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Steel corrosion

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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.



Steel corrosion is far beyond a "cosmetic treatment" with the so called
"rust inhibitors" or proper surface preparation,

or "caustic"/"acid" situations !

Hope this helps.

Desi J. Kiss, P.E.



On Tue, 17 Apr 2001 12:08:19 -0500 "Fountain Conner"
<fconner(--nospam--at)pcola.gulf.net> writes:
> This is a multi-step process; 'sounds like you may not have paid 
> attention
> the individual steps properly.
> 
> 1.  Coating selection -- I this a moist or corrosive environment?  
> Is
> abrasion a problem?  Do you want a long-life system, or simply a 
> cosmetic
> treatment?
> 
> 2.  Proper surface preparation is the key.  Without it, everything 
> else
> will be less-than satisfactory.  In your case, I think an SSPC-6 
> would be
> appropriate.
> 
> 3.  Assuming you have opted for long-term quality instead of 
> something
> temporary, I have assumed good surface preparation (see above), 
> followed by
> a multi-coat system.  I have also assumed a cyclical "caustic" 
> situation,
> followed by periods of  "acid" saturation.
> 
> I have also assumed you want something with a certain visual 
> quality.  This
> calls for a relatively impervious base with a "finer" top coat.
> 
> 4.  With the requirements outlined, it would seem that the most 
> appropriate
> coating system would consist of a base coat of tar, followed by a 
> topcoat
> of feathers.  And, incorporating mobility considerations, I would 
> recommend
> provisions for being transported  "out of town" -- "on a rail" might 
> be
> appropriate   ;-)
> 
> Irrespectfully,
> 
> Fountain Conner 
> 
> 
> ----------
> > From: Bill Polhemus <bill(--nospam--at)polhemus.cc>
> > To: seaint(--nospam--at)seaint.org
> > Subject: RE: Steel corrosion
> > Date: Tuesday, April 17, 2001 11:08 AM
> > 
> > Speaking for myself, my wife informs me that noticeable flakes are
> beginning
> > to form...
> > 
> > William L. Polhemus, Jr., P.E.
> 
> 
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