The Complete Guide to Acid Cleaning Copper Coils & Heat Exchangers
Table of Contents
What you will learn
📋 What You'll Learn
This guide walks you through the complete guide to acid cleaning copper coils & heat exchangers with detailed instructions.
Copper is the backbone of heat transfer — from HVAC condenser coils to oil field coolers, marine heat exchangers to radiator recore work. But copper demands different acid chemistry than steel. The wrong acid can pit, dissolve, or over-etch copper in minutes. This guide breaks down the four major acid options, tells you exactly which one to use for your type of buildup, and walks you through the complete cleaning process from tank setup to final rinse.
Why Copper Requires Different Acid Chemistry Than Steel
If you've cleaned steel with hydrochloric acid or sulfuric acid and figured the same approach works on copper, stop right there. Copper is a noble metal — it sits higher on the electrochemical series than iron, which means it resists corrosion differently. But it is also far more vulnerable to certain acids.
Nitric acid reacts violently with copper, producing toxic brown nitrogen dioxide gas. Concentrated hydrochloric acid dissolves copper into copper(II) chloride, causing pitting and thinning of tube walls. Sulfuric acid at high concentrations aggressively attacks copper. These acids are standard for steel cleaning — but they are dangerous choices for copper without proper inhibitors and strict concentration control.
The fundamental difference comes down to how copper oxides behave versus iron oxides. Iron rust (Fe₂O₃) is porous and flaky — it comes off easily with strong mineral acids. Copper oxide (CuO) and copper carbonate (Cu₂(OH)₂CO₃, the green "patina") form tighter, more adherent layers. You need an acid that dissolves the oxide layer without aggressively dissolving the base copper underneath.
This is why phosphoric acid, citric acid, and acetic acid are the copper-cleaning workhorses — they dissolve scale and oxides at controlled rates while leaving the base metal intact.
The 4 Acid Options for Copper Cleaning — Head-to-Head
Every acid has trade-offs. Here's how the four primary options stack up for industrial copper coil and heat exchanger cleaning.
Acetic Acid (Vinegar-Based)
The safest option. Low fumes, non-toxic, readily available. Works well on oxidation and light scale. Slowest of the four — but when combined with salt (NaCl) at 2–3%, the cleaning action accelerates significantly. Ideal for shops without heavy ventilation.
Hydrochloric (Muriatic) Acid
The fastest stripper. Industry standard in radiator and recore shops. Cuts through old solder, heavy flux, and stubborn scale in 1–2 hours. But it produces strong fumes, requires ventilation and PPE, and can pit copper if left too long or used too concentrated.
Phosphoric Acid
The smart choice for pre-solder prep. Removes oxidation and leaves a thin protective phosphate layer on the copper surface that resists re-oxidation. Moderate fumes. Widely used as a final dip before brazing and soldering. Excellent on rust, scale, and tarnish.
Citric Acid
The best pure descaler. Dissolves calcium carbonate and mineral scale faster than any other option here. Food-safe, low odor, powder form for easy mixing. Preferred for HVAC condenser descaling and water-side scale removal from heat exchanger tubes.
Side-by-Side Comparison Table
| Property | Acetic Acid | Hydrochloric (Muriatic) | Phosphoric Acid | Citric Acid |
|---|---|---|---|---|
| Recommended Concentration | 10–15% | 10–15% | 10–20% | 2–5% (by weight) |
| Speed of Action | Slow (4–12 hrs) | Fast (1–3 hrs) | Moderate (2–6 hrs) | Moderate (2–6 hrs) |
| Best For | Light oxidation, tarnish | Heavy solder, flux, corrosion | Oxidation, pre-solder prep | Mineral scale, calcium deposits |
| Copper Safety | Excellent | Use With Caution | Excellent | Excellent |
| Fume Level | Very Low | High | Moderate | Very Low |
| Cost (Bulk) | Low | Lowest | Moderate | Moderate |
| Leaves Protective Layer? | No | No | Yes (phosphate film) | No |
| PPE Required | Gloves, goggles | Full: respirator, gloves, goggles, apron | Gloves, goggles | Gloves, goggles |
Many experienced recore shops use a two-stage process: citric acid first to dissolve mineral scale, then phosphoric acid as a final solder-prep dip. This gives you the best descaling action followed by a clean, oxidation-resistant surface ready for brazing. It's slower than straight muriatic but produces better solder joints.
Matching the Acid to Your Buildup Type
Not all fouling is the same. The type of deposit on your copper coils determines which acid delivers the fastest, safest clean. Using the wrong acid on the wrong buildup wastes time and chemical.
| Buildup Type | What It Looks Like | Best Acid Choice | Why It Works |
|---|---|---|---|
| Calcium / Mineral Scale | White, chalky, hard crust — common on water-side surfaces | Citric Acid | Dissolves CaCO₃ rapidly at low concentrations without attacking copper |
| Copper Oxide / Tarnish | Dark brown or black discoloration | Phosphoric Acid | Removes CuO cleanly and leaves protective phosphate film |
| Green Patina (Verdigris) | Green crust, copper carbonate | Acetic Acid + Salt | Acetic acid naturally dissolves copper carbonate (that's how verdigris forms in the first place) |
| Old Solder / Flux Residue | Dull, crusty deposits around joints | Muriatic (HCl) | Fastest at dissolving tin-lead solder and rosin flux — monitor soak time carefully |
| Mixed Heavy Corrosion | Layered scale + oxidation + residue | Two-Stage: Citric then Phosphoric | Stage 1 strips mineral scale; Stage 2 cleans oxide and preps surface |
| Biological Fouling / Biofilm | Slimy, organic growth (cooling towers) | Biocide + Acid Flush | Kill biofilm with sodium hypochlorite first, then acid for scale underneath |
Step-by-Step: Industrial Copper Coil Cleaning Process
Whether you're running 260-gallon tanks for 1,000 lb cooler coils or a 5-gallon bucket for small HVAC evaporator coils, the process follows the same fundamentals. Scale it to your operation.
Pre-Clean & Degrease
Remove loose debris, oil, and grease before the acid bath. Heavy oil fouling blocks acid contact with the scale underneath. Use an alkaline degreaser or hot water pressure wash. For refrigeration coils with compressor oil residue, a solvent pre-wash may be necessary.
Prepare the Acid Bath
Fill your tank with water first, then add acid (never the reverse). For a 260-gallon tank: add acid to reach your target concentration (10–15% for most acids). Add salt at 2–3% if using acetic acid. Heat the solution to 120–140°F — warm solution works 2–3x faster than cold. Use acid-resistant tanks (polyethylene or polypropylene).
Submerge & Soak
Lower the coil into the bath completely. Soak times by acid type: Acetic acid: 4–12 hours (overnight for heavy buildup). Hydrochloric: 1–3 hours — check every 30 minutes. Phosphoric: 2–6 hours. Citric: 2–6 hours. A submersible pump recirculating the solution will cut soak time by 30–50%.
Rinse Thoroughly
Remove the coil and rinse with clean water immediately. For tube bundles, flush the inside of every tube. Any residual acid left on copper will cause rapid re-corrosion. Use high-volume water flow, not just a spray.
Neutralize
Prepare a baking soda (sodium bicarbonate) wash: roughly 1 lb per 10 gallons of water. Submerge or flush the coil in the neutralizing solution for 10–15 minutes. This stops all acid activity and returns the copper surface to a neutral pH. Verify with pH strips — target pH 7.0 or above.
Dry & Protect
Dry the copper as fast as possible — wet, freshly cleaned copper re-oxidizes within hours. Use compressed air or a warm, ventilated area. For recore prep, proceed to soldering/brazing promptly. For storage, apply a light corrosion inhibitor. Some shops do a final phosphoric acid dip (5%) to leave a protective phosphate layer.
Small HVAC coils (<50 lbs): 30–55 gallon drum. Medium coils (50–500 lbs): 100–150 gallon tank. Large industrial coolers (500–2,000+ lbs): 260+ gallon custom tanks. Always size your tank so the entire coil is fully submerged with at least 2 inches of solution covering the highest point.
Acid Mixing Guide by Tank Size
Use this table to calculate how much acid you need for your tank. All ratios assume you're buying concentrated acid and diluting to the recommended working strength. Refer to our dilution guide for the precise C₁V₁ = C₂V₂ calculation.
| Tank Size | Acetic Acid (30%) | Muriatic Acid (31%) | Phosphoric Acid (85%) | Citric Acid (Powder) |
|---|---|---|---|---|
| 55 Gallon | 18–27 gal (to reach 10–15%) | 18–27 gal (to reach 10–15%) | 6.5–13 gal (to reach 10–20%) | 9–23 lbs (to reach 2–5%) |
| 150 Gallon | 50–75 gal | 48–73 gal | 18–35 gal | 25–63 lbs |
| 260 Gallon | 87–130 gal | 84–126 gal | 31–61 gal | 43–108 lbs |
| Add Salt? (Acetic Only) | 5–8 lbs per 55 gal | — | — | — |
Application Spotlights
Recore Prep: Getting Solder-Ready Copper
When you're recoring coolers, radiators, or heat exchangers, the copper must be dead clean for reliable solder or braze joints. Any residual oxide, old flux, or scale means poor wetting, weak bonds, and leaks under pressure.
Recommended process for recore: Start with muriatic acid (10–15%) for 1–2 hours to strip old solder and heavy flux. Rinse. Follow with a phosphoric acid dip (10–15%) for 30–60 minutes to clean the oxide layer and leave a solder-friendly phosphate film. Rinse, neutralize, dry, and proceed to solder immediately.
For coils over 1,000 lbs, you'll need 260+ gallon tanks and a way to lower/raise the coils safely — most shops use overhead cranes or forklifts with custom fixtures.
HVAC Condenser & Evaporator Coils
HVAC coils primarily accumulate mineral scale on the water side and oxidation on the air side. The goal is restoring heat transfer efficiency, not preparing for solder.
Recommended: Citric acid at 2–5% for water-side descaling. For air-side oxidation and dirt, phosphoric acid at 10–15% works well. Many cooling tower operators run citric acid flush annually as preventive maintenance.
Oil Field Cooler Descaling
Oil field coolers deal with some of the harshest conditions — high heat, mineral-heavy water, hydrogen sulfide exposure, and continuous operation. Scale buildup directly reduces cooling capacity and increases operating costs.
Recommended: For heavy mineral scale, start with citric acid (3–5%). For mixed corrosion and scale, the two-stage approach (citric then phosphoric) produces the best results. In remote field locations where ventilation is limited, avoid muriatic acid entirely — stick with citric or acetic for safety.
Brewery & Food Processing Heat Exchangers
Copper heat exchangers in breweries and food processing require food-safe cleaning chemistry. Both citric acid and acetic acid (food-grade vinegar) meet this requirement and are commonly used in CIP (clean-in-place) systems.
Safety, Handling & Disposal
Working with industrial acids demands respect. Follow these non-negotiable safety protocols:
- Hydrochloric acid produces HCl gas at working concentrations. Use outdoors or in well-ventilated areas with a proper chemical storage setup. Wear an acid-rated respirator, chemical splash goggles, nitrile or neoprene gloves, and a rubber apron.
- Always add acid to water, never water to acid. Adding water to concentrated acid causes violent exothermic splashing.
- Never mix acids. Combining hydrochloric acid with other acids or oxidizers can produce toxic chlorine gas.
- Neutralize spent acid with sodium bicarbonate (baking soda) before disposal. Check local regulations — most municipalities require pH 6–9 for drain disposal of neutralized solutions.
- Keep eyewash stations and clean water accessible at all times. In case of skin contact, flush immediately with water for at least 15 minutes.
- Refer to the GHS safety data sheets for each acid you use.
5 Common Mistakes That Ruin Copper Coils
- Using too-strong acid concentration. More is not better. Exceeding 15% on most acids leads to base metal attack. Stick to the recommended ranges.
- Leaving coils in muriatic too long. HCl is fast — check every 30 minutes. An hour too long can cause pitting that weakens tube walls.
- Skipping the neutralization step. Residual acid trapped in tube crevices and fin joints continues corroding long after you pull the coil out. Always neutralize.
- Not drying quickly enough. Freshly cleaned copper is highly reactive. Leaving it wet in open air causes flash oxidation within hours, undoing your work.
- Using the same acid for every problem. Mineral scale and solder residue require different chemistry. Match the acid to the specific fouling type.
Shop Acids for Copper Cleaning
All four acid options in bulk — from quarts to 330-gallon totes. Ships nationwide.
Acetic Acid Glacial
Technical Grade · 1 qt to 330 gal tote
Light oxidation, tarnish, green patina. Add salt to accelerate. Lowest fumes.
Shop Acetic Acid →
Hydrochloric Acid 31%
Technical Grade · 1 qt to 330 gal tote
Heavy solder, flux, stubborn corrosion. Strips in 1-3 hours. Requires full PPE.
Shop Hydrochloric Acid →
Phosphoric Acid 85%
Technical Grade · 1 qt to 330 gal tote
Oxide removal, pre-solder prep. Leaves protective phosphate film on copper.
Shop Phosphoric Acid →
Citric Acid Monohydrate
USP Food Grade · 2 lb to 2,200 lb pallet
Mineral scale, calcium deposits. Dissolves CaCO3 without attacking copper. Low odor.
Shop Citric Acid →Need help choosing the right acid and concentration? Our technical team is here.
Browse All Acids →Frequently Asked Questions
What is the best acid to clean copper coils?
It depends on the type of buildup. For mineral scale, citric acid (2-5%) is the fastest and safest. For oxidation and pre-solder prep, phosphoric acid (10-20%) is ideal because it leaves a protective phosphate layer. For heavy solder and flux residue, hydrochloric acid (10-15%) works fastest but requires careful monitoring. Acetic acid (10-15%) with salt is the safest all-around option for light to moderate cleaning.
Can you use muriatic acid on copper?
Yes, but with caution. Muriatic (hydrochloric) acid at 10-15% concentration is commonly used in radiator and recore shops for stripping old solder and heavy flux. However, it can pit and thin copper tube walls if the concentration is too high or the soak time is too long. Check every 30 minutes and do not exceed 3 hours. Always rinse and neutralize immediately after.
What concentration of vinegar cleans copper?
For industrial copper cleaning, you need 10-15% acetic acid concentration, which is much stronger than household vinegar (5%). Add salt (sodium chloride) at 2-3% to significantly accelerate the cleaning action. For a 55-gallon tank, that's roughly 18-27 gallons of 30% acetic acid diluted with water, plus 5-8 lbs of salt.
How long do you soak copper in acid?
Soak times vary by acid type: Acetic acid takes 4-12 hours (overnight for heavy buildup). Hydrochloric acid works in 1-3 hours. Phosphoric acid needs 2-6 hours. Citric acid takes 2-6 hours. Heating the solution to 120-140 degrees F and using a circulation pump can reduce soak times by 30-50%.
Do you need to neutralize after acid cleaning copper?
Yes, always. Neutralization is critical. After rinsing with clean water, submerge or flush the copper in a baking soda (sodium bicarbonate) solution — about 1 lb per 10 gallons of water — for 10-15 minutes. Verify with pH strips that the surface reaches pH 7.0 or above. Residual acid trapped in tube crevices and fin joints will continue corroding the copper if not neutralized.
What is the best acid for cleaning copper heat exchangers?
For HVAC and heat exchanger maintenance, citric acid at 2-5% is the preferred choice for water-side mineral scale. For air-side oxidation, phosphoric acid at 10-15% works well. Many cooling tower operators run an annual citric acid flush as preventive maintenance. For heavy mixed fouling, use a two-stage approach: citric acid first for scale, then phosphoric acid for oxidation.
Can you use phosphoric acid on copper?
Yes — phosphoric acid is one of the best choices for copper. At 10-20% concentration, it effectively removes oxidation and tarnish while leaving a thin protective phosphate layer on the copper surface. This phosphate film resists re-oxidation, making phosphoric acid the preferred final step before soldering or brazing in recore work.
How do you clean copper coils for recore work?
For recore prep, copper must be perfectly clean for reliable solder joints. The recommended two-stage process: (1) Muriatic acid bath at 10-15% for 1-2 hours to strip old solder and flux residue, then rinse. (2) Phosphoric acid dip at 10-15% for 30-60 minutes to clean remaining oxide and leave a solder-friendly surface. Rinse, neutralize with baking soda, dry quickly, and proceed to solder immediately.
