Surface treatment of stainless steel
Stainless steel is used in fact due to its specific corrosion resistant features. This corrosion resistance is the result of its natural capacity to form a protective oxide film. This oxide film is however extremely thin and therefore very vulnerable. Many of the treatments that the stainless steel undergoes in the construction and sometimes also in the transport phase may result in damage to the oxide film. These will result in the stainless steel losing its corrosion resistance with the possibility of, depending on the application, (corrosion) damage arising. The main causes for the onset of (corrosion) damage are: Oxidation (discoloration) of the heated zone after welding and annealing after heat treatment: Application of heat, for example during welding, causes discoloration due to oxidation with an excessively thick oxide film and a chromium impoverished zone. These locations provide significantly reduced resistance to local corrosion (chloride cavity corrosion). Alien iron: Numerous sources may be stipulated whereby alien iron contamination of the stainless steel surface may occur. For example as a result of sanding, grinding, setting with material (tools) made of carbon steel or tools that are also used for processing carbon steel. Handling and transport of the material are also a frequent source of contamination. Pickling after the construction phase will restore the corrosion resistance and remove chromium impoverished zones and alien iron. It will restore the corrosion resistance to the basic materials original level. Pickling and Passivation Ceramic pearl blasting Electrolytic polishing Grinding and polishing Cleaning stainless steel for oxygen and ultraclean applications PICKLING AND PASSIVATION Pickling Pickling after the construction phase will restore the corrosion resistance to the same level as of the basic material. Pickling will remove the oxide scale (discolouration due to welding and annealing scale), areas of reduced chromium and foreign iron. Pickling will also remove the naturally occuring chromium oxide layer and render the surface active.
Pickling Immersion in pickling fluid for Austenitic stainless steel types. Passivation There are quite a number of misunderstandings with regard to the passivation of stainless steel. After pickling and complete removal of oxide scale/contamination, stainless steel will passivate spontaneously when exposed to air. No difference has ever been detected in the oxide layer quality resulting from chemical passivation relative to natural (air) passivation.
CERAMIC PEARL BLASTING Introduction Glass or glass pearl blasting is a common and widely used procedure for giving stainless steel a uniform finish after treatment. Glass blasting of stainless steel has however a number of significant drawbacks. The high breakdown rate of the glass balls means that in practice blasting is with a high percentage of broken balls, resulting in a relatively high surface roughness (2.0-3.0 micron). Glass blasted surfaces have shown in practice to be susceptible to attracting dirt and are difficult to clean. This combination of contamination of surfaces with high roughness together with difficulty of cleaning allows corrosion to set in even in a relatively mild (ambient) environment. Advantages of ceramic pearl blasting Blasting with ceramic pearls is free of the disadvantages described. The breakdown rate for ceramic pearls is a factor of 50 lower. This guarantees that processing will always be conducted with perfectly global pearls (circa 100 micron). This is immediately apparent in the visual result of surfaces that have been blasted with ceramic pearls: a uniform satin finish. Depending on the roughness of the untreated material a roughness less than 1.0 micron can be achieved. Applications Blasting with ceramic pearls is for this reason suitable for stainless steel products where appearance is important. Examples of this are stainless steel cladding for walls, shop and office fronts, staircases, lifts and road furnishings (cladding of viaducts, traffic signs etc.). Their susceptibility to corrosion under atmospheric conditions is significantly reduced by this treatment. Blasting with ceramic pearls has proved its suitability for the treatment of stainless steel surfaces in for example the food and harmaceutical industries. The lower roughness makes the surfaces much easier to clean. This is certainly true for industries where the contamination consists of microbes. Vecom Metal Treatment B.V. has facilities for ceramic pearl blasting. Using permanent magnets metallic iron is removed continuously. The material can when required be chemically passivated and rinsed with demineralized water after ceramic pearl blasting to remove for 100% residual blasting material. This treatment will also reduce susceptibility to staining (common phenomenon for glass pearl blasting). There are obviously benefits with respect to price, quality and delivery time associated with combining pickling, ceramic pearl blasting and perhaps chemical passivation at one location. In addition to its corrosion resistance (recovery of) the finish can be guaranteed too. The most important reason for pickling stainless steel is in order to remove welding discolouration and alien iron, thus restoring the corrosion resistance of the material completely. By then blasting with ceramic pearls a uniform, satin sheen can be achieved. Enumerating the benefits once more: The low breakdown percentage means that blasting will be conducted with perfect spherical pearls. Completely iron-free, therefore no risk of contamination. Elegant uniform finish (satin sheen) Depending on the initial state a roughness of Ra < 1,0 mm can be realized. Pickling/passivation and ceramic pearl blasting on one location saves time and (transport) costs Eminently suitable for the treatment of stainless steel products for the food and pharmaceutical industries.
GRINDING AND POLISHING Depending on the initial conditions it is possible to reduce the Ra value (roughness). The specification adopted for grinding is the grain of the grinding medium or the final roughness required in Ra value. The roughness is reduced in stages (grain 40, 80, 120, 180, 240, 400). Mechanical polishing of stainless steel may due to forcing of polishing paste into the surface result in that surface no longer being suitable for specific applications: pharmaceutical, medical and food industry and ultraclean application.
CLEANING STAINLESS STEEL FOR OXYGEN AND ULTRACLEAN APPLICATIONS Systems for pure oxygen applications require a total absence of organic contamination (oil, grease, fingerprints etc.). Even minimum traces of contamination may lead to spontaneous combustion of oxygen (read explosion). The cleaning of pipes, heat exchangers, air coolers etc. for pure oxygen applications requires a high level of purity on the part of cleaning products, rinse water etc. Vecom has many years of experience of cleaning stainless steel for pure oxygen application and meets the specifications of all internationally prominent companies. Apart from the cleaning procedure (in many instances ultrasonic cleaning) the final inspection of the material treated is also of crucial importance. Final inspection may be by UV light, wipe test or rinsing with chemically pure solvents. After every cleaning for pure oxygen or ultraclean application our laboratory prepares a certificate of completion. After cleaning and inspection the parts treated are packaged in accordance with special procedures to ensure safe handling and transport.