Bimetallic (galvanic) corrosion risks from contact with galvanised steel or aluminium
Bimetallic corrosion can only occur when two dissimilar metals are in ‘electrical’ contact and are bridged by an electrically conductive liquid.
The ‘cell’ produced can result in corrosion to one of the paired metals. This can be an issue when stainless steels are in contact with other metals, depending on the circumstances.
What is needed to set up the corrosion ‘cell’?
To set up a galvanic cell between two conducting materials, (metals or graphite), the two metals must have differing potentials or be more or less ‘noble’ than each other.
The more noble metal, (cathode), is protected as the less noble metal, (anode), sacrificially corrodes.
The table below is an example of these ‘metal to metal’ relationships, including graphite as conductive non-metal.
|ANODIC, (Least Noble)
|Carbon steel or cast iron
|Copper alloys, (brass, bronze )
|Nickel alloys, (Incoloy 825, Hastelloy B)
|CATHODIC, (Most Noble)
The further apart the metals are, in terms of relative potentials, the greater the driving force in a cell. So, for example, stainless steel in contact with copper is less likely to be a risk than when it is in contact with aluminium or galvanised (zinc coated) steel.
To complete the cell, a conductive liquid must bridge the contact metals.
The more electrically conductive the liquid is, the greater the danger of corrosion. Seawater or salt laden moist air is more of a risk than contact with rain water or towns water.
If the metals are dry, bimetallic (galvanic) corrosion cannot occur.
Corrosion risks with galvanised steel and stainless steel in contact
Galvanised steel in contact with stainless steels is not normally considered to be a serious corrosion risk, except possibly in severe, (marine type), environments.
In these situations, precautions such as insulating barriers are usually considered adequate to avoid bimetallic corrosion in most practical situations.
Corrosion risks with aluminium and stainless steel in contact
Aluminium and stainless steel together also appears to be a bi-metallic corrosion risk, from the ‘nobility’ table.
With this combination the affect of relative surface area on corrosion is important.
A large area of ‘cathode’ relative to ‘anode’ will accelerate the anodic corrosion. Although aluminium is anodic to stainless steel, large relative surface areas of aluminium to stainless steel can be acceptable, dependant on local conditions.
Stainless steel fasteners in aluminium plates or sheets are normally considered safe, whereas aluminium rivets or bolts holding stainless steel parts together is an unwise combination, as there is a practical risk of corrosion.
An example of the safe use of stainless steel and aluminium together is where stainless steel fasteners and hold down bolts are used to secure aluminium roadway or bridge parapet guards.
Even with no insulation between the metals, there should be little risk of corrosion.
In contrast, in a marine environment, severe localised pitting corrosion to the aluminium treads has been observed where un-insulated stainless steel bolts were used to secure the treads in place.
On the same ladder however, bolts with sound insulating washers did not show any pitting on the surrounding aluminium.
This illustrates the beneficial effect of breaking the corrosion cell by isolating the two ‘dissimilar’ metals in marginal cases.
Discolouration of stainless steel by corrosion products
Staining effects on stainless steels from corrosion products of the coupled metal can also be an issue.
Lead and copper are quite close on the nobility table to stainless steel and so the bimetallic corrosion risks should be small.
Any corrosion product, if washed onto stainless steel, may however result in problems not associated with the bi-metallic effect and so not be predicted from the tables.
Additional care in design should avoid such staining problems.
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