KEY INSIGHTS

• Rust conversion is phase-selective: Non-stoichiometric spinel (γ-Fe2O3/Fe3O4) converts at roughly 90%, goethite at about 30%, and akaganeite is barely affected. The rust converter doesn’t convert everything.
• Tannic and phosphoric acid still anchor the field: A 3% tannic acid + 10% phosphoric acid (3T–10P) combination was reported as synergistic in a study of FeOOH phases on cast iron.
• Other ingredients: Penetrants, wetting and dispersing agents, flash-rust inhibitors, film formers, and the resin system decide whether the converted layer is compact, adherent, and over-coatable.

BULLET POINTS

• Characterize the rust first: adherent vs non-adherent, phase composition, chloride load.
• Core system: tannic and/or phosphoric acid in an acidic window, with a penetrant and a waterborne film former.

KEYWORDS

Rust Converter, Acidic Rust Converter, Adherent Rust, Waterborne Rust Converter Primer, Iron Phosphate Conversion Layer

DETAILED DESCRIPTION

A rust converter is applied directly onto adherent rust and reacts with it, leaving a stable, over-coatable layer in place of a bare-metal surface. That makes it the practical route for large or hard-to-access steel — ship hulls, tanks, structures — where blasting is slow, costly, or environmentally unfriendly. It also means the formulation has to do reliable chemistry on a substrate it does not control. The sections below follow the decisions in the order a formulator meets them.

1. Start with the rust

Success depends on two things at once: the rust layer (amount of adherent rust, film thickness, structure, composition) and the converting compound (structure, concentration, solubility, pH, ability to chelate ferric iron).

Only adherent rust (AR) can be converted. Non-adherent rust (NAR) must be removed first. In steels exposed to chloride in dry–wet cycles, 55–90% of the corroded iron becomes AR, 3–18% becomes NAR, and up to 38% is lost. NAR is mainly α-FeOOH, γ-FeOOH, and hematite, with traces of β-FeOOH. AR contains α-, β-, and γ-FeOOH plus spinel phases. Steel type matters, as AR on carbon steel is mostly non-stoichiometric magnetite, while AR on weathering steel is mostly γ-, α-, and β-FeOOH with a little γ-Fe₂O₃.

Phase also controls how much a converter can do:

Phase	Morphology	Behavior toward a converter
α-FeOOH (goethite)	Granular	Relatively stable, non-detrimental rust
β-FeOOH (akaganeite)	Rod-shaped	Loose texture that stores moisture and anions; barely affected
γ-FeOOH (lepidocrocite)	Fine needles	Loose texture that stores moisture and anions; reacted primarily with the converter in one study, though its conversion yield was hard to assess
δ-FeOOH	Irregular, cotton-like, amorphous	Compact layer that improves corrosion resistance
Spinel (γ-Fe2O3/Fe3O4)	—	About 90% converted

2. Choose the converting agent

Tannins. Tannic acid chelates ferric iron into a crosslinked ferric tannate network, seen as a blue-black layer. Performance depends on tannin type, concentration, pH, reaction time, co-additives, application method, and the steel’s contamination grade. Quebracho tannin coordinates with iron only partially, and chestnut hydrolysable tannins do not complex iron at low pH.

Phosphoric acid. Forms water-insoluble iron phosphates on the steel. Polyphosphoric acid gives a smoother, more uniform film than simple H₃PO₄ because its higher viscosity slows the reaction with the substrate. In a polyphosphoric/tannic acid study, adding 50 g/L phosphoric acid to the tannic formula raised epoxy-to-rust adhesion from 1.93 to 11.63 MPa. The gain was attributed to a compact, micro-cracked conversion layer that anchors the coating. pH and phosphate content were the key factors.

Gallic and pyrogallic acid. These polyphenols are used alone or with tannic acid. Two modifications address tannic acid’s solubility and compatibility limits. (1) 2-Hydroxypropyl 3,4,5-trihydroxybenzoate (gallic acid esterified with 1,2-propylene glycol) forms an amorphous iron chelate layer. It is more water-soluble and more compatible with waterborne coatings than gallic acid. It can be applied without a cosolvent and avoids possible pitting. (2) Gallic acid/glycidyl methacrylate ring-opening emulsion gave the best adhesion and corrosion resistance at a 1:3 ratio. XPS and EDS showed the phenolic hydroxyls chelating iron into macromolecular compounds.

Lignin. Anticorrosive performance improves when lignin is treated with anhydrous NaOH or KOH, or their aqueous-alcohol solutions. One reported waterborne system combines ascorbic acid, an alkali-metal lignin sulfonate and a water-soluble polymer, with optional HEC or xanthan thickener and a corrosion inhibitor.

Other compounds. Pyridine derivatives (2-picolinic acid, 2-hydroxypyridine, 2-aminopyridine) gave more compact, homogeneous rust layers. They also gave much higher adhesion of an epoxy–polyvinylbutyral coating than painting over untreated rust. 3,4,5-Trihydroxy-2-[(hydroxyimino)methyl]benzoic acid has also been reported.

3. Set the ratio and the pH window

Tannin–iron chemistry changes with pH, which makes it a vital parameter.

• At 5–50 ppm tannic acid and pH below 4, tannic acid reduces dissolved ferric iron to the ferrous state.
• At pH 4 (typically 3-5), a blue-black precipitate containing ferric iron and tannic acid forms.
• At 500 ppm tannic acid and pH above 5, a ferrous complex forms.
• At pH 7 and above, a partially hydrolysed ferric tannate and hydrated polymeric ferrous tannate result.

Patents sit in the same region. A rust-converting agent specified at pH 3–6, and a conversion slurry at pH 2–5 that gives a micro-cracked film with active functional groups for bonding to the binder.

Regarding the ratio, the 3% tannic acid + 10% phosphoric acid (3T–10P) combination was reported as synergistic in the FeOOH study, with the β-FeOOH reaction products judged to be iron phosphate plus an iron–tannin chelate.

Another study cited the use of 35% phosphoric acid and 5% tannic acid (full formula in the table below). Its main conversion products were amorphous iron phosphates and tannates, with less tannate than phosphate expected because tannate kinetics are slower.

4. Build the supporting package

A commercial converter contains many components beyond the converting compound. The ranges below are those reported in individual patents, not a universal recipe.

Function	Materials reported	Reported range (wt %)
Penetrant/cosolvent	Isopropanol (IPA), t-butanol, ethylene glycol, glycerol, acetone/butanone	5–10 in a water-based conversion liquid; 11–13 (IPA or Ethanol) in a 30–35% H3PO4 based formulation; 12.5 each of IPA and t-butanol was mentioned in another literature
Wetting agent / surfactant	Sodium dodecylbenzene sulfonate, ethoxylated fatty alcohol	0.5–5
Defoamer	Octadecyl stearate, polydimethylsiloxane, tributyl phosphate	0.5–5
Thickener	CMC, sodium alginate, HEC, agar, polyacrylate, methyl cellulose	0.5–5; 5–10 in a vinyl acetate–acrylic based formulation
Film-forming aid	Ethylene glycol ethyl or butyl ether	0.5–5
Freeze–thaw stabiliser	Ethylene glycol, 1,2-propanediol, hexanediol	2–3
Preservative	Not specified	0.5–1
Catalysts and inhibitors	MnCO3, ZnCO3, ZnO; MoO3, KNO2; sodium benzoate, tartaric or citric acid; triethanolamine	roughly 0.05–2
Fillers and anticorrosive pigments	Modified wollastonite, mica, barite; aluminium tripolyphosphate; zinc phosphate; sericite, talc, nano-silica	Design-dependent

5. Fit the converter into a resin or primer system

Most recent converting-coating patents are based on water-dilutable resin systems because they are economical and environmentally friendly. A typical sequence is: pre-treat the rusted substrate to remove foul and NAR; coat the waterborne converting material; spray a waterborne anticorrosive primer, then flash-dry or bake; finish with a waterborne texture topcoat.

One patented preparation route builds the converting material in stages. Tannic acid, wetting, dispersing, antifoaming, flash-rust-proofing and thickening agents are stirred into distilled water to make an intermediate; a modified filler (wollastonite, mica, barite) is stirred in at high speed; separately, natural plant polyphosphate, potassium ferrocyanide and an acrylate ester are copolymerised into a resin emulsion; the emulsion and a film-forming agent are then added. The weight ratio of intermediate: resin emulsion: film-forming agent is 1–3: 15–24: 0.5–0.75.

A styrene-acrylic converting coating (formula in the table below) is reported at 16–20% solids, 60–80 µm fineness, 10–30 minutes surface dry, 60–100 minutes hard dry, 3–7 MPa adhesion and 11–15 MPa bending strength. In a polyester/amino resin primer, the converting agent is used at just 0.5–1.5 wt%, alongside 10–15% anti-rust pigment.

One patent takes the opposite approach: a neutral, water-thinnable rust-converting and inhibiting primer at pH 6.5–7.5 (preferably 7.2) containing no oxalic, tannic, phosphoric, or other acidic chelating agents. It is claimed to convert adherent rust to a passive form, dries in under an hour, can be over-coated within two hours, and can be top-coated with solvent-free two-component epoxy, polyurethane, and fluoropolymer systems. Its composition:

Component	Examples	wt%
Film-forming resin	Vinylidene chloride–vinyl chloride copolymer with a medium or long oil air-drying alkyd (soya or tall oil; soya-phthalic anhydride preferred)	20–40
Ionic surfactant	Amine salt of a long-chain fatty acid, e.g. triethanolamine oleate, optionally with a non-ionic EO/PO surfactant	3–5
Pigment	Red oxide and zinc oxide; also zinc phosphate, calcium borosilicate, sodium zirconium silicate, iron phosphate, magnesium silicate, barium metaborate, barytes	30–70
Coalescing agent	Diethylene glycol, carbitol, methyl carbitol	8–15
Anti-foam	—	0.5–1.5
Thickener/stabiliser	HEC or HPC, PVA, methyl cellulose, guar gum, clay, polyacrylamide	0.5–1.5
Pigment dispersant	Sodium or potassium pyrophosphate	0.4–0.6
Freeze stabiliser	Ethylene or propylene glycol	2.6–3.6
Drying accelerator	Co, Mn, Zr, Zn, Pb fatty-acid salts or naphthenates	1–4

6. Reference formulations

The formulations mentioned below is for reference only. It is hoped that these will guide you to reach the final goals quicker.

Formulation	Composition (wt %)
Classic phosphoric/tannic converter	Phosphoric acid 35%, tannic acid 5%, isopropyl alcohol 12.5%, t-butanol 12.5%, glycerin 10%, water 25%
Paint-grade converter	Tannic acid 27%, phosphoric acid 8%, Permax 803 48%, ethoxylated fatty alcohol 0.5%, propylene glycol 10%, tributyl phosphate 0.5%, potassium sorbate 0.5%, water 5.7% — reported complete rust conversion, good adhesion, flexibility, water and corrosion resistance
Water-based conversion liquid	Water 70–90%, phosphoric acid 5–20%, tannic and/or phytic acid 0.5–10%, penetrant 5–10%, film-forming promoter 0.5–5%, surfactant 0.5–5%, defoamer 0.5–5%, thickener 0.5–5%
Phosphoric-led converter	85% H3PO4 35–50%, organic acid (e.g. acetic) 8–10%, water 35–45%, catalyst (MnCO3, ZnCO3 and/or ZnO) 1–2%, antirust agent (MoO3 and/or KNO2) 0.2–0.8%, corrosion inhibitor 0.05–0.5%, activating agent 0.01–1%, penetrating agent 0.05–2%, iron metal powder 4–9%
Low-temperature converter (−30 to 40 °C)	H3PO4 3–5%, C1–C4 alcohol 79.5–86%, water 11–15%, triethanolamine 0.2–0.3%
Gallol blend	Pyrogallic acid 5–10%, gallic acid 5–10%, polyaniline emulsion 5–10%, fatty alcohol 5–10%, phosphoric acid 1–3%, citric acid 0.4–0.6%, water 60–70%
Styrene-acrylic converting coating	Styrene-acrylic emulsion 100%, ferric oxide 15–35%, tannic acid 10–30%, soft water 3–7%, graphite 5–25%, methyl cellulose 1–5%, silicon dioxide 2–8%, silica powder 8–12%, ethylene glycol 3–7%, tributyl phosphate 1–20%.
Tannin / oxalic / silver converter	Tannin extract 6.8–12.7%, oxalic acid 3.8–7.72%, silver nitrate 0.01–0.1%, HEDP (1-hydroxyethylidene-1,1-diphosphonic acid also called as etidronic acid) 0.15–0.67%, water or water/alcohol to balance. Optional sodium tripolyphosphate and sodium hexametaphosphate (0.06–0.18% each), glyoxal 0.4–0.57%, citric acid 0.9–3.5% as a storage preservative

The last formulation shows each component doing a separate job. Tannin is the converting agent that forms the insoluble iron-tannate film. Oxalic acid prepares the rust surface so the tannin can penetrate and react, rather than dissolving the rust away. Silver nitrate is a biocide that protects the converter and the treated surface from biological degradation, and it stays dissolved. HEDP drives deep penetration into rust pores and boosts the tannin’s converting power. The patent also claims a dry concentrate for transport — tannin 49–76%, oxalic acid 23–47%, silver nitrate 0.05–0.6%, HEDP 0.95–4% — diluted 1:5 to 1:6.25 with water before use. The resulting film is reported as 30–50 µm, dark blue-black and crystalline, with 100% conversion claimed at 300 µm rust thickness, 72 hours of water resistance and 96 hours of transformer and crude oil resistance.

7. Beyond the classical converter: tannate conversion coatings

A newer line of work uses tannic acid as a deposition chemistry rather than a rust-reaction chemistry, positioned as a chromate-free and phosphate-free alternative.

• TA/Zn film: Mixing 100 mL of 0.12 mM TA with 100 mL of 0.06 mM Zn(NO₃)₂·6H₂O for 60 s, then immersing iron for 170 s, gave a thicker, denser film than TA alone. EIS inhibition efficiency was 92.39% versus 68.38% for TA alone, and polarization data indicate anodic/barrier-type inhibition. In a separate saline study, 250 ppm TA + 750 ppm Zn(II) was the most efficient combination.
• Ce–tannic acid hybrid pigment: TA complexed with cerium(III) nitrate at pH 7, used at 1 g per 50 g epoxy-ester resin (40 ± 6 µm film), gave 63–81% inhibitive efficiency, peaking at 24 h. I_corr was about nine times lower than the blank. The mechanism is damage-responsive i.e., pH differences between anodic and cathodic sites weaken the Ce–O bond, releasing tannate and cerium ions at the respective sites.
• Tannic acid conversion coating: A TA + Zn(NO₃)₂ bath at pH 4 had a 2-minute optimum, with the highest Rp at day 7 and above 99% protection efficiency. At under 2 minutes, the film is too thin. At 10 minutes, hydrogen evolution and thermal-expansion mismatch crack the film, and protection falls below 25% at 7 days. Even the 2-minute coating loses efficiency over 7 days of continuous chloride exposure, so durability has a ceiling.

BIBLIOGRAPHY

1. Saji, V.S. “Progress in rust converters.” Progress in Organic Coatings, 127, 88–99, 2019.
2. Mei, L., Liao, L., Wang, Z., Xu, C. “Interactions between phosphoric/tannic acid and different forms of FeOOH.” Advances in Materials Science and Engineering, 2015.
3. Ocampo, L., Margarit, I., Mattos, O., Cordoba-de-Torresi, S., Fragata, F. “Performance of rust converter based in phosphoric and tannic acids.” Corrosion Science, 46, 1515–1525, 2004.
4. Heiss, A. “Rust converting-and-inhibiting water-thinnable paint primer.” US Patent 5,476,890, 1995.
5. “Rust converter, composition for producing same, and production method.” WO2016/171641A1, 2015.
6. “High-Performance Rust Converter with a Formula Based on Polyphosphoric Acid and Tannic Acid.” Advanced Materials Research, 150–151, 1277, 2010.
7. Kaghazchi, L., Naderi, R., Ramezanzadeh, B. “Construction of a high-performance anti-corrosion film based on the green tannic acid molecules and zinc cations on steel: electrochemical/surface investigations.” Construction and Building Materials, 262, 120861, 2020.
8. Wu, Y., Yan, R., Duan, Y., Qiu, J., Chen, T., Ma, H. “An environmental-friendly tannic acid/Zn conversion film with a good corrosion protection for iron.” Surfaces and Interfaces, 24, 101078, 2021.
9. Moghaddam, P., Amini, R., Kardar, P., Ramezanzadeh, B. “Epoxy-ester coating reinforced with cerium(III)–tannic acid-based hybrid pigment for effective mild-steel substrate corrosion protection.” Progress in Organic Coatings, 161, 2021.
10. Xiong, C., Wang, Z., Jin, Z., Lu, M., Huang, Y., Wang, P. “Preparation of tannic acid-based conversion coating and corrosion protection performances for steel in chlorinated simulated concrete pore solution.” Case Studies in Construction Materials, 21, 2024.