Methods of using phosphoric acid against rust

A common “disease” of metal (steel and steel alloys) is rust. It is formed under the influence of oxygen, carbon dioxide and water. Scientifically speaking, this is a chemical reaction that results in the formation of iron hydroxides and oxides. When rust first appears, it is necessary to clean the surface of the metal product and protect it, since, unlike the patina on bronze, rust on steel does not create a protective film.

Oxides or hydroxides (oxides) can be purified in several ways:

• mechanical;
• chemical;
• using sandblasting equipment.

Phosphoric acid is suitable for chemical purification of oxides, although mechanical and chemical treatment is most often combined.

What is phosphoric acid?

Orthophosphoric (phosphoric) acid is a water-soluble product of inorganic origin, produced in the form of an 85% aqueous solution of a syrupy, colorless consistency. It is used in various industries, including food. It is one of the main components in the production of phosphate chemical fertilizers and is also used in dentistry.

In the production of household chemicals, this acid is also used quite actively - it is present in almost all industrial products for combating rust. But metal primers are produced based on it, and the rust converter contains it as the main ingredient.

Precautionary measures

If you need to remove rust chemically, you first need to take care of your safety - prepare a respirator and rubber gloves. Phosphoric acid is an aggressive substance that can cause skin burns, and its fumes can cause burns to the respiratory tract and acute poisoning. In addition, it is fire and explosive.
All work must be carried out in well-ventilated areas, and the substance must not come into contact with the skin. If this happens, you need to rinse the affected area under running water. In case of a chemical burn over a large area, you must immediately contact a medical facility.

Rust removal

The advantage of using orthophosphoric acid for chemical cleaning of rust is that it not only removes the loose mass of oxides, but also creates a thin protective film. The mechanism for creating such protection is that the acid, corroding and absorbing iron oxide, phosphates the surface. Those who worked with this substance could observe that after processing the metal and drying, a grayish film, oily to the touch, forms on its surface, in place of the red coating.

Depending on the degree of corrosion and the size of the part or product being cleaned, you can choose different methods for removing oxides:

• etching of the part with complete immersion in the solution;
• single or multiple acid treatment of the surface with a roller or spray;
• application to metal with preliminary mechanical cleaning.

Full immersion etching of parts

If there is a sufficient amount of phosphoric acid and the required container, the easiest way to remove it is by complete immersion. To do this you need to do the following:

• degrease the part with any detergent and wash it;
• fill the container with a solution with the ratio: 100-150 g of 85% acid per 1 liter of water;
• immerse the part in the container and leave for 60 minutes, stirring the solution from time to time;
• remove and rinse;
• prepare a neutralizing solution in the following proportions: 50% water, 48% alcohol, 2% ammonia;
• rinse the part with the prepared solution, and then with clean water and dry immediately.

You cannot skip any of the above steps, as they are all interconnected. For example, if treatment with a degreaser is not carried out, then the etching will be uneven, since this acid does not corrode organic contaminants, and problem areas will have to be additionally cleaned. This method is suitable for parts with any degree of corrosion, however, the larger the layer of rust, the more time it takes to clean.

Advice

If the surface is not immediately dried after the final rinse, hydroxide will immediately form on it. Drying can be done by any method, including convection.

Applying acid to the surface

If the part is large, there is no suitable container or there is not enough acid, you can apply it to the metal surface using a sprayer, roller or brush with natural bristles. In this case, it is necessary to take into account the degree of corrosion.If the rust layer is large, you will have to use a combined method and first remove the surface layer of the loose mass mechanically. This can be done manually or using a grinder, on which an attachment is put on - a metal brush or a flap circle.

After mechanical cleaning, you need to degrease and apply an aqueous acid solution, being careful not to make any gaps. Two hours after application, you can rinse with a neutralizing solution, and then perform a final rinse and drying. For minor corrosion, mechanical repairs may not be necessary, although reapplication may be required.

Advice

An inhibitor, catapine, can be added to the acid solution, which inhibits the chemical process and prevents the reaction with the unoxidized metal. It is added in an amount of 1-2 g per liter of water.

Removing rust from the surface of bathtubs, toilets and washbasins

Orthophosphoric acid can also be used as household chemicals. Therefore, if you are faced with the question of how to remove rust from a toilet bowl, try phosphoric acid. It does an excellent job of cleaning traces of rusty water in toilets and enamel bathtubs. This product is not suitable only for acrylic bathtubs.

Application for earthenware and enamel surfaces:

• 100 g of 85% acid should be added to 500 ml of water;
• degrease the surface with any detergent;
• Use a brush with natural bristles to treat the contaminated surface;
• after a few hours (from 1 to 12 - depends on the accumulation of oxides), wash off the acid with a solution of soda - 1 tbsp. spoon per liter of water.

The advantages of this method of cleaning rust are that you don’t have to rub anything, so the enamel is not damaged.Information for housewives who use Coca-Cola to clean red plaque: it is phosphoric acid, present in small quantities in this drink, that gives this result. It is much more effective to use the active substance in the required proportion, and use drinks as intended.

Advice

When working with acid, do not forget about safety rules. Do not allow the chemical to come into contact with your skin, and if it does, wash immediately with warm water and soap.

Rust converter

A rust converter (rust modifier) ​​is a solution of the same orthophosphoric acid, but with special additives. Depending on the additives, these drugs are divided into several groups:

• primers,
• modifiers-stabilizers,
• rust converters.

An example of the first group is the EVA-0112 primer, which consists of two components - a base and 85% acid. It is used as a base for paint for steel products. The Tsinkar converter also contains orthophosphoric acid with the addition of zinc and manganese salts. Thanks to these additives, a hardened protective layer is obtained from the converted rust (metal alloying effect). Before using the converter, you must read the instructions, and the composition can only be used in accordance with the manufacturer’s recommendations.

Which is better to choose?

When choosing a rust remover, you need to focus on the place where it will be used. Full immersion cleaning of parts requires a large volume of acid. Here, the purchase of orthophosphoric acid of 85% concentration will be justified from both a practical and economic point of view.If you need to not only remove rust, but also create a protective layer under the paintwork, then a self-made solution will not work. In this case, it is better to buy a factory converter with all the necessary additives.

You also need to consider whether a layer of primer will be applied or not. Converter-modifiers increase the water-repellent and inhibitory properties of the primer, but are not primers themselves. But after treatment with the converter-primer, you can immediately coat the metal with paint.

In conclusion, I would like to focus on the fact that phosphoric acid creates a protective layer during the interoperative period. That is, without paint coating, the metal will be susceptible to corrosion.