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Metallurgical Considerations


New Austenitic stainless steels 300 series (i.e. 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 “deoxidizes 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).

This is implemented as a “mechanical finish” often prior to Electro-Polishing. 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.

Electro-Polishing 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 over-concentrate impurities.  It may also evidence a murky greenish-brown color.

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.


Inter-granular Carbide Precipitation or “Sensitization” is another major area of concern when selecting and working with these alloys.  The type 304 composition, in its fully annealed and quenched condition, has an Austenitic microstructure.  This signifies that its chromium, nickel, carbon, and iron are in solid solution with each other and cannot be separately distinguished under the microscope.  In this condition the 304 type possesses maximum corrosion resistance.


This characteristic annealed structure is susceptible to an important micro-structural change if the metal is subjected to temperatures between 800 and 1500 degrees F. for a sufficient time period. Within this temperature range chromium and carbon begin to form chromium carbides, and these precipitate out of the solid solution at the grain boundaries.  The rate at which the carbides develop depends on a number of factors, but generally the higher the carbon the more rapid the action. The metal temperature within the 800-1500 degree F. range is also important.


A heat source, applied as in welding at a spot or along a line, generates a range of temperatures. Around the spot and at a distance there-from, the heat gradient will include a circular band in which temperatures will dwell in the 800-1500 degrees F. range and slow cooling to normal temperature will follow this.  If the heat source is traversed along a line, there will be two bands at a distance from and paralleling it in which temperatures will likewise dwell in the 800-1500 degree F. range prior to slow cooling.  This results in Inter-granular Corrosion.  This alteration in properties due to thermal effects is sometimes referred to as Sensitization.  The steel is said to be “Sensitized”.


The carbide precipitates are harmful because they lead to lower corrosion resistance.  The type 304 is rendered susceptible to Inter-granular Corrosion, which simply means that corrosion may proceed rapidly along and between the grain boundary.  The tiny grains may actually fall apart in a severe corrodent because the grain boundary material is preferentially dissolved.  Quite frequently, the corroded area in a weld heat affected zone adjacent to a weld is visible to the naked eye.  It is manifested as a scuffed and roughened slightly darker appearance and is due to loss of metal in the heat-affected zone.


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 ferric ion (Fe+++) which is, itself, corrosive.  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 chlorines 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, aluminum, and other contaminants are prominent.

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