Steel Pickling With Sulfuric Acid Bath Control And Acid Consumption
Sulfuric acid (H2SO4, CAS 7664-93-9, EC 231-639-5, UN 1830) is a diprotic strong acid supplied to hot-rolled low-carbon steel descaling lines as 66°Bé (93.2 wt%), 60°Bé (77.7 wt%), or 50°Bé (62.2 wt%) feedstock. Steel pickling with sulfuric acid: bath control and acid consumption in continuous lines is governed by free acid reserve, ferrous iron accumulation, temperature, agitation, inhibitor dose, and rinse-water handling. The acid is normally diluted to 8–15 wt% H2SO4 before entering the bath and maintained at 60–80 °C. Higher free acid concentrations can suppress ferrous sulfate solubility and increase base-metal attack, while lower concentrations slow undercutting of mill scale and force line-speed reductions.
Scale on hot-rolled carbon steel is predominantly wüstite (FeO), with magnetite (Fe3O4) and hematite (Fe2O3) as secondary phases. The principal pickling reactions are:
FeO + H2SO4 → FeSO4 + H2O Fe2O3 + 3H2SO4 → Fe2(SO4)3 + 3H2O Fe3O4 + 4H2SO4 → FeSO4 + Fe2(SO4)3 + 4H2O Fe + H2SO4 → FeSO4 + H2↑
Incoming surface condition influences acid demand. Hot-rolled strip carrying heavy mill scale corresponding to ISO 8501-1 rust grade C or D consumes more acid per tonne than strip with light, uniform scale. Published industrial ranges for carbon steel continuous pickling are 12–25 kg H2SO4 (100% basis) per tonne of strip. The stoichiometric component is calculated from the phase distribution of the scale, while additional acid exits the bath by drag-out in the strip boundary layer, mist carryover, and rinse-water neutralization.
Which Bath Parameters Govern Acid Consumption in Sulfuric Acid Pickling?
Free acid concentration, ferrous iron concentration, temperature, and agitation are the dominant bath-side variables. When free acid is held at 10–12 wt%, the scale removal rate on low-carbon strip is stable, and acid consumption follows the mass of scale removed plus a small base-metal component. If free acid drifts below 8 wt%, the bath shifts from chemical descaling to slower electrochemical undercutting, increasing residence time and total acid use per tonne. If free acid exceeds 15 wt%, inhibitor consumption rises, base-metal loss increases, and ferrous sulfate saturation becomes more likely. Continuous lines therefore use automatic metering pumps with acid-resistant diaphragms to trim free acid after each titration.
A continuous bath with a volume of 80–150 m3 and a strip throughput of 100 t/h consuming 18 kg H2SO4/t on a 100% basis requires an acid feed of 1,800 kg/h of equivalent H2SO4. Supplied as 93.2 wt% concentrate, this corresponds to 1,932 kg/h of liquid acid. The feed rate must be trimmed continuously against titration data because acid consumed per tonne changes with coil-to-coil scale weight, bath iron, and line stop events. Published data for a specific site water and acid mass balance is limited, but the order of magnitude is representative of continuous hot-rolled strip lines.
| Scale phase | Reaction | H2SO4 demand |
|---|---|---|
| Wüstite, FeO | FeO + H2SO4 → FeSO4 + H2O | 1.365 kg H2SO4/kg FeO |
| Hematite, Fe2O3 | Fe2O3 + 3H2SO4 → Fe2(SO4)3 + 3H2O | 1.842 kg H2SO4/kg Fe2O3 |
| Magnetite, Fe3O4 | Fe3O4 + 4H2SO4 → FeSO4 + Fe2(SO4)3 + 4H2O | 1.694 kg H2SO4/kg Fe3O4 |
| Metallic iron dissolution | Fe + H2SO4 → FeSO4 + H2 | 1.756 kg H2SO4/kg Fe |
Bath Control Variables and Titration Frequency
Free acid is determined by acid–base titration to a methyl orange endpoint; ferrous iron is determined by redox titration with potassium dichromate using sodium diphenylamine sulfonate as indicator. Specific gravity, measured by hydrometer and corrected for dissolved ferrous sulfate, is a rapid in-line check but is not sufficiently selective for free acid control. Titration frequency on continuous lines is typically 20–30 min for free acid and 2–4 h for ferrous iron, with automatic sampling from the recirculating loop after a filter or strainer. The analytical values set the bleed rate, acid feed rate, and water addition rate.
| Control variable | Analytical method | Control range | Excursion consequence |
|---|---|---|---|
| Free H2SO4 | Acid–base titration to methyl orange endpoint | 8–15 wt% | Low: slow descaling and longer immersion; high: base-metal attack and FeSO4 crystallization risk |
| Ferrous iron | Redox titration with K2Cr2O7 | 40–80 g/L Fe2+ | Above 80 g/L: crystallization in cool zones and reduced pickling rate; below 40 g/L: low bath life and high fresh acid usage |
| Temperature | PTFE-sheathed thermocouple in recirculation loop | 60–80 °C | Below 60 °C: slower scale removal; above 80 °C: increased acid mist and inhibitor decomposition |
| Recirculation flow | Magnetic flowmeter on acid-resistant pump discharge | 2,000–4,000 L/min per 100 m3 bath volume | Low flow: local ferrous sulfate saturation; high flow: pump erosion and mist loading |
| Inhibitor concentration | Supplier-specific method or weight-loss coupon per ASTM G31 | 0.05–0.3 vol% | Overdose: residual scale islands and higher organic load; underdose: excessive base-metal loss and hydrogen generation |
| Drag-out and rinse return | Flow totalizer on first rinse stage | 5–15% of total acid consumption | High drag-out: increased acid feed and rinse-water iron load; low return: water balance disturbance |
In long pickling campaigns, the most common process upset is ferrous sulfate crystallization. Because FeSO4·7H2O solubility is temperature-dependent and suppressed by free acid, a bath that is stable at 70 °C and 10 wt% H2SO4 with 70 g/L Fe2+ may approach saturation when the bath cools to 40 °C during maintenance shutdowns. Crystals form first on tank walls, strainer screens, and heat exchanger surfaces, reducing heat transfer and clogging recirculation nozzles. Field observations from batch lines indicate that the onset of crystallization is often misdiagnosed as low free acid because measured acid concentration remains unchanged while the bath becomes turbid and scale removal slows. Corrective action is to lower ferrous iron by withdraw-and-replace bleeding, to maintain temperature above the solubility limit, or to adjust free acid only when the bath is under continuous feed; increasing free acid alone can worsen the saturation condition. Published data for the specific solubility isotherm of FeSO4·7H2O in 10–15 wt% H2SO4 are limited, but the phenomenon is a well-known operational constraint in long campaigns.
When Acid Recovery Is Limited to Neutralization Alone
When spent acid retains 8–12 wt% free H2SO4 and 40–80 g/L ferrous iron, neutralization without acid recovery becomes a sludge-management problem as much as a chemical consumption problem. Neutralization with lime to pH 6–9 produces gypsum and ferrous hydroxide; the precipitated solids are dewatered in filter presses to 25–40 wt% solids. This route is energy-efficient but discards unreacted acid, so the total H2SO4 purchased per tonne of strip rises. Acid retardation with ion-exchange resins, diffusion dialysis, and vacuum evaporation are available for sulfuric acid recovery, but they are applied selectively because the acid value is lower than that of hydrochloric acid and the capital intensity is high. Regardless of recovery route, the bleed rate must satisfy the ferrous iron control limit; if the bleed is set too low, the bath crosses the iron saturation line and crystallization forces an unplanned shutdown.
Because atomic hydrogen is generated at the steel surface, inhibitor additions are used to reduce base-metal dissolution and hydrogen evolution, but they do not eliminate the need for mechanical scale-breaking. In continuous sulfuric acid pickling, an effective inhibitor suppresses metallic iron dissolution while allowing acid to undercut the oxide layer. Overdosing creates a clean-looking surface that nevertheless retains residual scale islands; the defect is often discovered only after coating adhesion failure. For high-strength low-alloy steel with yield strength above 900 MPa, sulfuric acid pickling is restricted because absorbed atomic hydrogen can cause delayed cracking; sustained load testing per ASTM F519-17 is used to qualify process changes. For conventional low-carbon grades, immersion time is controlled so that base-metal loss remains below 0.5 wt% of strip weight. Chemical descaling of ferrous substrates falls under DIN 50938, which distinguishes inhibited acid pickling from alkaline descaling and mechanical methods.
Material Compatibility and Bath Life Are the Critical Boundaries
Continuous service with 8–15 wt% H2SO4 at 60–80 °C requires acid-resistant brick-lined or polypropylene-lined tanks, with pumps and valves in fluoropolymer-lined or high-silicon cast iron construction. Heat exchangers exposed to the bath are commonly graphite or fluoropolymer-tubed; stainless steel and titanium are unsuitable, and titanium in particular corrodes rapidly in hot sulfuric acid. Chloride ingress from rinse water should be controlled because chloride promotes pitting of metallic components and can contaminate recovered acid. Sulfuric acid pickling is not recommended for stainless steel descaling; chromium-containing scales require mixed nitric–hydrofluoric acid per ASTM A380-17. The operating window for carbon steel is therefore bounded on one side by free acid and temperature, on another by the ferrous iron saturation line, and on the third by ventilation capacity for acid mist.
