Specifications required for stainless steel architectural products

 

  • Material composition (304 vs. 316)
  • Material finishing (turning vs. turning & polishing)
  • Material treatment (pickling and/or passivating)

 

Please quote to research, develop and present a material and process standard for the finishing of all of our architectural stainless steel products, one that will provide us with the ultimate oxidation resistant stainless steel surfaces.

 

Background;

 

We sell several (architectural) stainless steel products (http://www.bollards.ca/All-Stainless-Steel, for example), produced by a variety of suppliers who employ their own processes and standards. We want to develop the ultimate specification for the sourcing and processing of these materials, which will be used to standardize this production across all product lines & vendors.

 

Some of our stainless products have displayed oxidation in the field.  We want to prevent such occurrences by determining the ultimate material selection (304 SS vs. 316 SS),  the ultimate process standard (knowing that the majority of our products do require welding and machine turning), the ultimate surface finish (i.e. mirror finish *, directional, non-directional, #4 satin brush finish, etc.), and if required, the ultimate post production treatment (sand blasting, pickling, and/or passivating).

* a mirror finish, if determined to be a wise oxidation resistant surface finish, has already proven somewhat undesirable as it poses a dangerous traffic distraction.

 

We have experienced similar problems with oxidation on some of our high grade stainless steel castings in the past, which we rectified by employing a rather complicated process (below) that we have used for the past several years with grand success.  Employing such a process on fabrications such as bollards, bollard covers and bike racks may be needlessly complicated, however, and may be cost prohibitive.

 

Process used to prevent oxidation of CF8 and CF8M Stainless Steel castings

  • CF8 is the equivalent of wrought 304 stainless steel
  • CF8M is the equivalent of wrought 316 stainless steel
  • Castings are to be produced under ASTM A743
    • All castings shall be solution annealed per table 1 in ASTM A743
    • All castings shall be subject to post production pickling, and/or passivating
    • Final shot cleaning shall use either stainless steel or glass beads - no steel shot shall be employed
    • Weld repairs are allowed if performed prior to solution annealing, pickling and passivating
    • All castings must have no visible rust after a 24-hour salt fog test.  The salt fog test shall be conducted per ASTM B117
    • All parts need to be INDIVIDUALLY wrapped and securely packaged for long distance freight in boxes or crates (heavy weight cardboard, wood or plastic), c/w cardboard sheets between layers of parts.
    • Packaging:  Protective Film + 3-5 layers foam tape (15-20mm thick) + plywood crates
    • No ferrous packaging of any kind can be used.
    • Parts cannot come in contact with any ferrous hand tools, storage vessels, carts, bench tops, etc.

 

References (some details found during our initial research on this subject);

 

  1. http://www.stalforbund.com/Fagboker/Rustfritt/Stainless_cleaning_no...
  2. http://www.ssina.com/finishes/sheetsamples.html
  3. http://www.plant-maintenance.com/articles/Tea_Staining.pdf
  4. http://www.brenclosures.com.au/pdfs/articles/Tea%20Staining.pdf
  5. http://www.nycote.com/products_nyclean.html
  6. http://www.everbrite.net/stainless_steel.html
  7. http://www.everbritecoatings.com/stainless-steel-restoration.htm
  8. http://www.stainless.com.au/maintenance.php
  9. http://www.assda.asn.au/index.php?option=com_content&task=view&...
  10. http://en.wikipedia.org/wiki/Brushed_metal#No._4_Finish
  11. http://en.wikipedia.org/wiki/Stainless_steel

 

Notes (some details found during our initial research on this subject);

 

No. 4 Satin Brush Finish

  • Satin brush finish achieved with 150 to 180 grit, fine directional polish.
  • This is a common finish and is most often seen on washroom accessories. Another example is the metal cover plate on a 3.5 inch floppy disk.
  • Welds of Stainless Steel: Exposed welds shall be ground smooth and polished to match the adjacent surrounding finish of the stainless steel.
  • Stainless Steel Tube or Pipe: Provide tube or pipe with a polished finish similar to NAAMM AMP 503 No. 4 finish. Tube or pipe shall receive a final polishing using grit no coarser than 180 grit. Ends shall be closed with matching material, welded, and ground smooth.
  • No. 4 Finish is characterized by short, parallel polishing lines, which extend uniformly along the length of the coil. It is obtained by mechanically polishing a No. 3 finish with gradually finer abrasives. Depending on the requirements of the application, the final finish can be anywhere between 120 and 320 grit. Higher grit numbers produce finer polishing lines and more reflective finishes. The surface roughness is typically Ra 25 micro-inches or less. This general-purpose finish is widely used for restaurant and kitchen equipment, storefronts and food processing and dairy equipment. If a fabricator needs to blend in welds or do other refinishing, the resulting polishing lines are usually longer than on product polished by a producer or toll-polishing house.
  • Surface finish designation like #4, #8 etc. refer to the grit of the polishing media used to create the finish (for example #4 uses a grit of between 120 and 150 to obtain the surface polish).
  • Grinding, polishing, and buffing operations are applied to stainless steel in much the same manner as to other metals. The differences which exist are related to properties of stainless steel.
    • More power is required to remove metal because of its higher strength.
    • The austenitic stainless steels have lower rates of heat conductivity than carbon steel. Thus, the surfaces may become hotter than the surface of carbon steel and heat tinting of the stainless steel surface may occur.
    • Two variables determine the amount of grinding and polishing required, namely, the initial surface condition and the desired finish. The rougher the starting surface, the coarser is the first grinding wheel. For the first grind on welds, it is common practice to use a wheel or a belt with a grit size of 20 to 60. If the article has a surface equivalent to a 2B sheet finish, it is often customary to start with a 100 grit. Succeeding operations should make use of increasingly finer grit size until the desired smoothness is reached

 

CLEANING and PASSIVATION OF STAINLESS STEEL

  • On the surface of stainless steels there is an extremely thin transparent film.
  • Nevertheless, it is tenacious, uniform, stable and passive. It imparts to the surface the property of passivity, normally associated with noble or inert metals and it is to this passive film that stainless steels owe their superior corrosion resistance.
  • The film will form spontaneously, or repair itself if damaged, both in air due to the presence of oxygen, or when immersed in solutions, provided there is sufficient oxygen or oxidizing elements present. The basic passivation treatment for stainless steel is exposure of a clean surface to air. However, there is much practical evidence which
  • ·shows that passivity, and therefore corrosion resistance, is enhanced if the passive film is formed by the action of oxidizing acid solutions.
  • Nitric acid is such an oxidizing acid, and is always used for passivation treatments.
  • Nitric acid does not corrode stainless steel, does not alter critically dimensioned parts and will not remove heat tint, embedded iron or other embedded surface contamination. Nitric acid passivation is most useful in enhancing the corrosion resistance of freshly machined surfaces.
  • The standard nitric acid passivating solution is made up and used as follows:
    • 0 to 15 percent by volume of nitric acid (HNO3) in water. Quickest and best passivation results if used at 150°F (65°C) for the austenitic (300 series) stainless steels, and 120°F(50°C) for the ferritic and martensitic(400 series) plain chromium stainless steels.
    • The immersion time is approximately 30 minutes, followed by thorough water washing.
  • It is, however, appreciated that it is not always possible to fully immerse fabrications in a hot passivating solution, and therefore, swabbing with cold acid solution is normally used where lower concentrations and temperatures of acid ease handling and application. It must be appreciated that longer contact times will be required.
    • For austenitic (300 series);15% nitric acid at 65/80°F (20/25°C).............................30 to 90 minutes
    • For ferritics (400 series);12% nitric acid at 65/80°F (20/25°C).............................30 to 45 minutes
  • The acid solution is swabbed on with sponges, soft paint brushes or fine nylon pads. Continual swabbing is necessary to ensure contact over the time period.
  • For the treatment of small localized areas proprietary passivating pastes are obtainable and can be used. Contact time should be in accordance with suppliers' recommendations, as the concentrations may differ from paste to paste, and the lower alloyed grades will require shorter times.
  • Thorough water rinsing MUST follow all passivating treatments.

 

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Brad Done | Vice-President, Sales & Marketing
Reliance Foundry Co. Ltd.
Unit 207, 6450 - 148 Street | Surrey, B.C., | Canada V3S-7G7
Direct: (604) 592-4323 - Toll Free: (888) 735-5680 - Fax: (604) 590-8875
Email: brad@reliance-foundry.com
Website: www.reliance-foundry.com

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