With particular relevance to corrosion resistance of screws exposed to weat=
her.

Basic tutorial on cathodic protection - but has a useful
electrochemical potential chart and comments
Conclusion - don't use nickel plated steel screws! If you want low
cost use zinc-nickel with up to say 10% nickel and if you want good
use stainless (at about 5 to 10 x the cost).
In a nickel plated steel screw the steel protects the nickel :-) ! -
any surface defect will produce an electrochemical cell with the whole
nickel area balanced by the small iron area at the defect. Such a
fault can drive a deep pinhole into material at the site of the
original defect. I'm told that plated tanks with bad electrochemistry
can develop pinhole leaks this way.

Chromium plating can be useful for cathodic protection although
electrochemically closer to steel. .

   http://www.gordonengland.co.uk/corrosion.htm

In the following re his comment on Zn-Al-Fe  - the ZN-Al does what it
is intended to do but ultimately gets consumed. Long term use requires
something which is iinherently corrosion resistant throughout.

________

Martin Ryder notes:

Plain steel bolts in an Aluminium structure (and Marine environment)
are a mistake. Same goes for Zn plated steel - Al takes out Zn, then
Fe takes out Al. I've seen it, and supported by the "GALVANIC SERIES
IN SEA WATER" chart in the link...

Cars from at least 1990's onwards have no exposed screws in light
fittings or trim. Those screws in older cars always seemed to rust,
and probably took some of the body panels along for the ride... My
1993  / 240K car still has the orginal exhaust system. Rarely used for
short trips, always gets a good run twice a week, but would love to
know what it's made of!

_________________________

This is an abstract only to a 1995 paper but has some usefuil comment.


http://www.onepetro.org/mslib/servlet/onepetropreview?id=3DNACE-95120932&so=
c=3DNACE&speAppNameCookie=3DONEPETRO

Also seems to work:

               http://www.onepetro.org/mslib/servlet/onepetropreview?id=3DN=
ACE-95120932
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