Aluminum Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals having different electrical potentials are electrically connected.
Aluminum and its alloys become the anode in galvanic cells with most metals. The rate of galvanic corrosion of aluminum coupled to a cathodic metal depends upon the degree of polarization of aluminum in the galvanic cell.
In a couple with chromium or stainless steel in atmospheric or other mild environments, the rate of aluminum galvanic corrosion is low.
In a couple with copper, however, the corrosion rate of aluminum is high.
Aluminium galvanic corrosion can result on fasteners. A compatible aluminum alloy is the safest, but often a higher strength steel fastener is used.
Zinc, cadmium, chromium, and tin coated fasteners may be used. Tin-zinc coatings are best for steel fasteners.
Cadmium is preferred over zinc if the coating thicknesses are equal; however, sufficiently thick plated cadmium fasteners may not be available.
Stainless steel fasteners may be used undermost conditions. Brass or plated brass items should be avoided.
Table below shows a galvanic series of aluminum alloys and other metals that represents the electrochemical behavior of each in seawater and in most natural waters and atmospheres and can be used to assess the likelihood of aluminium galvanic corrosion.
There is a wide range in the galvanic potential of the various aluminum alloys, some of the alloys show a significant effect of temper on the galvanic potential.
Permissible couples to avoid galvanic corrosion of aluminium are limited to a potential difference of 0.10 V on the galvanic series.
Aluminum is often used in a clad condition, and the cladding alloy may be slightly anodic or cathodic to the base alloy, depending on the application.
The usual clad product consists of a core alloy and a more anodic cladding alloy metallurgically bonded to one or both sides of the core alloy. Because the core alloy is cathodic to the cladding, any corrosion penetrates only to the cladding to-core interface.
It then spreads laterally and thus prevents perforation of the core.
This table is taken from: Atlas Steel Technical Note No. 7 "Galvanic Corrosion"
source: Material Deterioration Prevention Control Guide for Army Material - part one MIL-HDBK-735
Galvanic corrosion is a function of several different factors that need to be carefully evaluated when assessing the likelihood of this type of corrosion.
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