Descaling a Boiler and Heat Exchanger: Acid, Inhibitor and Step-by-Step Procedure

The boiler flue gas temperature creeps up, the temperature difference across the heat exchanger shrinks, and the pump works against ever higher resistance. In winter this ends one way: the boiler house can't keep up, and someone suggests "filling it with acid overnight". That is how equipment usually gets damaged — not by scale, but by descaling without a calculation and without an inhibitor.

Scale forms in any system that heats water containing hardness salts, even with proper water treatment. It is not a staff failure but routine maintenance. The only question is how to clean quickly, completely and without losing metal. A 0.8 mm layer already raises fuel use by 2%, and by up to 7% if the scale contains iron and silica (U.S. DOE, Steam Tip Sheet #7).

In short: first identify the scale type with a drop of acid and check the equipment material. Carbonate scale on carbon steel and cast iron is removed with a 3–10% hydrochloric acid solution containing an inhibitor at 20–60 °C, with circulation and concentration monitoring. Stainless steel, aluminium and galvanised steel are not cleaned with hydrochloric acid. After cleaning: neutralise, rinse, passivate; discharge only at pH 6.5–9.0.

How acid dissolves scale, and why without an inhibitor it dissolves the boiler too

Most scale is calcium carbonate. Hydrochloric acid turns it into soluble calcium chloride, carbon dioxide and water: CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O (Wikipedia, Hydrochloric acid). The scale fizzes, the gas escapes, the wall is cleaned.

The problem is that acid does not care what it dissolves. Once the scale is gone, it starts reacting with steel: Fe + 2HCl → FeCl₂ + H₂. The metal thins, and atomic hydrogen enters the steel and can cause hydrogen embrittlement (Wikipedia, Hydrogen embrittlement). An inhibitor adsorbs on the metal surface and slows exactly this reaction without hindering scale dissolution.

Numbers 🔬

• 1% and 2% — extra fuel use with 0.4 mm and 0.8 mm of scale; 3.5% and 7% if the scale contains iron and silica (U.S. DOE).
• An order of magnitude lower thermal conductivity of scale compared with steel (U.S. DOE).
• 73 g of pure HCl is needed per 100 g of calcium carbonate, releasing 44 g of CO₂ — from the reaction equation and molar masses (Wikipedia, Calcium carbonate).
• 50 °C — the threshold above which austenitic stainless steel in a chloride environment is prone to stress corrosion cracking (Wikipedia, Stress corrosion cracking).
• 6.5–9.0 — permitted pH of wastewater discharged to municipal sewers in Ukraine (Ministry Order No. 316, Annex 4).

Diagnosis: what kind of scale, and can acid remove it

Before preparing the solution, take a small piece of deposit (from a nozzle, cover or plate) and put a drop of hydrochloric acid on it. Within 2 minutes you will know whether acid cleaning will work at all.

The second question is the material. It matters more than the scale type.

Plate heat exchangers usually have stainless steel plates, so hydrochloric acid is not suitable for them even with an inhibitor — check the unit's data sheet.

Procedure: 7 steps

1. Calculate the amount of acid

The most common mistake is pouring "by eye": the acid is spent within an hour and the scale stays. Calculate from the deposit mass: area × thickness × density. Calcite density is 2.71 g/cm³ (Wikipedia); real scale is more porous, so the calculation includes a margin.

2. Prepare the working solution

The working concentration is 3–10% HCl depending on scale thickness; commercial SVK inhibited hydrochloric acid contains 20–27% HCl, so it is diluted with water. Always add acid to water, never the other way round. If the site already has technical acid without an inhibitor, the metal can be protected with a separate SVK pickling inhibitor.

3. Isolate the unit and set up circulation

Isolate the unit from the system, connect a pump, a solution tank and acid-resistant hoses. Bottom-to-top circulation displaces gas and prevents air locks. Keep the top point of the loop open for venting.

4. Clean at 20–60 °C

Heating speeds up dissolution, but above 60 °C the inhibitor in the SVK product no longer guarantees protection (the product's working range is 20–60 °C). Keep the temperature stable.

5. Monitor every 30–60 minutes

Measure acid concentration by titration, or at least pH. Falling concentration means the acid is working; below 2–3%, top up with fresh concentrate. Cleaning is complete when the concentration stops falling and gas evolution stops. A sharp rise in iron with stable acidity is a sign the acid has started on the metal: stop.

6. Neutralise and rinse

Drain the solution, flush the unit with water, then with a weak alkaline solution (e.g. soda ash) until the rinse water is neutral. Neutralise the spent solution separately: it may only be discharged to the sewer at pH 6.5–9.0.

7. Passivate the surface

After acid, the steel is "bare" and corrodes faster than before cleaning. Passivation restores the protective film. Passivation regimes and ongoing inhibitor protection are covered in Corrosion and Scale Inhibitors for Heat Exchangers.

Calculation: how much acid for one heat exchanger

Example for a carbon steel shell-and-tube heat exchanger.

The takeaway: with a unit volume of 50–100 l, one fill of solution will not be enough, and acid will have to be topped up several times. And 3 m³ of gas is why cleaning is done with open venting in a ventilated room.

What not to do

• Don't clean stainless steel, aluminium or galvanised steel with hydrochloric acid.
• Don't use acid without an inhibitor on working equipment — that is pickling, not cleaning.
• Don't heat the solution above 60 °C to "speed things up".
• Don't leave acid in the unit "overnight" without monitoring concentration.
• Don't mix with hypochlorite or chlorine-based products: hydrochloric acid releases chlorine with them (Wikipedia, Sodium hypochlorite).
• Don't discharge spent solution to the sewer without neutralisation.
• Don't work near open flames: acid in contact with metal releases hydrogen.

⚠️ Limitations. This procedure applies to carbonate and iron scale on carbon steel and cast iron. Cleaning of regulated steam boilers, equipment of unknown material and systems with sulphate or silicate scale must follow a programme agreed with a technologist and the equipment owner. Work with acid only in protective equipment (goggles or face shield, acid-resistant gloves, apron) and with access to water for rinsing.

Checklist before cleaning

• Scale type identified with the acid drop test.
• Unit material checked in the data sheet: carbon steel or cast iron.
• Acid quantity calculated from scale mass, with a reserve for topping up.
• Circulation loop assembled with an open top point for gas.
• Monitoring tools on hand: thermometer, pH meter or titration kit.
• Alkaline solution ready for neutralisation and a plan for effluent disposal.
• Staff in protective equipment, no ignition sources nearby.

Descaling takes 1–2 days once every few seasons; a botched descaling costs a new heat exchanger. To match the concentration to your scale type and equipment volume, see SVK inhibited hydrochloric acid (20 l canisters, 200 l drums, 1000 l IBCs) and our water treatment solutions, or send a request — we reply within 24 hours.

Read also

• Corrosion and Scale Inhibitors for Heat Exchangers: Selection and Optimisation
• How to Choose Reagents for Industrial Water Treatment
• Industrial Chemistry Glossary: 30 Terms for Technologists and Engineers