Technical center

Phosphoric acid, Wet-process acid. From a corrosion point of view, wet process phosphoric acid (WPA) is a rather complicated chemical. The reason is that it usually contains large amounts of impurities, some of which are activating and others will inhibit the corrosion process. The most detrimental impurities are hydrochloric and hydrofluoric and sulphuric acids, while ferric and aluminium ions are beneficial. The following four tables present corrosion test results performed in two series of synthetic wet process acids. In one of the series the detrimental impurities chloride and fluoride have been varied, while the content of the others, iron, aluminium and sulphuric acid, have been kept constant. In the other series the content of iron and aluminium was varied and all the others kept constant. The test acids also contained other compounds in constant quantities. There were: 54% P₂O₅ , 0.1% SiO₂, 0.2% CaO and 0.7% MgO. Due to the rather complex nature of WPA corrosion, the data presented must be regarded as examples only. In critical applications it would therefore be well advised to get additional information from corrosion expertise. The necessary data to determine the risk for corrosion are the composition of the acid and the maximum operation temperature. Phosphoric acid. Synthetic WPA acid Constant concentrations: 1.0% Fe₂O₃, 1.0% Al₂O₃, 4.0% H₂SO₄