How does water make rebar corrode? 2

[Kenton54]

I'm working on a Home Inspection book and I'd like to know (in laymen's terms more or less) why water corrodes steel. What is the chemical process? Oxidation?

-Kent

 

[dik]

Reinforcing steel exposed to air oxidizes (rusts) by converting the iron in the steel to iron oxide or iron carbonate (due to the CO2 in the atomsphere). The oxide formed is a loose material that has a greater volume than the original iron and because it is loose, it flakes off exposing new steel or iron to corrode. The process is ongoing until all the exposed iron or steel is consumed.

Rusting of reinforcing steel in concrete is a slightly more complicated matter involving a loss of protection afforded by the concrete. The loss of protection is two fold; the concrete protects the iron from the environment by covering it. In addition, the concrete, due to its alkalinity protects the steel from corrosion (It's a little more involved than that). The problem with corrosion of steel in concrete is that the rust products have a greater volume and cause the concrete to spall off exposing more steel. As briefly touched on, for the steel to rust in the first place, the alkaline protection must have been compromised.

It's possible to write pages about this issue and I hope the above brief outline will help you.

Dik

 

[Ron]

Kenton54...to expand a bit on dik's nicely done description, the water has a two-fold influence on the corrosion. First, water contains oxygen to assist in the oxidation process, and secondly water acts as an electrolyte in the process of galvanic corrosion, both of which occur in the deterioration of rebar. A third influence that water has under certain conditions is that it becomes the solvent that allows the carrying and contact of the rebar with certain water soluble oxidizing ions such as chlorides.

Water can also reduce the pH of concrete thus taking away the protective passivation it provides to the rebar when the high pH is maintained.

The corrosion of rebar can be on a microscopic or macroscopic level, usually with both occurring simultaneously. In the "micro" sense, small corrosion cells or batteries are set up on the surface of the rebar where variations in the steel occur and where certain constituents in the concrete (aluminum compounds for instance) create galvanic cells that remove material from the rebar. In the macro sense, oxidation caused by the dissolved oxygen in water or by the electrolysis of water releasing oxygen allows the premise that dik laid out.

As dik said, the corrosion of rebar can be rather complex and we've attempted to simplify and summarize it in a layman's nutshell.

 

[Kenton54]

Great explanations, guys. Exactly what I was looking for, Thanks.

Kent