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Buildings crack due to a number o
f reasons.
Traditional steel frame buildings were not made with construction joints to accommodate movement, whether it be from structural settlement, thermal changes, or general expansion and contraction of the materials. When micro-cracks first form on a building, they are not corrosion related. Over time however, these small cracks can allow oxygen, moisture and carbon dioxide to penetrate the masonry cladding and infill and provide a direct path to the steel frame.
As the steel frame begins to experience corrosion related deterioration, a direct correlation can be made between the size of the cracks and the amount of steel reverting to iron oxide (rust) at the steel frame. Stresses from the rust are then exerted through the infill material and masonry, cracking or displacing the cladding material.
Cracks may not just be cracks, but may be signifiers of larger corrosion problems which can eventually cause significant failures of the cladding materials, structural deficiencies, and pose a health and safety risk to pedestrians.
Echem consultants can provide corrosion rate testing, and through scientific calculations, predict when areas of the building will fail based on time to cracking models.
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The Corrosion Process for Embedded Steel in Masonry |
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Written by Admin
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Monday, 28 September 2009 18:46 |
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Steel embedded in an alkaline environment does not corrode as a result of the formation of a passive oxide layer on its surface. Corrosion can be initiated if the passive oxide layer is destabilized, this can occur in a number of ways. Aggressive species such as chlorides which are found in, for example, de-icing salts, sea water and marine environments penetrate the masonry and react to break down the passive oxide layer, initiating corrosion.
Carbonation of the masonry leads to a neutalization of the environment around the steel and the breakdown of the passive film. The carbonation reaction occurs with atmospheric carbon dioxide dissolves in the masonry port solution and reacts to form carbonic acid. This acid reacts with present alkaline calcium hydroxide to form neutral calcium carbonate. As the calcium hydroxide is consumed the pH of the masonry drops from approximately 13 to 9 at which stage the passive film of the steel begins to break down. Once the film is sufficiently destabilized corrosion occurs utilzing the available water and oxygen in the pore solution.
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Last Updated on Friday, 16 April 2010 14:40 |
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What is Impressed Current Cathodic Protection? |
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Written by Admin
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Monday, 28 September 2009 18:44 |
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Essentially, Cathodic Protection (CP) is the intentional application of a direct electrical current in the opposition to naturally occurring electrochemical corrosion of metal. Cathodic Protection is now a generally accepted and economical practive for reducing or eliminating the corrosion of metals, particularly steel.
Steel structures as varied as underground storage tanks, ships' hulls, oil well casings, hot water heaters, gas pipelines, concrete reinforcing, steel bridges and offshore drilling rigs are successfully protected by cathodic protection.
In order to mitigate the corrosion reactions, the steel component must be made cathodic, so that reaction
½O2 + H2O + 2e- → 2OH-
will occur and NOT
Fe → Fe2+ + 2e-
Correspondingly, an anodic reaction must occur on the surface of the anode which is provided for the Cathodic Protection (CP) system.
In Impressed Current CP systems, the anode is generally a conductive material which is not consumed. Typically an anode consists of a titanium substrate coated with a noble metal or metal oxide catalyst. In this case the anode reaction becomes the formation of oxygen from water:
2H2O → O2 + 4H+ + 4e-
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Last Updated on Friday, 16 April 2010 14:31 |
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