Environmental Engineering Reference
In-Depth Information
systems polarity of a couple can reverse. For example, iron-zinc couple in certain
domestic waters reverses polarity at temperatures over 80
°
C. Cathodic product
formed on zinc apparently causes this reversal.
3.3.4
Remedial Measures
1.
Avoid coupling of metals far apart in the galvanic series.
2.
Small anode-large cathode combinations should be avoided. Fasteners
should be cathodic to the parts being fastened.
3.
Whenever coating is applied on a galvanic couple, the cathodic member is
to be coated, and not the anodic member. This is because any discontinuity
in the coating on anodic member will only provide an unfavorable anode/
cathode ratio.
4.
Whenever possible, insulation should be provided between dissimilar metals.
5.
Anodic parts may be made thicker to ensure a longer life, e.g., water heaters
with copper tubes and heavy steel tube sheets.
6.
A third metal anodic to both metals of the couple may be installed.
3.3.5 Practical Examples
Practical examples of galvanic corrosion are varied and widespread. Rapid corro-
sion of steel rivets and bolts is commonplace when these are used to fasten copper
or Monel plates. Contact with graphite packing has led to the failure of steel
pump shafts or valve stems. Aluminum wire used as replacement of copper wire
for connecting the auxiliary anode (high-silicon cast iron) in an impressed current
cathodic protection system suffered rapid failure.
The incidence of galvanic corrosion at times is triggered by unexpected
sources. Dissolved copper from upstream may plate on downstream steel compo-
nents forming in situ galvanic couples. Chance contact of a vernier fuel line made
of aluminum alloy with helium pressurization line made of 301 stainless steel in
a liquid-fueled missile led to a leak formation in the aluminum pipe in a 16-
month exposure to a marine environment [2].
The failure of an automatic water sprinkler system was due to incompatible
metal combination and unfavorable area ratio [2]. The system had a copper plate
made of a cast copper alloy that was held in position by a clapper latch made
of malleable iron (Fig. 3.5). The malleable iron lip at the contact point had been
corroded to such an extent in only 21 months of service that it failed by plastic
deformation. Replacement of the latch material by silicon bronze prevented gal-
vanic corrosion and the latches were reported to be in satisfactory condition after
more than 14 years of service.
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