Wet  Phosphoric Acid (Wet-PAC)

The corrosion to austenitic stainless steel of SUS 316L and 316LN occurring due to carriage of Wet Process Phosphoric Acid is a very complex phenomenon. White Phosphoric acid (pure food grade) is not corrosive to stainless steel, however, the Wet PAC (fertilizer grade-green PAC and BK PAC) is perhaps the most aggressive and difficult of all chemicals transported in a stainless steel chemical tanker. The corrosion is affected by various impurities normally present in wet process acid. Only 317LN and duplex 2205 stainless steels can be said to resist this acid without too much damage and that too if the loading temperatures are within limits.

The tank surface should be smooth polished under 3.5 micron. Tank cleaning before and after loading requires special attention in order to avoid contamination. For example, after discharge the tanks should be cleaned by fresh water first, than seawater, then immediately again fresh water to remove all chlorides from tank. This is crucial to avoid elephant skin.

Please note acid / sea water mixture will result in pitting. After final cleaning with fresh water (pH7 neutral) tanks needs to be passivated in order to restore the passivity of the stainless steel as the phosphoric destroys the passive oxide film (coating) of the stainless steel.

Passivity

The corrosion resistance of stainless steels depends on the quality of a very thin oxide layer, the so called “passive layer”. Maintaining this passive layer is one of the most important aspects in maintaining the quality of stainless steel. Damage of the passive layer, in time, will cause severe corrosion. Therefore preserving the passive layer is a must. Passivation or Re-Passivation on regular base is highly recommanded to ensure a solid uniform Chromiun Oxide layer. High Passivity means better corrosion resistance.