TECHNICAL INTRODUCTION TECHNICAL INTRODUCTION Factors Affecting incompatible materials or incorrect a consistent and effective protection Galvanic Corrosion procedures are used. Fasteners are always of the structure, the anodes 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 metal Environmental Considerations will cause corrosion more quickly will corrode rapidly under corrosive As with all types of corrosion, the Dissimilar Metals 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 ions 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 steel material (which becomes an anode) 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 cathode). This is then known as a can be used in a structure and their materials are on the galvanic series, Related to the surface area and the use the electrolyte) 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 cathodic protection, 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 zinc appeal. Also, many industrial and of the anodic metal and decreased them that completes the galvanic cell. and aluminium 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 magnesium. This means that careful materials that are being developed, but anodes to protect steel structures such “success” of such applications leads 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, stainless steel, metals when they are coupled. larger its uncoated surface area, then corrosive locations. An example of copper, 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 iron 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 Nickel-chromium-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, Monel 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 Bronze items such as fasteners are particularly cathodic protection and the corrosion Ferritic stainless steel prone to premature corrosion if protection properties of various Brass 60/40 engineered coating systems are based on the principle of galvanic corrosion. Aluminium bronze Lead What is Galvanic Corrosion? Chromium Two dissimilar metals in electrical Increasingly cathodic Tin 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 screw 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 electrolytes 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.