Q&A / 

Paint and Primer as Rust Inhibitor

Rust Inhibitive Paints and Primers

Corrosion is a killer. It does it to steel, aluminum, zinc, magnesium, lead, and tin in cold blood. Often there are two accessories to the crime - contaminated water and air. You have witnessed this process no doubt. A shiny penny becomes dull brown and eventually green. Bright aluminum turns dull. Fresh, bare steel can be coated with a fine haze of rust in a matter of hours. I know, I've seen it happen in front of my own eyes.

States of Energy

When you look at any metal product, did you ever think that the metal is unstable? Probably not. I didn't think about it until I wrote this column. When metals like steel, copper, aluminum, etc. are refined, they are taken from a stable state (ores in the ground or rock) into an unnatural state. Think. You just don't dig into the ground and pull out I-beams......

Thus, once the metal is refined, it tries to go back to its original stable state. Different metals go back quicker than others. The ones that require the most energy to refine, go back faster to their natural states. In other words they corrode more readily.

Which metals corrode faster than others? It's easy. Here is a list that goes from the most likely to the least likely to corrode in sea water. The top of the list corrodes the fastest:

  • Magnesium
  • Zinc
  • Aluminum
  • Mild Steel
  • Lead
  • Tin
  • Nickel
  • Brass

______________________

  • Copper
  • Nickel
  • 410 Stainless Steel
  • Titanium
  • 304 Stainless Steel
  • Silver
  • Gold
  • Platinum

The line separates active metals (above the line) from the metals which are passive. As you know, you see on TV specials gold objects pulled from the ocean that look as brilliant as when the ship sank. Does that make it easier to understand?

The Corrosion Process

Steel rusts or corrodes because the steel is actually being eaten alive. The steel rusts, flakes off and disappears. Water which contains impurities (most water does, including rain water!), contacts the steel and creates a battery much like the one in your car.

The water contains ions which create a small electric current. The electric current pulls the iron atoms right from the steel. Furthermore, the steel reacts with air and oxidizes. This is the rust you see. Corrosion can be stopped. Simply do not allow the steel or other metal to come into contact with water and air. This is not as hard as it seems. Think of your car. Many of the body parts are steel which doesn't readily rust if left protected.

The Key is in Chemistry

Paint manufacturers all have chemists who work for them. They know that you can inhibit corrosion by controlling the chemistry at the surface of the metal. In other words, you can treat the steel with a coating which chemically alters the surface of the steel to make it less likely to give up the iron atoms. Rust inhibitive pigments in paint do this.

Some excellent rust inhibitive pigments are metals which are active. Zinc, chromate, lead, barium metaborate, etc. all make great chemical coatings which can protect steel. However, we all know lead is very poisonous. As such, it is tough to get paints that contain lead. Navy ships for years had paints which contained vast quantities of lead. It really protected the steel hulls from rust.

Check the Label on the Can!

Priming steel can't be done with just any paint. Look at the label and make sure you have a primer that contains rust inhibitive pigments. If the paint has them, it WILL say so on the label. If it doesn't, keep looking!

Be sure the primer you use is oil based. Oil based paints can simply coat the steel better. They flow better into the microscopic surfaces of the steel. This provides much better holding power. Not only that, oil paints are great vapor barriers. Water vapor and air itself simply can't pass through. This is vitally important if you want to stop the corrosion process. Good luck on your painting project!

Column B140

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