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Galvanic Corrosion Bimetallic Corrosion
Galvanic corrosion is a localised corrosion mechanism by which metals can be preferentially corroded. Sometimes is also called metal to metal corrosion or bimetallic corrosion.
This type or form of corrosion has the potential to attack junctions of metals, or regions where one construction metal is changed to another. Frequently this condition arises because different metals are more easily fabricated into certain forms; an example might be a door frame manufactured from aluminium extrusions (aluminium extrudes extremely well into architectural sections), but with a door handle fabricated from stainless steel tube to exploit its higher strength and abrasion resistance.
This type of corrosion is well known to most designers, specifiers and fabricators, but often the only rule in force for is "don't mix metals".
What Conditions are Needed
For galvanic corrosion to occur there are three conditions which must be met:
Condition 1. Metals must be far apart on the galvanic series
The galvanic or electrochemical series ranks metals according to their potential, generally measured with respect to the Standard Calomel Electrode (S.C.E.). The results are often viewed as a galvanic corrosion chart or galvanic corrosion table similar to that on the third page.
This chart says that the "anodic" or "less noble" metals at the negative end of the series - at the right of this diagram, such as magnesium, zinc and aluminium - are more likely to be attacked than those at the "cathodic" or "noble" end of the series such as gold and graphite.
The critical point is the difference in potential of the two materials being considered as a joined pair. A difference of hundreds of millivolts is likely to result in galvanic corrosion, but only a few tens of millivolts is unlikely to be a problem.
Condition 2. The metals must be in electrical contact
The two different metals must be in electrical contact with each other. This is of course very common. The two metals can be bolted, welded or clamped together, or even just resting against each other.
Condition 3. The metal to metal junction must be bridged by an electrolyte
An electrolyte is simply an electrically conducting fluid. Almost any fluid falls into this category, with distilled water as an exception. Even rain water is likely to become sufficiently conducting after contact with common environmental contaminants.
If the conductivity of the liquid is high (a common example is sea water) the metal to metal corrosion of the less noble metal will be spread over a larger area; in low conductivity liquids the corrosion will be localised to the part of the less noble metal near to the junction.
The Area Effect
The relative area of the anode and cathode has a pronounced effect upon the amount of corrosion that occurs due to Bimetallic Corrosion. A small anode (the less noble metal, such as aluminium) joined to a large cathode (the more noble metal, such as stainless steel) will result in a high current density on the aluminium, and hence a high rate of corrosion.
The corrosion is concentrated by the area difference. Conversely if the area of the anode is large compared to that of the cathode this dilutes the corrosive effect, in most cases to the extent that no problem occurs. It is common practice to use stainless steel fasteners to fix aluminium sheeting or signs, but if aluminium screws were used to fix stainless steel the screws may rapidly corrode.
An apparent contradiction of the area effect on metal to metal corrosion occurs when the component comprised of the two metals is only partly wetted. Consider for instance a stainless steel bolt in an aluminium plate; if water collects in the corner at the edge of the bolt but the remainder of the plate remains dry, the effective area of the less noble aluminium is only the wetted region, which may be only a similar size to that section of the bolt that is wetted .... thus it is quite possible for the aluminium plate to be galvanically attacked in the region immediately surrounding the bolt. Only the wet “area” counts.
Crevices & Stagnant Conditions
As shown in the electrochemical series chart there are two different potentials associated with each stainless steel grade. The less noble value shown in outlined boxes is that which applies if a crevice is formed between the two dissimilar metals or such as beneath bio-fouling.
Such a crevice could be from the design or fabrication of the component, and formation of biological films is more likely in stagnant or slow-flowing sea water. The result of these stagnant conditions is oxygen depletion and the less noble potential which can make the stainless steel susceptible to corrosion in conditions that might otherwise be considered non-corrosive.
Passive Surface Films and effect on Bimetallic Corrosion
Stainless steels naturally form passive surface films this is what makes them “stainless”. This film also reduces the amount of current available for corrosion, so slows the corrosion rate down compared to some other galvanic pairs.
Galvanic Corrosion Prevention
The methods for galvanic corrosion prevention are in general suggested by the above descriptions of the conditions necessary for its occurrence.
Don’t Mix Metals. If only one material is used in a construction the problem is avoided (Condition 1 is not present – no mixed metals). Be particularly aware of zinc plated or galvanised feasters in stainless steel sheets – a common substitution because of perceived cost savings or better availability. These less noble fasteners in the galvanic series are likely to be rapidly attacked.
Prevent Electrical Contact. It is often practical to prevent electrical contact between the dissimilar metals (removal of Condition 2). This may be achieved by the use of non conducting (eg rubber or plastic) spacers, spool pieces or gaskets, perhaps in conjunction with sleeves around bolts. For the same reason a gap may be left between galvanised roofing and a stainless steel down-pipe.
Corrosion potentials in flowing sea water at ambient temperature. The unshaded symbols show ranges exhibited by stainless steels in acidic water such as may exist in crevices or in stagnant or low velocity or poorly aerated water. The more Noble materials at the left side tend to be cathodic and hence protected; those at the right are less Noble and tend to be anodic and hence corroded in a galvanic couple.
Prevent the Wetted Junction.
The third Condition can be removed by ensuring that no electrolyte remains at the intermetallic junction - this may require extra attention to drainage or to protection from the weather. A good covering of paint or sealant over the junction can be effective to prevent metal to metal corrosion.
Use the Area Effect to avoid Bimetallicc Corrosion.
The area effect should also be considered in avoiding corrosion damage, particularly in selection of fastener materials.
Stainless steel fasteners can be used to hold aluminium structures, but the area effect will not apply if the wetted area shrinks over time due to evaporation. In the situation of 304 or 316 fasteners used in conjunction with other less noble structural materials, the fastener will be galvanically protected by the surrounding large, less noble area.
Positively Use Galvanic Protection to avoid Galvanic Corrosion.
The galvanic effect can also be used to provide corrosion protection. For example it is prudent to guard against possible crevices, perhaps associated with marine fouling, or simply under bolt heads, by specifying slightly more noble bolt materials. An example is the use of 316 fasteners in conjunction with 304 structural materials – the minor galvanic protection afforded the fasteners improves their corrosion resistance.