Acid pickling, surface contamination

The inherent solderabiUties of selected alloys are Hsted in Table 9. Class IV and V ratings with this particular flux indicate the presence of oxides or other surface contaminants that may be removable with more aggressive flux or acid pickling. 

Surface Preparation. Steel hot rolled at 800-900 °C acquires a tenacious oxide layer (mill scale) that is cathodic with respect to the steel to the extent of about 300 mV. In the presence of an electrolyte (seawater containing 3.5% salts, mainly sodium chloride) the steel would corrode and pit and roughen severely. The first process in new construction and refurbishment is therefore the complete removal of mill scale. A small amount of very light-gauge steel is prepared by acid pickling, but most steel for ship and off-shore construction is centrifugally or pneumatically blasted with steel shot that can be recycled or with expendable abrasive grit. Freedom from scale and soluble salt contamination are the main requirements, texture and profile are less important [11.13],... 

Descaling of hot worked stainless steels can be accomplished by mechanical or by chemical cleaning, or by a combination of both methods. The surface can be cleaned by sand blasting, but unless it is machined after blasting, only nonmetallic blast material should be used in order to avoid contamination and a reduction of the corrosion resistance. Following blast cleaning stainless steels are usually acid pickled and washed with water. Common etching acids are ... 

Tarnish films are readily removed by abrasive methods, and all but the heaviest films can be removed by acid pickling. Prolonged exposures at temperatures above about 600 °C (1100 °F), in combination with surface contaminants, result in heavier surface films that are not removed satisfactorily by acid pickling, but require descaling treatments for their removal. 

Platinum, palladium and the normal alloys of platinum used in industry are easily workable by the normal techniques of spinning, drawing, rolling, etc. To present a chemically clean surface of platinum and its alloys after fabrication, they may be pickled in hot concentrated hydrochloric acid to remove traces of iron and other contaminants —this is important for certain catalytic and high-temperature applications. In rolling or drawing thin sections of platinum, care must be taken to ensure that no dirt or other particles are worked into the metal, as these may later be chemically or elec-trolytically removed, leaving defects in the platinum. 

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