these reasons, the Container Owners Association (COA) put together a set of guidelines for the carriage of NOR cargo, in cooperation with leading shipping lines and leasing company owners, the TT Club and CINS (Cargo and Incident Notification System). A guide was published in August 2017, and is available to download from the COA, TT Club or CINS websites.

Without going into detail that is well covered in the guidelines, the occurrence of corrosion to parts of the interior of reefers has become notably worse in the past five years or so. The NOR cargo guidelines were prepared to raise awareness with shippers and owners of the potential risks that poor selection of NOR cargo can bring. Shipping lines cannot afford major repair bills after carrying an NOR cargo, neither do shippers want to be held responsible for these costs. It is best to avoid the problem in the first place by sensible selection of cargoes that have no effect on the interior of the reefer container, either physically or by the action of corrosive vapours, liquids or solids.

Publishing the NOR cargo guide is a start. Spreading the word across the wider container shipping community and in countries where English is not widely used is a challenge for the future. There may well be merit in having versions of the guide in other key languages, such as Chinese and Spanish.

Modified atmosphere

That this guide was prepared in the first place was in response to well-documented incidents of interior damage to reefers caused by unsuitable NOR cargoes. But, in recent years, there have also been growing problems with interior corrosion that appear to be unconnected to NOR cargo.

While the shipping line will know what type of refrigerated cargo is being carried, and will monitor machinery performance and temperature control during the journey, they may have little or no knowledge about treatments that are applied to the fruit or vegetable cargo prior to loading.

Certain fruits are more susceptible to decay over long transit times, even if the temperature is kept at the lowest optimal levels. Some fresh cargoes may decay due to respiration, resulting in them maturing before arrival. Others may decay due to the growth of mould. This is a large and technical subject, but, in simple terms, some cargoes can have their storage life extended by modifying the atmosphere inside the container by controlling oxygen and CO2 levels, or by the use of fumigation. Fumigation is now attracting attention as a likely cause of increasing levels of corrosion affecting reefer containers.

Fumigation of cargo prior to loading in the container can be effective in controlling insects and the presence of mould. However, use of fumigation inside the container after cargo is loaded is very likely to cause corrosion to the interior. Grape cargoes have used ‘controlled’ fumigation to prevent mould growth for many years, by laying thin pads over the top of grape cartons that emit small amounts of sulphur dioxide (SO2), and the cartons are wrapped in plastic to keep humidity levels high and to limit the amount of SO2 escaping into the container. But grape cargoes are increasingly seen as being connected to the rise in interior corrosion. Has something changed in the way grapes are loaded into the container or any fumigation treatments?

Avoiding grapes

Some leading shipping lines are avoiding grape cargoes because of the damage they have seen to their reefer containers, though, as other lines are now taking these cargoes instead, it seems likely that corrosion will be experienced by them too. Apparently, requests by lines to inspect the loading process at grape shippers have been turned down.

Grapes are not the only cargo linked to interior corrosion. Growth in demand for tropical fruits in Asia has seen the carriage of longans, lychees and durians increase. There are reports of relatively indiscriminate use of SO2 fumigation inside the container with these cargoes, which seems to have triggered corrosion.

Where SO2 has initiated corrosion to aluminium, the corrosion cannot be stopped by cleaning. Cleaning can temporarily remove white oxides and sulphites from the interior, enabling shippers to load cargo, but it does not repair the damage to the metal surfaces, and the white powders reappear after only a few weeks. Applying coatings to the metal is expensive and has not been shown to prevent corrosion.

Given that the machinery design and aluminium alloy used inside reefers has not changed for decades, and that interior corrosion was not a significant issue more than five years ago, the conclusion could be drawn that changes to practices by shippers is largely responsible for this issue.

Cooperation needed

A cooperative dialogue between shippers and shipping lines that carry their cargo would seem essential, as this problem is causing corrosion that costs many thousands of dollars to fully repair, and is clearly an unsustainable situation. Progress is being made on NOR cargo, with the recent publication of guidelines. For carriage of fruit cargoes where fumigation is being used, education will also be required if corrosion is to be eliminated, though it will likely require a change of approach, given the lack of transparency that apparently prevails between the shippers of cargo and the logistics providers investing in the specialist equipment to deliver it....

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This complete item is approximately 1000 words in length, and appeared in the October 2017 issue of WorldCargo News, on page 26.

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