Why is a pencil not used to mark aluminum?

Using a pencil to mark aluminum is discouraged primarily due to the potential for corrosion. This is because pencil lead is made of a combination of graphite and various clay materials, which can create a galvanic cell when in contact with aluminum. The presence of moisture can exacerbate this reaction, leading to localized corrosion where the pencil mark was made. Thus, the choice of marking tools is critical in maintaining the integrity of aluminum structures by preventing any possibility of corrosion that could compromise the aircraft's safety and longevity.

The other options, while they may present issues in specific contexts, do not address the most significant concern in using a pencil on aluminum materials. Permanent marks, smudging, and difficulty in erasing are secondary to the critical risk of corrosion, which is the primary reason for avoiding pencil marks on aluminum surfaces.