Corrosion considerations

Masonry Construction, Jan, 2005 by Norbert V. Krogstad

Q I am working on an older building that has considerable corrosion on the steel lintels above the windows. I noticed that the problem at the south and west elevations was worse than on the east and much worse than on the north.

Why did the more serious corrosion occur at the south and west locations?

A Corrosion rate is related to the temperature of the steel, with warm surfaces corroding faster than cool material in the same environment. I have observed many buildings that have significantly more corrosion on the south and west elevations than they do on the north or east sides.

The south elevation often has the most direct sunlight. The west elevation likely reaches a higher temperature than the east because it receives direct sunlight at a time of day when the air temperature is warmer. As a general rule of thumb, the corrosion rate for steel doubles with every 18 degrees Fahrenheit rise in temperature. This fact is taken from a paper published by Cutter and Slater in ASTM STP 1180 that references the book Corrosion Basics, an Introduction, National Association of Corrosion Engineers, Houston, Texas, 1984, page 348.

The corrosion rate is dependent on several other factors, which also may be influencing the observed differences between areas of the building. The amount of moisture in the masonry greatly affects the corrosion rate. Protected areas have significantly less corrosion than spots exposed to runoff water from roofs, for instance. Certain exposures of the building often see more rainwater due to prevailing winds during rainstorms.

Also, portions of the wall that undergo frequent wetting and drying have higher corrosion rates than areas that are continuously moist because these alternating cycles facilitate oxygen reaching critical areas. Another factor is the chemistry of the mortar in contact with the steel. Mortar initially has a very high ph. This alkalinity protects the steel from corrosion; however, this protection is eliminated over time when mortar carbonates due to its exposure to carbon dioxide.

Another factor that may affect the corrosion rate at different portions of the building is the presence of free chloride ions in the mortar. Chloride ions can enter mortar from the addition of calcium chloride or chloride-containing accelerators during the original mixing. Although this practice is not permitted by most project specifications today, it was fairly common during the 1960s and 1970s and is still sometimes done during cold weather.

Chloride ions also can be introduced from the use of muriatic acid or other hydrochloric acid-based cleaners during construction or subsequent treating of the masonry. Chloride within the mortar can greatly increase the corrosion rate of any embedded steel or other steel in contact with the mortar, depending on the chloride concentration.