The role of acids in cleaning formulations

When we talk about "Heavy-Duty Cleaning," the resources available at the time of developing a formulation capable of combining fast and efficient action are scarce. It is challenging to find ingredients with the same efficiency as highly reactive ones, such as alkalis/hydroxides like Sodium Hydroxide, also known as caustic soda, or strong acids like Hydrochloric Acid, also referred to as Muriatic Acid.

Their acidic or alkaline nature distinguishes them in relation to their function during cleaning, but here we will focus on strongly acidic products. To better understand how strongly alkaline/caustic products work, click here. You can also understand the function of a strongly alkaline or strongly acidic component by clicking here to read the article about the influence of pH on cleaning.

There are times when it is difficult to avoid using acid for efficient, quick, and inexpensive cleaning. But what makes these ingredients, mainly hydrochloric acid, nitric acid, and sometimes sulfuric and hydrofluoric acid, so effective?

Well, they are highly reactive and electrolytic compounds, and this gives them the following characteristics and behaviors regarding the cleaning process:

1. Corrosivity:

Its reaction with metallic ions causes a pickling process, that is, the removal of a film from the metal. This gives the impression that the product has "renewed" the surface, when in reality it has corroded it. The secret here, for an efficient pickling process, is to make the acid promote a corrosion process that is as uniform and controlled as possible. To achieve this, it is essential to combine the acid with surfactants and dispersing agents, such as Sodium Polyacrylate or some other acids that have this capacity, such as Phosphoric Acid and Citric Acid.

It is also important to work with an acid concentration appropriate to the amount of dirt or oxidation present on the metal surface, because the more concentrated the acid solution, the less controlled the corrosion reaction and the greater the chance of stains appearing. A classic example of this type of cleaning would be the use of hydrofluoric acid to clean the aluminum cargo boxes of trucks.

2. Solubilization of minerals:

Minerals, such as sand and limestone, have alkaline characteristics and therefore react with acidic ingredients. When this reaction occurs, salts are formed. Depending on the acid used in this reaction, the resulting salts will be more or less soluble. Salts formed from nitric, sulfuric, hydrofluoric, and hydrochloric acids, for example, are among the most soluble salts that exist. This is one of the reasons why these acids are so effective in some specific cleaning processes. A good example is the use of hydrochloric acid in cleaning rough floors, such as stone cleaners, as it dissolves all the mineral dirt present, greatly facilitating rinsing.

3. Degradation of organic materials:

In this specific case, sulfuric acid is the acid that has this distinctive characteristic. It reacts strongly with organic materials, degrading them into carbon dioxide and water in a powerful reaction that releases heat. This literally "carbonizes" the organic dirt. However, this behavior occurs more effectively when the acid is applied in its most concentrated form, as the reaction is slow when it is too diluted. For this reason, it is used in particular situations in professional cleaning.

4. Dispersion of solids:

Other acids with lower reactivity are used, as mentioned above, as dispersing agents for solids, such as phosphoric acid and citric acid, the most commonly applied due to their cost-effectiveness. These acids interact through bonds with water and their own molecules, forming a kind of "network" that holds the solid particles in suspension, preventing them from redepositing on the surface, thus facilitating rinsing.

In a pickling process, for example, microparticles are formed that can re-deposit in the surface pores, hindering the process. Although these acids are widely used for this purpose, some other dispersants are more efficient, among them, and most notably, Sodium Polyacrylate, which is very stable in strongly acidic environments.

To bring this to a more practical level, let's discuss some situations where the presence of these acids is quite frequent:

Car Cleaning

Options for a less aggressive and more environmentally friendly formulation indeed exist. However, these options still have a very high cost compared to acidic formulations and, therefore, end up being applied only in certain niches of this market.

In this type of cleaning, the most commonly found acids are hydrochloric, hydrofluoric, and sulfuric. It is difficult to find a compound as good for certain types of cleaning, specific vehicle parts, as these acids, such as cleaning wheels, tanks, and raw aluminum truck bodies. However, it is important to dispel some misconceptions about these applications.

Cleaning Aluminum Truck Bodies

Although widely used for cleaning aluminum parts in trucks, hydrochloric acid and sulfuric acid are not the most suitable options. These two acids cause a corrosion process that leads to darkening of the aluminum, which is undesirable. Furthermore, hydrochloric acid, if not properly rinsed, can tend to promote rust due to its chloride ion, reacting with the iron contained in the aluminum.

Watch the video here. This video shows Noxipon THZ, a thickening agent for cleaning truck bodies developed by Macler. We demonstrate the aluminum darkening process in this product.

To date, we have not found a more efficient compound for cleaning and pickling aluminum than hydrofluoric acid, which, strictly speaking, could be the only acid suitable for use in a formulation for cleaning truck bodies. However, both hydrofluoric acid and sulfuric and nitric acids have faced serious restrictions from ANVISA (Brazilian Health Regulatory Agency). Therefore, new formula structures have been applied for this type of cleaning, although they do not offer the same performance. Macler has been dedicated to researching alternatives for these cases, and some discoveries have already been made through customer service via SmartLab, its technical support dedicated to active clients.

Windshield Revitalizer

With the growth of the automotive market, a range of products and specific niches has emerged every day. One of the products that caters to the aesthetics niche is the "windshield revitalizer." This product aims to renew stains caused by encrustations and the action of acid rain, which are more visible in the area of the windshield where the wiper does not reach.

In this case, the formulation usually includes hydrofluoric acid as a pickling agent, which removes not only encrustations but also pickles the glass. Hydrofluoric acid is the acid with the greatest capacity to react with and dissolve glass. If used in the correct concentration with a balanced formulation, it promotes very uniform pickling that restores the translucent appearance of the glass.

It is important to point out that, despite being very efficient and having a low cost, acidic products are still widely used in contraindicated locations where the cleaning system would require greater caution, such as the mechanical parts of automobiles. These areas contain many aluminum or metal parts that suffer strong attack and corrosion, such as electronic components, small parts, and hose clamps that break easily due to the use of these products.

The choice of surfactants for acid cleaning in the automotive field must take into account that many of the surfaces to be cleaned are vertical. For this reason, the product should promote high foaming, so that this foam runs off slowly, increasing the contact time with the surface and allowing the acid more time to act.

In this sense, Sulfonic Acid is, among the options, the most widely used molecule, due to its high foaming capacity, good cost-benefit ratio, and high stability. Sodium Lauryl Ether Sulfate, Sodium Lauryl Sulfate, Betaine, and Amide, while exhibiting good foam formation, are not stable in acidic environments and are therefore contraindicated. Amine Oxides, despite their good foam formation and stability, protonate (acquire a positive electrical charge) in acidic environments and, therefore, tend to adhere to surfaces, hindering rinsing.

As an alternative to further increase the foaming power of Sulfonic Acid and, in more concentrated formulations, improve its solubility by increasing the stability of the formulation, Macler presents Prime AO40 as an additive to be used in combination with Sulfonic Acid. In some cases, it helps to improve not only stability but also viscosity.

Recognizing the limitations and difficulties in finding information about truck bed cleaner formulations, Macler, through SmartLab, has developed two formulation options with high foaming, good viscosity, and high performance. To access and benefit from this knowledge, simply click on the banners below:

Cleaning of floors and rough stones

Just as with cleaning metals, when cleaning a rough stone floor, acids promote a stripping effect by dissolving a very thin layer of the mineral.

The most reactive acid is hydrofluoric acid, which has a very strong pickling capacity. However, and because of this, this acid is the main cause of stains due to its excessive corrosiveness.

Sulfuric acid would be a good option to help remove organic materials adhered to the floor. However, hydrofluoric, sulfuric, and nitric acids are restricted by ANVISA (Brazilian Health Regulatory Agency) to specific professional applications and cannot be used in products for domestic and institutional use.

Considering this scenario, and if we want to move towards acid cleaning, we direct our attention to the use of hydrochloric acid, which still has good pickling capacity and, if used properly, also promotes a lower risk of stains compared to hydrofluoric acid.

It is important to emphasize that cleaning with strong acids is not safe for floors such as tiles and porcelain, presenting a risk of staining due to corrosion, especially if hydrofluoric acid is used. Another point of attention is that acid cleaning, regardless of the acid used, will always cause damage to the grout. The difference between acids is that the stronger the acid, the greater the damage to the grout and the sooner the floor will need to be regrouted.

Once again, the performance and uniformity of pickling will depend not only on the appropriate concentration of acid used, which should not be too high, but also on the combination of hydrochloric acid with suitable surfactants and dispersants. Therefore, Macler, through SmartLab, has combined its expertise on the subject and made available a suggested concentrated formulation with excellent performance. To access this material, click on the banner below:

Cleaning toilets and urinals

The logic behind acid cleaning of toilets and urinals is very similar to that of floor cleaners. However, the goal is to remove the calcium, magnesium, and iron deposits that accumulate on the toilet bowl walls and drain outlets. Acids, as explained earlier, can dissolve these minerals and are therefore very effective in this type of cleaning. In this case, hydrochloric acid is the most commonly used acid for this type of cleaning due to its cost-effectiveness in professional products and the ANVISA restrictions on the use of other strong acids, as previously mentioned.

The challenge in this type of formulation is to incorporate a high acid content, as it needs to act quickly, along with high viscosity, so that the product does not instantly run down the walls of the vessel, preventing adequate reaction time.

The best solution for achieving good formulations with these characteristics is the use of specific surfactants, in this case Ethoxylated Fatty Amine, more specifically Tallow or Coconut Fatty Amine with 2 moles of ethoxylation. However, these amines tend to become cloudy and generate phase separation at cold temperatures.

It is also very common to find bactericidal claims in this type of formulation. However, this effect is provided by the acid itself. The use of quaternary ammonium compounds is not suitable, as it breaks down the viscosity of the formulation, and those in acidic pH lose most of their bactericidal properties. In other words, it becomes an added cost that brings no benefit.

Recognizing this challenge faced by formulators, Macler developed an amine blend capable of stabilizing the formulation even at very low temperatures. Based on the study and development of this blend, called Isogen TH+, Macler developed a sample formulation that can be accessed via the banner below.

Industrial Cleaning / Surface Treatment

Another area where acid cleaning is widely used is in the treatment of sheet metal. In the treatment of steel coils for sheet metal production, as well as steel or cast iron parts, pickling is necessary to remove all oxidation/rust. Generally, a hydrochloric acid bath is used for this purpose, offering excellent performance in dissolving iron oxides and an unbeatable cost-benefit ratio. It can be enhanced with additives to improve its performance, such as dispersing agents like sodium polyacrylate and surfactants to improve penetration into the pores of oxidation.

Some classes of surfactants, such as Ethomeen T25 and the Etoquad T25 Products manufactured by the Dutch company Nouryon and distributed by Macler in Brazil, which belong to the class of tertiary and quaternary ethoxylated fatty amines, can help as corrosion inhibitors, protecting the raw metal from acid attack, since some metal alloys may exhibit a weakening of their mechanical resistance due to the action of the acid. Therefore, it is very important that, after the acid bath, an alkaline bath is performed to neutralize the acid, and, for parts that will be destined for storage or transport, some protective agent should be applied, such as oils or even polymers that form a film on the part.

Another very common method used to protect metal parts from rusting is the phosphating process. This process is generally done by applying a formula based on phosphoric acid and a surfactant. This process generates a layer of phosphate salt around the part, which protects it against oxidation for a certain period of time. Phosphating is not a permanent protection, so it is usually used as preparation for a subsequent step in the process, such as painting. If you are looking for a phosphating formulation, click on the banner below, which will direct you to material developed by Macler in its SmartLab.

Still on the subject of metal surface treatment, another frequently discussed topic is the cleaning and passivation of stainless steel. Stainless steel contains chromium and nickel, and its characteristic of exhibiting stainless action comes mainly from the chromium. When exposed to air, chromium reacts with oxygen, thus forming a layer of chromium oxide that protects the metal from corrosive agents.

Therefore, the cleaning and restoration process of stainless steel surfaces can be done chemically using acidic formulations. In these cases, the use of hydrochloric acid is not recommended, as it causes stains on stainless steel and rust on some alloys containing iron. The most suitable acid in this case is nitric acid. This acid can remove oxidation and expose the metallic chromium, allowing it to react again with atmospheric oxygen and passivate it.

Although nitric acid is very efficient in these cases, we know that combining it with a little hydrofluoric acid to accelerate its reaction and a little sulfuric acid to act on the carbon that is very commonly present in various alloys, helps to bring greater performance to the process. Here, again, the use of a surfactant is interesting to help in the uniformity of the acid's action on the surface. In this case, the use of non-ionic surfactants is recommended.

This same synergy between these acids is used in the formulation of Pickling Gel, a product used to restore the metallic appearance of stainless steel weld beads immediately after the welding process. The major challenge in developing this type of formulation is achieving high viscosity and gel formation. Because the acid concentration in these formulations is very high, as it needs to act very quickly, very few thickeners exhibit stability in this medium.

According to a study conducted by Macler, the thickener that showed the best stability in this case was Polyquaternium 37. Some Xanthan Gums showed good stability, however, they cannot be subjected to temperatures above 35°C and do not have a rheology suitable for the application.

Macler has developed a great combination of these acids, which can be explored through the formulation suggestions available by clicking on the banners below.

CIP Cleaning / Boilers

One last important topic regarding acid cleaning is pipeline cleaning through the CIP process. This is an automated industrial cleaning and sanitizing process that does not require the disassembly of equipment to remove residues, odors, and microorganisms from production equipment, especially in sectors such as food, beverages, and dairy. The system works by circulating water and cleaning products (acid and alkaline detergents and disinfectants) through pipes, sprayers, and other components, using appropriate pressure, temperature, and time for effective cleaning without the need for manual intervention.

In the case of acid cleaning, its purpose is to dissolve mineral deposits and encrustations, most commonly deposits rich in calcium, which are very present in the dairy industry, for example.

In the food and beverage industry, we encounter stainless steel equipment and piping. Therefore, nitric acid is the most suitable cleaning method for the reasons mentioned above when discussing stainless steel cleaning. The use of hydrochloric acid on carbon steel and cast iron piping is quite common, although it is a much more corrosive acid and therefore often requires the use of corrosion inhibitors to prevent it from corroding the raw metal and damaging it. In these cases, once again, Ethomeen T25 and Ethoquad T25, from the class of tertiary or quaternary ethoxylated fatty amines of tallow or coconut, are indicated for this type of protection. However, they may produce foam.

In this process, it is important to understand that the equipment used, especially the pumps, can be damaged if foam forms. Therefore, as mentioned several times, the use of surfactants is desirable, both for corrosion inhibition and to increase performance. However, in this case, a class of surfactants that does not promote foaming should be used, or an antifoaming agent should be added.

There are some types of surfactants that exhibit low foaming. The best known are Ethoxylated/Propoxylated Fatty Alcohols, but some short-chain fatty alcohols with low HLB can also exhibit low foaming. What matters most in this case is not the detergency power of the surfactant, but rather its ability to reduce surface tension, since it will not act in cleaning grease and oils through its emulsifying capacity, but rather it will help the acid penetrate deeper into the pores of the encrustations, so that the action of the acid becomes faster and more effective.

However, we encountered some difficulties when searching for this type of surfactant. Besides the low availability of this type of product in Brazil and its high cost, they also tend to be products with low solubility, meaning they don't dissolve well in water and end up requiring the use of hydrotropes to become soluble. Non-foaming hydrotropes are usually expensive and require relatively high concentrations to be effective. In this sense, and seeking a low-foaming solution that didn't require the use of hydrotropes and offered a good cost-benefit ratio, Macler developed the product Isogen SE32. A surfactant formula focused on applications requiring low foaming and easy application. The product can be used in extremely acidic or extremely alkaline environments, exhibiting excellent stability. In CIP cleaning, Isogen SE 32 can be used as a process additive, improving performance and aiming to reduce the time required for effective cleaning.

Another factor that contributes to CIP cleaning is the use of dispersing agents, such as the aforementioned Phosphoric Acid, Citric Acid, and, the highest-performing one, Sodium Polyacrylate.

It is very difficult to cover all situations related to acid formulations in a single text. Therefore, we have tried to gather here the main points related to this topic. But Macler's SmartLab has a team of professionals ready to assist its active clients in case of questions and specific development needs. Get in touch!