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Wet phosphoric acid. |
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| Normal, 54% phosphoric
acid is in itself not very corrosive to stainless steel. The impurities normally found in the acid can contribute to corrosion. The main ones are chloride and fluoride, which disrupts passivity, sulphate which lowers the pH, and solids, which can cause crevice corrosion when combined with an electrolyte. To minimise the chances of the product damaging the tanks, the following can be done: |
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| 1. Make sure the acid
temperature at loading is less than 50 deg. C 2. Always use fresh water for tank cleaning. 3. Stir the cargo using the cargo pump and drop-line during the voyage. 4. If the chloride concentration in the phosphoric acid exceeds 200ppm, then caution must be exercised. |
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| To emphasise the
importance of tank cleaning after discharge, if the cargo has left adherent deposits
behind, then these must be fully removed if any seawater tankcleaning has to be done. Tightly adherent deposits will absorb chlorides from sea water, and if not completely removed during cleaning, these will eventually initiate corrosion. This is true of all cargoes containing solid matter. |
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