The basic definition of corrosion is the deterioration of metal due to interactions with its surroundings. The explanation of how all corrosion happens is a little more detailed and involves anodes, cathodes, electrons and electrolytes. Interestingly, there are several types of corrosion that occur. In other words, the way in which a metal breaks down is different based on the type of attack. In the case of snow and ice removal, crevice corrosion is the main form of corrosion in concrete. Crevice corrosion is similar to pitting corrosion in that chloride ions combine with oxygen or oxidizing salts to form random pits in the metal.
There are two theories on how corrosion in concrete occurs:
1. Deicing chemicals enter the concrete and cause corrosion. Corrosion of the steel reinforcing rods (“rebar”) lead to expansive forces that cause cracking of the concrete structure.
2. Cracks in the concrete and existing pockets such as threaded joints allow moisture and salts to reach the rebar and cause corrosion.
Regardless of which is correct, or if both are correct, deicing chemicals are part of the equation. Comparing deicing products to determine which is most corrosive is complicated due to factors such as chemical concentration levels, effectiveness of inhibitors, and a shortage of in-field testing. Each deicing chemical has a different minimum concentration level needed for effectiveness, but not all products use the same concentration levels. Products offer various types of corrosion inhibitors that affect the overall corrosion rates making it hard to test apples to apples. Many of the existing scientific tests took place in a lab rather than in the field where other factors may affect the outcome. In fact, there is a shortage of in-field tests because it is so hard to create a controlled environment. Additionally, since one metal will react differently to a deicer than another, looking at one test result from one type of metal will not tell the whole story.
Even though the chemical makeup of deicers cause varying levels of corrosion, it is not just the amount of chlorides that cause the damage. While chloride levels may be a catalyst, moisture and oxygen levels influence the extent of damage. For example, high humidity areas have higher rates of corrosion than areas of low humidity. A book by Frank M. D’Itri called “Chemical Deicers and the Environment” helps explain this further. The link to read more of or order the book is at Google books.
Steps to take to limit corrosion when installing or replacing concrete
As in the previous article, the best defense against corrosion is concrete that has been installed correctly. The depth in which the rebar is placed in concrete affects the rate of corrosion. As you would expect, the deeper it is placed, the more it is protected and the slower the rate of corrosion. In addition, as the water/cement ratio increases, the less protected it is; therefore, the rate of corrosion also increases. Consolidation also affects the rate of corrosion. If vibration does not remove enough air bubbles, the concrete will be more permeable, thus more prone to spalling and corrosion. Specific recommended depths and details are mentioned in Mr. D’Itri’s book; however, this book was written in 1992 and newer studies and/or materials used in building are likely available.
Steps to take to limit corrosion on existing structures
There are ways to reverse the effect and limit future corrosion besides using deicers with lower corrosion rates. These include polymer sealants, overlays, cathodic protection,and electrochemical chloride extraction, but we are out of space to go into much detail about them in this article. The links will provide a little more explanation for now.