Why the paperwork package decides whether your corrosive cargo clears
Import coordinators, compliance officers, and QA/EHS teams rarely lose a corrosive shipment because the product is wrong. They lose it because the paperwork does not agree with itself. A drum marked for one concentration, a Safety Data Sheet written for another, and a shipper's declaration that cites a UN number matching neither will put the container on hold at the destination port while demurrage accrues. This guide walks through assembling a correct, audit-ready document set for shipping corrosive bulk chemicals from China by sea, using three products that cover most of the practical cases: sulfuric acid, hydrochloric acid, and ammonia solution.
The core principle is simple. The actual shipped concentration drives the classification, the classification drives the declaration and the labels, and every physical mark on the packaging must trace back to the same classification. When those elements line up, inspection is fast. When they diverge, the cargo waits.
How concentration sets the UN number, class, and packing group
Under the IMDG Code, the proper shipping name, UN number, hazard class, and packing group are not chosen by product family. They follow the specific substance and, for many solutions, the concentration band in which the shipment actually falls. Getting this right at the quotation stage prevents rework later.
Sulfuric acid (UN1830)
Sulfuric acid shipped at industrial strengths of 50%, 70%, and 98% is declared as UN1830, Sulphuric acid with more than 51% acid, or UN2796 for 51% and below, Class 8 (corrosive), Packing Group II. The 98% grade is the common electronics, battery, and process feedstock; the 50% and 70% dilutions move for water treatment and metal pickling. Because the hazard class is corrosive rather than toxic or a marine pollutant for most grades, the labeling burden centers on the Class 8 corrosive placard and correct UN marking. When you are comparing offers from a sulfuric acid supplier, confirm the exact concentration printed on the SDS matches the grade you intend to declare, because the 51% threshold changes the UN number.
Hydrochloric acid (UN1789)
Hydrochloric acid at 25%, 31%, and 36-38% is declared as UN1789, Hydrochloric acid, Class 8, Packing Group II for the stronger grades and Packing Group III for weaker dilutions, depending on the corrosivity data in the SDS. The 36-38% "fuming" grade releases hydrogen chloride vapor, so closure integrity and venting provisions matter more than for the acids that stay quiescent. When sourcing from a hydrochloric acid supplier, ask whether the quoted strength is the as-shipped assay or a nominal label value, since a shipment near a packing-group boundary needs the measured number to justify the declaration.
Ammonia solution: why strength changes the UN number
Ammonia solution is the clearest example of concentration rewriting the entire declaration. The same chemical family splits across three UN numbers:
- **UN2672, Ammonia solution**, relative density between 0.880 and 0.957 at 15 degrees C, more than 10% but not more than 35% ammonia. This covers the 20%, 25%, and 28% industrial grades and classifies as Class 8, Packing Group III.
- **UN2073, Ammonia solution**, more than 35% up to 50% ammonia, which moves into Class 2.2 as a non-flammable, non-toxic gas and is no longer a Packing Group substance at all.
- **UN3318, Ammonia solution**, more than 50% ammonia, which classifies as Class 2.3, a toxic gas.
This is why rising ammonia strength changes the applicable UN number and therefore the declaration, the packaging regime, the labels, and the vessel stowage. A 28% drum and a 40% consignment are not the same regulatory animal, even though both read "ammonia solution" on the commercial invoice. Ammonia solutions are also typically flagged as marine pollutants because of aquatic toxicity, so the consignment usually carries the marine pollutant mark in addition to the corrosive or gas labels. When evaluating an ammonia solution offer, pin down the ammonia percentage before anything else, because it decides whether you are shipping a Class 8 liquid or a liquefied gas with a far heavier compliance load.
Matching UN-certified packaging marks to the declaration and SDS
UN-certified packaging carries a coded mark that must correspond to the substance, packing group, and quantity being shipped. Three formats cover most corrosive bulk trade from China:
- **HDPE closed-head drums (1H1):** the 1 denotes a drum, H denotes plastics, and 1 denotes a non-removable head. These suit the acids and the lower-strength ammonia solutions at drum volumes.
- **Composite IBCs (31H1 with HDPE inner receptacle):** 31 denotes a composite IBC for liquids, with the rigid plastic inner receptacle housed in an outer frame, used for 1,000 L parcels.
- **ISO tank containers (T11):** the T11 portable tank instruction applies to the corrosive liquid grades such as sulfuric and hydrochloric acid moving in bulk. Gas-classified ammonia solutions above 35% require tank specifications appropriate to their class, not T11.
The audit check is mechanical. The UN marking on the drum or IBC must list a packing group equal to or more stringent than the one on the shipper's declaration, and the substance the packaging was tested for must be compatible with the product on the SDS. If the declaration says Packing Group II and the drum is marked only for Packing Group III, the packaging does not cover the cargo and the shipment is non-compliant on its face.
The complete consignment documentation set
A complete corrosive consignment leaving China by sea should travel with a consistent, cross-referenced package:
1. **Safety Data Sheet (SDS):** current, in the receiving country's required language, stating the exact concentration, UN number, class, and packing group.
2. **Shipper's dangerous-goods declaration:** the formal statement of proper shipping name, UN number, class, packing group, packaging type, number and kind of packages, and net quantity, signed by the shipper.
3. **Container packing certificate:** confirming the unit was loaded, braced, and segregated according to the IMDG Code.
4. **UN marking and labeling:** the UN specification mark on each package plus the correct hazard labels, orientation arrows where required, and the marine pollutant mark for ammonia solutions.
5. **Marine-pollutant screening:** a documented check of whether the substance and grade are marine pollutants, which determines label and declaration entries.
Every field that appears in more than one document must read identically. The concentration on the SDS, the UN number on the declaration, and the class on the label are the three values inspectors reconcile first.
How labeling and documentation errors trigger holds and demurrage
At the destination port, the dangerous-goods review is a document-to-cargo match. If the declared UN number does not match the label, if the packing group on the mark is weaker than the declaration, or if a required marine pollutant mark is missing, the container is held pending correction. Corrections on arrival are slow and expensive: the carrier will not release until the paperwork is right, container detention and demurrage clocks run during the dispute, and relabeling a sealed hazardous unit at a port is often not permitted without specialist handling. A single transposed digit in a UN number can convert a routine clearance into a multi-day hold.
Cross-checking paperwork on arrival
Receiving QA/EHS teams should treat arrival as a verification gate, not a formality. The practical sequence is to read the UN number and grade off the shipper's declaration, confirm the same UN number and class on the physical labels, confirm the packaging UN mark covers the declared packing group, and confirm the SDS concentration matches the grade on the commercial documents. For ammonia specifically, verify that the UN number corresponds to the concentration band, since a 28% parcel must show UN2672 and anything above 35% must not. Any mismatch is logged and quarantined before the material enters inventory.
Writing documentation and labeling acceptance into the purchase contract
The cheapest place to fix these problems is the contract. Acceptance terms should state that the declared UN number, class, and packing group must match the shipped concentration on the SDS; that packaging must carry a valid UN specification mark covering the declared packing group; that the correct hazard and marine pollutant labels must be affixed; and that the full document set must be provided before or with the bill of lading. Make document conformity a condition of acceptance, assign responsibility and cost for holds caused by shipper documentation errors to the shipper, and require the as-shipped assay certificate so the declared grade is backed by a measured value rather than a nominal label.
Using UN Comtrade import figures as a sanity check only
Public UN Comtrade import statistics are useful for one narrow purpose: a directional read on liquidity and plausible lead times, never a firm price or freight benchmark. For reference, China's recorded imports show sulfuric acid (HS 280700) at about 126.2 million kilograms for 2025 against a reported trade value near 15.5 million US dollars, hydrochloric acid (HS 280610) at about 6.1 million kilograms for 2023 against about 14.3 million US dollars, and anhydrous ammonia (HS 281410) at about 348.5 million kilograms for 2025 against roughly 133.0 million US dollars. Treat these only as scale indicators that a product category moves in meaningful volume. They are annual, aggregated, and historical, they mix grades and trade lanes, and they say nothing about the spot price, packaging premium, or ocean freight you will actually pay. Use your supplier quotations and a current freight rate for commercial decisions, and use Comtrade only to sense-check whether a quoted lead time or volume is broadly realistic.
Sources
- UN Comtrade public trade statistics, sulfuric acid (HS 280700), China imports, 2025: https://comtradeapi.un.org/public/v1/preview/C/A/HS?reporterCode=156&period=2025&cmdCode=280700&flowCode=M&partnerCode=0&partner2Code=0&customsCode=C00&motCode=0&maxRecords=1
- UN Comtrade public trade statistics, hydrochloric acid (HS 280610), China imports, 2023: https://comtradeapi.un.org/public/v1/preview/C/A/HS?reporterCode=156&period=2023&cmdCode=280610&flowCode=M&partnerCode=0&partner2Code=0&customsCode=C00&motCode=0&maxRecords=1
- UN Comtrade public trade statistics, ammonia (HS 281410), China imports, 2025: https://comtradeapi.un.org/public/v1/preview/C/A/HS?reporterCode=156&period=2025&cmdCode=281410&flowCode=M&partnerCode=0&partner2Code=0&customsCode=C00&motCode=0&maxRecords=1
