The most cost-effective method for obtaining a clean passive surface on stainless steel is by immersion pickling in a solution of dilute acids. Pickling in a solution containing halogens, such as hydrochloric acid, must be avoided at all costs.
Correct pickling in a hydrofluoric based acid mixture produces a uniform matt grey finish, the effect of chemical micro-etching. Certain types of stainless steel are not suitable for lengthy immersion times in the strong acid pickle for austenitic grades , as they are susceptible to acid attack. Typical materials, subject to over-pickling, are the sulphur containing machining grades (e.g. 303) and low nickel grades (400 series). Special pickling solutions are available for these less resistant materials.
A clean stainless steel surface will generate its own passive oxide layer when exposed to oxygen. It is this oxide layer that gives stainless steels their corrosion resistant qualities. It is, however, possible to accelerate the formation of this layer by chemical passivation.
Chemical passivation is not always necessary, but is important where surfaces are possibly starved of oxygen, as for example in the case of pipework or enclosed vessels. Where items are pickled and likely to be employed in corrosive atmospheres, such as in chemical processing plants, at marine and coastal locations, chemical passivation following pickling is highly recommended. Additional passivation following electropolishing is not necessary due to the excellent oxide layer formation present after electropolishing.
Pickling and passivation of stainless steel is not restricted to immersion in treatment tanks, although this method is widely used. Where fabrications are too large for immersion or require treatment in-situ, spray-on systems can be employed. The results are indistinguishable from the finish obtained by immersion methods. Where only localised areas require treatment, for example heat effected surfaces resulting from welding, paste products can be applied with a brush.
Cleaning, pickling and passivating products are available in liquid and paste form. Anopol also markets the WELDKLEEN machine, an environmentally friendly way of removing weld burn