It is not always essential to have chloride ions or aggressive industrial liquids to generate corrosion. Sometimes corrosion can result from something as gentle as the morning dew.

The top photo at the left shows the roof of one wing of a pair of multi-story buildings. Each essentially identical wing radiates from a central tower on 120° centerlines. The central tower contains the elevators and HVAC. It also supports each of the three wings.

In the top photo note the triagular "hanger beams." These attach to one of three cantelevered beams that runs back to the central tower. Notice at the end of each of the hanger beams is a dark vertical duct. Inside each duct runs a low alloy, high strength steel strap. The floors clamp to these straps for support. The floors then are not supported from below. Rather, they hang from the cantelevers above.

In the second and third photos note the corrosion on the hanger straps. This was typical of the hanger strap-hanger beam connections.

This corrosion was the result of what is known as differential cell corrosion. Corrosion is accelerated if a metal object is simultaneously exposed to two different environments. In this example, the upper end of the hanger strap was embedded in concrete. Below the hanger beam, the straps were run in sheet metal ducts packed with fire bricks.

When the air temperature would drop, moisture within the ducts would condense and create dew on the surface of the steel strap. The pH of quality concrete is about 12.5. The pH of dew around 6.5 or 6 units less than concrete. As the dew's pH is lower, that means it is more acidic than concrete. In fact, as pH is a logarithmic scale the dew is a million times more acid than concrete. This was enough to cause severe corrosion.

The corrosion was sufficient to require evacuation of both buildings and immediate shoring. Tinnea & Associates was the prime corrosion consultant on this work and provided the design of a cathodic protection system to correct the problem.