The Two Main Descaling Acids
The majority of chemical descaling products used in marine and industrial settings are based on one of two acids: phosphoric acid (H₃PO₄) or hydrochloric acid (HCl). Both effectively dissolve calcium carbonate scale, but they differ significantly in corrosion behaviour, material compatibility, safety profile and logistics.
Traditional marine descalers — including well-known brands like Unitor Descaling Liquid — are based on hydrochloric acid. Newer formulations like CFS Descaler use phosphoric acid with corrosion inhibitors to deliver effective descaling with significant advantages in safety, material compatibility and transport.
Head-to-Head Comparison
| Factor | Phosphoric Acid (H₃PO₄) | Hydrochloric Acid (HCl) |
|---|---|---|
| Scale Dissolution | Effective on CaCO₃, lime, light rust. Moderate reaction speed. | Fast dissolution of CaCO₃. Aggressive reaction. |
| Surface Passivation | Yes — forms iron phosphate (FePO₄) conversion layer, providing corrosion protection | No — leaves bare metal surface prone to flash rusting within hours |
| Titanium Compatibility | Safe at working concentrations | Risk of pitting and crevice corrosion |
| Stainless Steel | Safe — no chloride stress corrosion risk | Chloride ions from HCl can cause stress corrosion cracking |
| Copper / CuNi Alloys | Safe — protective layer reforms in service | Requires inhibitor; aggressive without it |
| Fume Generation | Minimal — low vapour pressure at working temperatures | Produces corrosive HCl vapour, especially above 30°C |
| Operator Safety | Less aggressive on skin; minimal inhalation hazard | Causes burns; HCl fumes irritate respiratory system |
| Transport Class | Formulation-dependent — CFS Descaler achieves favourable classification | Typically UN1789, Class 8 Corrosive — DG surcharges apply |
| Concentrate Economy | CFS Descaler: 4:1 concentrate (1L → 5L RTU) | Typically supplied ready-to-use or at lower concentration |
| Spent Solution | Calcium phosphate in water — common fertiliser component | Calcium chloride in water — contains residual chlorides |
The Passivation Advantage
The most significant difference between the two acids is what happens to the metal surface after descaling:
Phosphoric Acid
Reacts with iron oxide (rust) to form iron phosphate (FePO₄) — a stable, adherent conversion layer. This layer passivates the surface, inhibiting further corrosion and providing an excellent base for painting or coating.
Result: Protected surface, ready for coating.
Hydrochloric Acid
Dissolves the oxide layer and leaves bare, reactive metal. In marine environments, flash rusting can begin within hours of treatment — requiring immediate neutralisation, rinsing and coating to prevent re-oxidation.
Result: Bare metal, prone to flash rust.
Logistics & Cost Advantage
Transport classification directly impacts the total cost of ownership for marine chemicals. HCl-based descalers are typically classified as UN1789, Class 8 Corrosive — attracting dangerous goods surcharges on air, sea and road freight.
CFS Descaler's phosphoric acid formulation achieves a favourable transport classification, enabling:
- Air freight capability — critical for urgent vessel supply worldwide
- No DG surcharges — significantly lower freight costs per litre
- Simplified storage — no segregation requirements in ship chandlery or workshop
- 4:1 concentrate economy — one 20L drum of CFS Descaler produces 100L of working solution, further reducing freight cost per treatment
When to Use Each Acid
Choose Phosphoric Acid When:
- Descaling titanium, stainless steel or copper alloy equipment
- Working in confined spaces with limited ventilation
- Transport classification and freight cost matter
- You want surface passivation after descaling
- Pre-treatment before painting or coating
- Regular maintenance descaling on a schedule
HCl May Be Considered When:
- Maximum speed is critical and material compatibility is confirmed
- The system is mild steel only (no titanium, SS or copper alloys)
- Transport classification is not a constraint (shore-based facility)
- Adequate ventilation and operator PPE are available
Frequently Asked Questions
Is phosphoric acid safer than hydrochloric acid for descaling?
Yes. Phosphoric acid produces minimal fumes at working concentrations, does not generate corrosive HCl vapour, and is less aggressive on skin contact. It also passivates ferrous surfaces rather than leaving them vulnerable to flash rusting. Transport classification is typically less restrictive for phosphoric acid formulations.
Can phosphoric acid descale as fast as hydrochloric acid?
Hydrochloric acid typically dissolves calcium carbonate faster than phosphoric acid in laboratory conditions. However, in practical applications the difference is modest — CFS Descaler completes most heat exchanger descaling jobs in 1–4 hours. The passivation benefit and reduced corrosion risk of phosphoric acid often make it the better overall choice.
Which acid is better for titanium heat exchangers?
Phosphoric acid is strongly preferred for titanium. Hydrochloric acid can cause pitting corrosion on titanium alloys, particularly at elevated temperatures. Phosphoric acid-based descalers like CFS Descaler are safe on titanium, stainless steel and copper alloys at working concentrations.
What happens to the metal surface after phosphoric acid descaling?
Phosphoric acid reacts with iron oxide to form iron phosphate — a stable, adherent conversion layer that passivates the surface and provides short-term corrosion protection. This is a significant advantage over HCl descalers, which leave bare metal surfaces prone to flash rusting.
Try CFS Descaler — Phosphoric Acid Done Right
CFS Descaler combines phosphoric acid performance with a 4:1 concentrate economy, favourable transport profile and broad material compatibility. Available in 5L to 1000L pack sizes.