Principles of Aqueous Metal Cleaning And Surfactant Applications

Aqueous metal cleaning is primarily achieved through the functions of surfactants, such as emulsification, wetting, solubilization, penetration, dispersion, corrosion inhibition, and chelation.

Surfactants can modify (usually reduce) the interfacial tension between two phases. They are amphiphilic compounds containing both hydrophilic and lipophilic groups. The lipophilic group generally consists of hydrocarbon chains, while the hydrophilic group is usually composed of oxygen-containing functional groups such as hydroxyl, carboxyl, or ether groups. Therefore, the properties of surfactants depend not only on the size and shape of the hydrophobic moiety but also closely on the type of hydrophilic group. Based on their ability to ionize in water and the charge of the resulting active ions, surfactants can be categorized into four types: nonionic, anionic, cationic, and amphoteric. Among these, nonionic and anionic surfactants are the most commonly used in metal degreasing cleaners.

Parameters for Evaluating Surfactant Properties

Hydrophilic–Lipophilic Balance (HLB)

To describe the hydrophilic–lipophilic capacity of a surfactant and its balance, the HLB value is introduced. A low HLB indicates strong lipophilicity and poor water solubility, making rinsing difficult. A high HLB indicates strong hydrophilicity and good solubility, but less effective adsorption at the interface. For nonionic surfactants, HLB values typically range from 0 to 20; for ionic surfactants, they range from 1 to 40. For detergents used in wetting, penetration, and emulsification, an HLB of 13–15 is considered optimal.

Cloud Point

The cloud point is an important indicator for nonionic surfactants. It refers to the critical temperature at which a 1% aqueous solution becomes turbid upon heating and then turns clear again upon further heating or cooling. Above the cloud point, surfactant activity decreases, while below it, activity increases. The cloud point is influenced by the length of the lipophilic group, the type of hydrophilic group, and the electrolyte concentration in the solution.

Nonionic surfactants generally become less soluble in water as temperature rises because the hydrophilic groups (–OCH₂CH₂–, –OH) form hydrogen bonds with water molecules. As temperature increases, water molecules dissociate, and hydrophilicity decreases. Therefore, nonionic surfactants are the main component of low-temperature metal cleaning agents.

For ethoxylated nonionic surfactants (e.g., alkylphenol ethoxylates), longer carbon chains lower the cloud point, while a higher ethylene oxide molar number (n) increases the cloud point and enhances hydrophilicity. Hence, products with appropriate cloud points should be selected according to practical requirements.

Common Surfactants for Metal Degreasing Cleaning

Nonionic Surfactants

These surfactants do not ionize in water. Their hydrophilic groups usually consist of oxygen-containing moieties (ethers or hydroxyls), and their lipophilic groups are long hydrocarbon chains.

• Alkyl polyoxyethylene ethers: For example, dodecyl polyoxyethylene ether contains a lipophilic group –C₁₂H₂₅ and a hydrophilic group –O(CH₂CH₂O)₆H.

• Alkylphenol polyoxyethylene ethers (OP series): Commonly based on octyl (C₈) and nonyl (C₉) phenols. When the ethylene oxide addition number (m) is 8–12, they provide excellent wetting, emulsification, and penetration properties. For instance, polyoxyethylene octylphenol ether (TX-10) has an HLB of 12.8 and a cloud point of 61–67 °C. When m > 15,      emulsifying ability decreases.

• Fatty alcohol polyoxyethylene ethers (AEO series): With the general formula R–O(CH₂CH₂O)ₙ–H, where R = C₁₂–C₁₈, and n = 7–13. For example, AEO-9. These surfactants not only emulsify, disperse, and clean effectively but also provide good hard water resistance and wetting performance. Widely used in textile cleaning and dyeing.

• Alkanolamides (e.g., coconut oil diethanolamide, RCON(CH₂CH₂OH)₂): Used in synthetic cleaners for foam stabilization and enhanced detergency. Also applied in short-term rust prevention during metal degreasing.

Nonionic surfactants exhibit excellent stability in acidic and alkaline media, compatibility with other surfactants, and good solubility in a variety of solvents. They show weak adsorption on metal surfaces, leave minimal residue, and are therefore ideal materials for metal degreasing cleaning.

Anionic Surfactants

These ionize in water to produce negatively charged active groups, mainly including:

• Carboxylates: e.g., sodium oleate (C₁₇H₃₃COONa).

• Sulfates: e.g., sodium dodecyl sulfate (C₁₂H₂₅OSO₃Na, SDS).

• Sulfonates: e.g., sodium dodecyl sulfonate (C₁₂H₂₅SO₃Na).

• Alkylbenzene sulfonates: e.g., sodium dodecylbenzene sulfonate (LAS).

Among these, LAS is the most widely produced and used. Its performance depends on alkyl chain length and the position of the sulfonic group on the benzene ring. Linear alkyl chains generally provide stronger detergency than branched chains, while branched structures improve solubility.

For higher fatty alcohol ester surfactants, sodium salts generally have the best degreasing capacity. Ethoxylated sulfates such as AES (sodium fatty alcohol ether sulfate) and APEO-sulfates combine nonionic and anionic hydrophilic groups, offering strong detergency, degreasing efficiency, and good hard water resistance.

Anionic surfactants tend to adsorb on both metal surfaces and particulate contaminants (such as polishing compounds, grease, and wax), imparting the same charge and causing mutual repulsion. This reduces adhesion and facilitates soil removal. However, they often leave more residue on metal surfaces and thus show relatively weaker rinsing performance. Additionally, anionic and cationic surfactants should not be mixed to avoid forming insoluble precipitates.

Cationic and Amphoteric Surfactants

• Cationic surfactants (e.g., cetyltrimethylammonium bromide) produce positively charged ions in water. They generally have weak detergency but strong bactericidal properties, so they are not typically used for metal degreasing.

• Amphoteric surfactants contain both anionic and cationic groups in the same molecule, such as betaine (R–N⁺(CH₃)₂CH₂COO⁻). They exhibit both cleaning and antibacterial functions, but are more commonly used in high-end shampoos, baby cleansers, fabric softeners, antistatic agents, and disinfectants rather than in metal degreasing.

The relationship between surfactant structure and cleaning performance is notable: within solubility limits, longer hydrophobic chains provide stronger cleaning; linear chains perform better than branched chains; hydrophilic groups at terminal positions are more favorable than internal ones; nonionic surfactants with higher cloud points clean more effectively; and for polyoxyethylene nonionic surfactants, cleaning ability decreases as the ethylene oxide chain length increases.

Builder Agents (Additives)

Builders, also known as synergists, are used to enhance surfactant performance in degreasing, accelerate soil removal, and prevent redeposition on metal surfaces. Common types include:

• Chelating agents: Bind alkaline earth metal ions such as Ca²⁺ and Mg²⁺ into soluble complexes, preventing      secondary adsorption.

• Corrosion inhibitors: For example, benzotriazole is added when cleaning copper and its alloys; sodium metasilicate is added when cleaning aluminum, zinc, and their alloys.

• Water softeners: Such as sodium tripolyphosphate (Na₅P₃O₁₀), which converts calcium and magnesium ions into soluble phosphates.