Corrosion control extends infrastructure life
With increasing competition for scarce public works dollars, spending on corrosion control often ranks low on the priority list. The demand for new structures, such as roads, bridges, water treatment plants and solid waste disposal facilities, is rarely matched by an outcry for more maintenance.
Yet it is the very lack of attention to issues like corrosion control that is causing the nation’s infrastructure to deteriorate before its time, costing the public money and depleting resources.
In fact, neglected corrosion costs billions of dollars every year — $300 billion last year. Yet, current corrosion control technologies properly applied would have prevented one-third of that loss.
And corrosion is not just a budget issue. It can cause leaking in underground storage tanks, pipeline failures and unsafe bridge decks, all of which are a direct threat to public safety and the environment.
Corrosion is the deterioration of a material, usually a metal, that results from a reaction with its environment. It can be controlled, usually with protective coatings and linings, cathodic protection, materials selection and/or corrosion inhibitors.
Coatings and Linings. Often used in conjunction with cathodic protection, coatings and linings protect structures by providing a barrier between a structure’s metal surface or components and a corrosive environment.
Cathodic Protection (CP). CP involves the use of a direct electrical current to counteract the normal external corrosion of a structure that contains metal, such as a pipeline or a concrete bridge with steel reinforcing components. On new structures, CP can help prevent corrosion from starting; on existing structures, it can help keep existing corrosion from getting worse. Installing CP on infrastructure facilities can be economical, often costing less than 1 percent of the capital cost for building structures such as pipelines.
Materials Selection. Manufacturing with corrosion-resistant materials, such as stainless steels, plastics and alloys can help structures survive longer in a particular environment. (The two most common materials used in constructing infrastructure facilities — steel and steel reinforced concrete — can be severely affected by corrosion.)
Corrosion Inhibitors. Certain substanes, added either continuously or intermittenly in small amounts to liquids, such as acids, cooling waters and steam, can prevent serious internal corrosion.
Water and Wastewater Facilities
Corrosion control in water and wastewater facilities prevents failures, extends the life of structures and reduces costly downtime that occurs when structures are shut down for repair. With publicly owned utilities, taxpayer confidence is increased, since service is undisrupted and reliable, and crews are not constantly digging up streets to repair failed pipe.
Corrosion in water and wastewater facilities results from a number of factors, including:
* pipe walls that are too thin;
* use of paints with no proven corrosion-resistant history;
* removal of heavy metals — which act as corrosion “decoys,” thus protecting pipelines — from sewage effluent; and
* stray electrical currents.
When designing and constructing new water and wastewater systems, it is important to consider the potential corrosiveness of the intended environment.
Proper materials selection, corrosion resistant coatings and cathodic protection should be mandatory aspects of any new facility’s design.
For water and wastewater systems already suffering from corrosion, the damage can be mitigated through the use of cathodic protection, coatings and adequate maintenance.
Reinforced Concrete Structures
Steel-reinforced concrete is used in everything from shopping malls and office buildings to bridges, power plants, parking garages, highways and water facilities and can be severely damaged by corrosion.
Corrosion in concrete can be caused by carbornation, acid rain or the use of substances — such as calcium chloride — that make the concrete cure faster.
Normally, the steel rebar is protected from corrosion through a passive film that forms on the rebar surface during concrete curing. However, when chlorides from salts (mostly sodium chloride) penetrate concrete and come in contact with the reinforcing steel, the protective film is destroyed. Rust, the corrosion product, expands and creates pressure — up to 5,000 psi — within the concrete, causing cracking, which in turn allows more salts to contact the rebar. The corrosion gets progressively worse and, if left unaddressed, eventually will lead to structural failure.
Additionally, once the concrete is contaminated, corrosion is progressive, and surface treatment will have little preventive effect. The condition of the country’s bridges clearly demonstrates the effects of failing to use established, proven corrosion control techniques. The Federal Highway Administration’s 1992 National Bridge Inventory reported that 118,563 — 21 percent — of U.S. bridges are structurally deficient.
And, corrosion is often the culprit. Yet, many of those bridges suffering corrosion damage could be saved with minimum repair and the use of permanent corrosion control techniques. After a bridge is 25 percent to 40 percent deteriorated, it usually requires replacement.
However, up to that point, professional application of proven corrosion control technology can slow down, if not stop, the deterioration to a point that allows the bridge to last for many years. The primary goal when designing corrosion control for new reinforced concrete structures is to eliminate or minimize salt intrusion. This can be accomplished with known techniques, quality materials and good construction practices. Dense, high-quality concrete mix with adequate cover over the rebar should be used and complete drainage provided.
Once the concrete is contaminated with salt, corrective action must be taken to prevent further damage and total structural failure. Using surface coatings and sealants may prevent further contamination, but it will not solve the problem of the concrete already contaminated. In those cases, CP is the only proven technology for stopping the process. CP has been used on pipelines, storage tanks, ships, offshore platforms and water and wastewater treatment facilities for more than 50 years and on bridges for several decades. If installed by experts, it will delay the need to replace or close contaminated bridges.
Studies suggest that corrosion control in water and wastewater facilities can save $8.50 in maintenance and/or replacement costs for every $1 spent. Furthermore, corrosion control on bridges can extend structural life by decades and save $3 in maintenance and/or replacement costs for every $1 spent. Controlling corrosion in the infrastructure can prevent premature failure and lengthen useful service life, thus saving countless hours and dollars.
For more information or for technical assistance with corrosion control problems, contact NACE International, formerly the National Association of Corrosion Engineers, at (713) 492-0535.