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Acid and Pickling Tank Heating
In acid service the question is not which metal resists best. It is how to keep metal out of contact with the acid entirely.
Oxidizing versus reducing acid
This distinction decides the material, and getting it backwards is the most expensive mistake in acid tank heating.
Oxidizing acids — nitric, chromic — form and maintain a protective oxide film on titanium and on stainless. Titanium performs very well here and is the standard choice.
Reducing acids — hydrochloric, sulfuric, hydrofluoric, phosphoric — strip that film instead of maintaining it. Titanium is attacked, sometimes rapidly, and stainless pits. For these, the practical answer is to keep the metal out of contact: a PTFE-coated element, or quartz.
So specifying titanium because it is the expensive corrosion-resistant option can produce a faster failure than a cheaper PTFE-coated heater in the same tank. Concentration and temperature shift the boundaries in both directions, which is why a bath analysis matters more here than anywhere else.
Practical realities of a pickling tank
- Fume is corrosive. Terminal housings on open acid tanks fail from condensing vapor as often as elements fail from the bath. NEMA 4X and sealed terminals, positioned clear of the plume.
- Iron and scale accumulate. Pickling tanks build up a heavy sludge. Keep the element well above it, and choose a heater that lifts out for cleaning.
- Levels move. Drag-out and evaporation drop the level continuously. A low-level cutoff is not optional.
- Concentration changes with use. A bath specified fresh becomes a different bath as free acid drops and iron loads up. Specify for the range, not just the fresh analysis.
Sheath selection by acid
| Acid | Type | Usual sheath | Avoid | Max W/in² |
|---|---|---|---|---|
| Sulfuric acid, dilute | Reducing | PTFE-coated steel, quartz | Titanium, stainless | 20 – 30 |
| Sulfuric acid, concentrated | Reducing | Quartz, PTFE-coated | Titanium, stainless, steel | 20 – 25 |
| Hydrochloric acid | Reducing | PTFE-coated steel, quartz | Titanium, stainless | 20 – 30 |
| Hydrofluoric acid | Reducing | Quartz | Essentially all metals | 20 – 25 |
| Phosphoric acid | Reducing | PTFE-coated steel, 316 stainless | Carbon steel | 25 – 35 |
| Nitric acid | Oxidizing | Titanium, 316 stainless | Carbon steel, copper | 25 – 30 |
| Chromic acid | Oxidizing | Titanium, PTFE-coated | Carbon steel, stainless | 25 – 35 |
| Citric and organic acids | Mild | 316 stainless, PTFE-coated | Carbon steel | 25 – 40 |
| Mixed acid pickle | Mixed | Quartz, PTFE-coated, Hastelloy | — | Application specific |
Guideline values for typical concentrations and temperatures. Concentration, temperature and metal loading all shift the answer. Confirm against your bath analysis before ordering.
Acid tank questions
What heater material works in a hydrochloric acid pickling tank?
A PTFE-coated element or quartz. Hydrochloric acid is a reducing acid, so it attacks titanium and pits stainless; the practical approach is to keep metal out of contact entirely. PTFE-coated is the usual choice for cost and durability, with quartz as the fallback in hot or concentrated service.
Can titanium be used in sulfuric acid?
Generally no. Titanium relies on an oxide film, and reducing acids such as sulfuric, hydrochloric and hydrofluoric strip it, so corrosion can be rapid. Titanium is the right choice for oxidizing acids — nitric and chromic — and the wrong choice for sulfuric. This one gets specified backwards frequently.
How do you heat a hydrofluoric acid tank?
Quartz sheath heaters. Hydrofluoric acid attacks nearly all metals and also attacks glass, and quartz is one of the few practical materials that survives. Quartz is fragile, so the element needs proper support and the tank needs to be laid out so nothing can strike it.
Why does the terminal box on my acid tank heater keep corroding?
Acid vapor rising from the tank condenses inside the enclosure and tracks across the terminals. The element is usually fine when this happens. Specify a NEMA 4X corrosion-resistant enclosure with epoxy-sealed terminals and position it outside the vapor plume, ideally with the tank hooded and ventilated.
Does acid concentration change which material I need?
Substantially, and so does temperature. Many materials that are acceptable in cold dilute acid fail in hot concentrated acid. A pickling bath also changes as it is used, as free acid drops and dissolved metal builds up. Specify for the operating range rather than the fresh-bath analysis.
Should the heater go in the tank or outside it?
For acid service, keeping it out of the tank is usually better. An over-the-side heater avoids a penetration and lifts out for cleaning and inspection. An external circulation heater goes further — it can be isolated, drained and serviced without anyone working over open acid, and the wetted materials can be chosen independently of the tank.
Send the acid, the concentration and the temperature
Including how much the bath drifts in service. That is what decides whether PTFE, quartz or titanium is the right specification.