Acidic and alkaline metal cleaners

Acidic Cleaners

There are various types of dirt on metal surfaces, including both oil-soluble and water-soluble dirt, as well as metal rust scale, scale, and dust. Acidic cleaning methods play an important role in removing inorganic salt scale, alkaline oxides, and hydroxides from metal surfaces. In practical applications, the acids used include inorganic and organic acids, with hydrochloric acid and sulfuric acid being the most widely used.

 During the pickling process, the role of acid is not only to dissolve dirt but also involves a series of reactions with inorganic salt scale. This is manifested in two aspects: on one hand, acid can alter the composition of inorganic salts, partially converting them into soluble salts. For example, calcium carbonate and magnesium carbonate react with hydrochloric acid to form calcium chloride and magnesium chloride. On the other hand, some inorganic salts are converted into new insoluble salts, but these newly formed insoluble salts remain dispersed for a short period and are easy to remove. For instance, when sulfuric acid is used to clean carbonate scale, sulfate precipitates are generated. The reaction equations are as follows:

 CaCO₃ + H₂SO₄ = CaSO₄↓ + CO₂↑ + H₂O

MgCO₃ + H₂SO₄ = MgSO₄↓ + CO₂↑ + H₂O

 Additionally, acid radicals containing multiple coordination atoms, such as phosphoric acid and citric acid, can form chelates with divalent metal ions. The effect of acid on metal rust scale is primarily through acid-base neutralization reactions, converting the rust scale into water-soluble salts, as its main components are metal oxides and hydroxides. If the acid radical has a coordinating effect on the metal ion, it can further promote the dissolution of metal oxides.

 Composition of Acidic Cleaners

Acidic cleaners are primarily composed of acids. Commonly used acids include hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, phosphoric acid, sulfamic acid, citric acid, acetic acid, hydroxyacetic acid, and oxalic acid, among which hydrochloric acid and sulfuric acid are the most frequently used. To enhance cleaning efficiency, shorten cleaning time, reduce corrosion on the substrate, and minimize environmental pollution, acidic cleaners often contain additives such as corrosion inhibitors, wetting agents, defoamers, acid mist suppressants, and thickeners, many of which are surfactants.

 (1) Acids Commonly Used in Pickling Solutions

① Hydrochloric acid: As one of the three major strong acids, hydrochloric acid is a commonly used pickling medium. It is often the first choice when technical requirements are met. It is suitable for carbon steel, copper and copper alloys, and various non-metallic equipment, such as heat exchangers, reaction equipment, boilers, and heating systems. Hydrochloric acid is inexpensive, highly efficient in cleaning, and poses relatively low operational risks, but it is volatile and produces acid mist, which can harm personnel and equipment. Furthermore, chloride ions can easily cause localized corrosion of passive metals, leading to stress corrosion cracking in austenitic stainless steel. Therefore, it is not suitable for cleaning stainless steel and aluminum.

 ② Sulfuric acid: As a commonly used strong acid in industry, sulfuric acid is generally not chosen if hydrochloric acid is applicable. Sulfuric acid is mostly used for cleaning equipment made of special materials such as stainless steel and aluminum alloys.

 ③ Nitric acid: As an oxidizing acid, nitric acid exhibits oxidizing properties regardless of concentration, and can passivate metals such as iron, aluminum, and chromium. Concentrated nitric acid has stronger oxidizing power. Nitric acid can oxidize and decompose organic matter in dirt and quickly dissolve insoluble metal oxides and salts. However, it is unstable and decomposes upon exposure to light or heat, releasing nitrogen dioxide, which is harmful to human health and the environment. Nitric acid is generally suitable for passivatable metal materials that are not suitable for hydrochloric acid cleaning, such as aluminum and stainless steel.

 ④ Hydrofluoric acid (HF): Hydrofluoric acid is a weak acid and causes relatively less damage to the base metal, but it still has some corrosiveness. Hydrogen fluoride dissolves iron scale and other oxides quickly, offering high cleaning efficiency. It has excellent dissolution capability for silicon scale, especially. Even at low temperatures, hydrofluoric acid can dissolve silicon scale, a characteristic that other acids lack. Therefore, hydrofluoric acid is mainly used for cleaning silicon scale. In practical applications, hydrofluoric acid is often mixed with hydrochloric acid, sulfuric acid, nitric acid, or ammonium bifluoride.

 ⑤ Phosphoric acid: Phosphoric acid is weaker than sulfuric acid, nitric acid, and hydrochloric acid. Except for potassium salts, sodium salts, and ammonium salts, most phosphates are insoluble in water, which can affect cleaning effectiveness. Moreover, phosphoric acid is relatively expensive, so it is only considered for cleaning thick workpieces with special requirements.

 ⑥ Sulfamic acid (NH₂SO₃OH): Sulfamic acid is produced by the reaction of urea with fuming sulfuric acid or chlorosulfonic acid. It is colorless, odorless, and has low toxicity. As a cleaning medium, sulfamic acid has the following characteristics:

a. It is a monobasic solid acid, making it easy to transport;

b. It has low corrosiveness to metals and is suitable for cleaning various metal materials. It is the only acid that can be used to clean galvanized metals;

c. It has high solubility for most metal salts, and the products generated after cleaning are mostly soluble in water;

d. It reacts vigorously with calcium and magnesium salts, generating soluble alkaline earth metal sulfamates that dissolve well in water, thus effectively removing scale;

e. It dissolves iron rust relatively slowly;

f. It is relatively expensive.

 ⑦ Citric acid: Citric acid is the most commonly used organic acid in industrial cleaning, and its usage in chemical cleaning is second only to hydrochloric acid. Citric acid can dissolve iron and copper rust scale. On one hand, this is because the hydrogen ions dissociated from it can react with alkaline hydroxides; on the other hand, citric acid can chelate metal ions, thereby promoting the dissolution of metal oxides. It should be noted that during citric acid pickling, if the concentration of iron ions in the pickling solution is too high and the pH is greater than 4, ferric citrate precipitate may form, affecting cleaning effectiveness, so caution is required during use.

Due to the relatively high cost of citric acid, other pickling solutions should be prioritized in general cleaning tasks. Citric acid is mainly used to remove oxide scale from high-value equipment surfaces and can also dissolve carbonate scale.

In addition, other organic acids used in pickling solutions include acetic acid, ethylenediaminetetraacetic acid, oxalic acid, hydroxyacetic acid, and organic phosphonic acids, each with unique characteristics.

(2) Surfactants Used in Pickling Solutions

 During the pickling process, the oxide scale on the metal surface is dissolved, while the metal substrate is also subject to dissolution and corrosion. Moreover, due to the presence of insoluble oxides on the material surface, pitting corrosion can easily form, and the accumulated hydrogen diffusing into the metal can cause severe hydrogen embrittlement. To improve pickling effectiveness, a small amount of surfactant is often added to the pickling solution to accelerate the dissolution of the oxide layer, delay acid corrosion of the substrate, and inhibit the diffusion of hydrogen atoms into the substrate. This is because the added surfactant can form a thin film on the metal surface, providing some corrosion inhibition.

Additionally, the pickling process generates a large amount of acid mist, which not only harms workers' health and pollutes the environment but also severely corrodes factory buildings and equipment. To eliminate or reduce this hazard, acid mist suppressants can be added. Acid mist suppressants are typically composed of nonionic surfactants, carboxylates, and other surfactants. The mechanism of mist suppression is that surfactant molecules align directionally on the solution surface, forming a monomolecular film that reduces vapor pressure, thereby inhibiting acid mist generation. Common inhibitors include sodium dodecyl sulfate, OP emulsifier, and urea. Sodium dodecyl sulfate and OP emulsifier are suitable for gas suppression in heated sulfuric acid washing and room-temperature hydrochloric acid dezincing and decadmiation processes; urea is mainly used to suppress nitrogen oxides (such as NO and NO₂) generated during nitric acid pickling.

 Based on their functions, surfactants in pickling are often referred to as corrosion inhibitors, mist suppressants, or corrosion and mist inhibitors. Currently, there are various types of acid pickling inhibitors developed domestically and internationally, including specialized types for hydrochloric acid and sulfuric acid, as well as universal types. These products are usually compounded from surfactants such as thiourea and its derivatives, alkylphenol polyoxyethylene ether, and imidazoline. They include components that can accelerate the removal of oxide scale and dirt, form a thin protective film on the metal surface to prevent excessive corrosion and inhibit hydrogen atom penetration into the substrate, utilize the directional arrangement of hydrophobic and hydrophilic groups of surfactant molecules to form surface films and double-layer liquid films, and utilize foaming action to suppress acid mist volatilization. For example, a certain type of compounded corrosion inhibitor is a khaki liquid, soluble in acid and water, with an aromatic odor and low toxicity. It is suitable for pickling various materials such as steel, copper, and aluminum with hydrochloric acid, sulfuric acid, hydrofluoric acid, and organic acids. It has significant corrosion and mist inhibition effects, can prevent hydrogen embrittlement, improve material mechanical strength, and make the metal surface smooth and even.

 (3) Other Additives

 ① Corrosion inhibitors: To reduce the corrosion of metal by pickling solutions, inorganic or organic compounds containing O, N, S elements are often added as corrosion inhibitors, such as urotropine, aldehyde-amine condensates, thiourea and its derivatives, and pyridine derivatives. Urotropine is commonly used in hydrochloric acid, while Rodine (a mixture containing phosphorus dimethylthiourea, NaCl, dextrin, and saponin powder), thiourea, and pyridine derivatives are commonly used in sulfuric acid.

② Wetting agents: Wetting agents can repel gases adsorbed on solid surfaces and accelerate the contact between liquid and solid surfaces. They mainly include nonionic surfactants and sulfonate anionic surfactants, such as Pingpingjia, OP emulsifier, Tween80, etc.

 ③ Defoamers: Mainly include silicone oil, Span nonionic surfactants, and low molecular alcohols.

 ④ Thickeners: Used to increase the viscosity of the pickling medium and prolong its action time on the cleaning surface.

 Alkaline Metal Cleaners

 In metal cleaning, alkaline cleaners are more widely used than acidic cleaners and can be used alone or alternately/mixed with other cleaners. Alkaline cleaners are mainly used to remove grease and oil dirt, and can also remove inorganic salts, metal oxides, organic coatings, and protein dirt.  Alkaline cleaners have strong removal capability for animal and vegetable oils but are less effective for mineral oil dirt. To improve cleaning ability, a small amount of surfactant is often added. Compared with acidic cleaners, alkaline cleaners are more expensive and slower in derusting and descaling, but except for amphoteric metals (such as Al, Zn, Sn), they do not cause severe corrosion or significant dimensional changes of workpieces.

Cleaning Action of Alkaline Metal Cleaners

 (1) Cleaning of animal and vegetable oil dirt: Long before the advent of soap, humans used alkalis for degreasing, such as using plant ash (mainly containing alkaline substances like potassium carbonate) for washing clothes. Alkaline cleaners are highly effective for oil and grease dirt. On one hand, they convert oils into water-soluble fatty acid soaps through saponification reactions; on the other hand, animal and vegetable oils contain about 30% free fatty acids, which can undergo neutralization reactions with alkalis to generate fatty acid soaps. The latter act as surfactants to promote emulsification and dispersion of oil dirt.

Animal and vegetable oils are mainly composed of high-carbon fatty acid triglycerides, containing about 30% free fatty acids. The reaction with alkali is as follows:

 (2) Cleaning of mineral oil dirt: Although mineral oil dirt and animal/vegetable oils are both referred to as oily dirt, their structures are completely different. Mineral oil typically has a saturated chain hydrocarbon structure, belonging to non-polar substances. Their molecules do not contain polar groups such as carboxyl or sulfonic acid groups, so they can form micelles. These micelles have certain emulsifying, dispersing, and adsorbing effects on oily dirt. Therefore, adding alkaline substances such as silicates and polyphosphates to alkaline cleaning solutions can promote the removal of mineral oil dirt.

 (3) Cleaning of inorganic salt scale: For some inorganic salt scales that are difficult to dissolve in acid and water (such as calcium sulfate, calcium silicate, etc.), after reacting with certain alkaline cleaners, they can be converted into loose new salts that are easily soluble in acid, facilitating further removal by acid pickling. The chemical reaction equations are as follows:

 CaSO₄ + Na₂CO₃ = CaCO₃↓ + Na₂SO₄

 CaSiO₃ + Na₂CO₃ = CaCO₃↓ + Na₂SiO₃

(4) Cleaning of metal rust scale: Alkaline cleaners containing metal ion chelating agents can dissolve metal oxide scale through chelation. Additionally, strong alkaline cleaning solutions can react with amphoteric metal oxides or hydroxides (such as aluminum oxide, zinc oxide, zinc hydroxide, aluminum hydroxide, etc.) to dissolve them, for example:

 ZnO + 2NaOH = Na₂ZnO₂ + H₂O

 Zn(OH)₂ + 2NaOH = Na₂ZnO₂ + H₂O

(5) Cleaning of organic coatings: Strong alkalis can cause the organic coating to be removed to swell and soften, making it easier to remove.

 (6) Cleaning of protein dirt: Alkaline cleaners also have some removal effect on protein dirt. On one hand, dilute alkaline aqueous solutions have strong dissolving and dispersing effects on proteins, as some proteins are soluble in water. Amino acid molecules contain carboxyl and amino groups. When the numbers of these groups are unequal, the protein molecule has excess acidic or basic groups, enabling it to react with acids and alkalis. After alkali reacts with carboxyl groups, the protein molecule becomes negatively charged, repelling each other, reducing intermolecular forces, and increasing water solubility. On the other hand, under heating conditions, strong alkaline cleaners can promote the hydrolysis of protein macromolecules, generating small water-soluble amino acid molecules and dissolving. For example, insoluble proteins (such as hair) can dissolve in a 5% sodium hydroxide aqueous solution. It should be noted that metal surfaces have an affinity for alkalis and easily form an alkaline coating that is hard to rinse off completely. If necessary, objects can be immersed in a dilute acid solution to neutralize the alkali, then rinsed with water.

 Composition of Alkaline Metal Cleaners

 Alkaline cleaning solutions are mainly composed of alkaline substances and often use a combination of multiple alkalis. Commonly used alkaline compounds include sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium silicate, and various phosphates. Sodium hydroxide is a strong alkali that can neutralize acidic dirt, saponify animal and vegetable oils, and convert insoluble salts for subsequent acid pickling, for example:

 MgSO₄ + 2NaOH = Mg(OH)₂↓ + Na₂SO₄

 CaSO₄ + 2NaOH = Ca(OH)₂↓ + Na₂SO₄

However, sodium hydroxide is highly alkaline and corrosive to humans and metals, so caution is required during use. Sodium carbonate is the salt of a strong base and weak acid, hydrolyzing to exhibit alkalinity, but weaker than sodium hydroxide. Sodium bicarbonate is the acid salt of sodium carbonate and has even weaker alkalinity. They can loosen, disperse, emulsify, and saponify oils, exerting cleaning effects. Sodium silicate hydrolyzes to alkalinity close to sodium hydroxide. Not only can it saponify oils, but the silicic acid colloid produced by hydrolysis also has dispersing and stabilizing effects on dirt. The simplest phosphate is trisodium phosphate, and its acid salts include disodium hydrogen phosphate and sodium dihydrogen phosphate. Additionally, sodium tripolyphosphate and sodium pyrophosphate are commonly used. Their aqueous solutions are alkaline and have certain cleaning capabilities. Moreover, phosphates have significant dispersing effects, dispersing large dirt particles and chelating metal ions.

 In addition to alkaline substances, alkaline cleaners often contain a small amount of surfactants to reduce surface tension, emulsify and disperse dirt, significantly improving cleaning efficiency. Furthermore, a large amount of inorganic salt components can reduce the critical micelle concentration (cmc) of surfactants, enhancing their surface activity. Nonionic surfactants (such as alkylphenol polyoxyethylene ether, polyethers, etc.) have good wetting and penetrating abilities. Adding a small amount can improve the wetting and penetrating ability of alkaline components on oil dirt, scale, and rust.

 Alkaline cleaners also often contain small amounts of chelating agents, organic solvents, etc. Their formulations vary depending on the type of material to be cleaned, the composition of the dirt, cleaning quality and speed requirements, and cleaning conditions.  Alkaline cleaning agents often also contain small amounts of chelating agents, organic solvents, etc. Their formulations vary depending on the type of cleaning material, the composition of the dirt, the requirements for cleaning quality and speed, and the cleaning conditions.