After a stainless steel coil travels across the ocean for a month, opening the container reveals rust spots on the surface — the buyer's first reaction is "wrong material," but in the vast majority of cases the material is qualified: the problem lies in the micro-environment inside the container.
One-sentence judgment
First distinguish "contamination rust" from "pitting." Contamination rust is shallow rust spots caused by iron particles or chlorides attached to the surface; after wiping off, the surface underneath is smooth and clean. Pitting is a pit that goes inward and cannot be wiped off. The former is a packaging and transportation issue; only the latter requires suspicion of material or surface treatment.
Three causes
① Condensation (most common). Sea shipping has large day-night temperature differences, and the inner walls of the container and the coil surface repeatedly condense water. Condensation itself is not scary; what is scary is that it dissolves salt from the air — air on seawater routes contains chlorine, and the condensation becomes a dilute chlorine solution, exactly what 304/316L fears most.
② Iron contamination. Steel wire ropes used for packaging, nails, iron pieces on wooden battens, mixed loading with carbon steel parts, forklift forks — iron particles embed into the stainless steel surface, rust first when exposed to moisture, and the rust water then contaminates the surroundings.
③ Surface residue. Chloride ions left from incomplete rinsing after pickling, hand sweat and oil stains during processing, iron filings from cutting/grinding not cleaned off. These hidden hazards are already planted before container loading.
Four things that really work in packaging
- Vapor phase corrosion inhibitor (VCI) film + desiccant: calculate the desiccant amount based on container volume and voyage, not just toss in two packs casually; the film must completely cover; damage equals no coverage;
- Wooden battens must be dry: wood with high moisture content is a "reservoir," and wood treated with chlorine-containing preservatives must not be used (it will continuously release chlorine);
- Do not mix with carbon steel: placing carbon steel parts in the same container, condensation will carry iron rust onto the stainless steel;
- Store coils upright: when lying flat, water easily accumulates in the coil eye; upright placement lets water flow away; bottom dunnage should be ventilated, not fully in contact with the ground.
Evidence worth keeping during container loading
- Photos before loading (surface condition, packaging integrity);
- Temperature and humidity data logger inside the container (a few dozen yuan each, hard evidence for claims);
- Record of desiccant quantity and placement;
- Packing list and container number for traceability.
When rust spots appear on arrival, these items determine whether responsibility can be clarified — claims without records usually end up with each party bearing half.
Handling sequence after arrival
- First determine the nature: wipe a small area with a damp cloth/neutral cleaner; what wipes off is contamination rust; what cannot be wiped off and has depth is pitting;
- Contamination rust: clean with neutral cleaner + thorough drying, local passivation if necessary (nitric acid or citric acid), do not use steel wire brushes (will introduce new iron contamination);
- Pitting: measure depth, assess whether it affects use; if occurring in batches, check material (spectroscopy) and surface treatment records;
- Record and claim: take photos, mark locations, keep packaging materials, follow contract terms.
Common pitfalls
- Using chlorine-containing cleaners (such as certain rust removers, bleach), the more you clean the worse it gets;
- Grinding rust spots with carbon steel brushes/sandpaper, embedding iron into the surface, recurrence in the same spot after a month;
- Blaming only the supplier, not checking loading records, ultimately unable to define responsibility;
- Saving costs by omitting VCI film and desiccant — the money saved is usually not enough to compensate for one coil.
Tell us the route, voyage, packaging method and arrival condition, and we will give packaging recommendations; when needed, we can also cooperate with your acceptance method to issue surface condition records and material certificates.
