Galvanized Coil White Rust After Ocean Shipping: Causes and Prevention

Why Does White Rust Appear on Galvanized Coil After a Long Sea Voyage?

The story is always the same. The container is opened at Jebel Ali or Santos, the kraft paper is dry, the coil edges are unmarked, and yet the top laps of the coil are covered in a white, chalky powder that was not there when the coil was inspected at the mill. Some importers reject the whole lot. Others wipe it off, send the coil to a roll-forming line, and watch the powder clog the paint pre-treatment bath and streak the finished panel. Both reactions cost money, and in most cases both were avoidable six weeks earlier.

White rust is not a rolling defect and it is not a coating defect. It is zinc hydroxide, Zn(OH)2, produced when the zinc surface is exposed to moisture for long enough that corrosion starts before a protective zinc carbonate layer can form. The reaction is simple: zinc plus oxygen plus water gives zinc hydroxide; zinc hydroxide plus carbon dioxide gives the hard, grey, protective zinc carbonate that you want. If the coil is wet and there is no airflow, the first step runs and the second step never gets the chance to complete. The result is a soft, voluminous white deposit that can be several times thicker than the zinc it came from.

Two things matter commercially. First, white rust is cosmetic and reversible in the sense that the coating is still there underneath, but it destroys paint adhesion if it is not removed properly, and removal costs your customer labour. Second, red rust is a completely different failure: it is iron oxide, it means the base steel is exposed, and it is never acceptable on a Z275 coil that has been sitting in a dry container for forty days.

The Three Conditions That Actually Cause White Rust

White rust needs three things at the same time. Remove any one of them and the coil arrives clean.

• Liquid water or condensation on the zinc surface. Not high humidity alone, but condensation. This is what container sweat and cargo sweat produce when a warm, humid container is cooled rapidly overnight.
• No ventilation or air exchange. A sealed container with wet coils traps the moisture in place and lets the reaction continue for the whole voyage.
• Time and temperature cycling. A twelve-day crossing to Southeast Asia is forgiving. A thirty-five day crossing to West Africa or South America with a ten to fifteen degree day-to-night swing is not.

The practical conclusion is that you do not prevent white rust by asking the mill for better galvanizing. You prevent it by controlling moisture inside the container. A Z275 coating (275 g/m2 total, both sides) does not corrode in transit because the zinc is thin; it corrodes because the water never leaves.

What to Agree Before the Coil Leaves the Mill

The surface treatment specified on the purchase order decides how much time you have. There are three normal options and they are not equivalent.

Surface treatment	What it does	Typical specification	Notes
Bare / untreated	No film on the zinc	Mill finish, oil-free	Cheapest; highest white rust risk on long voyages
Chromate passivation	Thin conversion film slows zinc corrosion	Cr3+ (trivalent) passivation, 5-25 mg/m2	Cr6+ is being phased out on RoHS grounds
Chrome-free passivation	Organic / Ti-Zr film	Cr-free, 0.5-1.5 g/m2	Good for food-contact and EU markets
Oiled	Physical water barrier	Anti-finger or protective oil, 0.5-1.5 g/m2	Oil must be removed before painting

If you are buying coil for a paint line, request trivalent chromate or chrome-free passivation and confirm the film weight on the mill test certificate. If the coil is going straight to a cut-to-length line and will be stored indoors, bare coil is acceptable. For anything crossing more than twenty-five days of ocean, passivation plus desiccant is the default that we ship and the default we recommend.

Desiccant, VCI and Container Preparation by Voyage Length

Desiccant is cheap relative to a claim. The quantities below are working figures for a lined 20GP container stowed with coil, based on normal container sweat conditions.

Voyage length	Desiccant (per 20GP)	Lining	Notes
Under 10 days	10-20 kg	Kraft paper	Short intra-Asia crossings
10-20 days	20-40 kg	Kraft paper + VCI film	Middle East, Indian subcontinent
20-35 days	40-80 kg	VCI film, full wrap	Europe, East Africa
Over 35 days	80-150 kg	VCI film + roof liner	West Africa, South America

Use calcium chloride or clay desiccant bags rated for container use, hang them high in the container away from the steel, and never let a bag sit directly on the coil where it can weep. VCI (vapour corrosion inhibitor) film emits a protective vapour that reaches the coil surface and neutralises the moisture film; it works best when combined with desiccant rather than instead of it.

Loading and Stowage: Keeping Condensation Off the Coil

• Line the container roof first. A kraft or aluminium foil liner on the ceiling stops roof drip from landing on the top laps.
• Palletise or use wooden skids. Steel on a bare container floor sits in the coldest layer where condensation forms.
• Leave a small air gap between coils. Coils packed metal-to-metal trap moisture in the laps.
• Do not stow coils against the doors last without a check. Door-end cargo cools fastest and sweats first.
• Close the container with a fresh seal and record the number. A seal number on the loading report is your first line of defence in a dispute.

Inspection on Arrival: What Counts as a Claim and What Does Not

Judge the coil against a defined standard, not against a feeling. Use ASTM D610 rust grading for the percentage of surface affected, and separate white rust from red rust in the report.

Observation	Severity	Normal commercial treatment
Light white haze, under 5% area	Minor	Clean and use; no claim
White rust 5-20% area, edges only	Moderate	Deduction negotiated; still paintable after cleaning
White rust over 20%, or in the coil laps	Severe	Reject or major deduction; paint adhesion at risk
Any red rust on a dry coil	Serious	Claim; check coating weight by stripping test

Photograph the coil in the container before unloading, keep the desiccant bags, and record the seal number and the time of opening. If the receiver claims red rust, request a coating weight test rather than accepting a visual judgement.

Ready to move? Send us the coating spec (for example Z275 with Cr3+ passivation), the coil size and weight, the quantity in tons and your discharge port. We will come back with a quotation that includes the packing and moisture-control option suited to your voyage length, so the coil that leaves our mill is the coil your customer receives.