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I found a good
web page which gives a good overview of the 8 (!!!!) different kinds of
corrosion to which Stainless Steel is subject. Technically, I was
wrong to call it rust or oxidation. The SS keelbolt is susceptivle
to crevice or pitting corrosion, not rusting
2. Crevice
Corrosion - this is a problem with stainless fasteners used in seawater
applications, because of the low PH of salt water. Chlorides pit the passivated
surface, where the low PH saltwater attacks the exposed metal. Lacking
the oxygen to re-passivate, corrosion continues. As is signified by
its name, this corrosion is most common in oxygen restricted crevices, such
as under a bolt head.
3. Pitting -
See Galvanic Corrosion. Stainless that had had its passivation penetrated in a
small spot becomes an anodic, with the passivated part remaining a cathodic,
causing a pit type corrosion.
Fair winds, Judy
B
ps> HERE'S THE
WHOLE PAGE....
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MARINE FASTENERS
What Makes Stainless
"Stain-less"
A common misconception about stainless
steel is that is not affected by corrosion. While misleading, the
phenomenal success of the metal makes this common belief
understandable. One of New York City's most impressive landmarks is
the stainless steel clad peak of the Chrysler Building. Built in
1930 of 302 Stainless, a recent inspection revealed no signs of
corrosion or loss of thickness. The tallest manmade monument in the
US, the St Louis Arch, is entirely clad in 304 stainless steel
plates. Except for cleaning, the stainless exterior of this monument
has required no corrosion maintenance. Closer to home, housewives
work in stainless steel sinks that shine as bright as the day the
were installed. Everyday the average American will come into contact
with numerous examples of the success of stainless steel. And while
the name correctly signifies the rust resistant properties of the
metal, "stain-less" is not 100% "stain-proof" in certain
applications.
Types of Stainless
Corrosion
According to the DOD Technical
Bulletin Corrosion Detection and Prevention there are 8 separate
types of corrosion, with only a few having a major impact on
stainless steel. Please be advised the descriptions below are
extremely brief and written in laymen terms. Before acting on any
particular application, qualified advice particular to such
application should be obtained.
1. Uniform Attack - also known
as general corrosion, this type of corrosion occurs when there is an
overall breakdown of the passive film. The entire surface of the
metal will show a uniform sponge like appearance. Halogens penetrate
the passive film of stainless and allow corrosion to occur. These
halogens are easily recognizable, because they end with "-ine".
Fluorine, chlorine, bromine, iodine and astatine are some of the
most active.
2. Crevice Corrosion
- this is a problem with stainless fasteners used in seawater
applications, because of the low PH of salt water. Chlorides pit the
passivated surface, where the low PH saltwater attacks the exposed
metal. Lacking the oxygen to re-passivate, corrosion continues.
As is signified by its name, this corrosion is most common in
oxygen restricted crevices, such as under a bolt head.
3. Pitting - See
Galvanic Corrosion. Stainless that had had its passivation
penetrated in a small spot becomes an anodic, with the passivated
part remaining a cathodic, causing a pit type
corrosion.
4. Galvanic Corrosion - Placing
2 dissimilar metals in a electrolyte produces an electrical current.
A battery incorporates this simple philosophy in a controlled
environment. The current flows from the anodic metal and towards the
cathodic metal, and in the process slowly removes material from the
anodic metal. Seawater makes a good electrolyte, and thus, galvanic
corrosion is a common problem in this environment. 18-8 series
stainless fasteners that work fine on fresh water boats, may
experience accelerated galvanic corrosion in seawater boats, and
thus it is suggested you examine 316 stainless.
The simplified galvanic series chart
below will assist you in determining the potential electrical
activity between 2 metals. |
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Galvanic Series of
Metals and Alloys |
| Magnesium |
Anodic
More likely to be
attacked |
| Magnesium Alloys |
| Zinc |
| Aluminum 1100 |
| Cadmium |
| Aluminum 2024-T4 |
| Steel |
| Iron |
| Cast Iron |
| Lead-Tin Solders |
| Lead |
| Tin |
More
Noble
Cathodic |
| Brass |
| Copper |
| Bronze |
| Copper-Nickel Alloys |
| Stainless Type 430 (Passive) |
| Stainless Type 304 (Passive) |
| Stainless Type 316 (Passive) |
| Silver |
| Graphite |
| Gold |
| Platinum |
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Source: ITT
Harper
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Guideline for Selection of Fasteners
based on Galvanic Action |
|
Fastener Metal |
|
Zinc &
Galvanized
Steel |
Aluminum &
Aluminum
Alloys |
Steel and
Cast
Iron |
Brass, Copper,
Bronze,
Monel |
Martensitic
Stainless
(Type
410) |
Austentic
Stainless
(Types
302,
303, 304, 305) |
|
Base Metal |
|
|
|
|
|
|
|
Zinc &
Galvanized
Steel |
A |
B |
B |
C |
C |
C |
|
Aluminum &
Aluminum
Alloys |
A |
A |
B |
C |
Not
Recommended |
B |
|
Steel and
Cast
Iron |
AD |
A |
A |
C |
C |
B |
|
Lead-Tin
Plated
Sheets |
ADE |
AE |
AE |
C |
C |
B |
|
Brass, Copper,
Bronze,
Monel |
ADE |
AE |
AE |
A |
A |
B |
|
Ferritic Stainless
(Type
430) |
ADE |
AE |
AE |
A |
A |
A |
|
Austentic Stainless
(Type
302/304) |
ADE |
AE |
AE |
AE |
A |
A |
A - The corrosion of the base metal is
not increased by the fastener
B - The corrosion of the base metal
is marginally increased by the fastener
C - The corrosion of the
base metal may be markedly increased by the fastener material
D -
The plating on the fastener is rapidly consumed, leaving the bare
fastener metal
E - The corrosion of the fastener is
increased by the base metal
Note - Surface treatment and
environment can change activity
Source - "Stainless Steel
Fasteners A Systematic Approach To Their Selection" AISI
502-476-18M-CP
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5. Intergranular Corrosion - all
austentic stainless steels contain a small amount of carbon. At extremely
high temperature, such as welding, the carbon forces local chrome to form
chromium carbide around it, thus starving adjacent areas of the
chrome it needs for its own corrosion protection.
When welding, it is recommended you consider low carbon stainless such as
304L or 316L.
6. Selective Leaching -
Fluids will remove metal during a de-ionization or
de-mineralization process. This usually happens inside a
pipe and is rarely a fastener problem.
7. Erosion Corrosion - This
corrosion happens when the velocity of an abrasive fluid removes
the passivation from a stainless. Again, this is
almost exclusively limited to pipe interiors and rarely a fastener
problem.
8. Stress Corrosion
- Also called stress corrosion cracking or chloride stress
corrosion. Chlorides are probably the single biggest enemy of
stainless steel. Next to water, chloride is the most
common chemical found in nature. In most environments, the PPM
are so small the effects on stainless are minute. But in
extreme environments, such as indoor swimming
pools, the effects can be extreme and
potentially dangerous. If a stainless part is under tensile
stress, the pitting mentioned above will deepen,
and cracking may take place. If you are using
stainless steel bolts under tensile stress, in an environment
where chlorine corrosion is
likely, you should examine the potential for stress
corrosion cracking carefully.
According to a NACE International & CC
Technologies study, corrosion costs the United States
$276,000,000,000 annually. That's $276 billion and 4.2% of the nations
GNP. In the power generation and transmission industry alone, it
is estimated that nearly 8% of the typical electric bill is attributed
to the cost of corrosion. It is unknown how many lives
are lost annually due to corrosion but the number could be
frighteningly high. Extreme
examples thru the years include collapsed bridges and
jet airliner crashes.
Summary
No metal, except for gold
and platinum in their natural state, are completely
corrosion proof. But stainless steel has proven in thousands of
applications, that it is one of the most economical
solution's to combat the ever present elements that cause
corrosion.
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Fair winds, Judy B
1977 Catalina-27 Tall Rig, hull
#3459,Bijou, currently being stripped
and refit with all new systems.
Sailing San Francisco Bay
Area
Visit Bijou’s website at <http://www.blumhorst.com/catalina27/catalinahomepage.htm>
Commodore Emeritus, Potter Yachters of Northern
California
1985 West Wight Potter 19 #266,
Redwing
Danville/SF Bay, CA
Potter Yachters website at
<http://potter-yachters.org/>
Potter Forum on the
TrailerSailor website. http://bbs.trailersailor.com/forums/potter/index.cgi Judy
B's Potter Website at <http://www.blumhorst.com/potterpages>
In a message dated 7/19/2006 3:19:10 P.M. Eastern Standard Time,
[EMAIL PROTECTED] writes:
DOES
rust when there's no oxygen available
????? Oxidation without Oxygen
?????
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