During the manufacturing and transportation processes, metal components are highly susceptible to rusting. Rust is a mixture of oxides and hydroxides formed on the metal surface due to the reaction of oxygen and water. Iron rust appears red, copper rust is green, while aluminum and zinc develop white rust (commonly referred to as "white corrosion").
Machines in operation or storage inevitably come into contact with oxygen, moisture, or other corrosive substances in the air. These elements cause electrochemical corrosion on metal surfaces, leading to rust formation.
Causes of Metal Rusting
(1) Atmospheric Relative Humidity
At a given temperature, the percentage of water vapor content in the air compared to its saturation level is called relative humidity. Below a certain critical humidity, the rate of rusting is minimal. However, once the humidity exceeds this threshold — known as the critical humidity — the corrosion rate increases sharply. For many metals, the critical humidity ranges between 50%–80%, with steel being around 75%.
When atmospheric humidity surpasses the critical level, a thin film or droplets of water form on the metal surface. If harmful impurities in the air dissolve into this moisture, they create an electrolyte solution that accelerates rusting.
(2) Temperature and Humidity
Temperature and humidity together significantly influence metal corrosion:
Higher temperatures increase the water vapor content in the air.
Elevated temperatures intensify corrosion, especially in humid environments.
At low humidity levels, temperature has little effect on rusting. But when humidity exceeds the critical point, rising temperatures dramatically increase the amount of rust formed.
A temperature difference between the atmosphere and the metal can also lead to condensation on cooler surfaces, promoting rust development.
(3) Corrosive Gases
Airborne corrosive gases, particularly sulfur dioxide (SO₂), play a major role in metal corrosion, especially for steel, copper, and their alloys. SO₂ mainly originates from coal combustion. Carbon dioxide (CO₂), another combustion product, also exhibits some corrosive properties. In industrial areas, gases such as hydrogen sulfide (H₂S), ammonia (NH₃), and hydrochloric acid vapor further accelerate rusting.
Oxygen in the air is the most common and ever-present contributor to metal corrosion.
(4) Other Factors
The atmosphere contains numerous particulates like smoke, soot, chlorides, and acidic or alkaline salts. Some of these particles are inherently corrosive, while others act as condensation nuclei for moisture. Chlorides, in particular, are often referred to as the "arch-enemies" of metals due to their strong corrosive effects.
To prevent rusting, it is essential to block the contact between these substances and metal surfaces. Rust preventives (or corrosion inhibitors) are specifically designed for this purpose.
Classification of Rust Preventives
Rust preventives are chemical agents that form a protective film on metal surfaces to inhibit oxidation. They are widely used in industries such as automotive, electronics, mold-making, and machinery. Common types include:
1. Water-Soluble Rust Preventives
These can be dissolved in water to form aqueous solutions that protect metals after treatment. Their anti-corrosion mechanisms fall into three categories:
① Formation of a dense oxide film:
Some rust preventives react with metal to produce an insoluble oxide layer, preventing anodic dissolution or promoting passivation. Examples include sodium nitrite and potassium dichromate. Insufficient dosage may result in incomplete coverage, leading to localized corrosion due to concentrated current flow.
② Formation of insoluble salts:
Certain compounds react with metals to form non-soluble salt layers that isolate them from corrosive media. For example:
Phosphates react with iron to form iron phosphate.
Silicates react with iron or aluminum to form metal silicates.
③ Formation of insoluble complexes (chelates):
For instance, benzotriazole reacts with copper to form Cu(C₆H₄N₃)₂, which is insoluble in both water and oil, effectively protecting copper surfaces.
2. Oil-Soluble Rust Preventives
Also known as oil-soluble corrosion inhibitors, these are mostly long-chain organic compounds containing polar groups. Their molecules adsorb onto metal surfaces via polar groups, while the non-polar hydrocarbon chains orient outward and blend with oils, forming a protective barrier against water and oxygen.
They are categorized by functional group:
① Sulfonates, e.g., petroleum sulfonates of alkali or alkaline earth metals like barium petroleum sulfonate, sodium petroleum sulfonate, and dinonylnaphthalene sulfonic acid barium salt.
② Carboxylic acids and their soaps, such as stearic acid, oleic acid, oxidized petroleum esters, and naphthenic acids. Metal soaps offer better polarity and corrosion protection than free acids but have lower oil solubility and may hydrolyze in water.
③ Esters, including natural esters like wool wax and beeswax, and synthetic esters such as pentaerythritol monooleate and sorbitan monooleate, which provide excellent rust protection.
④ Amines, e.g., octadecylamine, though usually less effective alone. More commonly used are amine salts or complexes with organic acids, such as octadecylamine oleate and cyclohexylamine stearate.
⑤ Sulfur- or nitrogen-containing heterocyclic compounds, such as imidazoline phosphate ester salts, benzotriazole, and mercaptobenzothiazole, which are effective for both ferrous and non-ferrous metals.
3. Emulsifiable Rust Preventives
There are two types of emulsifiable rust preventives:
Oil-in-water emulsions (water-continuous), typically milky white.
Water-in-oil emulsions (oil-continuous), usually transparent or semi-transparent.
These products not only provide rust protection but also offer lubrication and cooling properties, making them ideal for use as cutting fluids in machining operations.
Traditional emulsifiers were made from saponified vegetable oils (e.g., castor oil). Modern formulations often use triethanolamine oleate, sulfonated oils, or nonionic surfactants.
To enhance rust protection, water-soluble inhibitors like sodium nitrite + sodium carbonate or sodium nitrite + triethanolamine can be added during dilution. Additionally, small amounts of mildew inhibitors (e.g., phenol, pentachlorophenol, or sodium benzoate) may be included to prevent microbial degradation of the emulsion.
4. Vapor Phase Rust Preventives
Also called vapor phase corrosion inhibitors (VPIs), these function by slowly volatilizing at room temperature and forming a protective gaseous layer on metal surfaces.
For ferrous metals, examples include ammonium carbonate, urea, hexamethylenetetramine (urotropine), and cyclohexylamine carbonate.
For copper, benzotriazole is widely used.
In practical applications, multiple rust preventives are often combined into composite formulas to enhance overall performance.
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