Pretreatment of Metallic Substrates

Many processes and techniques are used for pretreating metallic substrates (Table 8.1), [8.1]-[8.4]. 

Chemical Methods. Chemical methods include pickling, which can be used to remove scale and metal oxides. Sulfuric acid and hydrochloric acid are usually used to treat steel and zinc, while nitric acid, nitric acid-hydrofluoric acid, or sulfuric acid-hydrofluoric acid are used for aluminum. Although pickling in phosphoric acid is somewhat slower, it has the advantage that a thin conversion layer of the substrate phosphates is produced, which provides temporary corrosion protection 

Processes using aqueous solutions or organic solvents have become extremely important for removing organic impurities. In special cases, salt melts and treatments at elevated temperature in the gas phase are also employed. 

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