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ORIGIN AND CONTROL OF STAINLESS STEEL PARTICULATE

HISTORY

n order to understand stainless steel particulate it is first necessary to have a basic understanding of stainless steel as well as forming and finishing operations.

ugh there are five different categories of stainless steel and hundreds of various alloys therein, bio-pharmaceutical needs center on the AUSTENITIC alloys. This category of alloys, and in particular type 316L, is a best choice for machining, forming, and welding for use in depyrogenation, autoclaving and lyophilization operations.

In 1913 Harry Brearly of Sheffield, England accidentally discovered that adding chromium (CR) to iron rifle barrels gave the barrels stain (oxidation) resistance, hence, STAINLESS steel. Subsequent scientific investigation over the next 40 years showed that chromium atoms and their oxides are approximately the same size and fit together tightly on a metal surface to form a protective stable shield just a few atoms thick. Of all the ingredients in stainless steel, chromium has the least corrosion resistance and quickly forms a protective chromium oxide layer in the presence of oxygen.

MOLYBDENUM

In addition to chromium, it is also important to understand the 316L alloy element molybdenum (MO) with the sixth highest melting point of any element. First isolated in 1781, molybdenum was not used for over 100 years, when it was found to enhance structural iron protection from oxidation by chlorides common in seaside air environments. By the 1950s boat builders realized that 316L stainless steel protected boat fittings from salt air and saltwater making it a best choice for quality nautical hardware. Molybdenum readily forms carbides in stainless steel, adding strength and durability.
316L stainless steel alloy with alloy enhancing molybdenum features include:

  1. Additional hardness over common type 304 stainless steel (#316L: 217 Brinnel hardness vs. #304: 123 Brinnel hardness)
  2. Low carbon (C) content for optimal welding (hence the “L” in 316 L)
  3. Best surface protection in a low oxygen environment
  4. Additional high heat chromium oxide protection. (Note: all stainless alloys with chromium content over 18% have a high heat capability to withstand 870 degrees Celsius. 316L resists oxidation at higher heats than stainless alloys without molybdenum.)

HARDNESS

Converting the 316L Brinnel scale hardness of 217 to the Rockwell ‘C’ scale rates 316L stainless steel at a relatively soft 17Rc. In comparison, boron silicate glassware common to the industry is rated at 68 to 72Rc. This easily explains rapid wear and tear of stainless steel trays used in conjunction with vials and bottles. Other abrading factors include glass finish (extruded or molded), weight, size and traffic.

Boron silicate glassware is the hardest material to normally contact stainless steel vial trays. The second hardest material is stainless steel itself as stacked upon itself or stored on stainless steel rail supports in cabinets and carts.

PARTICULATE

Stainless steel particulate is generated by breaking off ultra thin ridges formed by scratching or roughening stainless steel. Ultra-thin ridges may also be formed by shearing and punch perforating.

SS Particulate Chart

Considering the obvious softness of stainless steel and the hardness of boron silicate glass, there is no way around generating some stainless particulate in use. This particulate, however, may be minimized in several ways.

OPTIMAL STAINLESS STEEL VIAL TRAY MANUFACTURING

  1. Use a starting finish of 2B on both sides of tray material. 2B is a mill finish with a 10Ra smoothness rating. Page Two of Three Pages.
  2. High speed turret hole punching provides a smooth annular finish at the trailing end of the punch and a sharp leading edge. Sharp edges may be softened by coining (reverse side peening) or electro-polishing.
  3. Shearing also creates a sharp leading edge. Laser blanking does not.
  4. TIG welding is an art form and varies from weld to weld. Laser fusion welding melts stainless into itself at temperatures to 28,000 degrees F. Crevasses, cracks and pin holes are non-existent.
  5. Electro-polishing passivates welds, softens sharp ridges, fills in depressions, and destroys all surface oxides. Electro-polishing reduces the surface of 2B stainless to a featureless 4-5 Ra finish making the resultant surface easiest to clean.

VIAL TRAY MAINTENANCE

Other than optimal stainless steel tray manufacturing, there are a number of steps to be taken to provide proper maintenance, minimize particulate, and extend vial tray life.

  1. A date, origin and lot code with consecutive numbering is the best basis for a meaningful stainless steel tray maintenance program.
  2. Establish routine scratch inspection. A scratch over .001” is easily felt and seen.. A Surface Roughness Meter may be used to record real scratch data, including frequency, height and depth of scratches. Routine testing can provide a projected time for tray replacement.
  3. At least half of scratching within vial trays can be avoided using ‘slip sheets’ while loading trays.
  4. HDPE sheets should be used on storage shelving to eliminate stainless to stainless contact.
  5. Radel R or Noryl engineered autoclavable plastics are a preferred track material in carts and cabinets.
  6. Stainless steel needs a continuous oxygen supply in order to maintain a strong chromium oxide protective layer. Stored stainless trays and lids need breathing space.
  7. Routinely clean all metal to metal wear parts. Entrapped particulate, especially on cart and cabinet rails, can be devastating.

IN SUMMATION

Stainless steel is a sacrificial metal. Much softer than the glassware it supports, stainless steel will eventually and unavoidably generate particulate. FDA inspectors are familiar with intrinsic stainless steel particulate. Quality manufacturing and a meaningful stainless maintenance program including routine inspection and cleaning will demonstrate awareness and responsibility in minimizing particulate risk.


 

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