Thursday, November21, 2019

Galvanic Corrosion


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            ASSOCIATION > T E C H N I C A L                                                                          FA Q s            No1

                                     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 galvanised
purlin 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 steels,
examples.                                      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 0.1 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
condensation. 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 significantly
increases 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 as
complication 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 will
initially benign water may cause quite

ASSDA TECHNICAL SERIES                                                                                          ASSDA
>FREQUENTLY ASKED QUESTIONS                                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
PAGE 1                                                                  Email Website
ASSDA TECHNICAL SERIES                                                                                                       ASSDA

generate 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 been
sacrificial 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. Although
underlying steel  and so galvanised                         low. If the behaviour of a copper/steel and     aluminium bronze is more active than
fasteners in stainless steel are not                         a stainless steel/steel couple is compared,     stainless steel, the conductivity of the
acceptable. However, a stainless screw in                    the copper/steel 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 require
although a mound of zinc corrosion                           similar potential separation of 0.35 volts.     replacement but only at times of major
product will accumulate around the                           Examples of acceptable galvanic pairs           overhaul.
fastener. 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 1:50 ratio to 1:1                        hang stainless steel piping externally          conductive and caused severe galvanic
during drying after a rainstorm. If                          around chemical plants. The surface area        attack of a 316 stainless lug. Graphite
contaminants are significant this means                      ratio is bad with large area of stainless       gaskets have caused similar problems.
that avoiding dissimilar metal pairs may be                  steel to small area of active zinc/steel 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 galvanic
effects 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 recommendations contained in this publication are necessarily of a                                         Development Association
general 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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