Reefer containers are regularly found with a white powder on aluminium components. For many years there has been a debate: Is this caused by cleaning techniques and materials, or by the use of low quality alloys?

A recent inspection of a number of reefer containers found that the diffusers on the machinery were corroded and exhibited white deposits. The surveyor found that there was no indication that the shipper's actions caused the white powder formation. He reported that '.with the randomness of the corrosion problem, it seems highly unlikely that one shipper was the sole cause of the problem'.

Metallurgic testing found that the aluminium alloy used on the machinery diffuser was type A380 (UNS A13800) rated among the lowest with respect to corrosion resistance compared to other die casting alloys.

But was this the real cause? The Institute of International Container Lessors (IICL) has published a technical bulletin on Reefer Cleaning Issues. This reports a growing number of the interiors of reefer containers affected by the use of SO2 as a fumigant for the transport of grapes. The findings conclude that 'there is no known cleaning agent to neutralise' the affects, but that immediate cleaning will assist.

The specific chemical in question is 'sodium metabisulfite' which has been used to control a fungus known as 'Botrytis Cinerea' during the carriage of many fruit products. When mixed with water, sodium metabisulfite releases sulphur dioxide (SO2), a pungent, unpleasant smelling gas that can also cause breathing difficulties in some people. The released sulphur dioxide makes the water a strong reducing agent which can result in a chemical attack causing corrosion to interior aluminium and stainless steel components within the container and machinery cooling unit.

Analyses by Carrier Transicold environmental specialists have identified the white powder as consisting predominantly of aluminium oxide. Aluminium oxide is a coarse crystalline deposit most likely the result of surface orrosion on the aluminium parts within the container. If left untreated over time, it may build up in thickness and eventually flake as a light-weight white powder. Carrier Transicold proceed to report in their December 2010 issue of Techline, that they have discovered a fully biodegradable and environmentally safe alkaline cleaning agent (Tri-Pow'r® HD) for the unit. This will assist in helping to remove the corrosive fumigation chemicals and dislodging of the corrosive elements. Both IICL and Carrier Transicold recommend that the interior of the container and the machinery unit should be cleaned as soon as possible after fumigation, although IICL feel that this condition is a non-standard use of the container and/or that failure to take corrective action when sodium metabisulphite is used will lead to continuing non-normal and excessive corrosion.

Returning to the original question, is this a result of the cleaning or the material used? The use of sodium metabisulphite as a fumigant and its effect on the aluminium alloy components is known; until there is a clear alternative, its use will continue. The designers of reefer containers and machinery should recognise this and consider eliminating the corrosion attack by using replacement housings made from materials exhibiting better corrosion resistance or treat existing diffuser housings with corrosion resistant coatings such as marine grade epoxy paints.

It would appear that the presence of aluminium oxide does not affect the cargo, but shippers may not accept containers exhibiting this 'defect'. The IICL report that they consider this irreversible condition will cause widespread damage and eventually render the container un-usable, resulting in premature disposal.