Stainless Steel Passivation & Cleaning
ASTM A380 - 06 Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel
Parts, Equipment, and Systems.
1.1 This practice covers recommendations and precautions for cleaning, descaling, and passivating of new stainless steel parts, assemblies, equipment, and installed systems. These recommendations are presented as procedures for guidance when it is recognized that for a particular service it is desired to remove surface contaminants that may impair the normal corrosion resistance, or result in the later contamination of the particular stainless steel grade, or cause product contamination. For certain exceptional applications, additional requirements which are not covered by this practice may be specified upon agreement between the manufacturer and the purchaser. Although they apply primarily to materials in the composition ranges of the austenitic, ferritic, and martensitic stainless steels, the practices described may also be useful for cleaning other metals if due consideration is given to corrosion and possible metallurgical effects.
1.1.1 The term passivation is commonly applied to several distinctly different operations or processes relating to stainless steels. In order to avoid ambiguity in the setting of requirements, it may be necessary for the purchaser to define precisely the intended meaning of passivation. Some of the various meanings associated with the term passivation that are in common usage include the following:
Passivation is the process by which a stainless steel will spontaneously form a chemically inactive surface when exposed to air or other oxygen-containing environments. It was at one time considered that an oxidizing treatment was necessary to establish this passive film, but it is now accepted that this film will form spontaneously in an oxygen-containing environment providing that the surface has been thoroughly cleaned or descaled.
Passivation is removal of exogenous iron or iron compounds from the surface of a stainless steel by means of a chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination but will not significantly affect the stainless steel itself. This process is described in a general way in and defined precisely in with further reference to the requirements of and Part II of the table on acid cleaning of steel. Unless otherwise specified, it is this definition of passivation that is taken as the meaning of a specified requirement for passivation.
Passivation is the chemical treatment of a stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancing the spontaneous formation of the protective passive film. Such chemical treatment is generally not necessary for the formation of the passive film.
Passivation does not indicate the separate process of descaling as described in Section, although descaling may be necessary before passivation can be effective.
1.2 This practice does not cover decontamination or cleaning of equipment or systems that have been in service, nor does it cover descaling and cleaning of materials at the mill. On the other hand, some of the practices may be applicable for these purposes. While the practice provides recommendations and information concerning the use of acids and other cleaning and descaling agents, it cannot encompass detailed cleaning procedures for specific types of equipment or installations. It therefore in no way precludes the necessity for careful planning and judgment in the selection and implementation of such procedures.
1.3 These practices may be applied when free iron, oxide scale, rust, grease, oil, carbonaceous or other residual chemical films, soil, particles, metal chips, dirt, or other nonvolatile deposits might adversely affect the metallurgical or sanitary condition or stability of a surface, the mechanical operation of a part, component, or system, or contaminate a process fluid. The degree of cleanness required on a surface depends on the application. In some cases, no more than degreasing or removal of gross contamination is necessary. Others, such as food-handling, pharmaceutical, aerospace, and certain nuclear applications, may require extremely high levels of cleanness, including removal of all detectable residual chemical films and contaminants that are invisible to ordinary inspection methods. The term "iron," when hereinafter referred to as a surface contaminant, shall denote free iron.
1.4 Attainment of surfaces that are free of iron, metallic deposits, and other contamination depends on a combination of proper design, fabrication methods, cleaning and descaling, and protection to prevent recontamination of cleaned surfaces. Meaningful tests to establish the degree of cleanness of a surface are few, and those are often difficult to administer and to evaluate objectively. Visual inspection is suitable for the detection of gross contamination, scale, rust, and particulates, but may not reveal the presence of thin films of oil or residual chemical films. In addition, visual inspection of internal surfaces is often impossible because of the configuration of the item. Methods are described for the detection of free iron and transparent chemical and oily deposits.
1.5 This practice provides definitions and describes good practices for cleaning, descaling, and passivation of stainless steel parts, but does not provide tests with acceptance criteria to demonstrate that the passivation procedures have been successful. For such tests, it is appropriate to specify one of the practices listed in Specification A 967.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. (For more specific safety precautions see , , Section , , and .)
2. Referenced Documents
A967 Specification for Chemical Passivation Treatments for Stainless Steel Parts
F21 Test Method for Hydrophobic Surface Films by the Atomizer Test
F22 Test Method for Hydrophobic Surface Films by the Water-Break Test
Fed. Std. No. 209e for Clean Room and Work Station Requiring Controlled Environments
While not offering all of the benefits of electropolishing, chemical passivation will improve the corrosion resistance of stainless steel. The presence of exogenous surface contaminates may disrupt the formation of stainless steels' naturally corrosion resistant surface layer. Surface contaminants, including grease, dirt, iron, and other metallic particles are inherent to the metal fabrication process. During a chemical passivation treatment, the surface of a part is chemically treated by a mild oxidant. The passivation treatment removes free iron and other foreign contaminants and promotes the formation of a chromium-rich corrosion resistant layer.
Passivation Treatments Explained
Passivation treatments are meant to stabilize the protective oxide film on the surface of "passive" stainless steel. Oxide forms an invisible and remarkably thin layer of film that is roughly 1/100,000th the thickness of human hair. Newly machined, polished or pickled stainless steel parts immediately acquire such an oxide film, simply from exposure to oxygen. Under ideal conditions, this protective oxide film completely covers all surfaces of the part.
Typically, shop dirt or particles of iron from cutting tools end up on the surface of stainless steel parts during machining. If not removed, these foreign particles reduce effectiveness of the original protective film. Passivation treatments help to counteract this process.
Passivation treatments are also effective in counteracting the effect of sulfides on stainless steel. Sulfides act as launching sites for corrosion on the surface of fabricated products. If the steel is properly passivated, sulfides are not going to be a problem.
Passivation treatments, most importantly, maximize the natural corrosion resistance of stainless steel. It removes surface contamination, shop dirt and particles from cutting tools that lead to rust or corrosion. Passivation treatments can ensure proper stainless steel performance and help avoid material malfunction.