Overview of Common Rust Preventives and Corrosion Inhibitors: Properties, Advantages, Disadvantages, and Application Tips in Metalworking Fluids
Borates / Borate Esters
Borates and borate esters are widely used water-soluble rust preventives, particularly effective for cast iron. They are applicable in fully synthetic cutting fluids, semi-synthetic cutting fluids, and water-based grinding fluids.
1. Borates:
Excellent rust prevention and low cost; however, they are prone to hydrolysis and reversible reactions in solution. As alkalinity is consumed, boric acid may precipitate and cause corrosion.
2. Borate Esters:
Provide high alkalinity reserve and stabilize system pH; exhibit good anti-wear and friction-reducing properties; also possess certain bacteriostatic and bactericidal effects.
Borates/borate esters alone are not ideal and need to be combined with other rust preventives. Borate esters blended with different carboxylates show excellent rust protection for ferrous metals.
Disadvantage:
Long-term use may lead to stickiness and affect cleaning performance.
Molybdates
1. Molybdates:
When blended with nitrites at a mass ratio of nitrite:molybdate = 2:3, and at a total concentration of about 5 mg/L, they provide good corrosion inhibition for carbon steel.
2. Sodium Molybdate:
When used with monoethanolamine at a mass ratio of 1:5, it shows excellent corrosion inhibition for steel.
In addition, molybdate + BTA (benzotriazole) + sodium benzoate + amine can achieve excellent protection for steel and cast iron.
Advantages:
① Good corrosion inhibition, especially when combined with other inhibitors to suppress pitting;
② Effective on carbon steel, copper, brass, and aluminum;
③ Low toxicity;
④ Broad application range, effective in both soft and hard water.
Disadvantage:
Requires relatively high dosage, leading to higher costs.
Sulfonates and Other Oil-Soluble Rust Preventives
1. Petroleum Barium Sulfonate T701
Available as 50% solid / 50% liquid. Heated solid T701 has good oil solubility; provides excellent rust prevention for ferrous and non-ferrous metals. Used in rust preventive oils, oil-based products, emulsifiable oils, and microemulsions.
2. Petroleum Sodium Sulfonate T702
Strongly hydrophilic; provides corrosion inhibition and rust prevention; also offers good emulsification. Used in oil-based products, emulsifiable oils, and microemulsions.
3. Synthetic Sodium Sulfonate
Produced by alkylbenzene sulfonation, neutralization, and decolorization. With an HLB value of ~12, it is strongly hydrophilic, effective as both rust preventive and emulsifier. Compared with petroleum sodium sulfonate, it is lighter in color, has broader emulsification range, and is easier to formulate—ideal for microemulsion formulations.
4. Zinc Naphthenate T704
Good oil solubility; provides rust prevention for steel, copper, and aluminum. However, its performance on cast iron is poor when used alone. Requires blending with T701 for best results. Applied in oil-based products, emulsifiable oils, and microemulsions. (Dosage: 3–10%)
5. Dinonylnaphthalene Barium Sulfonate T705
Provides outstanding rust prevention for a wide range of metals; excellent oil solubility, lubrication, anti-wear properties, and stability even under humid conditions. Widely used in petroleum and synthetic lubricants/greases, as well as rust preventives in petroleum ether cleaners and water-displacing fluids.
A typical oil-soluble rust preventive with excellent film-forming ability and humidity resistance, especially effective for ferrous metals. Applicable in rust preventive oils, oil-based products, emulsifiable oils, and microemulsions, often combined with petroleum sulfonates for enhanced protection.
Advantages:
Good oil solubility, versatile formulation compatibility, effective at low dosage; low-acid-value grades improve fluidity and storage stability.
Disadvantage:
Rust prevention for lead and cast iron is relatively weak; blending with other rust preventives (e.g., T701) is recommended for comprehensive protection.
Fatty Acid Salts, Esters, and Amides
1. Fatty Acid Salts
Oleic acid/alkanolamine salts, tall oil/alkanolamine salts, C21 dicarboxylic acid/alkanolamine salts, dimer acid/alkanolamine salts.
2. Fatty Acid Esters
Sorbitan monooleate (Span-80) and others.
3. Fatty Acid Amides
Alkanolamides, castor oil maleamides, heterocyclic boron amides, tall oil amides, etc.
Advantages:
Provide emulsification, cleaning, lubrication, and rust prevention. Perform well in high-oil systems.
Disadvantage:
Rust prevention is moderate in low-oil systems; also tend to generate foam and are prone to microbial degradation.
Caprylic Acid, Neodecanoic Acid, and Isononanoic Acid
These acids are mainly used in fully synthetic cutting fluids, particularly those containing polyethers. In alkaline systems, they can couple with polyethers, improving solubility and enhancing lubrication. They also provide some rust prevention and improve the film-forming ability of other inhibitors. In microemulsion and emulsion systems, they can act as balancing agents.
Disadvantage:
Strong odor and only moderate rust prevention when used alone.
Dicarboxylic Acids
1. Sebacic Acid (C10 Dicarboxylic Acid)
Produced from castor oil through saponification and thermal cracking. As a key derivative of castor oil, it is widely used in water-based rust preventive formulations.
Advantage:
Good hard-water resistance and relatively low foaming.
Disadvantage:
Moderate rust prevention; rising prices in recent years have reduced cost-effectiveness.
2. C11/C12 Dicarboxylic Acids
Long-chain dicarboxylic acids are not naturally occurring and can be synthesized chemically, though this process requires high energy and equipment input. Increasingly, microbial fermentation methods are used instead.
Advantage:
Good rust prevention in water-based systems.
Disadvantage:
Poor hard-water resistance, high foaming, prone to microbial degradation; prices have also risen significantly, reducing cost-effectiveness.
