Industrial Acids Selection Guide — Which Acid for Which Job

Direct answer

Choose an industrial acid by the anion you can live with, not by strength. Sulfuric acid is the cheapest source of hydrogen ions per tonne and the default for bulk pH depression, ion-exchange regeneration and effluent neutralisation — but it leaves sulfate, which precipitates calcium as gypsum scale. Hydrochloric acid dissolves carbonate and iron scale fastest and leaves a soluble chloride, which is exactly why it must never be used on stainless steel that will stay wet. Phosphoric acid is the slow, low-corrosion choice for food-contact plant and light rust conversion. Sulfamic acid is a dry, shippable, low-fume acid for descaling heat exchangers and cooling systems where chloride is banned. Oxalic acid removes iron staining from stone, timber and radiators where mineral acids would etch. Formic acid is a reducing organic acid for leather, textile and biogas duty. Sodium bisulfate is a dry granular acid used where handling liquid acid on site is not acceptable.

The five questions that decide the acid

1. What are you dissolving? Carbonate scale yields to almost anything. Iron oxide needs hydrochloric or a chelating organic acid. Sulfate scale (gypsum, barite) does not dissolve in acid at all — that is a conversion or mechanical job.
2. What metal is the vessel? Chloride and austenitic stainless steel are a stress-corrosion-cracking pair. Carbon steel tolerates inhibited HCl for short cleans. Sulfuric above 70 % passivates carbon steel; dilute sulfuric attacks it aggressively — dilution is the dangerous state, not concentration.
3. What anion enters your effluent? Sulfate loads gypsum scale downstream and is capped in many municipal discharge permits. Chloride raises conductivity and corrodes concrete rebar. Phosphate triggers nutrient limits.
4. Can the site handle a liquid acid? Dry acids — sulfamic, oxalic, sodium bisulfate — remove bunding, fume scrubbing and tanker unloading from the scope. They cost more per equivalent of acidity and buy back safety.
5. What is the temperature? Sulfamic hydrolyses to sulfuric above roughly 60 °C, losing the low-corrosion behaviour you bought it for. Formic and oxalic decompose in hot oxidising systems.

Practical selection table

Cost is per equivalent, not per litre

Compare acids on cost per kilogram of hydrogen ions delivered, adjusted for concentration and for the neutralisation cost of the resulting salt. A 98 % sulfuric at a given price is usually two to four times cheaper per equivalent than 33 % hydrochloric, before freight. Freight then matters more than price: you are shipping water in every dilute acid. Whenever the site can dilute safely, buy concentrated and dilute in place — always acid into water, never the reverse.

What to state on the RFQ

• Acid and concentration (e.g. sulfuric acid 96–98 %, HCl 31–33 %)
• Grade: technical, food, reagent, or electronic
• Iron and heavy-metal limits if the acid touches product or a catalyst
• Inhibited or uninhibited, if the duty is metal cleaning
• Packaging: IBC, drum, 25 kg bag, or bulk tanker with unloading arrangement
• Delivery point restrictions: hose length, pump on truck, bund capacity
• SDS, TDS and a recent certificate of analysis

Questions to ask before you buy

• What is the maximum chloride the downstream plant can accept?
• Is the storage tank material compatible at the delivered concentration and at the diluted working concentration?
• Who neutralises the rinse water, and with what?
• Does the site need a dry acid to avoid a bunded liquid installation?