The most functional method to obtain a decontaminated and homogeneous stainless steel surface is by means of immersion in a specific acid compound that removes a very thin surface film and consequently removes the contaminating agents that have deposited on it. Correct pickling in a solution containing fluorides results in an attractive even grey surface appearance, an effect of the chemical micro-pickling action.
The most cost-effective method for achieving a clean and passive stainless steel surface is immersion in an acidic pickling solution. Correct pickling in a pickling solution containing fluorides enables an attractive, uniform grey surface appearance to be attained, an effect of the chemical micro-pickling action. Some grades of stainless steel are unsuited to extended immersion in pickling solutions for austenitic steels because they are more sensitive to chemical attack.
Materials typically prone to over-pickling are those containing sulphur to improve the machinability of the material (Aisi303), and also those with a lower nickel content (400 series). Special pickling solutions are recommended for the treatment of these less resistant materials. A clean, contaminant-free stainless steel surface independently generates its own passive layer of chromium oxide when it comes into contact with oxygen in the air. It is precisely this passive surface layer that gives this material its excellent resistance to chemical attack and corrosion in general. Chemical passivation can also be used, however, to accelerate the formation of this protective layer. Chemical passivation is not always necessary, but is certainly recommended when surfaces are not well ventilated, for example inside pipework or in hermetically sealed reactors.
Our chemical pickling and passivation treatments are compliant with ASTM A380/06 and ASTM A967/99. Passivation of an electropolished surface is unnecessary due to the strong formation of surface oxide after the electropolishing stage (ASTM 912/02). Pickling and passivation treatments do not necessarily have to be carried out by immersion in tanks, although this is certainly the most widely used method. When the parts to be treated are large or the process needs to take place on-site, an interesting spray technique can be used; this involves spraying the entire component with a liquid gel using specific pumps for the rapid treatment of large surfaces.
The final finish is indistinguishable from that achieved by immersion. When only small areas need to be treated - for example near to welds - pickling pastes and gels can be applied with brushes. When stainless steel surfaces with not particularly stubborn oxides need to be decontaminated, a product can be used to completely remove them without markedly altering the colour of the treated surface.
Delmet can supply a wide selection of systems, products and tools to carry out these processes either automatically or manually. Also available at the operational headquarters in Gorgonzola is a sub-contract service equipped to immediately meet your production needs.