Phosphoric Acid Metal Prep: Safe and Efficient? Or Not?

If you have ever had metal products that are rusted, then you know how frustrating and difficult it can be trying to clean them up. Also, you know how difficult it can be to remove and prevent rust from recurring.

Usually, rust spots and rust stains can be extremely disheartening, especially when you don’t know what products are safe and efficient to use. But phosphoric acid is one of the popular products you can use to remove rust from metal surfaces. The acid is an odorless, non-volatile, and inorganic compound.

The acid is a solution of phosphoric acid and water, usually sold in a concentration of 85 percent. You can use it to remove rust from household metallic appliances and items around the house or, commercially, as a metal cleaning agent.

Mainly, phosphoric acid is a fantastic rust remover and leaves your rusted metal shining as if it’s new again. But is it safe and efficient? In this post, we will discuss in detail the use of phosphoric acid in metal prep, its safety, and its efficiency.

What is Phosphoric Acid Metal Prep?

Phosphoric acid metal prep is common practice in industries that entails the use of phosphoric acid to clean metal objects. Usually, metals are prone to rusting; hence treating the metals to remove the rust is crucial. Rust is a red oxide substance formed when oxygen, water, and iron react.

To remove the rust, diluted phosphoric acid is applied over the rusted metal surface, or the object is immersed in a diluted phosphoric acid solution. The acid removes the loose mass of rust and reacts with the oxides to create a thin protective layer.

While the rusted metal surface is reddish brown, after cleaning the metal object using phosphoric acid, a grayish layer is formed over the metal. The new coating of oxide phosphate is inert and hence prevents further rusting. Notably, a paint coating can adhere to the new surface.

Is Phosphoric Acid Safe on Metals?

Phosphoric acid, like other acids, is corrosive. However, phosphoric acid is a weak acid that does not cause significant damage to most types of metals.

But the acid is not suitable for use on all metals. While it is safe for use in cleaning stainless steel, you should not use it on some surfaces such as painted steel surface, aluminum, or on chrome surfaces. Notably, although there are no significant damages to the metals, the acid can be dangerous to the people handling it.

Therefore, to ensure safety, you should use the acid according to the manufacturer’s instructions. If used based on the manufacturer’s instructions and diluted to the correct proportion of 20%(200ml/liter) before application, the acid will be safe and efficient for cleaning metal surfaces.

Importantly, unlike other acids that are corrosive and cause further rusting, phosphoric acid prevents further rusting. When you use phosphoric acid on metal, a chemical reaction begins as the acid dissolves the upper layer of rust and oxidizes loose material on the surface to form an inert layer of protective metal phosphate.

The new layer acts as a tough and durable undercoat. Therefore, phosphoric acid, when used correctly, is safe on metals and protects them metals from rusting.

Is Phosphoric Acid Safe to Handle and Use

Although when phosphoric acid is used correctly, it is safe on metals, there are health risks still associated with exposure to the acid. For instance, the acid can cause burns on the skin or severe eye irritation when it comes into contact with the skin or eyes. Also, the fumes from the acid can cause respiratory problems.

In most instances, phosphoric acid fumes inhaled irritates the nose, causing coughing and wheezing. Therefore, phosphoric acid is not safe and should be handled carefully, with the proper safety and preventive measures in place.

Safety Measures

Since phosphoric acid is not safe and is still a popularly used and suitable metal surface cleaning agent, it is essential to take the necessary precautions when handling it. You must always take proper safety precautions when using this chemical.

Mainly, not following the manufacturer’s safety guidelines increases the risks to your health, especially if it gets into contact with your skin or eyes. The following are some of the safety measures you should observe:

1) Wear Rubber Gloves

If the chemical gets on your hands or under your fingernails, it could enter your body when ingested and cause some severe health problems later on. Therefore you should not handle the acid with your bare hands. Always wear clean rubber gloves when handling the acid to avoid burns or acid ingestion.

2) Wear Protective Clothing

When working with phosphoric acid, wear protective clothing to ensure the acid does not get into contact with your skin or with clothes that are in direct contact with your skin.

3) Wear Goggles

One of the most important precautions for using phosphoric acid metal prep is protecting your eyes. Mainly, if the acid gets into your eyes, it can cause eye irritation. Therefore, to ensure safety, you should wear goggles when handling the acid to protect your eyes.

4) Use the Acid in a Well-ventilated Area

This ensures you don’t inhale high quantities of hazardous acidic fumes. Inhaling the acid fumes can cause coughing, irritation in the nose, and wheezing; hence ensure you use the acid always in a well-ventilated room.

Other safety precautions include:

  • Put up a warning sign. This will help the visitors take necessary precautions when around the phosphoric acid metal prep area.
  • Wash your hands after a phosphoric acid metal prep process. Although the phosphoric acid may not get into contact with your hand directly because you have gloves on when handling it, it is essential to wash your hands after use.
  • If you experience skin irritation, clean the area with clean water.Sometimes you may experience irritation on your skin, especially if you are not adequately protected. To stop the itch from occurring, wash up thoroughly after using phosphoric acid.
  • Do not touch anything with bare hands when using phosphorus acid. Try not to touch anything with bare hands until you are through the acid metal prep process and have washed your hands.
  • Contact a health professional in case of accidental ingestion of the acid.In case you ingest the acid, don’t induce vomiting, instead inform a medical doctor immediately. Also, in case you are experiencing persistent irritation even after washing an affected area, contact a health professional for advice.

Are There Other Ways to Clean Metal Surfaces?

Phosphoric is an efficient rust remover product, especially on large surfaces. The acid reacts with the rust creating a protective layer that prevents further rusting. However, it may not always be possible to use phosphoric acid to clean your metal objects.

Are there alternatives? Yes, there are other ways and products you can use to remove rust before painting. For instance, instead of buying phosphoric acid to remove rust over a slightly rusted area, you can use baking soda, which is most readily available in your kitchen.

Other solutions you can use to remove rust include white vinegar and salt. These products come in handy for minor rust removal jobs at home.

Therefore, while phosphoric acid is the commonly used rust-removing product for commercial metal cleaning projects, you may explore alternative options for DIY rust removal tasks at home.

Benefits of Using Phosphoric Acid as a Cleaning Agent

Compared to other cleaning agents, phosphoric acid is one of the top rust removers. The following are the benefits of using phosphoric acid as a metal cleaning and rust-removing agent.

  • The acid removes the acid within a short time. Therefore, if you are looking for the fastest way to remove rust, phosphoric acid should be your top choice.
  • It’s a suitable metal cleaning agent for commercial projects. The acid is ideal for commercial metal cleaning projects. The acid is quite effective even when used over large metal areas.
  • Various ways of application. There are multiple ways to use acid to clean the metal surface. The methods include immersing the metal into the acid solution, spraying the acid solution, or applying the acid using a sponge. You can explore and choose the most suitable method based on the condition of the rusted area.
  • It reacts with the rust and produces a protective coating.The protective coating formed prevents further rusting. The phosphoric acid metal prep provides a lasting solution to the rusting menace.
  • Does not damage the metal. Unlike other acids that remove rust and then react with the iron and oxidize it further, phosphoric acid removes rust without damaging the metal underneath. It instead protects the metal from further damage.


The whole point of the phosphoric acid metal prep is to give the metal an excellent rust-free look before you apply another coating, such as paint. Overall, phosphoric acid is the safest metal cleaning agent that only requires proper handling, including wearing protective gear and working in a well-ventilated area.

If you are careful and use it according to the manufacturer’s instructions, phosphoric acid is usually safe and efficient.