Weather resistance of sealed anodized coatings
Aluminum is often used for architecturally pleasing building façades and other structures exposed to weathering influences. Lightweight and fairly easy to work, it is also durable and requires little to no maintenance over the long term when effectively protected by an anodized coating. But to ensure the reliability of the corrosion protection, the sealing of the coating must be verified.
Pure aluminum is actually highly susceptible to corrosion. However, in contrast to iron, it naturally forms a corrosion-inhibiting layer on its surface when it comes into contact with oxygen, which preserves it very well under normal environmental conditions: When this protective oxide layer is scratched away it is quickly replenished upon renewed exposure to oxygen. Nonetheless, for many applications, especially out-of-doors, this natural protection does not suffice because even the smallest entrapment of heavy metals can prevent the formation of a seamless oxide layer, allowing corrosion to get in and cause damage.
In order to improve on the natural oxide film, an electrolytic passivation process called anodic oxidation, or anodising, is often used. In an acid bath, oxygen released by an electrical current immediately reacts with the aluminum. It forms two oxide layers: a very thin insulating film (barrier layer) directly on the metal and, above that, a thicker, porous layer. Due to its microporous structure, the outer layer is vulnerable to contamination, requiring a post-treatment (hydration) that expands the coating and causes the pore walls to swell and close off at the surface (sealing).
Fig.1: Schematic diagram of the anodized film on aluminum before (left) and after (right) the sealing process
Although the ultimate quality of the sealing depends on a variety of factors including the condition of the oxide layer, the hydration time, the sealing bath temperature, the pH value of the solution, etc., it can be examined simply and non-destructively by measuring the electrical admittance (Y). Using FISCHER’s handheld ANOTEST® YMP30-S instrument, the coating’s admittance can be determined in situ according to standards such as DIN EN ISO12373-5 and ASTM B 457-67. An electronic reference is also available for cross-verification of the instrument.
Fig.2: Testing the sealing of an anodized façade with ANOTEST® YMP30-S
With the ANOTEST® YMP30-S, the sealing of anodized aluminum can be tested by measuring its electrical admittance. This makes it possible to determine the corrosion resistance of façades and other aluminum components exposed to weathering influences. For further information please contact your local FISCHER representative.
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