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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 an ORM–D material is permitted in domestic mail provided that all applicable requirements in 348 and DMM 601.10.19 are met.
Separate packaging requirements apply for liquids and solids.