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Re: Apt quote on Corrosion

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Mr. G. Vishwanath,

Thank you for the fine note.

I'd like to add some more info. regardind corrosion.  You may find it
interesting.
Furthermore designers sould be aware of all these conditionds.  
I hope it helps.

Sincerely,

Desi J. Kiss, MS, PE



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.

Also worth mentioning here are:

Liquid impingement.   This is material removal due to the action of an
impinging stream of fluid.    The mechanism of attack is removal of
protective
films which leads to an accelerated corrosion.

Liquid errosion.    This type is simmilar tp impingement, however the
exception is
that the fluid flow is parallel to the surface.  The mechanism is the
same: removal
of metal or films by mechanical action + corrosion of the "active" metal.

Slurry errosion.   This is metal removal due to the combined action of
wear and corrosion.
The source of wear and an accelerator of corrosion is abrasive particles
dispersed in the
slurry.

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.

In addition to these basic forms of corrosion of metals here are also
some
that are worth mentioning:

a) Dealloying.     This is a corrosion process whereby one constituent of
metal
alloy is preferentialy removed from the aloy, leaving an altered
microstructure.
A number of alloy systems are susceptible to this process such as yellow
brass.
The removal of zinc from brasses is called deincification.    The net
damage is in
most cases a mechanical failure,  because the metal remaining after
dealloyng
is weak and "spongy", an effect similar to intergranular attack
(corrosion).

b)Cuastic embrittlement:  A form of corrosion cracking.  It is the
embrittlement of a
metal by an alkaline environment.

c) Corrosion fatigue.   The reduction of fatigue strength by a "corrosive
environment"

d) Underdeposit attack.  This is a form of crevice corrosion.  Corrosion
under/around
a localized deposit on a mettal or alloy surface.

e)Exfoliation:  "corrosion cancer" .Scaling off a surface in
flakes/layers as the result of
corrosion.

f) Filiform corrosion: it occurs under organic coatings on metals as fine
hairlines.

g) Stray current corrosion:  A form of attack caused by electric currents
traveling through
irregulat patterns.

h) Microbiological corrosion: Corrosion activated by the presence and
growth of living
organisms.   It is most common in soils and cooling water systems. 
Biocides are the
remedy for this type of corrosion.

Onother subject is the corrosion of PLASTICS. 

In the case of plastics and ceramics "corrosion" occurs due to the fact
that the bonds between
the organic molecules making up the structure can be affected by various
corrosive environments.
Ceramics and plastics do NOT RUST.   However deterioration is in the form
of loss of properties.
Many of the corrosion processes that occur in metals are
electrochemical.( ie they require flow of
electrons).  Plastics and ceramics as well as composite materials are
poor conductors and are not
susceptible to electrochemical corrosion as metals.   They corode but not
as readily and corrosion
occurs simply because the environment will simly react chemically with
the molecules making up
these non-metal materials.





On Thu, 25 Jul 2002 00:47:34 -0700 (PDT) G Vishwanath
<gvshwnth(--nospam--at)yahoo.com> writes:
> Thanks to Desi Kiss and Richard Beldyk and others for
> that illuminating discussion on Corrosion.
> 
> I had the benefit of a quick "Refresher Course" by
> reading those posts.
> 
> I am preserving these mails to be read again.
> 
> I am reminded of an apt quote that I heard when I was
> learning about Corrosion.
> 
> "Corrosion is metallurgy in reverse."
> 
> How true!
> 
> Metallurgy produces steel from iron oxide
> Corrosion produces iron oxide from Steel.
> 
> Regards
> G Vishwanath
> Bangalore, India
> 
> 
> 
> __________________________________________________
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