Premature Failure of Cargo Hold Coatings by “Active” Cargoes
By Dr Jane Lomas & Dr Les Callow
“It’s a bulk carrier; of course the paint came off the cargo holds, what did you expect?”
Everyone who works with bulk carriers knows that there is no paint available on the market today that is capable of withstanding the type of physical damage inflicted in cargo holds by grabs, bobcats, bulldozers, etc, particularly where such damages result in the deformation of the steel itself. As a result of these impacts and abrasions, paint is damaged or removed from the cargo holds and also from the reverse side of the steel plating, in areas such as the ballast tanks, stools and outer hull.
If paint is expected to fail under severe impacts, what about the situation where the cargo hold paint is damaged by the cargo itself and how do certain cargoes accelerate the corrosion of the cargo holds? This article gives an outline of some of the factors influencing coating breakdown and steel corrosion when “active” cargoes are carried.
The type of coating failure produced by an active cargo is shown in photograph 1 (bulkhead). The defects have a characteristic “Christmas tree” pattern, as shown in photograph 2 (X trees) and typically have a high density of damages of between 60 and 250 defects per square metre, as shown in photographs 3 and 4 (many spots and big scrapes). The number of coating damages and sags varies depending upon the local conditions within the cargo hold. Factors such as coating type and the extent of cure, cargo type and shape and cargo corrosivity will all have an influence on the extent of damage. Other factors such as the method of loading the cargo, the sea states during the voyage and the quantity of water applied to or associated with the cargo are also important.
The paint damage is usually concentrated on bulkheads, hoppers and the face plates of the vertical stiffeners on the shell bulkheads. Damage to the coating often extends up to the top of the cargo line.
What is an “active” cargo? For this article, it can be defined as one that physically affects the cargo hold coating and directly influences the corrosion reaction on the underlying steel. Examples of active cargoes in this article are coal, coke, bauxite, sulphur and petcoke.
What makes these cargoes “active” is that they are ionically conducting, posess a large surface area with a significantly high moisture content or associated water and the cargoes act as good cathodes.
The corrosion reaction that is taking place on the steel is the anodic half of the process and generates electrons. The electrons are consumed by the reduction reaction of oxygen and water that takes place at the cathodic site. Normally the cathodic reaction is very slow and controls the rate of the anodic dissolution of iron, however, the extremely large surface area of the cargo removes this limitation. The corrosion reaction can occur at significantly increased rates when an active cargo is carried. The corrosion products formed can lever the paint off the steel, causing further fresh steel to be exposed to the aggressive environment. In severe cases, the corrosive solutions that develop can also result in the formation of pits.
Pitting corrosion occurs when there is a small anodic site (in this case the exposed steel) and a large cathodic site. Cathodic sites generally form on the intact coating, but some cargoes are capable of supporting the cathodic reaction. Pitting corrosion can be accelerated with active cargoes, which drives the corrosion process at a greater rate. This is frequently the situation that occurs with carbon based cargoes such as coal and coke. Products such as coke and petcoke have very large surface areas and can form excellent cathodic sites and pits may grow rapidly under such circumstances.
There are many cargoes transported by bulk carriers that will cause accelerated corrosion of the steel if they are in direct contact with the metal. However cargoes that are soft and of low density are unlikely to cause direct physical damage to the paint and therefore they will only accelerate corrosion of the steel and subsequent coating breakdown at sites of existing coating damage or failure. Hard, dry cargoes such as ingots or ferrochrome do not tend to produce this type of damage, but cargo cycles that include iron ore can exacerbate the damage caused by the previous cargoes.
How do the active cargoes cause coating failure? Two major factors are required, a suitably hard and abrasive cargo and a coating in a susceptible condition. The cargo hold coating may be susceptible to damage by active cargoes due to a number of factors.
1. An incorrect choice of paint (that is a coating that is not designed to withstand abrasions and impacts) will quickly fail in a cargo hold environment when active cargoes are carried.
2. If the application and/or curing conditions for the coating were not suitable, for example the curing temperature was too low or there may have been inadequate ventilation during the curing period, then the coating may remain soft when it enters service and will fail prematurely.
3. If the quality of the surface preparation of the steel is not sufficiently good, then contamination may remain on the steel and be over coated by the paint. When suitable conditions of water and oxygen are present, corrosion can quickly initiate and propagate at contamination sites, generating rust under the paint and ultimately levering the coating from the steel.
4. The presence of water in the coating can also result in plasticisation of some paints, making them easier to deform.
5. Increased temperatures can cause some types of coating to soften and again this will result in deformation and sags when in contact with active cargoes.
Active cargo corrosion occurs in a sequence of events that results in coating damage, often with the characteristic tree pattern, together with steel corrosion. The first stage of the damage to the paint occurs when the sharp, hard and angular cargo scratches into the coating due to settlement during both loading and the voyage. Eventually the cargo cuts through sites of weak paint to the steel and exposes the metal to the cargo environment.
It is first seen as a small puncture in the coating, which creates a pathway for water to reach the steel. Water associated with the cargo can permeate along the interface between the paint and the steel at the site where the coating has been damaged, resulting in the loss of adhesion of the coating.
When examined visually, the damaged area may appear to be small, but when it is investigated with a penknife, it is often found that the paint is loose around the damage site. When this loose coating removed, the true extent of the corrosion and associated coating delamination becomes apparent. Photograph 5 (large disbond) shows two damage sites that were initially very similar with only tiny punctures visible in the paint. It can be seen that the corrosion under the coating and the loose paint extend several centimetres from the visible damage site.
As the cargo settles, soft or weak paint can move with the cargo creating blisters and “sags” of the type shown in photographs 6 and 7 (scrape & sag and black tear/sag). The loose coating sag is removed by further settlement of the cargo, which in turn can cause the formation of another sag. Repeated “stick – slip” cycles of cargo settlement result in the tree shapes of photograph 2.
Coating problems of this type, particularly when they occur very early in the service life of a new vessel or very quickly after major cargo hold repainting work, usually give rise to a claim against the coating applicator and/or the paint supplier. Such paint damages can often be avoided by a careful choice of the coating system. The coating application and curing conditions are vital to get right in order to minimise the risk of paint failure and subsequent corrosion of the steel by active cargoes.
The risk of active cargo damage is highest in the period following the coating work. When possible, careful consideration needs to be given to cargo choice immediately following new building or major refurbishment. If such cargo hold coating damage does occur, it should be carefully quantified and assessed before further damage occurs.
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