Nothing – the two organizations have merged.

NACE International, formerly the National Association of Corrosion Engineers, was established by a group of engineers for the petroleum industry. NACE is headquartered in Houston.
SSPC, the Society of Protective Coatings (formerly the Steel Structures Painting Council), headquartered in Pittsburgh, formed after NACE and focused on structural steel.

The merger is still in progress however courses, standards, committees, etc. have been merged or are in the process of mergin.