Water marks and dark spots after pickling of hot-rolled or cold-rolled stainless are common shop-floor defects. Typical causes:

1) Incomplete rinsing: residual acid or salts dry on the surface and leave marks and spots. 2) Uneven drying: moisture stays on part of the surface, producing water lines and colour differences. 3) Under- or over-pickling: under-pickling leaves scale; over-pickling roughens the surface and makes it hold water. 4) Poor passivation: insufficient concentration or time leaves weak spots that rust early. 5) Incoming material: uneven scale thickness or localised rust on the coil can appear as dark spots after pickling.

Investigation and remedies:

• Check rinse-water quality (chlorides, hardness) and final rinse conductivity. • Verify drying method (air knife / dryer) and exit temperature. • Analyse dark spots metallographically or by EDS to distinguish residual scale, inclusions or corrosion pits. • Re-pickle and passivate if needed, and adjust process parameters.

Agree the surface grade and permitted defects at acceptance to reduce disputes.