When pickling carbon steel with sulfuric acid, two parameters determine most of the practical result: the concentration of the solution and the operating temperature of the bath. When these two factors do not match the thickness and type of scale on the part, the effect shows up quickly: immersion takes longer than expected, some areas of the part are left with incomplete oxide removal, and the bath wears out faster, so acid has to be replenished more often.
Sulfuric acid (H2SO4) is a strong, diprotic mineral acid: each molecule can release two H+ ions in the reaction that dissolves iron oxides. This property directly affects the dissolving capacity of the bath over its service life. It matters both for anyone sizing a new process and for anyone already running a pickling line who wants to understand why efficiency drops as the bath ages.
Bath concentration and scale removal
The mill scale formed on carbon steel consists mainly of iron oxides in different oxidation states, usually a combination of wüstite, magnetite and hematite, in layers whose thickness and adhesion vary with the part's thermal history. Sulfuric acid reacts with these oxides progressively: the acid first attacks the interface between the oxide and the base metal, causing the scale layer to flake off, while at the same time dissolving the oxide in direct contact with the solution.
In the surface treatment literature, sulfuric acid pickling baths typically run at concentrations of 5% to 20% by volume, depending on scale thickness, the immersion time available on the line and the temperature used. Higher concentrations tend to speed up the reaction, but they also increase acid consumption per batch and can intensify attack on the base metal once the scale is fully removed. The result is over-pickling: loss of material and unwanted surface roughness.
These values are literature references only. The right concentration for each pickling line depends on process-specific variables (type of steel, scale thickness, part geometry, available cycle time) and must be defined and validated by the plant's own engineer or technical lead, not by the input supplier.
Why concentration drops with use
Each pickling reaction consumes part of the free acid in the solution, converting it into dissolved ferrous sulfate. With continued use, the free acid concentration falls steadily while the concentration of iron salts rises, even with no physical loss of volume. That is why sulfuric acid pickling baths need periodic acid additions to keep the working concentration within the range defined for the process. This replenishment is different from simply making up the volume lost to evaporation.
Bath temperature and reaction rate
The rate of the chemical reaction between sulfuric acid and iron oxides increases with temperature, as is typical of acid dissolution reactions. At room temperature, sulfuric acid reacts with carbon steel scale more slowly than other mineral acids do. That is why industrial operations usually heat the bath, to achieve cycle times compatible with production.
In industrial practice, sulfuric acid baths for pickling carbon steel commonly operate at roughly 60°C to 90°C. In this range, the oxide removal rate increases significantly compared with room temperature operation. Higher temperatures, however, bring side effects that have to be built into process design: water evaporates faster, which concentrates the dissolved salts more quickly; exhaust systems become necessary to deal with the increase in acid mist; and tanks and fittings not lined for this temperature range wear out faster.
As with concentration, the ideal operating temperature depends on the type of steel, the scale thickness and the safety and exhaust conditions available at the plant. The reference values given here reflect practices documented in the surface treatment literature and do not replace the technical definition of the process by the person responsible for the operation.
The safety rule for diluting the acid
Whatever concentration or working temperature is set for the process, diluting concentrated sulfuric acid always follows the same safety rule: the acid must be added slowly to the water, never the other way around. Because Sulfuric Acid 98% is highly hygroscopic and its dissolution in water is strongly exothermic, adding water to the concentrated acid can cause a sudden release of heat and acid splashing, a direct hazard to anyone handling the product.
What shortens the life of a sulfuric acid bath
The life of a sulfuric acid pickling bath rarely ends for a single isolated reason. In day-to-day operation, three factors account for most of the loss of efficiency over time.
Buildup of dissolved iron. Each pickling cycle transfers iron from the part into the solution as ferrous sulfate. As this content rises, the bath's ability to dissolve new oxide decreases, the required immersion time grows and, at high concentrations, ferrous sulfate can crystallize at the bottom of the tank, forming a deposit that interferes with operation and with bath density readings.
Dilution from water drag-in. Parts that reach the pickling tank still wet from earlier steps (rinsing, degreasing) carry water into the bath, gradually diluting the free acid concentration. This drag-in is cumulative and, if it is not offset in the replenishment routine, it distorts concentration readings and masks the real saturation state of the solution.
Contamination by oils, greases and suspended solids. Oily residues from earlier process steps (machining, forming, storage) can form a film on the surface of the bath, which hinders direct contact between the acid and the oxide to be removed and also compromises the final quality of the pickled surface.
Each of these factors reduces bath efficiency on its own; together, they explain why pickling baths that seemed efficient for a while start to demand longer and longer cycle times.
What is the ideal temperature for pickling carbon steel with sulfuric acid?
There is no single temperature that applies to every process. The surface treatment literature usually cites ranges between 60°C and 90°C for sulfuric acid baths on carbon steel, but the exact value depends on scale thickness, available cycle time and the exhaust conditions at the facility. The final setting must be validated by the process engineer or technical lead, taking into account the specific characteristics of the production line.
Why must sulfuric acid always be added to water, and never the other way around?
Because dissolving concentrated sulfuric acid in water is a strongly exothermic reaction. When water is added to concentrated acid, the heat released can build up abruptly at the contact surface, causing local boiling and acid splashing. When the acid is added slowly to the water, the heat generated spreads through a larger volume of liquid, and the dilution stays controllable and safer.
What are the signs that a sulfuric acid bath is losing pickling efficiency?
The most common signs are a steady increase in the time needed to remove scale, deposits or crystals forming at the bottom of the tank, changes in the color and viscosity of the solution, and the need for more and more frequent acid additions to get the same result. Together, these signs point to a buildup of dissolved iron and a drop in the free acid available to react with the oxide.
Boreto & Cardoso: technical supply for pickling processes
Boreto & Cardoso has supplied industrial chemical inputs since 1972, with headquarters in Santana de Parnaíba, a branch in Limeira and ISO 9001:2015 certification (GSC Certificadora, registration BR.087.25). When supplying sulfuric acid for pickling processes, the company provides complete technical documentation: a Certificate of Analysis (COA) and a Safety Data Sheet (SDS) for every lot. This lets the customer trace each shipment received and correlate variations in process performance with the documented history of the input used. Defining the process parameters remains the customer's technical responsibility; document traceability serves as support for the operation.
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Note to the Reader This article brings together technical information drawn from different sources, with the aim of broadening knowledge about the raw materials mentioned. It is informational material only and does not represent any recommendation by Boreto & Cardoso regarding application, formulation or direct use. Boreto & Cardoso®, with more than 50 years of experience, works strictly as a supplier of chemical inputs for a wide range of applications, and each user is responsible for verifying the suitability and compliance of their own application.