Metallurgical

In an ideal world 316L will spontaneously create a protective oxide layer, but we are not so lucky, a lot of the stainless steel to-day is made from recycled sources and aluminium is often added to American Austenite to control grain size and deoxidise the melt.

As can be seen from this, before we start fabrication we may have left our ideal world.

The alloy is then extruded into piping, Made into plate for the fabrication of tanks/vessels, forged or cast into valve bodies, Pump housings, Impellors and Backing plates.

Before we continue we should take a look at the material we will be dealing with and the processes it under goes to become a component in a system.

This 316L alloy typically contains the following constituent elements:

Chromium         16-18%
Nickel                 10-14%
Molybdenum      2-3%
Manganese        2% Max.
Carbon                 0.03% Max.
Iron the balance and is usually found to be 68%

The three nobel elements are usually found to be at the lower end of the specification due to their cost and at certain periods of world politics, may even be in short supply.

SURFACE FINISHING

The next process is to surface finish the above items, this is achieved by applying a hard abbrasive usually in diminishing grit sizes using Grinders, Buffing wheels and the use of mandrels in piping. Diamond dust or certain Zirconium compounds may be used but often the choice is the less expensive Corrundum. Corrundum is an impure form of Aluminum Oxide and rated 9 on the MOHS hardness scale of 10.

These Aluminum Oxide fragments, together with their binders and parafins get impregnated into the microstructures as impurities. Where rouge has formed these fragments are almost always a component portion.

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The following stage is usually Electropolishing, this step is a smoothing process which does not remove "valleys" but reacts with the "peaks" making them rather uniform in relation to one another. The "valleys" still contain the abbrasive partical fragments.

Additionally if the electrolyte solution is not changed on a frequent basis it will concentrate impurities and these impurities can and do deposit on polished surfaces.

FERRITE

Casting, Machining and Cold working can cause a disparate structure. 316L possess a crystaline structure and orientation that renders it virtually non-magnetic.

If magnetism is detected in a 316L component it indicates that a portion of the structure is non-austenitic in composition. This is generally Delta-Ferrite and can be detected with a magnet and measured using a ferrite meter to quantify the percentage. If a 316L component with a high ferrite content is in contact with a corrosive media such as WFI or Clean Steam incipient corrosion will usuallly occur.

WELDS

Too much heat or inadequate purging gas can cause heat tint, or an oxide scale in the heat affected zone (HAZ). This heat tint is a Blue or Brownish discolouration within the weld area, It is a stressed zone with somewhat "discimilar metallurgy" that can create Galvanic action, it may also be slightly depleated of Chromium. It can also exhibit a high ferrite content, all of which may lead to corrosion.

OTHER FACTORS

Atmospheric dust can find their way into a system when pipework, vessels, WFI stills and PSG's are left open to the environment. These dusts can often contain free iron from grindings of proximal mild steel installations such as catwalks etc. This free iron on the surface of 316L will create tiny galvanic cells, each of which is corrosive.

SUMMARY

New stainless steel surfaces may look shiny and clean. They may however conceal much debris and a "metallurgical zoo" of contaminates. These appear as components of corrosion products, "rouge" . Prominate among these are ferric oxide and aluminium.

Corrosion once begun is continous and self catalysing forming the ferric ion (Fe+++) which is itself corrosive. The higher oxidising potential is passed from one atom to the next, ongoing until stopped.