Water marks and dark spots after pickling: common causes
Water marks and dark spots after pickling usually come from incomplete rinsing, acid residue, uneven drying or poor passivation, and can also trace back to incoming scale and process parameters.
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.
