Corrosion Damage in Reinforced Concrete

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Reinforced concrete is an extremely important material of construction. One reason for the success of this combination is the similarity of thermal expansion properties of carbon steel and concrete. Another is that the extremely high pH of the cement content (typically pH > 13 for new concrete) passivates the steel surfaces against corrosion activity.

The main causes of corrosion of steel in concrete are chloride attack and carbonation. These two mechanisms are unusual in penetrating the reinforced concrete without significantly damaging it structurally.1   Instead, these chemical species pass through the pores in the reinforced concrete and deplete the alkalinity that passivates the steel surfaces. In contrast, deterioration processes resulting from chemical attack on concrete by acids and aggressive ions, such as sulfates, destroy the integrity of the concrete before the steel is affected.    In carbonation, carbon dioxide (CO2), often from the atmosphere, reacts with calcium hydroxide (Ca(OH)2) to form calcium carbonate (CaCO3) and water. This consumption of hydroxide ions reduces the pH of the reinforced concrete and may deplete the passive layer at the surface of the reinforcing steel.

The penetration of chlorides to the steel surface also breaks down the passive layer rather than attacking the steel directly. Relatively minor metal losses that do not compromise the structural integrity of the steel can generate a volume of corrosion product that can spall and crack the surrounding reinforced concrete. This accelerates the penetration of aggressive species. This is a key factor in the corrosion of bridge decks. Forty percent of the bridges in North America are at least 40 years old, and reinforced concrete is the primary material of construction.2   Structures in coastal areas are subject to salt spray atmospheres. It was recognized by the mid-1970s that the deterioration of concrete-bridge structures in cold climates was caused by the corrosion of the reinforcing steel in the concrete, which, in turn, was induced by the intrusion of chlorides from deicing salts into the concrete.

Reinforced concrete bridge decks constitute the weakest link in North America’s infrastructure network. According to a 1997 report, of the 581,862 bridges in the U.S., about 40% were either functionally or structurally deficient. Most of these bridges were severely deteriorated with extensive loss of serviceability and reduced safety, such that some of the bridges had to be load-posted so that overweight trucks would be required to take a longer alternate route.3   In spite of the clear association with bridge damage, the benefits provided by deicing salts are too great in terms of reducing vehicular accidents and minimizing traffic disruption.4   Therefore, its use is not likely to decrease in the future.

In areas of strategic importance such as highway belts of most modern cities, the total cost of repairs is greatly amplified by adding the indirect costs of traffic disruptions. A number of fundamental measures can be taken to address the problem of reinforcing steel corrosion; for example, creating a barrier between the concrete and/or the rebar and the existing environment or applying cathodic protection to the rebar structure. For new structures, it is believed that much progress will be made toward effective corrosion control as life-cycle costing strategies are adopted, as opposed to awarding contracts solely on the basis of lowest initial capital cost outlays. With such a vision, using alternative methods of reinforcement, such as fiber-reinforced polymer composites, corrosion-resistant stainless steel rebar, or epoxy-coated rebar to reduce the exposed steel surface, could prove to be a cost-effective route even if the initial cost were significantly increased.

References

  1. J.P. Broomfield, Corrosion of Steel in Reinforced Concrete (London, U.K.: FN Spon, 1997).
  2. Z. Lounis, “Maintenance Management of Aging Bridges: Economical and Technological Challenges,” Canadian Civil Engineer 19 (2002): p. 20-23.
  3. Corrosion Protection: Concrete Bridges,” U.S. Dept. of Transportation, FHWA-RD-98-088, Washington, D.C., 1998.
  4. Highway Deicing: Comparing Salt and Calcium Magnesium Acetate,” National Research Council, Transportation Research Board, Special Report 235, Washington, D.C.. 1991.

This article is adapted by MP Technical Editor Norm Moriber from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), p. 190,196-197.

Open Recirculated Cooling Water Systems

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Open recirculated cooling water systems remove the heat picked up in a plant by evaporative cooling. This may be done by a spray pond, for example, combining air-conditioning needs with aesthetic consideration in industrial parks. The most common type of evaporative cooling, however, is effected in cooling towers of one type or another.

Cooling towers may operate on natural draft, as in the case of wind-cooled towers for small home air-conditioning systems or the large concrete hyperbolic towers used in power-generating stations. In process plants, the towers are more often aided by fans, either forced or induced-draft operations, to improve the cooling capacity.

There are certain fundamental considerations that should be understood in relation to open recirculated systems. First is the concept of cycles of concentration. If three cups of boiling water in a tea kettle were allowed to boil away to one cup, the residual cup would contain a three-fold concentration of soluble water salts, assuming that only steam (i.e., pure H2O) was driven off. The water would be said to be at three cycles of concentration. If the two cups of evaporated water were replaced and again allowed to boil down to one cup, the remaining water would be at five cycles of concentration. In this fashion, the soluble salts would soon become unmanageable. In practice, the percentage of replacement water is much smaller, but the increased concentration of salts still must be addressed.

To prevent this accumulation from becoming unacceptable from the standpoint of scale and corrosion, a small amount of blowdown (bleeding of the system) is maintained to control the number of cycles of concentration from evaporation. This means that make-up water must be added to equal the evaporation and blow-down losses, but this is a minor amount compared to the volume of the total system.

For example, if we need 19,000 Lpm (5,000 gpm) of cooling water in a system, the cost for treatment in a once-through design would be excessive. However, in a recirculating system, the make-up may be as little as 2 percent, 380 Lpm (100 gpm), of which perhaps only 95 Lpm (25 gpm) may need to be treated with inhibitors. This brings chemical treatment into the range of economic feasibility, as compared with a once-through system.

Not only are there tangible limits, imposed by water chlorinity and hardness, as to how far one can concentrate the soluble salts in the water, but the savings effected by a recirculating system compared to a once-through system are maximized at about four to six cycles of concentration. Below this range, treatment costs become prohibitive. At high cycles (e.g., eight to 10), the additional water savings generally are not commensurate with the increased difficulty of effective treatment. If the blowdown is shut off entirely, there is still an effective upper limit of concentration dictated by water losses from drift or windage. The normal upper limits might be about 20 to 22 cycles of concentration for a mechanical draft tower.

The advantages of water savings provided by the cooling tower impose certain inherent disadvantages as well. The water becomes saturated, ensuring its full corrosion potential; its natural alkalinity may increase beyond the tendency to form protective surface scales and actually obstruct water flow. The air-scrubbing action can contaminate the water with airborne materials, notably dust fines, which form silt in the tower basin, and spores of slime, algae, and fungi that can reproduce in the warm nutrient-rich water of the system.

This article is adapted by MP Technical Editor Norm Moriber from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 138-140.

Evaluation of Polymeric Materials

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Polymers include a wide range of natural and synthetic materials that are formed from repeating building blocks that are typically connected by covalent chemical bonds between carbon atoms. These bonds can form linear chains or cross-bonds between chains. Structural polymeric materials (commonly called plastics) cannot be evaluated by the same criteria applied to metals. These materials often degrade from the effect of absorbed species from a solution at rates defined by Fick’s laws of diffusion. A number of interactions between a polymer matrix and the absorbed compound(s) are possible, such as breaking of the polymer chain, breaking of cross-links between chains, oxidation of the polymer, reactions with terminal groups of the chains, or simply lubricating the area between chains to weaken the mechanical properties of the material. This deterioration of the material that results from a reaction with its environment falls under the general definition of corrosion.

One of the most important aspects of the behavior of polymeric materials is that polymers can selectively absorb one or more constituents from an environment. This means that a continuous supply of only parts per million of an aggressive chemical in a stream can produce results that are similar to exposing the polymeric material to a 100% concentration of that chemical.

Many of these polymeric materials are used as resins in composites (i.e., combinations with glass fibers or other strengthening material), and the variables introduced, in addition to chemical resistance of the polymer matrix itself, might include the following:

  • Extent to which the fibers are wetted by the polymer
  • Strength of the fibers
  • Mode of manufacture (i.e., whether fibers are single filaments wound about a mandrel or fabrics impregnated with the resins)
  • Characteristics of the layers of the fabrication (i.e., whether glass fibers are exposed to the corrosive agents)
  • Internal or external strengtheners or stiffeners
  • Percentage of resin to fiber, and many other variables

Among the many types of tests that can be applied to polymeric materials are the following. Obviously, not all of these tests are necessary in evaluating polymeric materials for every use.

  • Visual (changes in appearance vs. unexposed samples)
  • Tensile strength
  • Flexural strength
  • Ability to withstand flexural cycling
  • Impact resistance
  • Notch sensitivity
  • Heat stability
  • Abrasion resistance
  • Static build-up
  • Coefficient of expansion
  • Chemical stability
  • Weight changes
  • Hardness
  • Dimensional change
  • Volume change
  • Flame propagation rate
  • Biological effects
  • Creep rates
  • Stress cracking resistance

For many purposes, the change in hardness of a polymer during an exposure can be as good an indicator of the resistance of the polymeric material to the environment as any other mechanical test. However, working with these materials at ambient temperatures is similar to working with metals at high temperatures. Thus, creep rates at given stress levels while exposed to the environment are probably the best of all corrosion tests if the costs of the various tests are to be prioritized.

Whatever the test procedure employed, a test period of at least 100 days should be used. The properties of a polymeric material do not change linearly with time. Thus, appraisal of the properties at various times during the test period should be considered.

This article is adapted by MP Technical Editor Norm Moriber from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 632-634.

Materials In High Temperatures

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In some applications, such as underground piping, temperatures within the range from 90 °C to 150 °C may be considered high temperature. The focus of this article is significantly higher temperatures, primarily above 650 °C.

Ordinary iron-carbon alloys, including carbon steels, have been used successfully from room temperature up to temperatures of about 480 °C, and for short periods of time up to 590 °C. At temperatures beyond this, ordinary steels tend to corrode rather heavily, and the formation of surface oxide scales is ineffective at reducing these rates. However, it is possible to resort to cementation (surface-diffusion treatments) on carbon and low-alloy steels to render them more resistant to corrosion. Depending upon the corrosiveness of the medium involved and the exposure temperature, one or several of the following may be used: siliconizing, aluminizing, chromizing, or a somewhat different method of surface treatment such as plating, plasma spraying, cladding, and so forth.

The most common alloys used for exposure at high temperatures are the iron/nickel/chromium alloys, often referred to as stainless steels (SS). The materials used may be cast or wrought. For most furnace operations, the cast metals are used.

If medium- or high-alloy steels are used, chromium-bearing SS are generally recommended. Both the straight chromium steels (400 series) and the chromium/nickel/steels (300 series) are used successfully up to approximately 870 °C. At still higher temperatures, the higher nickel and higher chromium alloys of the 300 and 400 series are used, and at temperatures in excess of 1,100 °C, only the alloys with more than 20% Cr can be used with relative safety, particularly for short or intermediate periods of time.

During long-term use at high temperatures, a phenomenon known as creep is experienced, during which relatively low stress may cause a metal to deform very slowly and possibly rupture. The percent elongation of a metal component is called strain, and the strain per unit time – typically not a constant at high temperatures – is called creep. Although this is not a corrosion phenomenon, it is nevertheless a factor that must be understood by anyone considering or recommending alloys for use at very high temperatures. It is on the basis of the creep to be expected at the operating temperature that furnace parts are designed to withstand the mechanical stress.

At still high temperatures, it may be necessary to use refractory metals that have good high temperature mechanical properties, plus melting temperatures greater than 1,650 °C, as shown in Table 1. Alloys of chromium, niobium, molybdenum, tantalum, rhenium, or tungsten will therefore be considered. Although titanium and zirconium melt above 1,650 °C, their high temperature mechanical properties are rather unsatisfactory, so they are not generally considered in the category of refractory metals. Although pure vanadium melts near 1,900 °C, the low melting point of its oxide makes it a poor choice for high temperature applications. While alloying with metals that melt at higher temperatures will generally produce a higher-melting material, the resulting properties and high-temperature performance of specific candidate alloys must be fully understood.

Metal ° C ° F
Aluminum 659 1,218
Beryllium 1,280 2,340
Carbon 3,600 6,512
Chromium 1,890 3,430
Cobalt 1,495 2,723
Copper 1,083 1,081
Gold 1,063 1,945
Iridium 2,454 4,449
Iron 1,539 2,082
Lead 327 621
Magnesium 650 1,202
Manganese 1,245 2,273
Molybdenum 2,625 4,760
Nickel 1,455 2,651
Niobium 2,468 4,475
Osmium 2,700 4,892
Platinum 1,773 3,224
Rhenium 3,180 5,756
Rhodium 1,966 3,571
Silicon 1,430 2,605
Silver 961 1,761
Tantalum 2,996 5,425
Tin 232 450
Titanium 1,668 3,034
Tungsten 3,000 5,432
Vanadium 1,900 3,452
Zinc 419 787

This article is adapted by  Norm Moriber from Corrosion Basics–An Introduction, Second Edition, Pierre  R. Roberge, ed. (Houston, TX: NACE International, 2006), p. 218-219

Content reprinted with permission from Materials Performance Magazine.

Inline Inspection Electronics Lab Expansion

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Mears Integrity Solutions is proud to announce the expansion of its West Road Inline Inspection Electronics lab in Houston. Rick Jacob, Mears’ manager of the lab, and of engineering services explains further.

Q: WHAT IS THE IMPORTANCE OF A MEARS ENGINEERING LAB TO SUPPORT THE INLINE INSPECTION OPERATION IN HOUSTON?

All operations for the Inline Inspection division are run out of our Houston facilities. That’s where our inspection tools are prepared for specific inspection runs prior to mobilization to the job site and are refurbished after the runs are complete. The Engineering Lab in Houston provides three functions in support of these operations:

  • provide detailed technical engineering support to operations;
  • provide repair facilities in the event of an electrical or mechanical failure of a tool; and
  • provide the facilities to successfully perform a root cause analysis in the event of a failure during an inspection run.

Q: CAN YOU EXPLAIN WHAT THE PROCESS IN THE EVENT WE HAVE A FAILURE DURING AN INSPECTION?

A failure during an inspection run is considered a very serious event and is a top priority for the division. Considerable effort goes into preparing each tool for an inspection run. The tool is then set up to record data collected during the run, loaded into the launcher, and launched into the pipeline, typically being driven ahead of the product running through the line. At the end of the run, the inspection tool is received in a trap and removed so the data can be downloaded for analysis.

Many things can go wrong when a sophisticated mechanical and electrical device is put into a pipeline and driven substantial distances through all the pipeline features, such as bends, tees, and valves. The tools are subjected to substantial pressure and driven at high velocities. In most cases the tools perform the inspection without problems and provide our analysts with good information; however, even after careful preparation, things sometimes go wrong and cause failures.

Procedures are in place to respond to a failure during a run. Once the possibility of a failure is recognized, an Inspection Failure Investigation, or IFI, is immediately implemented. The goal of the IFI is to collect as much forensic information as possible relating to the failure, analyze that information, and determine the root cause of the failure. Once the root cause is determined, it is used to establish short or long term solutions, or both, so that the lessons learned can be implemented to minimize the chances of the failure occurring again.

The Electronics Lab is an important part of that process. The lab provides the facilities to review the data, dismantle and inspect relevant parts of the tool, measure the electronic signals being generated within the tool, and perform advanced troubleshooting and other testing on the tool to determine the cause of the failure.   Once the cause of the failure is found and repairs or other solutions are put into place, the lab provides the test facilities to verify that the problem is resolved.

Q: WHAT VALUE DOES THIS BRING TO THE CLIENT?

Obviously, successful pipeline inspections are very important to our clients. Substantial man-power is devoted and expense incurred to inspect a pipeline. Every effort must be made to ensure our clients that we can successfully inspect their pipeline and do so while minimizing that man-power and expense. We use a First Run Success (FRS) metric to measure our ability to achieve these objectives.   A high FRS rate indicates a high probability of successful pipeline inspection with a single run without the need to repeat an inspection due to a failure. Our goal is to maintain a high FRS rate for our customers. While efforts are focused on performing successful inspections with a single run, occasional failures are unavoidable. Mears has invested substantial funds to provide the facilities and man-power so our engineering team can analyze forensic information relating to a failure and develop real solutions to mitigate the possibility that a failure will recur. This constantly improves our FRS rate and provides better service to our clients. We are constantly working to improve the design of our tools and our inspection procedures.

By using the facilities of our Electronics Lab, we can ensure our clients that we can develop real solutions to real-world issues relating to our inspection tools, procedures, and processes. This constant quest to improve the quality of our services ultimately provides value to our clients by maximizing the all-important FRS rate.

Q: TELL US MORE ABOUT THE FUTURE…

Current plans for 2015 include investing in additional staff to support the Electronics Lab in Houston. We will be hiring two additional engineers to work out of the Houston facilities as well as an Electronics Technician to work exclusively in the lab.

We have planned for a major expansion of the lab this year and look forward to enhancing the resources available to us to further improve supporting our clients and the Operations Department here at Mears Inline Inspection.

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