In general, a metal alloy (e.g., an aluminum-copper alloy) may be susceptible to pitting corrosion (also referred to as pitting). Pitting corrosion may refer to as an extremely localized corrosion that impairs the metal alloy, e.g., by the formation of holes. Pitting corrosion may be induced by stoichiometric inhomogeneities leading to an anodic-cathodic coupling, which induces localized galvanic corrosion in the spatial scale of the stoichiometric inhomogeneities. Therefore, pitting corrosion may also occur in otherwise corrosion-resistant alloys.
By way of example, in case of intergranular corrosion (also referred to as intergranular attack) the boundaries of crystallites of the metal alloy may be more susceptible to corrosion than their insides, e.g., when the grain boundaries are depleted (also referred to as grain boundary depletion) of the corrosion-inhibiting elements such as chromium. In nickel alloys and austenitic stainless steels, where chromium is added for corrosion resistance, the formation of chromium-depleted zones adjacent to the grain boundaries may be induced by precipitations of chromium carbide at the grain boundaries. The grain boundary depletion may induce local galvanic coupling, causing local galvanic corrosion.
Alternatively or additionally, a chemical activating environment may induce or enhance pitting corrosion. By way of example, wet chemical treatment of an aluminum-copper alloy may result in galvanic deposition of copper from the solution and aluminium corrosion. Alternatively or additionally, pitting corrosion may be induced or enhanced by an electrical current flowing through the metal alloy.
Conventionally, pitting corrosion may be reduced by an artificial passivation (e.g., using a protection nitride) of the metal alloy. However, the artificial passivation may obstruct electrical contact of electronic components, e.g., if the metal alloy provides a contact pad. For electrical contacting, the artificial passivation may be locally removed by dry etching to expose the metal alloy, e.g., for bonding the exposed region of the contact pad. In this case, the dry etching may also attack the inherent passivation (e.g., aluminium oxide) of the metal alloy and, thereby, increase the risk of pitting corrosion proximate the electrical contact. Alternatively, the bonding parameters may be adapted for bonding through the artificial passivation, which may result in a weakened electrical contact. Further, if the artificial passivation includes a metal, the artificial passivation may form a galvanic element with the metal alloy, thereby, inducing galvanic corrosion itself.