Publication 52 - Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instruction 8A
A corrosive is any liquid or solid that causes visible destruction or irreversible alteration in human skin tissue at the site of contact or a liquid that has a severe corrosion rate on steel or aluminum.
A Class 8 corrosive material that qualifies as a Limited Quantity air or Limited Quantity surface material is permitted via air or surface transportation in domestic mail provided that all applicable requirements in 348 are met.
Separate packaging requirements apply for liquids and solids.
The following labels and text markings must be placed on the address side of the mailpiece unless specified in 221.1 and 325.1.