DB4 wrote:
> I'm getting lost on some of the logic behind this thread....
>
> Why would stainless steel promote any greater re-action on a mechanical
> fixing (against aluminium) than mild steel? The reasoning being suggested
> is the chromium content but don't many motorcycle engine casings use
> stainless screw sets on 'tricked up' machines? My motorcycle engine casings
> where fitted out with stainless screws sets 7 years ago but don't show any
> signs of re-action?
>
> Regards,
> Dennis
>
>
> On 13/6/06 18:25, in article [email protected],
> "SpamTrapSeeSig" <[email protected]> wrote:
>
>> In article <[email protected]>, Steve Taylor
>> <[email protected]> writes
>>> Andrew Renshaw wrote:
>>>> The Chromium stops any rust.
>>> And the salt promotes chloride cracking in stainless steels.
>> And the chromium promotes rust in the surrounding mild steel.
>>
>> Regards,
>>
>> Simonm.
>
A few years ago someone really knowledgeable answered this in this same
forum; I kept their post, but sadly not their name, but here it is again
anyway:
<quote>
Aluminium corrosion
Galvanic corrosion can occur when dissimilar metals are electrically
connected in the presence of an electrolyte (i.e. conductive solution,
e.g. salt water). Galvanic attack can be uniform in nature or localised
at the junction between the alloys depending on service conditions. In
such a process, one of the metals acts as cathode where oxygen is
reduced and the second metal acts as anode that suffers corrosion
(oxidation).
To predict the risk of corrosion and determine which metal will corrode
in a given electrolyte, metals/alloys are classified following the
corrosion potential (natural potential) they develop in contact with an
electrolyte.
The following table gives those potentials for some typical
alloys/metals in contact with seawater at 25C - I know thats an unheard
of sea water temp in the UK, but here in North Eastern Australia in mid
winter the water temp has only just dropped below 22C.
The dissolution potentials of various metals in sea water are;
Graphite + 200
Stainless Steel - 100
Brass and copper - 350
Low-alloy steel - 600
Low-alloy steel and cast iron - 600
Aluminium alloys - 750
Zinc - 1000
Magnesium - 1600
This classification stays valid for many types of metals and liquids
solutions that they might come into contact with. As a consequence,
aluminium will corrode when contacted with stainless or normal steel
(commonly used in road transport), or copper (sometimes). Aluminium will
be protected in contact with zinc that corrodes instead of the aluminium.
In most cases (except in frequent contact with salty solutions), the
combination "aluminium/steel" only generates a detrimental effect on the
aesthetic appearance of the aluminium structure but not on the
mechanical properties of the assembly. Indeed, aluminium corrosion in
contact with common mild steel is very slow - years instead of minutes.
This is why connections with steel chassis which are expected to last a
long time are usually made using steel fasteners.
Without suitable precautions, the combination "aluminium/stainless
steel" should be avoided taking into account the big difference of
corrosion potential of both metals (-100 : -750). However, aluminium
alloy components are often assembled using stainless steel bolts. It's
important to remember that stainless steels have surfaces that can be
passivated (eg washing in nitric acid) and this inhibits or eliminates
galvanic corrosion until the oxide created at passivation is worn off by
friction. Turning a nut onto a bolt can be sufficient at times.
In case of frequent immersion or wetting (e.g. inside road tankers or
the bodies of Landrovers), aluminium and other metals should be isolated
from one another to prevent aluminium corrosion (an elastomer gasket on
flat surfaces and a grommet through holes or a specially formulated
coating such as an epoxy resin with a glass filler which is hard enough
to withstand friction).
Possible origins of galvanic corrosion in an older Landrover:
1. Steel bolts or rivets on aluminium panels
2. Aluminium body on a steel chassis
3. Stainless steel bolts or rivets on aluminium panels
4. Aluminium body on a galvanised steel chassis
5. Other bolt on accessories - bull bars, etc.
6. Multi-aluminium-alloy components (condenser, radiators, coolers,
welded or brazed assemblies)
Possible effects near the metallic contacts
1 & 2 Less aesthetical appearance (the white powder of aluminium oxide)
3 to 6 Significant decrease in the thickness of aluminium components
over short periods of time depending on the electrolyte strength - salt
water, etc.
Prevention or Remedial Actions
1 to 5 Insulate dissimilar materials
3 & 4 Use stainless steel with passivated surfaces only if they oxicde
will not be removed
5. Use special coatings for the application - no one coating will
prevent all corrosion
6. Good corrosion design (choice of right combination of aluminium alloys)
If the insulation is not properly done, galvanic corrosion will often be
combined with crevice corrosion.
I have quoted from various sources and books on corrosion and tried to
bring it together.
Hope this helps.
</quote>
Karen
--
"I'd far rather be happy than right any day."
- Slartibartfast
