Abstract: EDTA chelation cleaning and conventional acid cleaning are not competing for the title of the stronger chemistry; the question is which one suits a given item of equipment. Chelation holds calcium, magnesium and iron in the liquid phase as a complex, runs at a higher pH and carries no chloride, so it is safer on stainless steel, copper alloys and thin-wall tube bundles. Acid cleaning dissolves carbonate scale through the acid itself: fast and cheap, but risky on thin-wall carbon steel and on stainless steel that is sensitive to chloride. Five criteria settle the choice - corrosion allowance, fouling type, material, waste treatment, and schedule and cost. Acceptance does not change with the route: it follows GB/T 25146-2010, with corrosion rate and corrosion amount from Table 2, removal and cleaning rates from Table 3, and the chloride content of the cleaning fluid held below 25 mg/L wherever stainless steel is present.
1. The Short Answer: When to Choose EDTA, When to Choose Acid
Chelant cleaning is used on equipment that cannot tolerate a hard acid attack; acid cleaning is used on equipment whose fouling is simple and whose material can take it. The two are not substitutes for one another. They are filtered through the same five conditions on one item of equipment: corrosion allowance, fouling type, material, waste treatment, and schedule and cost. If any one of them crosses a prohibited line, the route changes rather than being pushed through with a stronger inhibitor package.
Three cases favour EDTA chelation cleaning: (1) the heat transfer surface is stainless steel, copper alloy or a thin-wall carbon steel tube bundle, so the wall thickness margin is small and the allowance for metal loss is narrow; (2) the fouling is a mixed deposit of iron oxide and carbonate scale where the iron fraction is high, so an acid route would load the cleaning liquor with dissolved iron; (3) the system contains stainless steel and the site cannot guarantee control of chloride in the cleaning liquor - clause 5.2.9 of GB/T 25146-2010 requires the chloride content of the cleaning fluid to stay below 25 mg/L where stainless steel is present, which an HCl system cannot hold reliably.
Three cases favour acid cleaning: (1) the deposit is carbonate scale, identified by the qualitative test in Table 1 of the same standard - largely soluble in 5% hydrochloric acid with strong gas evolution and little residue - with CaCO3 at 60% or more; (2) the material is a thick-wall carbon steel shell and tube sheet, with no copper, aluminium or austenitic stainless steel in the system; (3) schedule and cost are tight and the site has neutralisation, passivation and waste collection in place.
Mixed deposits are handled in sequence: where the deposit carries an oil film or carbonaceous layer, neither route can go in directly. Break the oil film first with an alkaline or solvent wash, then decide between acid and chelant by the fouling under the oil. If the film is left in place, neither the acid nor the chelant reaches the deposit, and the operation wastes chemistry and releases acid mist.
2. Criterion One: Corrosion Allowance Decides Whether Acid Can Be Used
Corrosion allowance is the first gate in the selection. The smaller the wall thickness margin left for corrosion, the more care an acid route needs, and the governing figures are the two lines in Table 2 of GB/T 25146-2010: the corrosion rate K, which is the loss of mass per unit area per unit time, and the corrosion amount A, which is the cumulative loss over the whole cleaning cycle.
| Material | Corrosion rate K, laboratory g/(m²·h) | Corrosion rate K, field measurement g/(m²·h) | Corrosion amount A g/m² |
|---|---|---|---|
| Carbon steel | ≤2 | ≤5 | ≤80 |
| Stainless steel | ≤1 | ≤1.5 | ≤20 |
| Copper | ≤1 | ≤1.5 | ≤20 |
| Copper alloy | ≤1 | ≤1.5 | ≤20 |
| Aluminium and aluminium alloy | ≤1 | ≤1.5 | ≤20 |
The table has to be read in pairs. The field limit for carbon steel, 5 g/(m²·h), looks generous, but the cumulative corrosion amount behind it is only 80 g/m². Raise the cleaning temperature, or lengthen the circulation time, and the rate can stay inside its limit while the cumulative loss closes on the ceiling. A thin-wall tube bundle is designed to a minimum wall thickness in the first place, so spending most of that 80 g/m² margin shortens equipment life. That is why thin-wall items are steered towards a chelant or organic acid system, where a lower corrosion rate compresses the cumulative loss.
Stainless steel and copper are held to a tighter line: 1.5 g/(m²·h) and 20 g/m² by field measurement. For these materials an inhibitor selection report is not enough - clause 5.2.11 requires a corrosion monitoring coupon of the same material as the equipment to be fitted for the duration of the work, and the acceptance decision is taken on the measured coupon data.
3. Criterion Two: The Fouling Type Decides the Cleaning Medium
Fouling is identified with the qualitative methods in Table 1 of GB/T 25146-2010 from a deposit sample taken through the inspection opening; quantitative analysis overrides the qualitative call. Two questions are asked at once: whether this deposit dissolves at all, and what removal rate the acceptance has to show, since the removal and cleaning rate targets in Table 3 are banded by fouling type.
| Fouling type and identification | Fit with acid cleaning | Fit with EDTA chelation | Removal / cleaning rate target |
|---|---|---|---|
| Carbonate scale: grey-white, largely soluble in 5% HCl with strong gas evolution, CaCO3 60% or more | High, first choice; the reaction is fast and the chemicals are cheap | Moderate; it complexes but reacts slowly and the chemicals cost more | ≥95% / ≥95% |
| Iron oxide scale: brown, slowly soluble in 5% HCl giving a yellow-green solution, iron oxide 80% or more | Moderate; Fe³⁺ must be controlled, and above 1000 mg/L a reducing or complexing agent is added or the bath is replaced | High; iron is held in the liquid phase as a complex | ≥95% / ≥95% |
| Sulphate scale: yellow-white or white, very little gas evolution and poor solubility in 5% HCl, CaSO4 40% or more | Low; a single acid will not dissolve it, it must be converted first | Moderate; it also has to be converted before complexing | ≥85% / ≥85% |
| Silicate scale: grey-white, insoluble in 5% HCl, SiO2 20% or more | Low; cold hydrochloric acid is effectively useless | Low; conversion or another system is required | ≥85% / ≥85% |
| Oil fouling: black, an ether extract of the crushed sample turns yellow-green, oil content 5% or more | Not applicable; degrease first | Not applicable; the oil film blocks the complexing reaction | ≥95% / ≥95% (by fouling type after degreasing) |
| Other types: carbonaceous, polymer and process deposits | Decided by a sample test | Decided by a sample test | ≥85% / ≥85% |
Two rows in this table cause most of the mistakes. Iron oxide scale looks like an acid job, but on an acid route the dissolved iron climbs quickly, and clause 5.2.10 requires a reducing or complexing agent, or a bath change, once Fe³⁺ passes 1000 mg/L - a bath change pushes both chemistry and schedule up, so a high-iron deposit often runs more smoothly on a chelant system. Sulphate and silicate scale are given a removal target of only 85%, which is the standard telling the reader that neither dissolves completely in a single chemical route: they have to be converted by an alkaline boil, or combined with mechanical and physical methods, before chemical cleaning is applied.
4. Criterion Three: Material Versus Prohibited Process
Material, fouling type, recommended process and prohibited process belong in one table, worked through item by item against the equipment list. Where a match cannot be found, stop and re-check the deposit sample and the material.
| Part and material | Typical fouling | Recommended process | Prohibited process and reason |
|---|---|---|---|
| Thick-wall carbon steel shell and tube sheet | Carbonate scale | Inhibited acid cleaning, neutralisation, rinse and passivation | Strong acid without inhibitor, or an over-temperature long circulation: neither the corrosion rate nor the corrosion amount passes Table 2 |
| Thin-wall carbon steel tube bundle (heat recovery, air preheater) | Carbonate scale with iron oxide | Chelant or organic acid complexing, with circulation time and temperature controlled | Long hydrochloric acid circulation: the rate may stay inside its limit while the corrosion amount closes on 80 g/m² |
| Austenitic stainless steel (condensers, exchanger tubes, tank internals) | Carbonate scale, iron oxide scale | EDTA chelation, or chloride-free acids such as sulphamic and citric acid | Chloride-bearing systems such as HCl: above 25 mg/L chloride breaches 5.2.9 and risks pitting and stress corrosion cracking |
| Copper tube and copper alloy (condensers, HVAC coils) | Carbonate scale | Mild acid cleaning with a copper inhibitor, or a low-ammonia chelant system | A high-ammonia EDTA system without a copper inhibitor: the copper-ammonia complex keeps dissolving copper and leaves copper plating on steel |
| Aluminium and aluminium alloy (plate-fin exchangers) | Carbonate scale | Neutral or mildly alkaline system with contact time controlled | Strongly alkaline systems: aluminium is amphoteric and an alkaline wash attacks the base metal |
The three prohibited lines in this table are hard requirements and are not relaxed for schedule pressure: no stainless steel system may be exposed to uncontrolled chloride, no copper surface may meet a high-ammonia system without a copper inhibitor, and no aluminium part may go into a strongly alkaline bath. The reverse case matters too - where a thick-wall carbon steel system includes one stainless steel exchanger, the cheapest answer is not two chemical packages but one chloride-free chelant or acid route across the whole system, with a single inhibitor package holding every material.
5. Criterion Four: Waste Treatment Decides Whether It Can Be Discharged
This criterion is the one most often left out of the selection, and it often decides it. The two routes produce waste of completely different character, so the disposal path differs too.
Acid cleaning waste: mainly acidic water with metal ions in the free state. Neutralisation to a suitable pH followed by coagulation and settling removes most of the metal, so the path is short and cheap; but inhibitors and some complexing agents raise the COD, and an HCl system brings a high chloride load, which has to be assessed before the water is reused or sent to a biological stage. Where Fe³⁺ passes 1000 mg/L during cleaning, clause 5.2.10 requires a reducing or complexing agent, which adds further reducing and complexing species to the water.
Chelant waste: the metal is held as a complex. Raising the pH does not precipitate it, so the usual neutralise-and-settle step does nothing; the complex has to be broken first, by oxidation or displacement, before the metal can be precipitated, or the whole batch is collected and consigned to a licensed contractor. EDTA itself is poorly biodegradable, so the liquor carries a high COD and a low biodegradability ratio; in quantity, its disposal cost and compliance risk are higher than for acid waste. That runs against the intuition that chelation is the gentler option - gentler refers to the equipment, not to the environment.
Both routes need the same record set: waste collected by stream, measured, and handed over with the transfer documents retained, one set before and one after the work. On a site with tight effluent limits, or without a discharge permit, the waste volume should be calculated before the medium is chosen rather than after a disposal contract has to be arranged in a hurry.
6. Criterion Five: Schedule and Cost
The advantage of acid cleaning is speed and price: the chemicals are cheap and the reaction with carbonate scale is fast, so a single exchanger is usually scheduled in shifts. The stages around it cannot be skipped, though - neutralisation and rinsing, passivation and waste transfer all take time, as does protecting floors and walls.
Chelant cleaning costs more per kilogram and uses more of it, and the circulation time is longer: the chelant has to work through the deposit by circulation, holding flow and a suitable temperature band while it complexes the metal ions one by one, and a slow reaction means a longer schedule. The total cost is higher than a conventional acid wash, and what it buys is three things - a low corrosion risk on thin-wall and mixed-material systems, less dismantling work, and the ability to circulate in a closed loop with less uncontrolled release.
The cost question reduces to one sentence: where the wall thickness margin is generous, the fouling is simple and no sensitive material is present, schedule it as an acid wash; where the margin is thin, the materials are mixed, or stainless steel and copper are in the circuit, schedule it as a chelant clean, and treat the extra chemical cost as the price of the corrosion risk and the dismantling time saved. Either way the quotation should show waste disposal as a separate line rather than burying it in the chemical cost.
7. One Set of Acceptance Criteria for Both Routes
Acceptance is not decided by the route but by one set of indices, all from GB/T 25146-2010. Removal and cleaning rates follow Table 3 by fouling type: carbonate scale, iron oxide scale and oil fouling must reach 95% or better, while sulphate scale, silicate scale and other types must reach 85% or better. Corrosion rate and corrosion amount follow the field measurement column of Table 2, with a coupon of the same material as the equipment fitted under clause 5.2.11. After cleaning, clause 5.2.8 requires no secondary rust, no copper plating and no over-pickling, with a complete passive film formed; the carbon steel film is checked by the red-spot method for at least 5 seconds and the stainless steel film by the blue-spot method with no more than 8 blue spots in 10 minutes (5.3.4). Where stainless steel is present, chloride in the cleaning fluid must stay below 25 mg/L (5.2.9).
Four records should be kept for quality acceptance: the deposit identification and quantitative result, the chemical and process log including temperature and circulation time, the coupon corrosion rate and corrosion amount, and the removal rate inspection record. The deposit sample is taken and the corrosion allowance calculated before the route is chosen, so that sampling record is itself the basis of the selection.
8. Frequently Asked Questions
Will EDTA cleaning corrode carbon steel?
It corrodes within a controlled allowance. The carbon steel corrosion rate is still held to no more than 5 g/(m²·h) and the corrosion amount to no more than 80 g/m² by the field measurement column of Table 2, GB/T 25146-2010. A chelant system runs at a higher pH and carries no chloride, so general corrosion on carbon steel is clearly lower than in an HCl bath under the same conditions, but a mild chemistry is not a licence to skip monitoring: fit a coupon of the same material, calculate the chemical charge from the deposit quantity, and accept the work on the coupon data.
How is EDTA cleaning waste liquor treated?
Break the complex first, then precipitate, or consign the whole batch. The metal in chelant waste is complexed, so neutralising and settling will not remove it; the complex has to be opened by oxidation or displacement so the metal returns to a precipitable form. EDTA is poorly biodegradable and the liquor has a high COD, so disposal costs more than acid waste, and the waste volume should be counted into the quotation when the route is chosen.
Can a thin-wall tube bundle be acid cleaned at all?
Yes, but only with a shorter circulation, a lower liquor temperature and coupons fitted throughout. A thin-wall bundle has the smallest corrosion allowance, and against the 80 g/m² cumulative limit in Table 2 the same corrosion rate retires the wall sooner; so such bundles are preferred on a chelant or organic acid route. Where acid cleaning is chosen, keep the concentration low, the time short and the testing frequent, and never mix a chloride-bearing acid into a circuit that also contains stainless steel parts.
What process suits equipment with both stainless steel and copper tube?
One chloride-free chelant or organic acid route, with a dedicated copper inhibitor. Stainless steel is intolerant of chloride, which clause 5.2.9 caps at 25 mg/L in the cleaning fluid, while copper is intolerant of ammonia, and chelant systems often use ammonia-based assistants; running both in one circuit means protecting the copper surface with an inhibitor and confirming after cleaning, under 5.2.8, that no copper plating has formed.
Where does EDTA cleaning cost more than an acid wash?
In three places: chemical unit price, charge per job and circulation time. The chelant complexes metal ions stoichiometrically, so a heavier deposit needs more chemical, and the unit price already exceeds hydrochloric or sulphamic acid. A slower reaction means the circulation pumps, the heating and the crew are occupied longer, and site schedule and energy use rise with it. What the chelant buys is a low corrosion risk, less dismantling and closed-loop circulation, and if the site also has to handle a complexed waste stream that cost belongs in the quotation as well - so quote chemicals, schedule and waste disposal as three separate lines.
Related Reading
Further reading on the adjacent selection questions and the same acceptance route:
- Organic versus Inorganic Acid Cleaning - medium selection within the acid route
- Sulphamic Acid versus HCl Comparison - corrosion and chloride differences between the two acids
- Copper Tube Condenser Cleaning and Corrosion Protection - inhibitor pairing for copper systems
- Passivation After Cleaning - finishing a chemical clean and checking the passive film
- Equipment Scaling Diagnosis Tool · Maintenance Cycle Calculator
The selection criteria, corrosion rate and removal rate indices and acceptance requirements for EDTA chelation cleaning and acid cleaning in this article follow GB/T 25146-2010 Acceptance Standard for Chemical Cleaning Quality of Industrial Equipment (corrosion rate and corrosion amount in Table 2, removal and cleaning rates in Table 3, chloride limit of 25 mg/L for systems containing stainless steel in clause 5.2.9). Where a high-pressure water jet is used after chelation or acid cleaning to open hard shells and blocked tubes, the safety of the operation follows GB/T 26148-2025 Safety Specification for High-pressure Water Jet Cleaning Operations, and the machines are selected under GB/T 26135-2020 High-pressure Cleaning Machines.
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