Over the last few months, an increasing number of incidents of phosphoric acid damage to stainless steel ship’s tanks have been observed.
Typical phosphoric acid damage is manifest as general steel wastage and/or elephant skin. Phosphoric acid is a crude product and therefore the varieties are legion. Generally, stainless steel tanks found in chemical tankers are resistant to phosphoric acid attack; however, excessive fluoride within the phosphoric acid can initiate corrosion attacks allowing for continuation of the attack by the phosphoric acid itself. Excessive fluoride in phosphoric acid is usually inhibited by the presence of iron and aluminium and therefore it is only in those cargoes which have insufficient iron and aluminium that the fluoride-initiated attack will take place. Recently, however, more incidents of ship’s tank damage have been observed with high levels of iron and aluminium which are clearly not fluoride-initiated attacks, but have been caused by some other element. Further research has lead to the belief that chlorides (600-800ppm) initiated the corrosion. Although chloride corrosion attacks are typically in the form of pitting, it has come to light that that if chlorides initiate the corrosion, phosphoric acid will then continue to attack so that the resulting damage is in the form of general steel wastage and/or elephant skin.
There are of course other corrosion stimulators and inhibitors in phosphoric acid:
Spoils passivity, may cause pitting
Passivates the steel
Lowers pH, liberates HF action
May cause crevice corrosion
The obligation upon charterers is the implied term not to load dangerous cargoes. Dangerous cargoes are those that are likely to cause damage or delay to the vessel. However, it is always easier to bring claims on behalf of owners against charterers as timecharterers cover a range of cargoes that are rarely individually specified. It is more difficult when vessels are voyage chartered to carry a named cargo of phosphoric acid, such as Phosphoric Acid – 52/57% Concentration, Max Loading Temperature 45°C, Max 1% Sediments, SG 1.63/1.7; because the cargo may meet that specification, but nevertheless be exceptionally aggressive. In such a case, it will be necessary to establish that the cargo was of an entirely different or unusual character from that which was agreed. It is to be noted that stainless steel manufacturers have generally imposed a new, reduced safe loading temperature for phosphoric acid of 35°C.
Source of Information:
Russell Ridley, (Partner specializing in Chemical Tankers)
Sinclair Roche and Temperley
Tel +44 (0)20 7452 4000
Fax +44 (0)20 7452 4001