Corrosion in Reinforced Concrete Structures in the Middle East

Corrosion of reinforced concrete structures, both underground and above ground are a significant drain on the economy of most Middle Eastern countries. The majority of reinforced concrete structures in the Arabian Peninsula are chloride contaminated. As buildings and structures age, the chloride levels increase due to both chloride loading from atmospherically carried chlorides, and from capillary action which transports chloride laden ground water into concrete structures, where the water evaporates concentrating the salt above ground level. Allowing reinforced concrete structures to corrode freely results in buildings and structures that require repair or demolition due to structural failure.

One scenario of concrete damage due to corrosion happened in one of the Port in the Emirates which was constructed during 1970 and consists of pre-cast reinforced concrete beams and slabs with in situ concrete topping, supported by tubular steel piles. The first signs of deterioration were recorded after 7 years, evidenced by cracking of the lower corners of the pre-cast beams. Observing this, a series of detailed inspections were carried out.

An impressed current cathodic protection system incorporating metal oxide coated titanium anode was used to prevent further deterioration. The main advantage of impressed current cathodic protection (ICCP) lies in its much greater output capacity as compared to galvanic anode systems. Therefore, whenever corrosion protection is required for large poorly coated or bare structures, ICCP would be the system of choice. ICCP systems requires the use of an external DC power supply and metal anode in direct contact with concrete. This is achieved by embedding a durable conductive anodic overlay. This method is called reinforced concrete cathodic protection (CP).

Corrosion: Understand It to Fight It

Knowing what your operations are up against is crucial in preventing costly, potentially dangerous, damage.

CORROSION COSTS

There have been many studies about the cost of corrosion.

NACE International, formerly known as the National Association of Corrosion Engineers (nace.org, Houston), conducted research from 1999 to 2001 that found direct corrosion costs in the United States amounted to $276 million (about 3.2% of the country’s GDP). In March 2016, NACE released a study that estimated worldwide corrosion amounted to $2.5 trillion (about 3.4% of the GDP) and indirect costs doubled that.

In the 2016 study, NACE estimated that between 15% and 35% of those corrosion-related costs could be eliminated using current technology. Researchers also offered that, comparing corrosion costs in 1975 with those in 1999, an intelligent approach to automobile design and elimination of corrosion appeared to have reduced the cost to American consumers by about 52%.

HOW CORROSION OCCURS

Energy is needed to convert mined ores into useful metals. Corrosion is the natural result of those metals trying to revert back to their original states. Consider, for example, that there’s very little difference between the rust from corroded steel and the iron ores that were originally refined to make that steel.

Fig. 1 Depicting a steel bar in liquid, this diagram shows how corrosion occurs. The liquid contains water. When iron ions (Fe) off the bar unite with oxygen in the water, different kinds of rust can form.

he actual corrosion process is an electrochemical reaction. Depicting a steel bar in a liquid, Fig. 2, shows how this reaction takes place. In the diagram, corrosion is attacking the anode, with iron ions being released into the solution, while hydrogen is being generated at the cathode. Water (H2O), is made up of two hydrogen ions and one oxygen ion. The iron ions from the anode (the Fe symbols) will ultimately unite with oxygen in the water, whereupon several different types of rust can form.

At the cathode site of the piece, atomic hydrogen is being released. Most of those hydrogen ions then mate with another hydrogen ion and form molecular hydrogen, the readily flammable gas we’re used to thinking about. But some of the ions remain solitary and they are the cause of the many forms of hydrogen damage including hydrogen embrittlement, cracking, and blisters.

For wet corrosion, a liquid must be present to provide the complete circuit required by the electromechanical reaction. Electrons that flow from the cathode to the anode have to eventually return to the cathode, and they do so by traveling through the liquid.

Chemicals such as road salt are in the silt. As the moisture in it evaporates, the chemical concentration increases. The chemicals, in turn, make the water more electrically conductive and significantly increase the rate of corrosion.   

Temperature is a third important factor in corrosion. Below freezing, ice can’t conduct corrosion currents. But, as the temperature increases, the corrosion rate increases. A good example is the rapid attack on hot piping with moist insulation. The exact solution chemistry has a major effect, but up to about 175 F (80 C), the corrosion rate usually rapidly increases, then drops off and ceases when the liquid vaporizes.

TYPES OF CORROSION

Uniform corrosion causes about 80% of all corrosion. It occurs where anode and cathode sites relatively uniformly swap position. Examples include the railroad-bridge-support column shown in Fig. 1, buried steel water lines, nooks and crannies on vehicles where deposits build up, and machine frames and bases in damp areas.

Pitting corrosion manifests as isolated areas of attack. With carbon steel, it may take years before leakage occurs while stainless-steel pitting might progress at a rate of 0.001 in. (0.025 mm)/day. Steel examples frequently include water and wastewater tanks. Stainless-steel examples include external areas with dirt deposits on them.    

Galvanic corrosion occurs when two chemically different metals are joined. One is always the anode and continuously attacked, protecting the other piece. A common example involves a joint between steel and copper pipe, where the steel will always be attacked. 

Fig 2. A common example of galvanic corrosion involves a joint between steel and copper pipe where the steel will always be attacked. This joint was submerged in water for only nine months before damage occurred.

Figure 3 shows a bronze fitting and a steel pipe that had been submerged in water. Perforation of the freshly cut pipe threads happened in only nine months.

Selective leaching is essentially galvanic corrosion within a metal. The common industrial application involves buried cast-iron water or waste lines where the graphite in the iron acts as a cathode, and the iron is eaten away, leaving a weak and brittle graphite pipe. When initially excavated, the pipe may appear almost undamaged, but sandblasting will rapidly remove the graphite leaving proof of the mechanism. (A frequent problem with buried-pipe replacement is that the new piece is always anodic to the older sections. The new one will rapidly corrode and leak, and personnel will blame the material, not knowing that the actual problem is their lack of corrosion knowledge.)

Crevice corrosion occurs in a small gap between two pieces of metal. It allows a corrosion mechanism to act in a way that’s similar to pitting corrosion. Although it’s not a common industrial mechanism, it can happen with poor joint control on welded assemblies.

Intergranular corrosion involves galvanic attack at the grain boundaries within a   metal. It’s usually associated with a poor choice in materials of construction for  chemical processes.

Erosion corrosion is a combination of actions. Corrosion results in an oxide on a metal’s surface. The oxide, though, slows the attack because it prevents fresh corrodent from reaching the surface. If there’s a fast fluid flow that scrubs the oxide off the surface, corrosion continues at a very rapid rate. A common site for erosion corrosion is the outer radius of piping elbows in steel lines with untreated waters and flow rates exceeding approximately 10 ft./sec. (3 m/sec). It’s also been seen in pumps as a result of poor choices of construction materials.

The previous seven categories/types are basically different-looking versions of galvanic corrosion. Two other corrosion types—stress-corrosion cracking and hydrogen damage—result in metallurgical damage leading to often hard-to-detect catastrophic failures.

Stress corrosion cracking (SCC) can occur with almost any metal and is the result of a combination of stress, a chemistry that attacks the metal’s structure, and a susceptible metal. Industrially, although it is sometimes seen with nitrates and steel, the most common situation involves 300 series (austenitic) stainless steels and chlorides.

KEEP IN MIND

The battle against corrosion is never ending. In summary, if an area is wet and metal isn’t protected, there will be corrosion. What’s worse, the seriousness of the damage caused by this scourge may not be recognized for years. 

Causes of Stress Corrosion Cracking In Pipelines

Stress corrosion cracking (SCC) is a type of environmentally-assisted cracking (EAC), or the formation of cracks caused by various factors combined with the environment surrounding the pipeline. SCC occurs as a result of a combination between corrosion and tensile stress. Corrosion is related to the susceptibility of the material to the environment, while stresses may be residual, external or operational.

The most obvious identifying characteristic of SCC in pipelines, regardless of pH, is the appearance of patches or colonies of parallel cracks on the external surface of the pipe.

SCC is usually oriented longitudinally, and the dominant stress that causes it is usually internal pressure. Here we'll take a look at some different types of stress corrosion cracking, and how they occur. 

Conditions that Lead to Stress Corrosion Cracking (SCC)

The occurrence of SCC depends on the simultaneous achievement of three conditions.

1. A Potent Cracking Environment
The conditions at the pipe surface are referred to as "the environment." This environment may be isolated from the surrounding soil by the pipe coating, and the conditions at the pipe surface may be different from those in the surrounding soil.

The four factors controlling the formation of the potent environment for the initiation of SCC are the type and condition of the coating, soil, temperature and cathodic current levels.

• Pipeline Coating Types: SCC often begins on the pipeline surface at areas where coating disbondment or coating damage occurs. The ability of a coating to resist disbonding is a primary performance property of coatings and affects all forms of external pipeline corrosion. Coatings with good adhesion properties are generally resistant to the mechanical action of soils from wet/dry cycles and freeze/thaw cycles. They also are better able to resist the effects of water transmission and cathodic disbondment.

• Soil: There are several factors relating to soils that influence the formation of an environment that's conducive to SCC. These are soil type, drainage, carbon dioxide (CO2), temperature and electrical conductivity. The amount of moisture in the soil also affects the formation of stress corrosion cracks.

• Cathodic Protection: The presence of cathodic protection (CP) current is a key factor in the formation of a carbonate/bicarbonate environment at the pipeline surface, where high pH SCC occurs. For near-neutral pH, SCC CP is absent.

• Temperature: Temperature has a significant effect on the occurrence of high pH SCC, while it has no effect on near-neutral pH SCC.

2. A Material that Is Susceptible to SCC
In addition to a potent environment, a susceptible pipe material is another necessary condition in the development of SCC. A number of pipe characteristics and qualities are considered to determine if they are possibly related to the susceptibility of a pipe to SCC. These factors include the pipe manufacturing process, type of steel, grade of steel, cleanliness of the steel (presence or absence of impurities or inclusions), steel composition, plastic deformation characteristics of the steel (cyclic-softening characteristics), steel temperature and pipe surface condition. (For examples of susceptible materials, see Hydrogen Embrittlement Issues with Zinc and Causes and Prevention of Corrosion on Welded Joints.)

3. A Tensile Stress that's Higher than Threshold Stress
When tensile stress is higher than threshold stress, this can lead to SCC, especially when there is some dynamic or cyclic component to the stress. (For more on this topic, read The Effects of Stress Concentration on Crack Propagation.) Stress is the "load" per unit area within the pipe wall. A buried pipeline is subject to different types of stress from different sources. The pipeline’s contents are under pressure and that is normally the greatest source of stress on the pipe wall. The soil that surrounds the pipe can move and is another source of stress. Pipe manufacturing processes, such as welding, can also create stresses. These are called residual stresses.

Types of Stress Corrosion Cracking

SCC in pipelines is further characterized as "high pH SCC" or "near-neutral pH SCC." Note that the "pH" here refers to the environment on the pipe surface at the crack location, not the pH of the soil itself.

High pH Stress Corrosion Cracking (Classic Type)
High pH SCC occurs on the external surface of pipelines where the electrolyte in contact with the pipe surface has a pH of 8-11 and the concentration of carbonate/bicarbonate is very high. This electrolyte is found at disbonded areas of coatings where the CP current is insufficient to protect the pipeline. This type of SCC may develop as a result of the interaction between hydroxyl ions produced by the cathode reaction and CO2 in the soil generated by the decay of organic matter.

This form of SCC is temperature-sensitive and occurs more frequently at higher temperature locations above 100°F (38°C). This is why there is a greater likelihood of SCC immediately downstream of the compressor stations where the operating temperature might reach 150°F (65°C).

The high-pH form of SCC is intergranular; the cracks propagate between the grains in the metal, and there is usually little evidence of general corrosion associated with the cracking. These cracks are very tight, narrow cracks.

Near-Neutral pH Stress Corrosion Cracking (Non-Classic Type)
A near-neutral pH SCC environment appears to be a dilute groundwater containing dissolved CO2. The source of the CO2 is typically the decay of organic matter and geochemical reactions in the soil. It has been found that low pH SCC occurs in environments with a low concentration of carbonic acid and bicarbonate ions with the presence of other species, including chloride, sulfate and nitrate ions.

Typically, the SCC colonies initiate at locations on the outside surface, where there is already pitting or general corrosion. This damage is sometimes obvious to the unaided eye, while at other times it is very difficult to observe.

The near-neutral-pH form of SCC is transgranular; the cracks propagate through the grains in the metal and are wider (more open) than they would be in the high-pH form of SCC. In other words, the crack sides have experienced metal loss from corrosion. Near-neutral-pH SCC is less temperature-dependent than high-pH SCC.

How Crack Growth Occurs

Stress corrosion cracking in pipelines begins when small cracks develop on the external surface of buried pipelines. These cracks are not visible initially, but as time passes, these individual cracks may grow and forms colonies, and many of them join together to form longer cracks.

The SCC phenomenon has four key stages:

1. The initiation of stress corrosion cracks

2. The slow growth of cracks

3. The coalescence of cracks

4. Crack propagation and structural failure

This process can take many years depending on the conditions of the steel, the environment and the stresses to which a pipeline is subjected. Consequently, failure as a result of SCC is relatively rare, although failures can be very costly and destructive when they do occur.

Sources:

https://www.corrosionpedia.com

https://pgjonline.com