Efflorescence and rust staining on the underside of a basement's steel beam suggest:

Two different materials are leaving two different records on the same assembly, and read together they date the exposure. Efflorescence is mineral salt left behind when water passes through masonry and evaporates at the surface, so its presence means water has been moving through the adjacent masonry — not once, but enough times to deposit visible salt. Rust on the steel means the beam's protective coating has been compromised and the steel has been wet. Neither is an instantaneous event; both accumulate. So the finding is that moisture has been reaching this location over a period, and the questions that follow are where it comes from and whether the section loss matters. The inspector reports the evidence and the likely persistence, notes any measurable scaling or delamination, and refers evaluation of structural significance rather than judging remaining capacity.

Efflorescence records water moving through masonry and evaporating, and rust records the steel having been repeatedly wet. Both accumulate over time, so together they indicate a persistent moisture source rather than a single event.

Efflorescence is crystallized salt deposited as water migrating through masonry evaporates at the surface. Corrosion of structural steel requires moisture and oxygen, and significant section loss can reduce capacity, which is assessed by a structural engineer rather than by visual inspection.

Salt and Rust Both Take Time. Neither appears from one wet week — together they say the water has been coming back.

Trace the water before assessing the steel. A beam pocket in a foundation wall, a plumbing leak above, or condensation on a cold beam each produce this and each has a different remedy.

• •Reading rust staining as evidence of a protective coating
• •Accepting corrosion on structural steel as normal appearance
• •Confusing a durability finding with a sizing question