Adhesives, electrolytic corrosion

Another important factor is the corrosiveness of the adhesive. This may be especially important in those cases where the PSA has direct contact with the bare wire, the electronic component, or the silicon wafer in a dicing operation. In those cases where an electrical current is running through the device, electrolytic corrosion processes may occur, especially if moisture can penetrate into the adhesive or bond line. 

Bonding permits the joining of dissimilar materials, and since adhesives are generally dielectric materials, their use minimizes the possibility of electrolytic corrosion when different metals are joined. 

D 3482 Test Method for Determining Electrolytic Corrosion of Copper by Adhesives... 

ASTM D3482-90 (2000) Standard practice for determining electrolytic corrosion of copper by adhesives. 

A continuous intact film of water-resistant paint forms an effective electrical resistance to the flow of a corrosion current (a resistance of over lO flcm through the film is easily achieved). Underfilm corrosion can then only occur if a channel of electrolyte connecting anode and cathode can be established by local adhesion failure between the coating and the metal substrate. 

A1 is thermodynamically unstable, with an oxidation potential at 1.39 V. Its stability in various applications comes from the formation of a native passivation film, which is composed of AI2O3 or oxyhydroxide and hydroxide.This protective layer, with a thickness of 50 nm, not only stabilizes A1 in various nonaqueous electrolytes at high potentials but also renders the A1 surface coating-friendly by enabling excellent adhesion of the electrode materials. It has been reported that with the native film intact A1 could maintain anodic stability up to 5.0 V even in Lilm-based electrolytes. Similar stability has also been observed with A1 pretreated at 480 °C in air, which remains corrosion-free in LiC104/EC/ DME up to 4.2 However, since mechanical... 

Figure 4. Schematic illustration of corrosion induced paint adhesion loss. In the initial stage (A), iron is oxidized and oxygen reduced at a site of electrolyte penetration as corrosion proceeds (B and C), a rust deposit forms and the anodic and cathodic reaction sites become separated. Cathodically produced hydroxide progressively disrupts paint adhesion. Reproduced from Ref. 25, copyright 1980, American Chemical Society.

Too much enphasis has be given to adhesion under dry conditions. However, corrosion is only possible if enough water is present in the ooating/metal interface to provide the electrolyte for the corrosion elements to operate. This condition is hardly imaginable without a previous significant reduction or even the loss of adhesion. Therefore "wet adhesion" is considered to be of crucial inportance to corrosion protection by organic coatings (9). 

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