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.
Under certain specific conditions, particularly involving chlorides (such as sodium chloride in sea water) and exacerbated by elevated temperatures, small pits can form in the surface of the steel. Dependent upon both the environment and the steel itself these small pits may continue to grow, and if they do, it can lead to perforation, while the majority of the steel surface may still be totally unaffected. Pitting corrosion is a form of extremely localized corrosion that leads to the creation of small holes in the metal. The driving power for pitting corrosion is the lack of oxygen around a small area. This area becomes anodic while the area with excess of oxygen becomes cathodic; leading to very localized galvanic corrosion. The corrosion area tends to bury into the mass of the metal, with limited diffusion of ions, further pronouncing the localized lack of oxygen. This kind of corrosion is extremely insidious, as it causes little loss of material with small effect on its surface, while it damages the deep structures of the metal. The pits on the surface are often obscured by corrosion products.
Pitting corrosion is a localized form of corrosion by which cavities or "holes" are produced in the material. Pitting is considered to be more dangerous than uniform corrosion damage because it is more difficult to detect, predict and design against. Corrosion products often cover the pits. A small, narrow pit with minimal overall metal loss can lead to the failure of an entire engineering system
Pitting corrosion, which, for example, is almost a common denominator of all types of corrosion can produce pits with their mouth open (uncovered) or covered with a semipermeable membrane of corrosion products. Pits can be either hemispherical or cup-shaped. Contamination of stainless steel is not just a “cosmetic” problem; in the long term, those contaminants can cause localized damage (pitting) that can be very harmful.
Additional Information: 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.