Dr E. Ramanathan PhD
Chemistry of Acid Inhibitors
Acid inhibitors are chemicals added to acid solutions (like HCl or H₃PO₄) to reduce the corrosive attack on base metals (e.g., steel) without significantly affecting the acid’s ability to remove rust or scale.
Mechanism of Action
Acid inhibitors work by adsorbing onto the metal surface and forming a thin protective film that:
- Reduces metal dissolution
- Minimizes hydrogen evolution
- Inhibits localized corrosion (pitting, underfilm attack)
They are generally polar molecules with electron-rich atoms or groups (N, O, S, π-bonds) that interact with the metal surface through:
- Physisorption (electrostatic interaction)
- Chemisorption (covalent/coordinate bonding)
Types of Acid Inhibitors with Examples
| Type | Common Compounds | Mechanism |
|---|---|---|
| Organic Inhibitors | – Thiourea, Urea- Benzotriazole- Alkyl amines- Imidazolines | Form adsorbed films via lone pairs and π-bond systems |
| Surfactant-based | – Cetyltrimethylammonium bromide (CTAB)- Quaternary ammonium salts | Micellar film formation, lowers acid-metal contact |
| Plant Extracts | – Polyphenols, tannins, flavonoids (e.g., henna, green tea) | Eco-friendly adsorption via phenolic OH, COOH |
| Inorganic Inhibitors | – Antimony salts, arsenates, chromates (historical use) | Passive film formation, now restricted due to toxicity |
| Polymer-based | – Polyacrylamide, Polyethylene glycol (PEG) | Forms barrier layers on surface |
Example: Thiourea as Acid Inhibitor
Structure: NH₂–CS–NH₂
- Sulfur and nitrogen atoms coordinate with Fe surface.
- Forms a compact, adsorbed layer.
- Very effective in HCl-based pickling or derusting.
Ideal Properties of an Acid Inhibitor
- High inhibition efficiency
- Stability in acidic media
- Compatibility with other bath components
- Low toxicity and biodegradability (for eco-friendly formulations)
Let me spare some more details on this interesting chemistry of inhibitors.
Detailed Chemistry of Acid Inhibitors
1. Organic Inhibitors – Chemistry & Mechanism
These are the most commonly used and effective in acid pickling, cleaning, and derusting.
Key Functional Groups:
- Amino groups (–NH₂, –NR₂)
- Thio groups (–SH, –C=S)
- Hydroxyl (–OH), Carboxyl (–COOH)
- Aromatic rings (π-electrons)
These groups interact with the d-orbitals of Fe atoms forming:
- Coordinate covalent bonds (chemisorption)
- Electrostatic interaction (physisorption)
Examples:
| Compound | Functional Group(s) | Adsorption Type | Notes |
|---|---|---|---|
| Thiourea | –NH₂, –C=S | Chemisorption | Highly effective in HCl, forms stable Fe–S bond |
| Benzotriazole | N-rich heterocycle | Mixed (π, N lone pairs) | Effective for Cu, also used in Fe |
| Alkyl amines | –NH₂ with alkyl tail | Physisorption + film formation | Surfactant action adds hydrophobicity |
| Imidazolines | N-heterocycle + long chain | Chemisorption + barrier | Used in oilfield acidizing and HCl pickling |
2. Surfactant-Based Inhibitors
Quaternary ammonium compounds (cationic surfactants) adsorb on negatively charged steel surfaces and form a hydrophobic layer, reducing acid-metal contact.
- Example: Cetyltrimethylammonium bromide (CTAB)
- Mechanism: Tail aligns outward, forming micellar-like film over surface
3. Plant Extracts & Natural Inhibitors
Derived from tannins, polyphenols, and flavonoids. Functional groups: OH, COOH, aromatic rings.
- Mechanism: Natural polyphenols donate electrons and form hydrogen bonds or π–π stacking on steel.
- Example: Henna (lawsone), green tea (catechins), pomegranate peel.
Eco-friendly, biodegradable, and increasingly used as sustainable alternatives.
4. Inorganic Inhibitors (legacy, less used)
| Compound | Action |
|---|---|
| Chromates | Form Cr(III/VI) oxide layer on surface (passivation) – now banned in many regions |
| Antimony salts | Form protective Sb⁵⁺ compounds on steel surface |
| Arsenates | Similar to chromates, very toxic |
5. Polymer-Based Inhibitors
| Polymer | Mechanism |
|---|---|
| Polyethylene glycol (PEG) | Adsorbs via OH and forms hydrophilic barrier |
| Polyacrylamide | Binds via amide groups, slows ion transport |
| Polyvinylpyrrolidone (PVP) | Adsorbs via N and C=O |
Used in multi-functional systems (e.g., rust remover + flash rust inhibitor).
Comparative Inhibition Efficiency
| Acid | Organic Inhibitor Efficiency | Inorganic Inhibitor Efficiency |
|---|---|---|
| HCl | High (especially thiourea, amines) | Moderate to high (if allowed) |
| H₃PO₄ | Moderate (due to inherent passivation) | Less used |
| Citric acid | Organic inhibitors not always needed | Often self-limiting corrosion |
The chart above compares the inhibition efficiencies of various acid inhibitors in HCl and H₃PO₄. Thiourea shows the highest efficiency in HCl, while natural and polymer-based inhibitors perform moderately well in both acids.
Formula-1
Formulating a Thiourea-Based Acid Inhibitor System (for HCl Derusting)
Thiourea is a powerful organic corrosion inhibitor suitable for hydrochloric acid-based rust removers and pickling baths. Here’s how to incorporate it into a commercial-ready concentrate.
Formulation – Thiourea-Based Derusting Stock Concentrate (100 L)
| Component | Quantity (kg or L) | Purpose |
|---|---|---|
| Hydrochloric Acid (33%) | 50 L | Primary rust/scale removal |
| Phosphoric Acid (85%) | 20 L | Passivation and rust removal |
| Thiourea (pure) | 1.0 – 2.0 kg | Acid corrosion inhibitor |
| Surfactant (non-ionic, e.g., NP-9) | 1.5 kg | Wetting, degreasing |
| Chelating Agent (NTA·3Na, 40% soln) | 3.0 L | Binds metal ions, bath stability |
| Water (soft or DI) | Balance to 100 L | Carrier |
Usage (Dilution)
- Dilute 1:10 with water to prepare working bath.
- Immersion time: 5–15 min at room temp or up to 40°C.
- pH of working bath: 1.5–2.5.
Thiourea Handling Notes
- Acts via sulfur and nitrogen atoms adsorbing on metal.
- Inhibits both anodic (Fe dissolution) and cathodic (H₂ evolution) reactions.
- Should be used fresh; avoid long-term exposure to heat/light (decomposes to toxic compounds).
- Handle with PPE—thiourea is toxic if ingested or inhaled.
Optional Additives
| Additive | Function |
|---|---|
| Sodium Gluconate | Rust looseners, metal ion control |
| Film Forming Polymers | Extra flash rust protection |
| Isopropyl alcohol (1–2%) | Reduces surface tension, drying |
Formula – 2
Thiourea-Based Derusting System – Non-HCl, Neutral pH Version
This formulation is non-HCl, environmentally safer, and operates around neutral pH (~6–7). It’s suitable for light-to-moderate rust removal on mild steel, with low metal attack and low hydrogen evolution.
Formulation – 100 L Stock Concentrate
| Component | Quantity (kg or L) | Purpose |
|---|---|---|
| Citric Acid (50% solution) | 30 L | Mild organic acid for rust dissolution |
| Sodium Gluconate | 3.0 kg | Complexing agent, rust loosener |
| Thiourea (pure) | 1.5 – 2.0 kg | Acid inhibitor, corrosion protection |
| Non-ionic Surfactant (NP-9 or similar) | 1.5 kg | Wetting, oil/grease removal |
| Triethanolamine (TEA) or Monoethanolamine | 1.0 – 1.5 L | pH buffer and film-former |
| Deionized/Soft Water | Balance to 100 L | Carrier |
Operating Parameters (after 1:10 dilution)
- pH: 6.0 – 7.0
- Temperature: 40 – 60°C (can be used at ambient for longer cycles)
- Immersion Time: 10 – 30 minutes
- Agitation: Recommended
- Post-treatment: Water rinse + optional passivation or phosphating
Advantages
- No HCl fumes or acid attack
- Safer for operators, plant equipment, and environment
- Mild on base metal, prevents flash rust due to thiourea
- Compatible with multi-metal systems (MS, GI, Al with caution)
Important Notes
- Thiourea is stable in this buffered system at neutral pH.
- Avoid mixing with strong oxidizers.
- Shelf-life: ~2–4 weeks in sealed containers.
Formula-3
Plant Extract-Based Boosters for Neutral Derusting Systems
Plant extracts are rich in polyphenols, tannins, flavonoids, and other bioactive compounds that act as natural corrosion inhibitors and rust dispersants. They offer a green, biodegradable alternative or supplement to synthetic additives like thiourea.
Common Plant Extracts Used
| Extract Source | Key Actives | Functions |
|---|---|---|
| Henna (Lawsonia) | Lawsone (1,4-naphthoquinone) | Antioxidant, surface film formation |
| Green Tea | Catechins, tannins | Antioxidant, metal chelation |
| Pomegranate Peel | Ellagitannins, gallic acid | Strong antioxidant, acid stability |
| Guava Leaves | Flavonoids, phenolic acids | Adsorptive film formation, rust binding |
| Tamarind Extract | Tartaric acid, polyphenols | Rust loosening, mild acid chelation |
Booster Usage in Formulation
You can use plant extract concentrates or powdered extracts in neutral pH derusting formulations. Here’s how:
Formulation Supplement (per 100 L stock):
| Component | Dosage | Function |
|---|---|---|
| Henna Extract (10% soln) | 2.0 – 3.0 L | Corrosion inhibitor |
| Pomegranate Peel Extract | 0.5 – 1.0 kg | Film-former, chelation |
| Green Tea Extract | 0.5 – 1.0 kg | Free radical scavenger |
| Citric or Gluconic Acid | Adjust for synergy | Aids in mild derusting |
- Add after surfactants and pH buffers in final formulation.
- Maintain pH ~6.0–7.0.
- Store away from sunlight (polyphenols degrade with UV).
Advantages
- Fully biodegradable, non-toxic
- Suitable for eco-label or green chemistry products
- Compatible with thiourea, or can be used standalone with chelators
Formula – 4
Plant-Extract-Only Neutral Derusting Formulation (100 L Stock Concentrate)
A fully green, acid-free system for light-to-moderate rust on mild steel. Biodegradable and safe for handling.
Formulation (Stock Concentrate for 1:10 dilution)
| Component | Quantity (kg or L) | Purpose |
|---|---|---|
| Pomegranate Peel Extract (powder) | 1.0 kg | Polyphenolic corrosion inhibition |
| Green Tea Extract (powder) | 0.8 kg | Tannin-based antioxidant and rust binder |
| Citric Acid (50% solution) | 25 L | Mild acid, rust removal aid |
| Sodium Gluconate | 3.0 kg | Chelating agent for Fe²⁺, rust dispersant |
| Non-ionic Surfactant (NP-9) | 1.5 kg | Wetting, penetration of extract actives |
| Monoethanolamine (MEA) | 1.0 L | pH buffer (keeps system neutral) |
| Deionized/Soft Water | Balance to 100 L | Carrier |
Operating Conditions (after dilution 1:10)
- pH: 6.0 – 6.5
- Temperature: 40 – 60°C
- Immersion Time: 20 – 40 minutes
- Agitation: Recommended for powder-based extracts
- Post-treatment: Rinse + optional phosphating or passivation
Performance Notes
- Removes loose rust and some bonded rust layers.
- Leaves a passive antioxidant layer (from tannins).
- Surface turns slightly darker—indicating plant layer adherence.
Storage & Handling
- Use UV-opaque containers.
- Shelf-life: 2–4 weeks (concentrate), longer if extract stabilizers are added.
- Optional preservative: 0.1% benzalkonium chloride or sodium benzoate.
