Understanding Cathodic Corrosion Protection for Underground Tanks - Part 1

Underground Storage Tanks offered key benefits for communities and industries when it comes to appearance, cost and security. They are also protected from the effects of fire. Mainly the reason why most of the owners preferred to install it underground.

With more owners and customers choosing to hide their propane tanks with popular underground installations, the need to protect steel vessels from moisture and chemicals in the soil has never been greater. UG tanks can rust and cause problems for the owners.

Last year, the Environment Department of Dubai Municipality had release a code for installation of Fuel storage Tank requiring each underground (UG) tanks installed must have a cathodic protection system or similar internationally recognised method to help prevent corrosion of the tank. There are also requirements for testing the effectiveness of the system on a specified schedule, to document the testing, and to maintain the test results for a specified time.

Why Do Underground Tanks Need Cathodic Protection?

• To minimize the rusting of steel tanks and connected piping which can cause a release of a regulated substance.

• To eliminate the cost associated with environmental clean-ups caused by a release from the UST system.

• To extend the operational life of the UG tank.

Preventing Corrosion

Protecting underground tanks from corrosion is easily achieved by the use of two commonly applied protection methods: external coating and cathodic protection. These two methods are complementary and should be used in conjunction with the other.

Cathodic protection prevents corrosion at those defects by applying DC current from an external source, forcing the tank to become cathode. Application of sufficient DC current to the tank will prevent any corrosion from occurring. The two general types of cathodic protection systems are sacrificial and impressed current. Sacrificial systems are used when the amount of current required for the protection is small, such as in underground propane tanks. Impressed current systems are more commonly used for large structures such as large diameter pipelines. Electrical isolation of the tank from metallic piping systems and electrical grounds is critical for the cathodic protection system’s effectiveness.

Sacrificial systems work by creating a galvanic connection between two different metals. The most common anode material is magnesium, which when coupled to steel results in DC current flow from the magnesium to the steel. The open circuit potential of steel is about -0.50 volts referenced to a copper sulfate electrode. The open circuit potential of magnesium is about -1.55V to -1.80V. By connecting the two metals together, the difference of 1 to 1.25V volts results in current flow to the tank that overcomes the natural corrosion cells that exist on the tank. With this current available to the tank, no corrosion occurs.

BATTLING CORROSION ABOARD SHIPS - Part 3

Strategies for Prevention

Preventive measures will decrease maintenance costs, early system failures, and an overall improve service life of ships granted they are detected in enough time.

a. Modifying the Corrosive Environment

Hence, Inhibitors are salt water or electrolyte solutions which are controlled. Anodic inhibitors migrate to the anode creating a protective barrier which as a result, stops corrosion in its tracks. Therefore, Cathodic Inhibitors migrate to the cathode which prevents the absorption of oxygen or hydrogen.

In addition, Cathodic protection uses sacrificial metal to act as the anode which creates a protective barrier. This metal deteriorates first instead of the ships protected layers.

For example on the hull of a ship, zinc plates prevent corrosion of any sub-surfaces.

b. Protecting the Corrosive Environment

As a result, in the hope of protecting the vessel coat it with three different paint solutions.

Binders – Therefore, the binder is a film forming component of paint.

Pigments/Extenders – these powders mix with the binder at various particle sizes and enhance the overall effectiveness of the binders.

Solvents -Eventually evaporate from the surface and help for easy application.

In conclusion, it is important to have the proper plating before the equipment is put to work. As a result the equipment will last longer and will become resistant to the elements.

Battling Corrosion aboard Ships - Part 2

Impressed current cathodic protection (ICCP) systems are the ultimate state-of-the-art, long-term solution to corrosion problems, and are recognized as a superior alternative to sacrificial anode systems, which require frequent replacement. Impressed current cathodic protection systems are preferred by ship owners because they reduce fuel cost and maintenance.

ICCP systems work by supplying a controlled amount of DC current to submerged surfaces using highly reliable mixed metal oxide anodes and zinc reference electrodes. This electrical current, constantly monitored and regulated by the system itself to prevent the electrochemical action of galvanic corrosion before it begins.

For more than 25 years, sea-going vessels of every type and size – oil tankers, LNG carriers, cruise ships, pleasure craft, workboats, semi-submersibles, and more – have benefited from the 24-hour protection provided by Impressed current cathodic protection systems against the costly, corrosive effects of electrolysis.

Reference Cell / Electrode
ICCP systems are controlled to assure optimum protection. This control is obtained by inserting a third electrode between the anode and the cathode. The third cell/electrode is insulated and does not receive any anode current. This cell/electrode is freely corroding and it becomes the starting point — or reference — in eliminating corrosion. Cell/Electrodes constructed of Zinc are used exclusively with the ICCP system.

Zinc Reference Electrode

Power Supply Unit / Control Panel
Each standard ICCP system utilizes a solid-state controller which monitors and controls the protection as measured by the Zinc Reference electrode. Anode current automatically increases when the electrode potential falls below the designated control value. An over- and under-potential alarm is provided with the system package. We also offer optional digital control, state-of-the-art technology with every system. The computer controller (shown below is more accurate and provides central control, monitoring, data storage and hard printout.

Mixed Metal Oxide Anodes
Mixed Metal Oxide anodes of are used exclusively for ICCP systems. ICCP anodes are manufactured in Linear Loop , Elliptical and Circular designs with insulating holders. They are available in a single unit capacity of 75 to 225 amperes, as required for various installations.

Impressed Current Corrosion Protection System – MMO-TI Linear Strip Anode– MMO-TI Disk Anode

System Advantage

·         Increased life of rudders, shafts, struts and propellers as well as any other underwater parts affected by electrolysis

·         Anodes are light, sturdy and compact for easy shipping, storage and installation

·         Anodes, reference cells and automatic control systems maintain just the right amount of protection for underwater hulls and fittings, unlike standard zinc anodes, which can’t adjust to changes in salinity or compensate for extreme paint loss

·         Automatic control equipment ensures reliable, simple operation

·         Optimum documented corrosion protection at minimum overall cost

·         Only one installation required for the life of the vessel or structure

·         Increased dry-dock interval

Battling Corrosion aboard Ships - Part 1

All areas of the marine industry fight a constant battle against corrosion.

The shipping industry is one that continually faces the corrosion challenges stemming from marine environments, particularly seawater.

Because seawater contains a significant concentration of dissolved salts and is very corrosive to steel, infrastructure and assets in or near marine environments are particularly susceptible to corrosion. Efforts to mitigate corrosion in marine environments continue as industries develop and implement solutions to prevent asset degradation.

The most frequent forms of corrosion found on chemical tankers are uniform corrosion, pitting corrosion, crevice corrosion, galvanic corrosion, and microbiologically influenced corrosion. 

The most efficient system for combating underwater corrosion is 'cathodic protection'. The basic principle of this method is that the ship's structure is made cathodic, i.e. the anodic (corrosion) reactions are suppressed by the application of an opposing current and the ship is thereby protected.

CATHODIC PROTECTION USING SACRIFICIAL ANODES: THE BASICS

 How does corrosion take place in ships?

 Ships are made of steel; whose main component is iron. Iron is an electrochemically positive element, i.e., it has a tendency to give up electrons to become a free ion. Sea water is composed of oxygen and hydrogen, and it produces electrochemically negative hydroxyl ions which can accept the electrons given by Iron. This way the Iron ions combine with the hydroxyl ions of water to form Iron Hydroxide. This is called the oxidization of Iron, and this oxide is what we call as the brown color rust.

 The basic idea of using sacrificial anodes is to use a metal like Zinc/Aluminium and create its contact with the surface to be protected.

The simplest picture which comes to mind is simply using a flat bar of the metal and fix it to the surface to be protected. This is actually the method commonly used to protect the outer ship’s hull.

To protect steel successfully using cathodic protection, it is therefore only necessary to lower its potential by around a quarter of one volt (250mV). 

Cathodic protection using sacrificial anodes produces a decrease in the potential of the ship by connecting the vessel to a metal which takes up a reversible potential of less than –850mV (S.C.E.) and allowing the sacrificial metal to produce the electrons rather than the corrosion reaction of the steel.

Anode Classification

Anodes can be classified based on their shape, size, material, mounting method and method of securing to the surface to be protected.

 The following are some widely used shapes for anodes:

• Flat or block shaped

• Cylindrical or semi-cylindrical

• Tear-drop anodes

• Bracelet anodes

• Disc shaped

• Tubular anodes

 Anodes can be of different shapes based on their applicability. The selection of the shape of anode depends on several factors. Some of these factors are:

• shape of the surface to be protected,

• availability of space,

• accessibility,

• ease of installation

• special considerations, e.g., effect on resistance for small boats 

For example, flat anodes are used mostly for flat, large surfaces like the ship’s hull. Tear-drop anodes are used in high speed boats where streamlining of water is important as flat anodes will increase the boat’s resistance. Bracelet anodes are used for pipelines and propeller shaft, while tubular anodes are used for cables. There are no fixed rules here though, and the choice depends on the availability, cost and flexibility in design. For example, cylindrical anodes can also be used to protect pipelines, and it is not necessary to use bracelet anodes if they are costlier.

Anode Material

Usually for marine applications, Zinc or Aluminium anodes are deployed. Zinc has been traditionally used for corrosion protection, though Aluminium is now widely used. The two properties which measure performance of an anode are listed below.

1.       Closed Circuit Potential – the first parameter, Closed Circuit Potential signifies the ease with which the anode will be corroded. The more negative the value, the more readily the anode will get corroded. Generally, a potential of less than -0.08 Volts is required for cathodic protection of shipbuilding steel to be effective.

2.       Electrochemical Capacity (in Amp-hr/kg) – The second parameter, the Electrochemical Capacity, signifies the rate at which the anode material will be consumed.

The two parameters for Zinc and Aluminium are listed in the table below:

Properties of Anode Materials (Source: DNV RP-B401)

We can see from the above table that Aluminium has a higher closed circuit potential – so it will more readily start working compared to Zinc. It also has higher Electro-chemical capacity compared to Zinc, and will be longer lasting for the same anode size.

Further, in fresh water application, Zinc tends to develop a calcareous coating on the anode surface, which prevents their effective working.

However, Zinc anodes have sometimes been found more reliable in environments with low oxygen, e.g., marine sediments or areas with high bacterial activity. Thus, while Aluminium is the more efficient one, Zinc may be more effective in some cases.

Further, Aluminium anodes, if falling from a height on oxidized steel, can create sparks. Thus they are not recommended to be used inside cargo tanks of tankers. The maximum height above tank bottom which they must be placed is 28/W meters, where W is the weight of the anode in kgs.

Hence, the selection of the material depends on the type of environment it is going to be used, and should be carefully carried out.

Anode Mounting Method

The next important consideration for installation of anodes is the mounting method, i.e., the configuration of the tubular insert, and the positioning of the anode vis-à-vis the surface to be protected.

Based on mounting technique, there are two major types of anodes which are used in ships:

1.       Flush mounted anodes – in this type of anode, the anode material (Aluminium or Zinc) is in direct contact with the surface to be protected. The insert is generally a flat bar which can be welded or bolted to the surface.

Flush Mounted Anode

2.       Slender stand-off anodes – In these types of anodes, the anode material is not in direct contact with the surface to be protected, and there is a gap (hence the name stand-off). The insert is generally a tubular one which can be welded or bolted to the surface.)

The benefit of a stand-off design is that it is a more compact design, and the anode material is better utilized in a stand-off design. This is quantified by a parameter called ‘anode utilization factor’. This is the fraction of the anode material which is actually utilized over the lifetime of the anode. For flush anodes, this is around 80%, while for stand-off anodes it is 85 to 90%. Thus, stand-off anodes are better utilized over their lifetime.

Further, in case of flush anodes, due to constant contact between the anode material and the surface, the surface may suffer from embrittlement caused by deposition of ions from the anode material to the cathode (the protected surface).

That said, stand-off anodes protrude from the surface on which they are installed. When used on external hull of a vessel, these affect the streamlined shape of the vessel, and lead to increased drag and higher powering requirements. In comparison, flush anodes are closer and more compliant to the vessel’s geometric shape and have lower effect on resistance. Thus, flush anodes are usually preferred on outer hull due to their low drag properties.

Both Flush mounted and slender stand-off anodes are further classified into Short and Long, depending on their ratio of length to width. The length affects the resistivity of the anode and thus its current capacity.

References:

https://thenavalarch.com/ship-corrosion-cathodic-protection-sacrificial-anodes/

Interesting Facts About Corrosion

• Corrosion is a natural mechanism that occurs to all metals

• There are 6 main types of corrosion

• Corrosion failures cost the world in excess of over $2 trillion dollars

• Corrosion can result even in the absence of oxygen

• Corrosion protection is best addressed in the design stage

• Cathodic protection works to make the corrosion reaction thermodynamically unfavorable 

Towers, Corrosion & Sustainability - Part 3

Cathodic protection is an extremely powerful technology in that it has the power to almost completely stop corrosion on the structure it is deployed on. Most corrosion protection methods are passive in nature – such as paints. Cathodic protection is very much active. It attacks the corrosion problem at its heart.

Towers, Corrosion & Sustainability in Dubai’s Economy - Part 2

In our last notes, we discussed how a leaky basement is a recipe for disaster waiting to happen. Why? Because clearly all the elements required for an effective corrosion cell (the basic building bloc of corrosion) to easily form and self replicate are present….in abundance.

But really, how bad can this corrosion get? Is it something that will make my building collapse? Is it something to worry about? Is it something to think about? How will it affect the sustainability of the building?

Lots of questions…

For starters, let us discuss how sustainability and corrosion are interlinked. If your building can be made to last an additional 20 years by stopping corrosion, then the technology that allows you to do this is a sustainable technology. This is the link between corrosion mitigation and sustainability.

What about our leaky basement? What can we expect in terms of how bad the corrosion can get.

The biggest risk posed to any structure from corrosion would be if the corrosion were to initiate in a ‘structurally sensitive’ area and remain accelerated and localized in that area. You can imagine what this would like; a gradual but accelerated reduction of the reinforcing steel diameter in an extremely short span of time. This is the nightmare situation for any building. It happens often. I have seen corrosion of structural columns in basements in under 5 years after completion that have caused massive cracking. It’s quite ugly and frightening. What does this tell us about how bad things can get? It’s very difficult to point to a specific rate of corrosion because corrosion is mix of so many factors – electrical, electrochemical reactions, temperature, moisture, pH and much much more. So in essence, it is difficult to predict – but who is prepared to take the chance? Would you? I wouldn’t! It is something to worry about.

The thing about corrosion is that it grows and gets worse as time passes – much like a tumor would. As a matter fact, in Australia, it is nicknamed  ‘concrete cancer’ - and unless you move quickly to snuff it out, its going to destroy the building and cost you an arm an a leg.

Many owners have gone about attending to leaky basements in a strange kinda way. As soon as leak is seen, it is plugged by crack injection. Sometimes the situation is so bad that the top layer of concrete is removed and reinstated – all in the hope of …..wait for it…removing the salt chlorides. Now why would I care so much about the chlorides. Primarily because it is a corrosion initiator and when present in excess of a certain amount, corrosion is guaranteed.

If only we had a technology that would stop corrosion in an inexpensive way irrespective of how much chlorides were present! Well it exists. Cathodic Protection. In our next part on this blog, we will discuss how this technology works for basements.

Cathodic Protection of Reinforced Concrete Diaphragm Walls

The Middle East with its high temperatures and extremely high saline content of its sea-water & ground water forms an extremely aggressive environment in terms of corrosion. When constructing structures that are exposed to this environment the challenge is how to ensure corrosion durability.

For diaphragm walls that are cast in situ, this is a special challenge for two reasons.

The first reason is that during construction, some methods employed involve the boring of the excavation, lowering the steel cage into the bored excavation and then casting. The excavation is typically inundated with bore -hole chemicals to ensure that the excavation does not collapse. During this period, it can be expected that chlorides from the surrounding groundwater permeate the excavation walls. Therefore it follows that the reinforcing steel cage will, once it is lowered, be immediately exposed to potentially high levels of chloride.  

The second reason that diaphragm walls are a challenge in terms of ensuring durability, is the exposure after construction. On one side the reinforcing steel is exposed to highly saline sea-water, while on the soil side, the wall is exposed to highly saline ground-water. The wall is seemingly sandwiched between to very corrosion environments.

By deploying a well-designed cathodic protection system using impressed current, the corrosion of the reinforcing steel can be stopped or significantly reduced to a negligible amount. Cathodic protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. A simple method of protection connects the metal to be protected to a more easily corroded "sacrificial metal" to act as the anode. These systems (also known as sacrificial cathodic protection)  can be designed with a service of approaching 20 years.

For longer service life systems, the ‘sacrificial metal’ method above is not sufficient. Metals made from coated titanium cast into the concrete matrix. These provide a means to create electrochemical cell, by introducing direct current onto the steel to be protected, in this case the reinforcing steel. The source of the direct current is typically located close to the structure in an electrical room. These systems can be designed in excess of 50 years.

Once deployed, these systems can be monitored with extreme ease remotely. Many asset owners fret about the upkeep of these systems. In fact, the upkeep is insignificant. With IoT devices, Ducorr has already developed systems that can be monitored from your smart phone or tablet, sending you updates at intervals you chose.

These systems are extremely powerful durability tools providing a hands-on and active approach for managing corrosion – almost unheard off a few decades ago.

EARLY CORROSION DETECTION TO AVOID COSTLY REPAIRS OF CONCRETE STRUCTURES

The protection of assets from corrosion is a key commercial, safety and environmental issue.

Deterioration of concrete structures can become a challenge for the owners of structures such as bridges, walkways, high rise buildings, etc. It is important to identify these defects on time and plan appropriate repair strategies. Concrete deterioration can occur through scaling, disintegration, erosion, corrosion of reinforcement, delamination, spalling, alkali-aggregate reactions, and cracking of concrete. Moreover, corrosion of reinforced steel is the main cause in modern concretes.

Successful contractors understand the importance of adding value to their clients' assets/structures. One of the best ways to do this is to offer additional services that provide a cost-effective benefit to the client. Contractors can provide value added service to their clients through the application of cathodic protection. Cathodic protection system stops the corrosion cycle in concrete by utilizing an electrical current. It can be an add-on service for the concrete contractor and a cost-effective benefit to the client.

How does cathodic protection work?

In the simplest terms, a small DC electrical current is discharged off of an anode and flows through the concrete to the reinforcing steel. This protective current prevents corrosion from occurring. A small power supply unit converts AC power available at the site to DC power to provide the negative charge, which is used to arrest the natural corrosion process. Typically these systems use very little power -- not much more than a conventional 120 Watt electric light bulb. The contractor has a wide range of decorative top coats available to finish the process while meeting the aesthetic requirements of the project. For more than 20 years, this proven technology has been employed successfully on numerous installations in coastal environments.

Contractors should be encouraging their clients to consider cathodic protection when major repair projects are undertaken. The first reason is the most important -- quite simply, cathodic protection stops the repair cycle by preventing further corrosion. When the client/owner completes a major concrete repair only to find that more corrosion is occurring just a few years later, there is an unhappy client eager to blame the initial repair contractor. Cathodic protection stops future corrosion which in turn stops the vicious restoration cycle.

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

Corrosion due to Accelerated Low Water Corrosion (ALWC)

Accelerated Low Water Corrosion (ALWC) is a relatively new phenomenon, and an extreme form of aggressive corrosion, that majority of the time occurs slightly above Lowest Astronomical Tide (LAT) level, and is reported to have occurred along submerged sections. The occurrence is on unprotected steel in tidal areas. The cause of this is due to bacterial activity, and is therefore a microbial influenced corrosion (MIC). This occurs when sulphate resisting bacteria, an anaerobic bacteria, grow on steel forming a colony, if growth is sustained for long enough it forms a biofilm. This patch of bacteria does not directly consume the steel; however, it promotes and aggressively increases the rate of corrosion as it makes the ideal environment for it.

According to CIRIA C634 this process is random, and a successful method for predicting its occurrence has not been developed. Cases of ALWC have been reported from around the world in all tidal areas, and cause of its occurrence has not been truly understood. Its high variability is baffling as variation occurs in the local geography, where some piles are found with ALWC and some piles within the same vicinity are found to be free of it. The time scale is variable also as it is a multi-stage process and not linear like in table 2, which underestimates ALWC, as the rate of corrosion varies depending on the micro-environment. However, once the biofilm has formed rates of metal wastage is very high, making it possible to see patches within a couple of years. As a rule of thumb localised corrosion rates are 1.5 to 3 times more than the general uniform corrosion rates.

Currently the only reliable method of detecting ALWC is by visual inspection together with residual wall thickness measurements. ALWC occurs as localised patches of damage, identified by a characteristic, poorly-adherent orange corrosion product over a 'soupy' black underlayer associated with rapid metal thinning. 

The strategy for management of ALWC will depend on whether the structure is new built or an existing structure. The corrosion protection measures that are currently applicable to ALWC are those based on conventional corrosion control methods such as cathodic protection (CP) and coatings of various types. 

Concrete Cancer

The Middle East is well known for the presence of a very aggressive salty water table that sits barely a few meters below the surface. As we all know, salt and water coupled with heat are the perfect blend to create corrosion nightmare of concrete structures.

Some Facts

Concrete Cancer, often identified by flaking concrete or rust stains, which originate deep within the concrete is a serious problem caused by corroding/rusted reinforcing steel from within the concrete. As steel rusts it can expand up to 7 times its original size causing the surrounding concrete to crack. As the steel pushes the concrete away, more water gets to the steel expediting the process.

The process is generally due to:

·       Presence of large quantities of water and salt

·       The ends of reinforcing being too close to the surface allowing water to seep through concrete and react with the steel

·       Poorly treated reinforcing steel being used in the original pour of the slab

·       Fractures in the concrete allowing water to penetrate the concrete and react with the steel

What do we do?

Spalled concrete can be a safety hazard. Concrete cancer and delaminated concrete should be treated immediately as deferring the treatment will inevitably lead to increased problems into the future.

Similarly, treating the visual aspects such as rendering over the steel are short-term solutions as the rusting process will continue below the surface causing the steel to again displace the concrete and in some cases, rust so badly the steel eventually needs replacement. This approach – we call it the ‘make up’ approach – is aesthetic. In essence, the ugly bits are removed and given a nice clean looking finish, however the underlying problem is very much still present. Within a short time, the area adjacent to the area repaired is cracking and breaking and requires repair. You are back to square one.

The Real Stuff…

The appropriate and effective treatment necessary is cathodic protection – an electrochemical method of arresting corrosion for an extended period of time – ranging from 5 years to 50 years.

Ducorr’s SHIELD™ technology is easy to install into dilapidated atmospherically exposed concrete areas and achieve excellent corrosion protection. The system uses permanent power to provide sustained protection by simply making the corrosion reaction impossible to occur. There’s lots of thermodynamic theory behind, which would be too long for this article – but in essence cathodic protection is the ONLY method that address corrosion at an elemental level eliminating the possibility of any further damage.

The Dubai Water Canal is key infrastructure project that involves the construction of water canal that routes just east of Sheikh Zayed Road to the Jumeirah beach. The canal mainly consists of block wall construction. However, in a minor section of the canal, the construction incorporates a reinforced concrete diaphragm wall. The project specification requires that the reinforcing steel of this diaphragm wall be protected from corrosion using cathodic protection designed and installed by DUCORR.

Contact us to deploy your system now.