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Galvanic Corrosion and Dissimilar Metals

Galvanic Corrosion and Dissimilar Metals

TECHNICAL INTRODUCTION TECHNICAL INTRODUCTION

Factors Affecting incompatible materials or incorrect a consistent and effective protection Galvanic procedures are used. Fasteners are always of the structure, the should be Galvanic Corrosion and There are a number of factors that much smaller in surface area than the placed at appropriate intervals. determine the occurrence and severity structures they are used for, so a fastener of galvanic corrosion. Some couples that is anodic to a structural Environmental Considerations will cause corrosion more quickly will corrode rapidly under corrosive As with all types of corrosion, the Dissimilar than others depending on the conditions. AS/NZS 2312 recommends environment of a metallic couple is materials that are coupled, the that “fasteners should always be of very influential. The galvanic action Introduction will flow from the more active a solid component or as a coating such environment and the design. the same metal as the structure or be of the metals shown in Table 2 One of the challenges in designing material (which becomes an ) to as with electroplate or galvanizing. All cathodic to the structure.” below is indicative of measurements structures or any metallic article is the the less active material (that becomes of these common building materials The further apart the coupled considering immersion in seawater (as problem of galvanic corrosion. Basically, a ). This is then known as a can be used in a structure and their materials are on the , Related to the surface area and the use the ) so it can be considered this is the risk of dissimilar metals being metallic couple. For further information interactions can be complex. then usually the potential difference of anodes for , is environment-specific. However, it is a in physical contact and bridged by an on the relative potentials of metals and between them will be larger and this the effectiveness of such protection still a good guide for many applications electrolyte in a corrosive environment. their measurement, refer to AS 4036. It is also worth noting that galvanic will result in a higher current which over a large area. The protection will where metallic couples may present an corrosion can only occur if the metals in turn should result in more severe progressively decline with increasing issue. It is important to remember, that The design of structures utilising The cathode and the anode can be are electrical contact or “coupled”. corrosion of the anode. This is only a distance from the anode due to the the information in the table is a guide dissimilar metals and other materials considered as the positive and negative Electrical contact means that there is broad assumption and is not always voltage dropCOMPAT associatedIBILIT withY OF theDISSIMILAR METALSonly; it describesIN CONTACT what IN is MARINElikely to occur is becoming more common. This can terminals of a battery as batteries usually a metallic pathway to allow electrons the case, but it does provide a guide resistance of the metal. To maintainENVIRON MENTif Smetals (IMM areERS coupled.ED) be commonly seen in architectural work on the principle of galvanic to flow from the anode to the cathode. to the designer of where galvanic applications where the different corrosion. The current generated under Another requirement is that there is corrosion has at least the potential to aesthetic and physical features of varied the condition of a metallic couple will an electrolyte that bridges the metals become a problem. For example, there materials add to the overall visual generally cause increased corrosion to allow the flow of ions between is only a small potential between appeal. Also, many industrial and of the anodic metal and decreased them that completes the . and so galvanized bolts infrastructure applications seek to make corrosion of the cathodic metal. Further, the electrolyte need not be can be safely used to join structural use of the different physical properties a liquid; it can be any material that aluminium in atmospheric and of various materials. The galvanic series provides an allows the current flow between the freshwater environments. indication of which metals will become metals, such as damp soil or concrete. It is not always the case that anodic or cathodic in a metallic couple. The ratio of the area of the anode and galvanic corrosion from the use of Note that a metal can be either anodic An example of the galvanic series the cathode can influence the rate dissimilar metals is problematic. This or cathodic depending on which other from AS/NZS 2312 is shown in Table 1 of corrosion of the anode. This is an includes inland rural areas and other metal it is coupled with. For example, below. Note that this table is far from important characteristic in the use environments of low corrosivity, zinc will be anodic to most commonly exhaustive and does not include the of cathodic methods of protection. distant from the coast or industrial used metals, but it will be cathodic to numerous alloys and new metallic An example of this is the use of zinc pollution. Unfortunately, the seeming . This means that careful materials that are being developed, but anodes to protect steel structures such “success” of such applications to attention is required when a structural it does provide an indication of the as pipelines and ships. The larger the design practices and material selection design incorporates different metals relationships between different common steel structure, or more importantly, the that are not appropriate for more such as steel, zinc, , metals when they are coupled. larger its uncoated surface area, then corrosive locations. An example of , aluminium and others, either as the larger the zinc anode or anodes this can be seen in the selection and should be to protect it effectively use of fasteners. Inland rural locations PARTIAL GALVANIC SERIES via galvanic corrosion or sacrificial are forgiving of designs with little or corrosion as it is commonly called. no thought of the different materials Relationship to mild Although this discussion deals with Metals and alloys in galvanic series that are used and such designs often steel and cast dissimilar metals, impressed current perform satisfactorily. cathodic protection systems powered Austenitic stainles steel by direct current electricity can mimic While it is well known that dissimilar --iron alloys this phenomenon by allowing for metals in contact can cause corrosion an increase/decrease of the applied Nickel issues, it is worth discussing why this galvanic current where active surface occurs and how to mitigate galvanic Gunmetal area can vary. corrosion in the design process. Also, consideration must be given to the fact A consequence of the effect of area on Copper that galvanic corrosion in not always the galvanic corrosion is that smaller an undesirable phenomenon. In fact, Phosphor items such as fasteners are particularly cathodic protection and the corrosion Ferritic stainless steel prone to premature corrosion if protection properties of various 60/40 engineered coating systems are based on the principle of galvanic corrosion. Aluminium bronze What is Galvanic Corrosion? Chromium Two dissimilar metals in electrical Increasingly cathodic contact will generate a current when they are in the presence of an Mild steel, cast iron electrolyte. The electrolyte can be water I ncreasingly anodic Aluminium alloys or some other solution or material that Zinc is conductive. The galvanic current is Figure 1 – Corrosion on a galvanized generated because there is a potential Magnesium in a coastal location. Note the corrosion difference between the metals and of the zinc at the point of contact with the Table 2: Compatibility of Dissimilar Metals in Contact in Marine Environments (Immersed), the negatively charged electrons or Table 1: “Partial Galvanic Series”, Table 3.1, AS/NZS 2312. bare steel. Table B1, AS 4036. p.24 CORROSION & MATERIALS April 2013 www.corrosion.com.au p.25 TECHNICAL INTRODUCTION TECHNICAL INTRODUCTION

Galvanic corrosion is greatest when In contrast to immersion in an In some circumstances, an interesting Designing for the Prevention and other small components of The above discussion is only an the bimetallic couple is immersed in electrolyte, bimetallic couples in condition known as “polarity reversal” of Galvanic Corrosion structural assemblies. introduction as galvanic corrosion a very conductive electrolyte such as an atmospheric environment will can occur. Polarity reversal is known in Since the cause of galvanic corrosion is is a complex issue. It is suggested that seawater. Seawater has a relatively high normally have the corrosion restricted zinc/steel and aluminium/steel couples. the generation of a galvanic current in It is also recommended that the interested readers reference the two conductivity and it allows an easier in the area close to the junction of It occurs because the galvanic potential a bimetallic couple, the most effective flow of rainfall and other potential standards mentioned, AS/NZS 2312 path for the ions to travel from the the dissimilar metals. The of the zinc or aluminium anode method of prevention is to eliminate the electrolytes does not wash from a and AS 4036 for further reading anode to the cathode. Immersion in an in atmospheric environments have a changes under certain conditions. use of dissimilar metals in the design of cathodic metal onto an anodic metal. and understanding. electrolyte means that the corrosion much greater resistance than when the The most notable example of polarity structures. If this is not possible, then AS/NZS 2312 gives the example of of a bimetallic couple in such an dissimilar metals are fully immersed in reversal between zinc and steel is isolation of the dissimilar metals is then small amounts of copper oxidation References environment is generally uniform over an electrolyte such as seawater. This is in warm to hot potable water. This required. This can involve painting of products washing onto zinc (hot dip AS/NZS 2312: 2002 Guide to the the anode. because, for example with rainwater, was first recorded in the 1930’s and one or both of the metals with a suitable galvanized steel) and aluminium. protection of structural steel against it sits as a relatively thin layer on has been further researched. Polarity engineered coating system or using This can occur with rainwater acting atmospheric corrosion by the use of In atmospheric environments, galvanic the surface of the metal structure. reversal is predominately due to the a physical non-conductive separator as both the transport medium of the protective coatings corrosion is greatest in coastal and The reduced amount of electrolyte potential of the zinc changing as the (insulation) between them. For example, metal salts and an effective electrolyte. marine locations due to the deposition increases the resistance of the path for steel remains relatively stable. insulating washers should be used Normally, the corrosion will be of a AS 4036: 2006 Corrosion of Metals – of salt water spray. The corrosion rate the transfer of current and thus the with stainless steel when fixing cosmetic or superficial nature, resulting Dissimilar Metals in Contact in Seawater of a bimetallic couple (or any structural galvanic corrosion is reduced. Where The common rule of thumb is that aluminium cladding. in spotting or staining of the anodic metal for that matter) will be far greater moisture bridging by an electrolyte can the polarity reversal occurs when the metal, however, if allowed to continue Zhang, X: Corrosion and in areas of surf and high seas rather be minimised or prevented by use of a water is at around 60°C, although as In many applications, total isolation unfettered for prolonged periods, of Zinc, Plenum Press, NY, 1996 than those adjacent to sheltered bays spacer or keeping the contact surface with many issues concerning corrosion, of the metals from the environment or significant corrosion may occur due and estuaries. This is due to the greater dry, corrosion will also be minimised this is by no means definitive. The themselves is not possible or practical, to the formation of galvanic cells as Editor’s Note availability of salt water in the air. or eliminated. conditions under which reversal occurs either physically or economically. In metallic copper forms. PD6484:1979 Commentary on Corrosion can depend on the composition of the such instances, designs that minimise at Bimetallic Contacts and its Alleviation, Rural and inland environments It should be noted that metals can water. Zinc/steel couples in hard waters the effects of galvanic corrosion should Conclusion from BSI is also an excellent reference. are generally more forgiving than exhibit different galvanic potentials reverse more easily than those in soft be considered. Galvanic corrosion is one of the most coastal atmospheres or areas of high when they are located in different waters. Zinc forms a passive film in hard common corrosion issues in the design Emmanual Pimentel urban development and industrial electrolytes since each electrolyte waters and this, coupled with a higher A good start is to check the tables in of steel structures. It is useful to have activity. This is because the available has a different conductivity. This can temperature, is one of the main reasons AS/NZS 2312 and AS 4036 and avoid an understanding of the mechanisms contaminant and electrolytes are result in a metallic couple behaving for the polarity reversal. Polarity using dissimilar metals together when behind its occurrence and also how less common. Fresh water has a unexpectedly in comparison to reversal can be a major problem because they are wide apart in the galvanic to design structures to protect against significantly lower conductivity what would be assumed in the in zinc is normally used for its anodic series. AS 4036 also provides further it. Of course, an understanding of than seawater and its effect on the galvanic series. For example, relationship to mild steel. If polarity advice on which types of environments galvanic corrosion is also necessary for galvanic corrosion is reduced unless zinc in many forms is often used to reversal occurs, a corrosion protection will exacerbate the galvanic corrosion the utilisation of its beneficial effects. contaminants are introduced which protect mild steel because under most system utilising zinc is rendered useless of particular bimetallic couples. Galvanic corrosion principles are the increase its conductivity. This is conditions it is anodic to mild steel. at best, and at worst exposing the steel foundation of cathodic protection possible in some agricultural However, in different environments, to accelerated corrosion and possible As mentioned previously, avoid systems, either using engineered situations through the use of the potential difference between zinc failure. It is understandable then that coupling small anodes and large protective coatings, physical sacrificial chemicals and fertilisers. and mild steel can change and so does hot water tanks are no longer made of . This will involve paying anodes or electronic systems. its sacrificial capability. galvanized steel! particular attention to fasteners

Figure 2 – an insulator being used Figure 3 – example of a large anode/ to prevent dissimilar metal small cathode with a stainless steel corrosion between stainless steel fastener being successfully used on wire and a galvanized balustrade in a galvanized steel bridge rail in a a coastal environment. coastal environment.

p.26 CORROSION & MATERIALS April 2013 www.corrosion.com.au p.27

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