Coating defect

Zinc salts (white rust)

White, grey or black powdery deposits on galvanizing or on zinc primer, worst where items were stacked wet.

Zinc salts (white rust): the surface as you see it, and a schematic cross-section.

Causes

Zinc reacts with moisture and air; without air flow the products stay loose and soluble.
Wet storage of fresh galvanizing in tight stacks.
Zinc primer left uncoated in the weather.

Prevention

Store galvanized items dry and ventilated.
Topcoat zinc primers within the maker's times.

How it is assessed

Visual and by touch: loose, powdery salts must go; tightly adherent ones may stay if the specification allows. ASTM A123 accepts light or medium wet storage stain on new galvanizing, because it weathers away. Before painting, loose zinc products are removed (ASTM D6386).

Telling it apart

Efflorescence: Efflorescence is salt from concrete or masonry. White rust is a zinc product on galvanizing or zinc primer.
Chalking: Chalking is the binder of a paint breaking down. White rust grows on bare zinc.

Repair

Remove loose salts by washing and scrubbing with a stiff brush or by sweep blasting and washing, then coat. The specification and the coating manufacturer have the final word on the repair procedure.

6 practice items in the members area mention zinc salts (CIP 2: 2 · FROSIO written: 4).

Try it in an exam-style question. The free readiness check shows the question style, explains every answer and gives you a dated study plan.

Take the free readiness check See the tracks

About this page

Written from coating practice and the published rating standards, in our own words; the drawing is schematic. Checked on 2026-10-14; errors go in the corrections log. CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.