Etching for bonding polymer material to a metal surface

A method for forming multiple interlocking structures at an external surface of a part that includes a metal substrate overlaid by a primary metal oxide layer, the method comprising:

• forming first interlocking structures at locations corresponding to primary metal oxide defects at an external surface of the primary metal oxide layer by exposing the part to a metal etching solution, wherein the first interlocking structures include an opening having a first width at the external surface that leads to a primary recessed portion having an undercut geometry with a second width greater than the first width;
• subsequent to forming the first interlocking structures, allowing formation of a native oxide layer by removing the part from the metal etching solution and exposing the part to a native atmosphere;
• and
• forming second interlocking structures at locations corresponding to native oxide defects of the native oxide layer by exposing the part to the metal etching solution.