Pickling for 300-series stainless steel items, along with nickel and other alloys, is the method of choice for removing weld and heat scale. The item is thoroughly cleaned to remove oily or greasy residues. Then, a pickling product, like those offered by QWP, is applied to the affected area. Pickling paste is applied to a welded seam and surrounding area. Pickling spray or immersion in a pickling bath can also be used. Pickling products contain strong acids, including nitric and hydrofluoric acid, along with other additives. Safety precautions, as described in each product’s MSDS and Product Data Sheet
must be used to prevent contact with the skin or eyes, ingestion, and inhaling fumes whenever pickling products are used.
The pickling operation continues for a time period recommended by the manufacturer in a Product Data Sheet
for that product. After the prescribed time, which depends on a number of factors, pickling paste is neutralized with an application of neutralizing paste, which reacts with the pickling paste to convert it from an acidic state to an alkaline state. The residue is then removed by rinsing with cold drinking-quality or deionized water. Spray pickling is generally removed with high pressure rinsing, with passivation occurring naturally. Bath pickling uses a second passivation or rinse bath to complete the pickling & passivation process.
Once the pickling process is complete, passivation occurs as a natural process in the presence of oxygen in the air. The chromium content of the items surface oxidizes quickly, restoring the passive layer on the surface. Passivation is a natural result of the pickling process.
The effectiveness of the passivation process can be tested. There are several tests, but one of the most common is the Copper Sulfate Test, as described in ASTM 380/967. After the pickling procedure is completed, a few drops of a copper sulfate solution are put on the surface of the pickled area. After several minutes, if the area remains colorless, the passive layer has formed. If it turns a reddish color, it means that there is still free iron on the surface and the passive layer has not formed.
Since the pickling products and the rinse water runoff are contaminated with chromium and other materials, methods must be used to capture these materials, which cannot be allowed to enter the sewer system or other drains. Local hazardous waste contractors can provide proper transportation, treatment and disposal.