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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.
A Class 8 corrosive material that qualifies as a mailable limited quantity or ORM-D material is permitted in domestic mail provided that all applicable requirements in 348 are met.
Separate packaging requirements apply for liquids and solids.