Stainless steels are used in a wide variety of applications due to their excellent corrosion resistance properties. In order to achieve this, it is necessary to carry out appropriate cleaning and finishing processes during fabrication, thereby preventing any impairment in steel surfaces.
Passivation is carried out to eliminate iron from the surfaces of stainless steels by developing a passive surface layer. The presence of carbon steel, low alloy steel, mild steel, cast iron or any iron particles on the surfaces will tend to develop pitting corrosion of the cells between the stainless steel and free iron. This major problem occurs because of the contamination resulting from grinding swarf, or scraping carbon steel fixtures or tools.
Pickling is a process of reducing the high temperatures of stainless steels, from hot working, heat treatment or welding, by means of acid treatment. This process also aids in removing steel or iron particles and red rust from the corroded steel surfaces. Corrosion particles formed after rusting of free iron cannot be removed using passivation. Besides producing undesirable aesthetics, rusting of surfaces due to high temperatures also minimizes the corrosion resistance of the steel surface layer.
Electropolishing is a process of brightening the surface of stainless steels through electrochemical dissolution of high points. It is exactly the opposite of the electroplating process, but it is carried out using equipment similar to that of electroplating.
Electropolishing can produce a smooth finish. However, preliminary trials must be carried out to determine the polishing and surface condition parameters. The process can also produce frosted surfaces in some cases.
Grinding and Polishing
Although stainless steels can be easily buffed, polished and ground, certain properties of these steels need to be modified to achieve the best results. For instance, high strength is required for loading abrasive media, and low thermal conductivity is required to improve surface heat. This, in turn, causes surface smearing or heat tinting, and even re-hardening of martensitic grades, or sensitization of austenitic stainless steels in extreme conditions.
Polishing methods using coarse abrasives will generally have a greater impact on the corrosion resistance of stainless steels. Polishing with chromium or aluminum oxide to attain smooth finishes, such as No.7 and No.8 buffed finishes, eliminates surface imperfections and fine pits, and enhances corrosion resistance.
It is essential to remove iron contamination, in order to achieve good corrosion resistance of polished stainless steel surfaces. Abrasives and polishing compounds need to be free of iron, and the devices used for stainless steel processing should not be used for other metals. Under circumstances where these conditions cannot be satisfied, corrosion resistance of stainless steels can be improved using cleaning/passivation treatment.
Sand blasting is carried out with garnet sand or clean silica. Cut wire, grit and shot blasting must be performed with stainless steel media with corrosion resistance similar to that of the metal to be cleaned.
Wire brushing can be used to remove light tints formed from heat. Wire Brushes are made from stainless steels and they should not be used on any materials other than stainless steels.
Barrel Finishing and Vibratory Finishing
Barrel and vibratory finishing methods are extensively used for mechanically polishing small sections of fasteners, such as pipefittings and bolts and screws, by using abrasive media. The mechanically cleaned products thus produced are not as corrosion-resistant as the ones produced by the pickling methods. Mechanical cleaning leaves behind certain scale residues, and it can be employed before acid pickling.