Hamilton Form, Ltd.
7009 Midway Road
Fort Worth, TX 76118

Phone: 817 590-2111
Fax: 817 595-1110
sales@hamiltonform.com

PCI NPCA CPCI

Frequently Asked Questions

What is rust?

Rust, like everything else in the world, is made up of chemicals, and chemicals are made up of atoms. The formation of rust is actually a chemical reaction. A chemical reaction occurs when atoms join together or break apart to form new combinations of atoms. Two things are necessary for this reaction to occur: oxygen and moisture. When iron combines with oxygen, it forms iron oxide, or rust. Because oxygen and moisture are readily available in the atmosphere, if no preventive measures are taken, steel, which is primarily made up of iron, will rust.

As steel rusts, it puffs up because iron oxide is a larger molecule than iron. The puffing causes cracks and voids that expose more metal to the environment, resulting in more rust. The rate of rusting is usually higher in the first year of atmospheric exposure than in subsequent years, but can increase significantly depending on the degree of pollution and moisture in the air. The real issue is that once rust is formed, there is mobile oxygen in the metal, and the oxygen can move deeper into the metal causing further rust.

Some things cause steel or iron to rust faster than others:

  • Water will cause iron and steel to rust faster. Water is necessary to facilitate transport of the electrons. Iron will rust in air after a long time, since there is moisture in the air, but the presence of water speeds the process.
  • An electrical conductor helps chemical reactions, like rust, occur faster. Salt water accelerates rust faster because salt water is a better electrical conductor than plain water.
  • Similarly, water lower in PH will contribute to more rapid rusting than high PH water. This is important because in areas where acid rain is more prevalent, metals will rust faster than in areas with pure rain water. Pure rain water has a pH of about 5.6 from dissolved carbon dioxide, a normal constituent of air; whereas, acid rain can be as acidic as to have a pH of 3. Acid rain is not only a problem in urban areas because winds can blow polluting gases from industry and car exhaust great distances before they return to the earth as acid rain.
  • Like most chemical reactions, heat speeds the process. Concrete creates heat as it hydrates, making the environment more favorable for rust to develop.
  • Dissimilar metals rust faster than single metals because of electrochemical reactions. Steel rusts faster than iron because steel is made up of iron and other metals. Joints between dissimilar metals can also rust very quickly.

All of this illustrates the importance of having a rigorous program in place to protect steel forms from rust, because we have yet to find a precast plant where steel forms are not exposed to air, moisture and heat.

Rust Protection

Rust inhibitors and protective coatings for steel are available from several industry sources. These coatings form a protective barrier on the surface of the form. Some release agents are formulated with rust inhibitors, but may or may not be concentrated enough to prevent rust. It is best to discuss options and the pros and cons of different formulations with one of these sources. When concrete cures it shrinks. It is not uncommon for condensation to occur between the form and the concrete. This is a prime area for rust to occur if the metal is not protected. Remember, whenever the surface of the form is exposed to air and moisture, it is vulnerable to oxidation. That means exposed steel surfaces need protection.

Why Now?

From time to time, we have heard of steel forms that have been in service for a long period of time, when suddenly rust appears on a section of the form. If this happens in your plant, start a systematic investigation to determine the cause of the rust. Look for changes in the product, material or process that may have triggered the rust. Have you changed the release agent you use? Have you changed mix materials? Have you added steam or heat? Any of these changes can triggered a reaction that can cause rust.

Also check for chlorides, acids and other contaminants. Contaminants may have been introduced from any number of sources: admixtures, contaminated water, chemicals in compressed air used in production, or even airborne particles outside the plant. In testing a section of form where rust suddenly occurred on a form that had been in service for many years, chlorides (salts) were detected on the metal. Salt could have been introduced through aggregates because the quarry cut into a vein of salt. Trucks or railcars used in hauling the aggregates could have been contaminated from a previous load of road salt. Recycled wash water could have contained salt. The mix water could have been treated with a water softener or other treatment. If rust suddenly appears, something probably occurred that introduced a contaminant in the process and the steel started to rust.

Removing Rust

A number of different strategies can be used to clean rust from steel. Below is a list of some options. Before you do any of these, make sure you research the method and proceed with caution to protect your investment.

Sanding

Surface rust can be cleaned from steel. A light brushing with sand paper, a steel brush or steel wool will usually clean surface rust. Be careful when removing rust. Use only soft, fine grain sand paper or use rubber sanding materials. Be especially careful when using power sanders or grinders. Do not use "hard rock" grinding disks which will cut into and damage the steel surface. Sanding and grinding can also create heat that can damage a steel surface.

If you do sand or brush the surface of your form, be aware that this type of cleaning "activates" the metal, making it more prone to rust. Immediately after metal is cleaned, it is essential that some form of rust proofing is applied to protect the metal.

Shot Peening

A technique used to refine or repair steel surfaces that will also remove rust is called shot peening. This process can be used to improve the metal finish and durability of a form surface after installation, before it goes into service. The process uses small beads or "shot" to blast the metal surface. Each piece of shot that strikes the metal acts as a tiny hammer that creates a small dimple in the material. In order for the dimple to be created, the metal must yield in tension. Below the surface, the metal tries to restore the surface to its original shape. Overlapping dimples redistribute the surface creating a highly stressed uniform surface high in compressive strength. Uniform bead blasting helps blend steel seams and imperfections and work-hardens the surface. Shot peening too hard or uneven will damage the surface of the metal. If you are considering shot peening use an experienced, trained professional. Shot peening is not a do-it-yourself job.

Bead Blasting

Bead blasting using materials other than shot can be used to clean and remove rust from older forms. If you decide to bead blast, only use non-abrasive materials. Some precasters have used crushed corn cobs with good results. Do not use a material such as walnut shells that can stain. If your form requires a heavy cleaning, you could try light sand blasting, but be very careful, sand cuts into steel and can easily pit and damage the surface. Any type of blasting will remove some good metal as it cleans and removes rust. After any blasting, the metal is exposed, making it essential to apply some form of rust protection applied immediately afterwards.

Acid Etching

A chemical technique for removing rust is etching with phosphoric acid. Some rust cleaning products contain phosphoric acid. A common product which contains phosphoric acid is the soft drink Coca-Cola. Phosphoric acid has a unique property of dissolving iron oxide quickly while etching iron very slowly. A unique advantage of phosphoric acid is that it leaves a fine coating of iron phosphate behind which helps prevent rust. Additional protection however, is still required.

Any type of acid treatment must be taken with extreme caution. Acid cleans rust; then slowly attacks the bare metal. Many times because of time constraints, language barriers or simple errors, acids are left on the forms and cause deep pitting and etching that ruins the smooth surface of the form and makes the form likely to have performance issues during its usage.

In addition, because of harm to underground water supplies, it is not acceptable to rinse any caustic chemical into the ground where it could be absorbed by underground water supplies or run off into a lake or stream. Before using any product, check the ingredients.

No matter what process you use to clean rust, be careful. Anything you do that removes material from the form will affect the finish. Be careful that you don't create heat when sanding, buffing or using abrasives on your form. Heat distorts metal. Bead blasting can be effective in cleaning your form, but if it is done too hard or unevenly, it will destroy the surface of the form. Always err on the side of caution when you work on the surface of a form.

An Ounce of Prevention

The best solution is to protect your forms from rust before it occurs. Talk to chemical suppliers about the best rust inhibitor products for your plant. Different products work better in different areas of the country. Variations in temperature and humidity, atmospheric contaminants, elevation, seasonal changes and the processes used in your plant should all be considered when making a product choice.

Finally, always make sure forms are properly cleaned and maintained. Apply a good long-term rust protective coating to your forms before storage. Never store forms filled with concrete. It's easy for condensation to form between the form and the concrete that will accelerate rust development. If rust does develop, begin a systematic investigation of all possible sources of the cause - and eliminate it.


RESOURCES

Find out More about Rust and How to Deal with It

An excellent article written about rust on steel forms was published in the Concrete Producer Magazine; May 2007. Go to www.concreteproduceronline.com and search for the article: Formwork Rust: Reasons and Prevention.

A great website with detailed information about all sorts of rust is The Corrosion Doctors. Go to: www.corrosion-doctors.org Cresset Chemical Company makes rust removal products and products which can be applied to protect forms for storage. Contact your local sales representative or go to www.cresset.com for more information.

A.L. Patterson carries a number of different rust inhibitors, form oils and release agents. Go to their website www.alpatterson.com or call 1-800-332-7090.