Metallurgical Considerations
The following is a description of the metallurgical considerations that form the basis of the R.W. Evans and Associates three-stage cleaning and passivation process, in addition to an outline of the procedure used. Specific details regarding the substances and application methods used vary from system to system. The three-stage process described here is the basis of a pending United States patent.
New Austenitic stainless steels 300 series (ie 304, 304L, 316, 316L) are made largely from recycled sources of "lower alloys" such as 400 series Ferritic or Martensitic materials to which nickel, additional chromium, and in the case of 316, molybdenum is added.
To American Austenite, aluminum is often added to control grain size and "deoxidize the melt." This can form corrosion-causing inclusions.
Surface finishing takes the form of an abrasive (applied with a mandrel in piping, buffing pads with vessels, etc.) most commonly in the form of aluminum oxide termed "corundum," having a hardness rating of 9.0 on the MOHS Hardness scale of 10 (diamond being 10). Example:
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| Migrational Rouge - 10,000X magnification showing the cubic structure of aluminum oxide - residual from mechanical polishing using an abrasive called "corundum." This can and, in the subject, has become a portion of the migrational rouge content itself. |
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This is implemented as a "mechanical finish" often prior to Electropolishing. There may be many impurities also within the abrasive itself, ferric (iron) oxide, or "rust" being one example. All of these impregnate within the stainless microstructure and can be an initial cause of corrosion.
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| Migrational Corrosion (Rouge): non-magnified from a "tee" exposed to Hot Water-For-Injection (HWFI) |
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Electropolishing itself can introduce an array of impurities from within the electrolyte solution (which may itself address a multiplicity of alloys). This process of "anodic dissolution" removes metal from surfaces and into solutions. If not carefully controlled and reconstituted at regular intervals it tends to overconcentrate impurities. It may also evidence a murky greenish-brown color.
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| Before & After cleaning - 3500X magnification illustrating Clean Steam System corrosion (LEFT) - shows surface grain of metal with corrosion present. (RIGHT) - after corrosion removal, shows substantial damage caused by corrosion itself. |
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Welding introduces another variable in the formation of oxides within the heat-affected zone (HAZ).
Heat-tint or weld-tint, the blue or brownish discoloration within the weld area, is a stressed zone with somewhat "dissimilar metallurgy" that can devolve galvanic action and subsequent corrosion. It may also be slightly Chromium depleted. It may further exhibit a high ferrite content.
Moreover, piping and vessels, as well as other component items, including capital equipment as WFI Stills and (Clean/Pure) Steam Generators, left open to the environment, are subject to deposition of atmospheric dusts including free iron from grinding of proximal "mild" steel installations, such as catwalks, etc. This "free iron" on surfaces creates tiny galvanic cells, each of which is corrosive.
The 300 series contains about 68%-70% of its content as iron which, as a solvent, has dissolved Chromium, nickel, (and with 316) molybdenum, when molten. On cooling, it "freezes" into a crystalline Austenitic structure which has unique properties, one of which is that it is non-magnetic. Corrosion, once begun, is continuous and self-catalyzing forming the ferring ion (Fe+++) which is corrosive itself. The higher oxidation potential is passed from one iron atom to the next, ongoing until stopped.
Oxidizing medium such as WFI, Clean Steam, and corroding media such as halogens (including chlorides as sodium chloride or saline solution, which remove chromium), exacerbate this effect by many orders of magnitude.
Therefore, new stainless surfaces, although shiny and clean in appearance, may conceal much debris and a "metallurgical zoo" of contaminants. These subsequently appear as components of corrosion products, "rouge", of which ferric oxide and aluminum and other contaminants are prominent.
(The reader is referred to "Corrosion Investigation of 316L Stainless Steel Pharmaceutical WFI Systems", Pharmaceutical Engineering, July/August 1991, Coleman/Evans, for a more detailed treatment of corrosion products.)
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