Aqueous hydrogen chloride is traded as hydrochloric acid under CAS 7647-01-0 and EC 231-595-7. The standard commercial grade is 37 wt%, with a density of 1.18–1.19 g/cm³ at 20°C, and the product carries harmonised CLP classification Skin Corr. 1B H314 and STOT SE 3 H335. The specific concentration limit for skin corrosion is 25%. Because hydrochloric acid is both a high-tonnage inorganic acid and a harmonised classified substance, suppliers must synchronise product registration, mixture notification, and shipping documentation with the two compliance deadlines for 2026: China Hazardous Chemicals Law and EU Registers. Missing either deadline affects customs release, poison-centre registration, and legal sale of HCl-containing mixtures in the respective jurisdictions.

The 2026 obligations are not identical. China imposes product-level registration and licensing under the hazardous chemicals catalogue, while the EU imposes mixture-level notification to poison-centre registers and, for new hazard classes introduced under Commission Delegated Regulation EU 2023/707, an update of existing notifications. For hydrochloric acid, the practical compliance burden concentrates on classification data, impurity profiles, and concentration-specific thresholds for skin corrosion, respiratory irritation, and transport.

What changes for hydrochloric acid under the 2026 China hazardous chemicals deadline?

In the People’s Republic of China, hydrochloric acid is listed in the Catalogue of Hazardous Chemicals under CAS 7647-01-0. Manufacture, import, storage, and sale require a hazardous chemicals safety production licence or operating licence under State Council Decree No. 591. Bulk and packaged shipments are assigned UN 1789, Class 8, Packing Group II. The 2026 registration phase requires the legal entity to complete hazardous chemical registration with the local emergency management bureau before first import or production and to align the product code with customs declaration data for HS code 2806.10.

The safety data sheet must be prepared under GB/T 16483-2008, and the precautionary label under GB 15258-2009 must display the GHS pictograms for corrosion and health hazard. For concentrations at or above 25%, the skin corrosion category applies; below 10%, skin corrosion/irritation classification changes but STOT SE 3 H335 remains relevant for the substance. Imported 37 wt% hydrochloric acid arriving in ISO tank containers requires exact concentration data on the Chinese SDS and label because discrepancy between the declared concentration and the hazardous chemical product code is a common customs hold on bulk chemical lines.

For EU poison-centre registers, a mixture containing hydrochloric acid at or above the relevant generic concentration limit for skin corrosion, skin irritation, or STOT SE must be notified to the ECHA PCN portal before placing on the market. The submission requires a UFI printed on the label. Hydrochloric acid at 37 wt% is classified Skin Corr. 1B with H314 and STOT SE 3 with H335; the specific concentration limits are 25% for Skin Corr. 1B, 10% for Skin Irrit. 2, and 10% for Eye Irrit. 2. A 2026 update obligation applies to existing notifications when the mixture composition changes or when new hazard classes under EU 2023/707 are introduced. The REACH registration dossier for hydrochloric acid must separately record use descriptors, tonnage band, and exposure scenarios for downstream uses including steel pickling, water treatment, and chemical synthesis.

Steel Pickling Bath Parameters and Acid Recovery Thresholds

Continuous and semi-continuous carbon steel pickling lines use hydrochloric acid at 18–20 wt% and 60–65°C. The primary oxide dissolution reaction is FeO + 2 HCl → FeCl₂ + H₂O. Bath productivity is controlled less by acid concentration than by accumulated ferrous chloride. In production-scale push-pull and continuous pickle lines, when FeCl₂ exceeds 110–130 g/L, descaling time increases by more than 15% at 60°C, and free acid titration alone becomes an unreliable control variable. Process control therefore requires both free-acid titration and iron analysis on each shift, with bath overflow directed to acid recovery when iron loading reaches the threshold.

Acid regeneration is performed in vertical spray roasting furnaces operating at 850–950°C with a hydrogen chloride absorption column. The thermal decomposition converts ferrous chloride to HCl and Fe₂O₃, and the recovered acid returns to the pickling bath at 18–20 wt% with residual iron typically below 0.1 wt%. This recovery step is not merely environmental; it closes the acid loop and reduces fresh 37 wt% hydrochloric acid consumption. Temperature control in the furnace is a narrow process window. Below 820°C, iron chloride conversion can be incomplete, and above 950°C refractory wear accelerates. A packed water scrubber downstream of the absorber limits HCl emission, with stack acid mist controlled to the site permit limit.

Cation exchange regeneration in demineralised water plants uses hydrochloric acid at 5–8 wt% applied to strong acid cation resin at 4 bed volumes and a linear velocity of 4–8 m/h. HCl is preferred over sulfuric acid where feedwater calcium is high because calcium chloride remains highly soluble and avoids calcium sulfate scaling on laterals and resin beads. Regeneration efficiency is measured by sodium leakage after service and regeneration; poor distribution at velocities below 3 m/h creates channelling and increases sodium leakage above the typical 0.1–0.3 mg/L target for mixed-bed polishing.

When HCl is used in oilfield carbonate stimulation, spent-acid scaling constrains the injection programme

Matrix acidizing of carbonate reservoirs uses hydrochloric acid at 15 wt% or 28 wt% depending on permeability contrast and bottomhole temperature. The reaction with calcite generates calcium chloride and carbon dioxide. In spent acid flowing back from the near-wellbore zone, calcium chloride concentration can exceed 10 wt%; commingling with formation water containing sulfate can precipitate calcium sulfate scale in tubing and surface separators. A scale risk assessment should therefore be run before pumping, using produced-water sulfate data and spent-acid calcium loading, rather than relying only on acid solubility tests.

Corrosion control in oilfield acidizing requires inhibitor packages capable of maintaining N80 steel coupon loss below 0.05 lb/ft² at 60–90°C in 15 wt% HCl. Laboratory qualification uses an acid immersion apparatus with live acid, coupons, and inhibitor added at the field concentration. Hydrogen sulfide from sour carbonate zones increases the stress corrosion cracking risk if the inhibitor does not provide adequate film persistence during shut-in. Operational boundaries include limiting shut-in time and avoiding mixing HCl with fluoride-containing preflushes if free acid can liberate hydrogen fluoride in the tubing.

Food-grade hydrochloric acid is listed as GRAS for direct food use under FDA 21 CFR 182.1057. The FCC monograph for hydrochloric acid sets a minimum assay of 35.0–38.0% HCl and limits for arsenic, heavy metals, and oxidizable substances. Batch-to-batch variance at food-grade plants arises mainly from iron and arsenic carryover from the absorber acid; distillation of 37 wt% hydrochloric acid strips volatile impurities but does not remove nonvolatile metal chlorides. For starch hydrolysis and pH adjustment in beverage and dairy processing, the product is therefore selected on certified food-grade provenance rather than general technical-grade assay alone.

Storage and materials selection for 37 wt% hydrochloric acid are governed by vapour pressure, not pH alone

Storage of 37 wt% hydrochloric acid at ambient temperature requires materials resistant to both liquid-phase corrosion and vapour-phase attack. Rubber-lined carbon steel is standard for large tanks, but field failures are more common at nozzle flanges and vapour-space manways than below the liquid line. Hydrochloric acid vapour condenses with atmospheric moisture to form thin acidic films that attack unlined carbon steel. A practical correction is to specify PTFE-lined dip pipes, rubber-lined flange faces, and headspace venting through a scrubber, with tank headspace relative humidity kept below 30% where possible.

Fibre-reinforced plastic, polyvinyl chloride, polypropylene, and polyvinylidene fluoride are used for smaller tanks, piping, and metering systems. Chemical resistance of thermoplastics to hydrochloric acid is concentration-dependent and should be evaluated under chemical immersion testing such as ASTM D543-14. Polypropylene is generally suitable for dilute acid below 10 wt% at ambient temperature, but chlorinated or highly oxidising impurities can shorten service life. Fuming hydrochloric acid above 30 wt% generates hydrogen chloride vapour that attacks copper, brass, zinc, and galvanised steel in electrical conduit and instrumentation lines.

Vent scrubber design for hydrochloric acid storage and loading must handle peak displaced vapour during tanker unloading. A packed column scrubber using 5–10 wt% sodium hydroxide at pH above 8 absorbs hydrogen chloride as sodium chloride. The scrubber recirculation pump should be interlocked with the unloading pump so that unloading cannot proceed without caustic circulation. Scrubber blowdown is discharged to neutralisation, and the scrubber liquid pH is maintained above 8 because pH excursions below 7 sharply reduce hydrogen chloride absorption efficiency. Ambient air monitors around HCl storage and transfer stations are commonly set to alarm at 2 ppm hydrogen chloride, below the OSHA ceiling limit of 5 ppm (7 mg/m³) for hydrogen chloride.