The corrosivity of phosphate ores, used in the production of wet process phosphoric acid (WPA) depends on two main factors: the chloride content and the interaction between HF formed in the WPA reaction slurry with SiO2, Al2O3 and MgO present in the ore. During the production of WPA, HF or F- (free fluoride ion) is highly corrosive but complexed fluoride compounds, e.g. H2SiF6, are non-corrosive towards the austenitic stainless steels and Ni-base alloys currently used in WPA service. Consequently, it is a common practice in the WPA industry to add silica- and aluminacontaining clays, or acid-soluble silicates, to reduce the corrosion effects. The quantitative relationship between fluoride and its complexants is expressed by a complexing ratio in chemical equivalents: F/(SiO2 + Al2O3 + MgO). This ratio was applied for the evaluation of the corrosivity of phosphate ores and for the calculation of addition of complexants to the phosphate ore or the WPA reaction system. The effect of clay addition on corrosion in an industrial WPA plant is illustrated.