Carpenter

Fabricating Carpenter Stainless Steels

June 2006  

Cleaning and Passivating Carpenter Stainless Steels

Cleaning Processes 

Practically all finishing operations require that fabricated stainless parts be subjected to some type of cleaning operation. These include painting, enameling, electroplating, metallizing, buffing, and polishing.  It is generally necessary to clean after welding, brazing, and machining operations. Heat treating often requires cleaning both before and after that operation.

The specific method of cleaning to be employed depends primarily upon the surface contamination present. However, there are other considerations such as design, subsequent operations, cleaning equipment required, shop operating conditions, production volume, cost, and some special precautions.  

The following methods can be used to clean stainless steels depending upon the surface contamination:

• Soak alkali cleaning
  
• Soak emulsion cleaning
  
• Soak acid cleaning
  
• Machine cleaning
  
• Electrocleaning
  
• Barrel cleaning, tumbling
  
• Steam cleaning
  
• Petroleum spirit cleaning
  
• Vapor degreasing
  
• Acid pickling
  
• Electrolytic pickling
  
• Salt bath cleaning, descaling
• Blast cleaning
  
• Brushing
  

 

Scale or Foreign Contamination

Cleaning operations can generally be divided into two categories: (1) methods for removing foreign contamination such as lubricants, paints, shop dust, polishing compounds, etc., and (2) methods for removing surface oxide or scale resulting from forging, heat treating, welding, etc.

The principles and methods used for removing foreign contamination from stainless steel are similar to the methods used for other metals. Space does not permit a description of all the metal cleaning methods. However, this section does describe some of the methods used for removing scale and oxide. Carpenter has developed most of this information in its own mill through experience with descaling and pickling processes.  

Excellent information has been published by the A.S.T.M. as shown in Standard Recommended Practice for Cleaning and Descaling Stainless Steel Parts, Equipment, and Systems, Designation A-380.  Topics covered in detailed include descaling, pickling, and passivating processes.

Descaling

Both mechanical cleaning and molten salt baths can be used for removing heavy scale from stainless steel.

Mechanical cleaning methods include dry blasting with cast iron grits, metal shot, cut wire or sand. Wet blasting will provide a better finish but is applicable mainly to small parts or when a light scale is to be removed. Brushing and tumbling might also be included as mechanical descaling methods.

Salt bath descaling methods have been found most advantageous for removing scale from large production lots of stainless steel. These methods involve molten salts operated at temperatures ranging from approximately 700º to 900ºF (371º to 482ºC). The sodium hydride caustic soda process is quite versatile; scale is removed by a reducing process so that base metal is not removed. Alternatively, the Hooker and Kolene processes oxidize the scale to a form in which it is more easily removed by acid pickling. 

Acid Pickling

Most descaling methods must be followed by acid pickling for the complete removal of surface oxide. Perhaps the most versatile acid bath for removing scale from all types of stainless steel is a solution containing 10% by volume sulfuric acid, with or without an inhibitor, operated at approximately 150º to 180ºF  (66º to 82ºC). Following a descaling treatment, this sulfuric acid pickling bath will effectively remove scale from most stainless steels, although it will react somewhat slowly with the austenitic stainless grades. A 50% by volume solution of hydrochloric acid (all acids are mixed with water), with or without an inhibitor, operated at 150º to 160ºF (66º to 71ºC), will clean all stainless grades considerably faster, although closer control is required.

Sometimes a two-bath operation is used with the austenitic stainless grades. Either the sulfuric or hydrochloric acid pickling solutions are followed by a bath consisting of approximately 10% by volume nitric acid and 2% by volume hydrofluoric acid, operated up to 120ºF (49ºC). The nitric-hydrofluoric pickle bath is used to remove the last traces of scale retained after sulfuric acid pickling processes. 

After descaling and between each pickling tank, a water rinse is always used. This may be in the form of a water blast to remove traces of scale and prevent contamination from one bath to the other.

Nearly all pickling operations impart a dark "smutty" surface on stainless steel, which can be removed in a cold 20% by volume nitric acid bath. This final pickling process both brightens and passivates the stainless steel surface. 

Acid Brittleness

Pickling can cause "hydrogen brittleness" or "acid brittleness" because of hydrogen absorption. Some hard and highly stressed parts are susceptible enough to suffer cracking during the pickling process. While hard but not highly stressed parts will seldom crack in the acid bath, they may crack in service when subjected to stress. Consequently, steels should not be "overpickled." Hardened articles should receive a stress-relieving temper before pickling and "bake" after pickling. The baking process consists of heating the part to about 300º to 400ºF (149º to 204ºC) for several hours to remove hydrogen and restore ductility.

Examples of pickling techniques most likely to cause cracking are hot hydrochloric acid and electrolytic techniques, which liberate large amounts of hydrogen. When employing these methods, high-strength grades such as Type 420 or the 440 series and the precipitation hardenable martensitics should not be exposed for long periods. 

Passivating

The non-rusting properties of stainless steels are attributable to a very thin, invisible oxide film that completely covers the surfaces of the parts and prevents corrosion from taking place. Theoretically, a freshly machined, polished or pickled part will acquire this film rather quickly from the atmosphere. In practice, however, such fabricated parts may be contaminated with small particles of foreign matter, which must be removed to impart full stainless properties. As an example, a slight amount of material worn off the cutting tools may be transferred to the stainless parts during machining. Under certain conditions, a thin coating of rust may appear on the part. This is corrosion of the tool steel and not the parent metal.

The primary purpose of a passivating treatment is to remove surface contamination, usually iron, so that the optimum corrosion resistance of the stainless steel will be maintained. Passivation is not a scale removal treatment.  

Basic procedure in passivating consists of cleaning the work with a commercial degreaser or cleanser, immersing it in a solution containing nitric acid, rinsing and drying it. The importance of cleaning prior to acid bath immersion cannot be over-emphasized. In some instances, this step is omitted, assuming the acid bath will give the necessary cleanness. Cleaning should not be skipped because the acid might not remove all of the residual cutting fluid, resulting in possible chemical reactions with the residual cutting fluid known as "flash attack." These unwanted reactions may cause serious deterioration of the surfaces that passivation is designed to protect.

After degreasing and thorough water rinsing, passivation of the stainless steels should take place according to the following table: 

Passivating Stainless Steels

The addition of sodium dichromate or use of 50% nitric acid solution increases the "passivating potential" of the bath so that undesirable local attack is less likely.

The free-machining grades differ from the regular grades of stainless steels because they contain a large number of nonmetallic inclusions throughout their microstructures which create microscopic discontinuities in the machined part surfaces. Even normally efficient water rinses can leave residual acid in these discontinuities after passivation. This acid can then attack the surface of the part unless it is neutralized or removed. Work in Carpenter's research and development laboratory has shown that the following passivating procedure for free-machining grades will produce resistance to subsequent superficial rusting. This procedure is known as the Alkaline-Acid-Alkaline, or A-A-A, passivation method.

Other Important Considerations

Maintain an effective passivating solution to prevent localized attack. Tap water is usually adequate for diluting the acid, although high chloride contents (greater than several hundred ppm) could be deleterious in a borderline situation. Nitric acid concentration should be checked periodically using a simple titration procedure, which can be provided upon request.  

When high production rates cause a heavy flow of material through a passivating bath, it is probably best to maintain a definite schedule for replacing the bath to avoid a significant decrease in the "passivating potential," which can result in corrosive attack of the work piece. You should also use a control sample of the same composition as the material to be passivated to test the bath. If the sample is attacked, it is time to change the bath before additional parts are passivated.

The temperature of the bath should be within the specified temperature range. A room temperature bath has a lower "passivating potential" than a warm bath and is, therefore, more likely to cause local attack. 

It is good practice to passivate only one grade of stainless steel at a time. Not only can mix-ups be prevented but you can avoid galvanic reactions.

Parts that were improperly heat-treated may lead to attack in a passivating bath. Furthermore, high-carbon, high-chromium grades must be hardened to render them corrosion resistant. Stainless steel parts that have been carburized or nitrided should never be passivated. These surface treatments lower the corrosion resistance of stainless steel, thus opening the way to attack in the passivating tank.