Stainless steel care, cleaning, surface prep and passivating

Stainless steel is corrosion resistant because it has the ability to form and maintain a thin, transparent, protective film of chrome oxide over its surface. Under normal conditions this film is self-healing should parts of it be destroyed.

As long as the film of chrome oxide is maintained, the stainless steel behaves like gold, silver or platinum, or in other words, it has a passive behavior. Stainless steel can also develop active sites if the protective film is destroyed by scratches, nicks, stock deposits or contamination by mild steel or non-ferrous inclusions.

A cell develops between the active and passive areas if a suitable electrolyte is present. In the closed loops of today’s paper machines, the white water system provides a very good electrolyte for these cells. The more dissolved the solids and the higher the temperature of the electrolyte, the faster the rate of corrosion.

NOTE: Electropolishing is being used more and more in the manufacturing of stainless steel equipment. The care, cleaning, and passivating procedures to be used for electropolished surfaces will be discussed later in this article. Equipment which Valmet recommends the mill passivate after installation and prior to start-up is also listed in a separate section of this article.

Care and cleaning

Cleanliness and care in handling, storage and installation are extremely important for the prevention of corrosion to stainless steel. Among the preventive measures that should be observed are the following:

• Holding stainless steel fabrications in original crates or wrappings pending the start of installation.
• Providing clean indoor storage places such as clean racks, shelves and platforms, and using covers where possible.
• Providing storage places well removed from sources of mill dirt or other contamination. Fine particles of scale from carbon steel fabrication or fragments of other metals undergoing work should be prevented from collecting on exposed metal.
• Avoiding walking over stainless material with shoes that are dirty or that may contain iron hobnails.
• Handling stainless material with clean gloves or handling cloths to guard against residues of human persiration (finger marks).
• Avoiding scratching with sharp tools or abrasives containing iron.

Surface preparation

The most important step in the passivating procedure is the proper cleaning of the surface to be passivated. Wash new surfaces with soap or a mild detergent and hot water and wipe with a soft cloth. Clean old surfaces with a bristle (nonmetallic) brush and a detergent or cleaning compound. Two cleaning agents which have proven satisfactory are: Oakite Highlite (cleaning agent) or 3M Scotchbrite (non-metallic pad). Flush with clean water before applying passivating mixture.

NOTE: Pad type cleaners are not recommended on new surfaces and are to be used only when deposits on stainless steel can not be removed with soap or a mild detergent. Use these cleaning agents sparingly. However, material adhering to the surface must be removed to properly passivate the base material.

Deep scratches, nicks or pits must be repaired by welding, grinding and polishing before beginning the passivating procedure. Contact Valmet for welding procedures and materials to be used for your particular application or contact Valmet to have a service engineer present to supervise work to be done.

See ASTM A380 “Standard Practice for Cleaning and Descaling Stainless Steel Parts, Equipment, and Systems.” The above publication will also provide you with a list of ingredients needed for a passivating solution.

Application

The passivating mixture can be applied with a “dock brush”, rubber squeegee or other non-metallic applicator. Allow the solution to remain on the coated areas for about 15 minutes. Flush with clean water (preferably hot) to completely remove the passivating solution.

In areas not accessible with a paste type passivator, it is feasible to use the same formula excluding the barium sulfate.

Electropolished surfaces

An electropolished surfaces gives good protection against corrosion if properly shipped, handled and installed. Cleanliness and removal of all contaminents from the surface is extremely important both at the time of installation and during operation.

CAUTION: If there are signs of corrosion at any time on the electropolished surface, the surface should be cleaned with water and nitric acid only, and wiped with a soft cloth. Ammonium Biflouride will etch the surface destroying the electropolished finish.

Do not use any brushes, pads or abrasive material on the electropolished surface. These may scratch the polished surface and make the stainless steel more susceptible to corrosion.

Equipment passivated at time of installation

The stainless steel equipment being shipped from Valmet has been carefully handled and isolated from contaminants at the time of manufacture. These areas are also well protected during shipping. During any period of storage and at the time of installation, there is a high potential for the stainless steel surfaces to become contaminated before start-up (especially the external surfaces).

Valmet recommends passivating for the following stainless steel equipment at mill installation:

Headbox

• All surfaces exposed to stock flow (Nitric acid wash only if electropolished).
• All external stainless steel.

Fourdrinier/Former

• Forming board, foil boxes, forming shoe
• Suction boxes
• Savealls
• Major framework
• Deflectors
• Flo-Vac
• Roll mountings
• Handrails
• Control cabinets

Press

• All cross machine framework
• Savealls
• Uhle boxes
• Doctor backs
• Major framework (stainless steel cladding)
• Crosswalks
• Swing arms
• Handrails

For more information on the care of stainless steel on your paper making machinery, contact your Valmet representative.