Factors influencing crystallisation from mixed acid pickling baths for stainless steel

During pickling of stainless steel metal fluorides are generated in the acid solution. If the concentration gets too high, precipitation and crystallisation can occur. Different factors that may influence the crystallisation in pickling bathes of mixed acid (HF/HNO3) have been investigated in laboratory experiments and evaluated also with multivariate methods. The results are compared with some findings reported in literature. Some of the results are: In order to predict if crystallisation occurs it will be needed to know values of several parameters like retention time, temperature, concentration of nitric acid, hydrofluoric acid and iron. Single knowledge of e.g. the iron concentration is not sufficient. Higher concentrations of iron, hydrofluoric acid and nitric acid increase the probability for crystallisation. Time is an important factor for crystallisation. Pickling bathes from different companies showed some difference in the crystallisation behaviour. Presence of seed material, both metal fluoride crystals and iron oxide increases the crystal growth rate. Other parameters that were investigated are: Kinetic energy by shaking, the age of the test solution, increased pressure and aeration (intensified contact with air)