Cathodic Protection Systems in Steel Frame façade restoration
Corrosion, in
steel frame or reinforced concrete buildings, and the related corrosion-induced concrete deterioration, are difficult areas to address in façade restoration . Particularly in historic façade restoration, the treatment of corroding steel presents difficulties, as replacement of the usually underlying members is typically not an option because of the destructive effects on the historically or aesthetically important exterior, or the massive cost such an undertaking would require. One of the newest methods for treating corrosion in façade restoration scenarios actually has its genesis in early 19th century discoveries of electric principles.
Cathodic protection was first reported on in 1824 to the Royal Society in London by Sir Humphry Davy, a renowned chemist and mentor of Michael Faraday. In a series of papers to presented at the Royal Society, he described the corrosion of copper sheathing on British warships, and a solution he devised: "sacrificial" anodes made from iron were attached to the copper hull sheathing below waterline, which dramatically reduced the rate of corrosion. Thomas Edison also had a hand in Cathodic protection development, experimenting with "impressed current" cathodic protection on ships in the 1890's, though he was ultimately unsuccessful due to a lack of a suitable source of electricity and anode materials. It would be 100 years after Davy's pioneering work before widespread use of Cathodic protection would take place. In the 20th century, CP technology found applications in the maritime industries as well as in the protection of underground metal structures, such as oil pipelines and tanks.
Cathodic protection plays upon the fundamental electrochemical processes that cause corrosion. Only a few metals exist in a pure state in nature: gold, silver and platinum among them. Most metals used in engineering are derived from their ores, or natural oxides and sulfides. The metal is purified and made usable by smelting, in which the pure metal absorbs tremendous amounts of energy to free it from the ore. This energy remains within the metal, and in this state is unstable, and continually tries to revert back to its more stable oxidized state. This is the process we call corrosion. The oxidation process is electrochemical in nature and depends upon the flow of electrons in the corroding metal due to electrochemical reactions. A metallic surface exposed to an aqueous electrolyte (water or water vapor in air) possesses sites for an oxidation (or anodic chemical reaction) that produces electrons in the metal, and a reduction (or cathodic reaction) that consumes the electrons produced by the anodic reaction. This is called an electrochemical cell. The anodic reaction dissolves the metal to form iron ions and free electrons. The electrons flow over to the site where the cathodic reaction takes place, producing hydrogen gas and hydroxyide (OH-) ions. The iron ions of the anodic reaction combine with the hydroxides of the cathodic reaction to eventually form iron oxide: rust. The corrosion cycle is complete.
The impact of iron and steel corrosion in façade restoration is evident in steel structural members as well as in the reinforcing steel bars in structural concrete. Over time, reinforced concrete develops fissures and cracks that admit water and oxidation corrosion can then take place on the reinforcing iron/steel.
Cathodic protection works by reversing or negating the flow of electrons in the "electrochemical cell" on the steel surface, substantially reducing the corrosion process. Two types of Cathodic Protection (CP) exist for façade restoration purposes: galvanic or Sacrificial Anodic CP (SACP) and Impressed Current CP (ICCP). SACP works by connecting a metal with a higher anodic potential than the steel to the steel structure, creating a current from the anode to the steel. Thus an external electrochemical cell is set up, with the steel as a uniform cathode and the "sacrificial" anode attached to it, usually an alloy of aluminum, zinc or magnesium, which is corroded instead of the steel. Façade restoration engineers can consult ASTM standards for the composition and manufacturing of galvanic anodes. SACP advantages include: ease of installation, lack of ongoing maintenance, no external power source needed, low risk of "overprotection".
Impressed Current Cathodic Protection is generally used for larger structures where the natural anodic potential of a galvanic system is not adequate to provide complete protection. ICCP uses anodes that are connected to a DC power source, with regulating and monitoring capabilities built into the system. While this provides more precision and flexibility in the system, it also introduces more complexity and failure potential. There are also the problems of overprotection of the steel and subsequent premature depletion of the anodes, along with hydrogen buildup and its embrittlement of the steel and welds. These competing qualities need to be balanced when a protection system is being designed in a façade restoration scenario.
While having a long history of use in the United Kingdom in historic structures, SACP and ICCP have mostly been relegated to maritime and bridge protection in the US. But use in historic renovation and façade restoration is increasing, as the technology matures and its efficacy is recognized. Particularly in façade restoration of historic structures, steel corrosion is a primary cause of masonry deterioration, and CP systems can bring tremendous value to the owner. For example, metal dowels and cramps are traditionally used to secure copings, parapets and cornices on masonry facades, which are prone to movement and displacement. Thinly faced, narrow-jointed stonework, hallmarks of historic ashlar walls, also used metal cramps to tie the facing back to the brick or rubble core. Corrosion on these metal supports is problematic because iron oxide, the main component of rust, occupies 6 times the volume per weight that steel or iron does, causing displacement of contiguous masonry components. Replacement of these metal pieces constitutes a major façade restoration project, involving potentially huge costs and large amounts of time. There also exist aesthetic concerns as removal of any facade material necessitates finding a matching repair piece, which can be difficult or impossible, damaging the visual integrity of an historic building. There also exists the real possibility that removal of a corroding piece of steel can actually accelerate corrosion nearby. Plus, removal of obviously corroding pieces does nothing to ensure that soon to fail areas of the façade are protected. Cathodic protection offers an alternative approach that is less intrusive, less costly and mitigates all of the above concerns.
In an historic façade restoration example, a SACP setup would have anodes placed in close proximity to the corroding metalwork, and electrically connected to it. The anodes would then be electrically connected to the stone in which the metal is embedded, and the current loop is completed by the pore water in the masonry. This directs the ionizing effects to the electrolyte-metal interface where corrosion occurs. SACP systems such as this are adequate for limited steel structures.
For larger amounts of steel, the ICCP system is most appropriate. It was used in the façade restoration of the bronze sculpture base of the Wellington Arch in Hyde Park, London. Built in the 1820's of masonry, concrete and steel I-beams, it was discovered during recent renovations to have significant corrosion in the steel supports under the concrete-Portland stone base supporting the sculpture. Portland stone, being a sedimentary rock, has a neutral pH, offering no corrosion protection for the I-beams. In contrast, concrete, being alkaline, helps to inhibit corrosion. The corrosion of the I-beams had caused a vertical displacement and cracking of the stonework and delamination of some areas of concrete which accelerated and spread the corrosion process. Conventional restoration would call for replacement of the I-beams, but this would necessitate the precarious lifting of the historic sculpture, an expensive procedure, as well as doing nothing to stop the very processes which caused the conditions in the first place. A new cycle of corrosion would take place as soon as the new steel was in place. As far as preservation values, such disruption would significantly and detrimentally alter the structure.
The ICCP system installation began with a titanium mesh cast into the surface of the concrete slab above the
steel frame to be protected, to effect efficient transfer of the current through the concrete to all parts of the steel. The Portland stone underlayment was treated by installing titanium rods embedded in the mortar pointing of a brick course under the stone at 300mm intervals. Wiring from the beams and anodes ran back to an instrumentation cabinet in an interior roof space which provided remote control and monitoring capabilities. The structure was able to remain largely intact, with little or no visual impact, vitally important in an historic façade restoration.
Cathodic protection is a relatively new tool in the façade restoration engineers arsenal. But it is a rapidly developing and maturing technology that will have broader applications in the future. Understanding the fundamentals of corrosion, and the implications that has for understanding the proper remediation system in a façade restoration project, is essential for the professional.This is advised by our tech staff with your technical data, such as current density, working temperature, Ions concentration and so on.
1. Low Density and High Strength
2. Excellent Corrosion Resistance
3. Good resistance to effect of heat
4. Excellent Bearing to cryogenic property
5. Nonmagnetic and Non-toxic
6. Good thermal properties
7. Low Modulus of Elasticity
1) Chemical industry
2) Petrochemical industry
3) Machining field, Automobile field, etc.
4) Desalination of sea water
5) Textile printing and dyeing
6) Mobile phone component field.
