Wednesday, June20, 2018

Galvanic Corrosion

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What it is and how to avoid it
Contact between dissimilar metals active galvanic effects as the liquid in the When two metals are connected and in
occurs frequently but is often not a crevice under a bolt or clamp becomes contact with a conducting liquid the more
problem The aluminium head on a more concentrated Water may be active metal will corrode and protect the
cast iron block the solder on a excluded by design or the use of adhesive noble metal Zinc is more negative than
copper pipe galvanising on a steel sealants steel and so the zinc coating of and the steel fastener
in an steel will corrode to protect the steel at
aluminium sheet are common Metal to metal contact scratches or cut edges The stainless
Galvanic corrosion can only occur if the including 304 and 316 are more positive
dissimilar metals are in electrical contact than zinc and steel so when stainless steel
The contact may be direct or by an is in contact with galvanised steel and is
What causes galvanic external pipe or wire or bolt If the wet the zinc will corrode first followed by
dissimilar metals are insulated from each the steel while the stainless steel will be
corrosion protected by this galvanic activity and will
other by suitable plastic strips washers or
For galvanic or dissimilar or electrolytic sleeves then galvanic corrosion cannot not corrode The rate of galvanic attack is
corrosion to occur three conditions must occur Paint is not a reliable separator from governed by the size of the potential
be met direct contact although painting all of the difference
the metal join must be wet with a noble metal is quite effective Painting the As a rule of thumb if the potential
conductive liquid active metal causes drill holes at coating difference is less than 01 volt then it is
there must be metal to metal contact defects unlikely that galvanic corrosion will be
the metals must have sufficiently Potential differences
different potentials If all three conditions are met then
All metals dissolve to some extent when galvanic corrosion is probable and the rate
Wetting the join they are wetted with a conductive liquid of corrosion will be influenced by the
The degree of dissolution is greatest with relative area and the current density
The conductive liquid or electrolyte could
active or sacrificial metals such as delivered by the noble metal
be rainwater or even water from
magnesium and zinc and they have The greater the conductivity
most negative potential In contrast noble
the more severe the galvanic effects Salt
or passive metals such as gold or graphite Relative wetted
or industrial pollution the conductivity of water so
are relatively inert and have a more surface area
positive potential Stainless steel is in the
galvanic effects are normally more severe If a noble metal like stainless steel has a
middle although it is more noble than
near the coast or in heavy industrial areas large surface area in contact with the
carbon steel The potential can be
Low conductivity pure rainwater will only electrolyte while the sacrificial metal such
measured with a reference electrode and
cause slight galvanic effects One as galvanised steel has a very small
used to construct a galvanic series is that during evaporation surface area in contact with the
shown on page 2 ASTM Standard G82
water films become more conductive so electrolyte then the stainless steel benign water may
Australian Stainless Steel Development Association ACN 061 226 051
Level 15 215 Adelaide Street Brisbane Q 4000
Telephone 07 3220 0722 Facsimile 07 3220 0733
ASKED a large corrosion current which Available current density
In the water industries galling between
will be concentrated on a small area of stainless steel threads and nuts has
metal The galvanised steel will Stainless steel has an effective passive film avoided by using aluminium bronze nuts
corrode quickly first the zinc then the so the available corrosion current is quite on stainless steel studs or bolts steel
and so galvanised low If the behaviour of a coppersteel and aluminium bronze is more active in stainless steel are not
a stainless steelsteel couple is compared stainless steel the conductivity of However a stainless screw in
the coppersteel coupling is a more water and hence the corrosion rate is
galvanised steel is frequently used significant galvanic problem despite the generally quite low The nuts will a
mound of zinc corrosion similar potential separation of 035 volts replacement but only at times of major
product will accumulate around the Examples of acceptable galvanic pairs This is because the ratio of
include One unacceptable case was a gasket with
wetted noble fastener in an active metal Galvanised steel pipe hangers are used to a carbon black loading so high it was
might change from a 150 ratio to 11 hang stainless steel piping externally conductive and caused severe drying after
a rainstorm If around chemical plants The surface area attack of a 316 stainless lug are significant this means
ratio is bad with large area of stainless gaskets have caused similar avoiding dissimilar metal pairs may be
steel to small area of active zincsteel but
a preferred option to prevent galvanic the rainwater is usually of quite low
attack conductivity and 20 year service life is
As a rule of thumb if the wetted area of normal
the corroding metal is 10 times the wetted
area of the noble metal then are not serious although the larger
the ratio the less the effect
T E C H N I C A L FA Q s PAGE 2 No1
Australian Stainless Steel
The technical contained in this publication are necessarily of a Development nature and should not be relied
on for specific applications without first ACN 061 226 051
securing competent advice Whilst ASSDA has taken all reasonable steps to ensure Level 15 215 Adelaide Street Brisbane Q 4000
the information contained herein is accurate and current ASSDA does not warrant
the accuracy or completeness of the information and does not accept liability for
Telephone 07 3220 0722 Facsimile 07 3220 0733
errors or omissions Email Website

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