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Re: Stainless Steel Rust

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Glen,

Please note that approximately $ 100 billion are lost in the U.S.
annually due to corrosion.

Corrosion of metals is driven by the basic thermodynamic force of a metal
to return to the oxide or sulfide form, but it is more related to the
electrochemistry of the reaction of a metal in an electrolytic solution. 


There are eight basic forms of corrosion:


1. Stress-corrosion cracking.  Cracking caused by the simultaneous action
of a tensile stress and a specific corrosive medium is called
stress-corrosion or exfoliation.  The stress may be a result of applied
loads or "locked-in" residual stress. 

2. Erosion-corrosion.   Deterioration at an accelerated rate is caused by
relative movement between a corrosive fluid and a metal surface; it is
called erosion-corrosion.   Generally the fluid velocity is high and
mechanical wear and abrasion may be involved, especially when the fluid
contains suspended solids.  Erosion destroys protective surface films and
enhances chemical attack.

A special kind of erosion-corrosion is cavitation, which arises from the
formation and collapse of vapor bubbles near the metal surface.  Rapid
bubble collapse can produce shock waves that cause local deformation of
the metal surface.

Another special form of erosion-corrosion is fretting corrosion.   It
occurs between two surfaces under load that are subjected to cycles of
relative motion.   Fretting produces breakdown of the surface into oxide
debris and results in surface pits and cracks that usually lead to
fatigue cracks.

3. Crevice corrosion.   An intense localized corrosion frequently occurs
within crevices and other shielded areas on metal surfaces exposed to
corrosive attack.   This type of attack usually is associated with small
volumes of stagnant liquid at design details such as holes, gasket
surfaces, lap joints, and crevices under bolt and rivet heads.

4. Galvanic corrosion.    The potential difference that exists when to
dissimilar metals are immersed in a corrosive or conductive solution is
responsible for galvanic corrosion.   The less-resistant (anodic) metal
is corroded relative to the cathodic metal.

5. Intergranular corrosion.   Localized attack along the grain boundaries
with only slight attack of the grain faces is called intergranular
corrosion.   It is especially common in austenitic steel that has been
sensitized by heating to the temperature range 950 to 1450 degrees F.  
It can occur during heat treatment for stress relief or during welding,
when it is known as weld decay.

6. Uniform attack.  The most common form of corrosion is uniform attack. 
  It is characterized by a chemical or electrochemical reaction that
proceeds uniformly over the entire exposed surface area.   The metal
becomes thinner and eventually fails.

7. Pitting.  Pitting is a form of extremely localized attack that
produces holes in the metal.  It is an especially insidious form of
corrosion because it causes equipment to fail after only a small
percentage of the designed-for weight loss.

8. Selective leaching.   The removal of one element from solid-solution
alloy by corrosion processes is called selective leaching.  When
selective leaching occurs, the alloy is left in a weakened, porous
condition.


Hope this helps.

Desi J. Kiss, P.E.



> Glenn,
> 
> Here are some example that I have witnessed on the staining/rusting 
> of stainless steel.
> 
> Seen at several waste water treatment facilities.
> 
> The bends have residual stresses caused by fabrication, cause some 
> of the austenitic phase to change to a ferrite phase. One ends up 
> with local galvanic corrosion cells (each phase has a different 
> electrical chemical potential).
> 
> Chlorides are present. Attaching the SS, causing intergranual 
> corrosion, and breaking the protective chromium oxide layer. ( Most 
> any halide ion will break down the protective layer.)
> 
> Ozone may be present. ( I witnessed 304L SS 2" sch 80 pipe fail in 
> about 4 hours in a stream with a high amount of ozone at a WWTP). 
> This is why glass piping is generally used.
> 
> Sulphuric acid and sodium/potassium hydroxide may be present used to 
> adjust the PH.
> Cold working the SS will generally result in an increase rate of 
> attack in acidic environments.
> 
> Some single cell organisms excrete iron sulphide/oxides that cause 
> staining.
> 
> At another pharmaceutical facility I observed steel wool being use 
> to clean the SS. Looked good until it got wet. (When I pulled up in 
> the parking lot the next morning with the overnight dew, ... well 
> you can imagine what it looked like.)
> 
> In petro-chemical & refinery operations,  I general take a small 
> bottle of Nitric acid to test the material to see if it is Carbon 
> Steel or Stainless Steel ( they look the same after a short period 
> in these atmospheres).
> 
> In another application (Cryogenic freezers) the liner is generally 
> type 304 Stainless steel.  The decontamination process of using a 
> bleach solution caused corrosion. 
> 
> In another case, the rust staining was caused by water with iron 
> ions dripping on the SS.  As the H2O evaporated , it left the ions 
> behind, cause the stain.  The paint on the overhead structural steel 
>  had failed.
> 
> Improper heat treatment can cause the SS to become susceptible to 
> intercrystalline corrosion. ( I have seen cases where individual 
> crystals popped out.) 
> 
> Just remember Stainless Steel only looks shiny in relative benign 
> environments.  Stainless steel does corrode, something quite 
> catastrophically. Corrosion (primary SCC) is The Number One cause of 
> chemical plant equipment failure. Ninety-five percent (95%) of 
> chemical equipment failure is caused by corrosion of the 18-8 family 
> of Stainless Steel. 
> 
> Rich
> 
> 
>
-------------------------------------------------------------------------
-------
> 
> Richard M. Beldyk, PE, NSPE, CWEng, CPE
> Professional Engineer - Delaware 6788, Maine 9987, Maryland 26594, 
> Michigan 6201048734, Ohio 66691, Oklahoma 20523, Pennsylvania 
> PE061240, Virginia 37138
> Registered Member - International Registry of Professional Engineers 
> - Certificate IR101
> AWS Certified Welding Engineer - Certificate  0009Eng
> AFE Certified Plant Engineer - Certificate 4582
>  
>  


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