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AUSTRALIAN STAINLESS STEEL DEVELOPMENT ASSOCIATION T E C H N I C A L FA Q s No1 METAL CORROSION What it is and how to avoid itContact between dissimilar metals active galvanic effects as the liquid in the When two metals are connected and inoccurs frequently but is often not a crevice under a bolt or clamp becomes contact with a conducting liquid the moreproblem The aluminium head on a more concentrated Water may be active metal will corrode and protect thecast iron block the solder on a excluded by design or the use of adhesive noble metal Zinc is more negative thancopper 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 ataluminium sheet are common Metal to metal contact scratches or cut edges The stainless steelsexamples 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 isWhat 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 becorrosion protected by this galvanic activity and will other by suitable plastic strips washers orFor galvanic or dissimilar or electrolytic sleeves then galvanic corrosion cannot not corrode The rate of galvanic attack iscorrosion to occur three conditions must occur Paint is not a reliable separator from governed by the size of the potentialbe 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 significant 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 rateWetting 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 densityThe conductive liquid or electrolyte could active or sacrificial metals such as delivered by the noble metalbe rainwater or even water from magnesium and zinc and they have The greater the conductivity most negative potential In contrast noblethe more severe the galvanic effects Salt or passive metals such as gold or graphite Relative wettedor industrial pollution the conductivity of water so are relatively inert and have a more surface area positive potential Stainless steel is in thegalvanic effects are normally more severe If a noble metal like stainless steel has a middle although it is more noble thannear the coast or in heavy industrial areas large surface area in contact with the carbon steel The potential can beLow conductivity pure rainwater will only electrolyte while the sacrificial metal such measured with a reference electrode andcause slight galvanic effects One as galvanised steel has a very small used to construct a galvanic series ascomplication is that during evaporation surface area in contact with the shown on page 2 ASTM Standard G82water films become more conductive so electrolyte then the stainless steel willinitially benign water may cause quiteASSDA TECHNICAL SERIES 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 0733PAGE 1 Email Website TECHNICAL SERIES ASKED a large corrosion current which Available current density In the water industries galling betweenwill 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 nutscorrode 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 thanfasteners in stainless steel are not a stainless steelsteel couple is compared stainless steel the conductivity of theacceptable However a stainless screw in the coppersteel coupling is a more water and hence the corrosion rate isgalvanised 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 majorproduct will accumulate around the Examples of acceptable galvanic pairs This is because the ratio of include One unacceptable case was a gasket withwetted noble fastener in an active metal Galvanised steel pipe hangers are used to a carbon black loading so high it wasmight change from a 150 ratio to 11 hang stainless steel piping externally conductive and caused severe galvanicduring 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 problemsthat avoiding dissimilar metal pairs may be steel to small area of active zincsteel buta preferred option to prevent galvanic the rainwater is usually of quite lowattack conductivity and 20 year service life isAs a rule of thumb if the wetted area of normalthe corroding metal is 10 times the wettedarea of the noble metal then are not serious although the largerthe ratio the less the effect T E C H N I C A L FA Q s PAGE 2 No1 Australian Stainless SteelThe 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 051securing competent advice Whilst ASSDA has taken all reasonable steps to ensure Level 15 215 Adelaide Street Brisbane Q 4000the information contained herein is accurate and current ASSDA does not warrantthe accuracy or completeness of the information and does not accept liability for Telephone 07 3220 0722 Facsimile 07 3220 0733errors or omissions Email Website wwwassdaasnau
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