Asset Integrity Management for Offshore and Onshore Structures

By Mohamed A. El-Reedy

Oil and gas assets are under constant pressure and engineers and managers need integrity management training and strategies to ensure their operations are safe. Gaining practical guidance is not trained ahead of time and learned on the job. Asset Integrity Management of Offshore and Onshore Structures delivers a critical training tool for engineers to prepare and mitigate safety risk. Starting with a transitional introductory chapter, the reference dives into integrity management approaches including codes and standards. Inspection, assessment, and repair methods are covered for offshore, FPSO, onshore and pipelines. Suggested proactive approaches and modeling risk-based inspection are also included. Supported with case studies, detailed discussions, and practical applications, Asset Integrity Management of Offshore and Onshore Structures gives oil and gas managers a reference to extend asset life, reduce costs, and minimalize impact to personnel and environment.

• Bridge between the theory of integrity management into oil and gas application
• Understand the strategies and techniques to mitigate corrosion affect, assessment, inspection, and repairs from real-world examples
• Manage a variety of assets including offshore, subsea, pipelines, and onshore

• Language: English
• Publisher: Elsevier Science
• Release date: May 11, 2022
• ISBN: 9780323859011

Mohamed A. El-Reedy

Mohamed A. El-Reedy's background is in structural engineering. His main area of research is reliability of concrete and steel structures. He has provided consulting to different engineering companies, the oil and gas industries in Egypt, and international oil companies such as the International Egyptian Oil Company (IEOC) and British Petroleum (BP). Moreover, he provides different concrete and steel structure design packages for residential buildings, warehouses, and telecommunication towers, and for electrical projects with a major oil and gas construction company in Egypt. He has participated in Liquified Natural Gas (LNG) and Natural Gas Liquid (NGL) projects with international engineering firms. Currently, Dr. El-Reedy is responsible for reliability, inspection, and maintenance strategy for onshore concrete structures and offshore steel structure platforms. He has performed these tasks for a hundred structures in the Gulf of Suez and the Red Sea. Mohamed earned a PhD in structural engineering, a M.Sc degree in materials and concrete technology, and a B.Sc. in civil engineering, all from Cairo University.

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Preface

Most oil and gas companies first began to develop asset integrity management approaches at the end of 1980 and the beginning of 1990. At that time, around 40% of the facilities worldwide were aging. Many oil and gas projects began in the 1950s and the numbers increased in the 1960s, so many facilities were over 30 years old, and doubts were arising about the direction of maintenance approaches from the point of view of equipment economics and reliability.

Mechanical engineers had been pioneers in maintenance philosophies and approaches, due mainly to their dealings with rotating equipment; however, due to the differing nature of structures and other static equipment, they were a step behind in their development in this area. The run-to-failure or corrective maintenance approaches were proving to be expensive in general and could affect production and increase the risk of investments; therefore there was a new direction to develop a system to maintain the liquid inside the equipment with zero leakage. From that philosophy came the approach of asset integrity management.

This term refers to managing the integrity between different entities with different scopes, to achieve general maintenance of an asset; thus this task cannot be done by one entity. However, you will find many companies and other entities on the market who will tell you that they are doing asset integrity management. Take care: this is largely a marketing issue. Even the best companies can achieve management of only two or three elements but not all, as the integrity of the asset should be managed between the owner, the operator (if different), the surveillance and inspection company for onshore and offshore, integrity experts to diagnose the problems, the engineering firm for evaluation and assessment using complicated analyses like pushover or finite element, the engineering firm or integrity experts to define the structure of the static facilities and its fitness for service, the owner or operator integrity team so the owner can define a maintenance plan, and software developers to collect and archive all these data.

So, in the end, the owner is responsible for managing the integrity between many entities to maintain the asset.

This book is a blend of practical coverage and theoretical background of integrity management elements for pipelines, tanks, and offshore and concrete structures. The corrosion phenomena is discussed in detail, as it is the main driver of any deterioration. The structure probability of failure and its reliability as part of integrity management is also discussed. Inspection methods and up-to-date techniques are presented in detail, and assessment and evaluation of offshore and onshore structures, tanks, and pipelines are illustrated. Ways to protect structures from corrosion are also examined in this book. The integrity management system and procedures, in addition to qualitative and quantitative risk assessment with real-world examples, are presented in the last chapter.

In this book I have attempted to cover all aspects of structure integrity management systems from a practical point of view with a strong theoretical background, which is my approach for all my books.

Mohamed A. El-Reedy, PhD, Cairo, Egypt

Chapter 1: Corrosion effects on offshore and onshore structures

Abstract

Corrosion phenomena are discussed in detail in this chapter. The characteristics of corrosion of offshore structures are examined as well as the corrosion of the steel bars inside concrete, which also affects the integrity of onshore structures. Corrosion in seawater and pipeline corrosion are also discussed. The different types of corrosion are presented as well as practical approaches to maintenance.

Keywords

Corrosion; Rust; Steel in concrete; Chloride attachment; Carbonation process; Corrosion rate; Polarization

1.1: Introduction

Corrosion is the main factor affecting deterioration of structures offshore and onshore, and specifically in plants near the coast, which includes most oil and gas plants worldwide. The main driver in structural integrity management is corrosion; repair projects are even referred to as brown field projects, as an indication of corrosion effects. Thus it is mandatory to know the basics and the features of corrosion, which are the main topics of this chapter.

Many textbooks examine corrosion phenomena through in-depth presentations of chemical reactions and electrical theories. However, the objective of this chapter is to describe corrosion in a simple form that matches the requirements of engineering aspects, in order to discuss the effects of corrosion on onshore and offshore structures.

In general, the main factors in any type of corrosion are the environmental conditions surrounding the structure. The variability and characteristics of these factors cannot be defined easily. Practically, the main characteristics of the environment are that it is variable and its condition can change over time.

The local environment at the metal surface is the microenvironment that affects the metal. The real corrosion damage is mainly affected by the local environment reactivity. Therefore local effects like flow, pH cells, deposits, and galvanic effects should be considered in experimental investigations with the nominal environmental conditions to define the lifetime prediction.

In general, corrosion has begun when rust is present on any steel surface. The main two drivers of corrosion are chemical reactions and electrical current. Once corrosion of the steel occurs, in a hole that contains water, electrons are obtained according to Eq. (1.1), which represents an anodic reaction. The nature of offshore structures is that they are exposed continuously to saline water throughout their lifetime. Therefore water is the main factor that causes corrosion in offshore structures. It is thus important to look at the reactions of iron (Fe) with water (H2O). The reaction between steel ions and water in the presence of oxygen accumulates a solution at anodic areas in an amount that is chemically equivalent to the reaction at cathodic areas as well; these reactions are presented in Fig. 1.1. In general, the corrosion process on the steel surface conforms to the anodic chemical reaction at the anodic area as per the chemical reaction in Eq. (1.1).

si1_e    (1.1)

Fig. 1.1

Fig. 1.1 Corrosion process on the steel surface.

If the electrons accumulate on one part of the steel rather than the other part, another chemical reaction will occur as a combination of the electrodes with oxygen and water. This is called a cathodic reaction and its equation is given as:

Cathodic Reaction:

si2_e    (1.2)

In Eq. (1.2), it is clear that the presence of (OH−) occurs due to a cathodic reaction. The hydroxide ions affect the alkalinity and slightly reduce the effect of chlorides. Based on that, it is obvious that the presence of water and oxygen is the main cause for corrosion.

As shown in the preceding equations and Fig. 1.1, the first step in the corrosion process is the anodic and cathodic reaction, as the hydroxide ions (OH−) will react with ferrous iron (Fe² +) as result of chemical Eq. (1.1). This reaction will produce ferrous hydroxide, which will react with oxygen and water again and produce ferric hydroxide. The equations of the chemical reactions are as follows:

si3_e    (1.3)

si4_e

   (1.4)

si5_e

   (1.5)

From these chemical reactions, the ferrous hydroxides (Fe(OH)2) will react with oxygen and water to produce ferric hydroxides (Fe(OH)3) and this component is rust; its chemical name is hydrated ferric oxide and the chemical term is Fe2O3·H2O.

Saturated Fe(OH)3 is almost neutral in pH. Magnetic hydrous ferrous ferrite, Fe3O4 _ nH2O, can form a black intermediate layer between hydrous Fe2O3 and FeO. This is the reason behind the fact that rust films normally consist of three layers of iron oxides in different states of oxidation.

1.2: Corrosion effects on offshore structure

1.2.1: Corrosion in seawater

Many industries are affected by seawater, such as shipping, FPSO, offshore oil and gas facilities, power plants, and coastal industrial plants. Seawater is used in many applications, such as cooling purposes, firefighting systems, oilfield water injection, and desalination plants. For many years a great deal of research and development has been carried out, studying corrosion problems in these systems; however, after all these studies have been done on the behavior of materials in seawater, failures are still occurring.

Dissolved materials concentrations in the sea vary greatly with location and time, because rivers dilute seawater, as do rain and melting ice, and seawater can be concentrated by evaporation. The most important properties of seawater are as follows:

•Remarkably constant ratios of the concentrations of the major constituents worldwide

•Sodium chloride (salt) exists with high salt concentration

•High electrical conductivity

•Relatively high and constant pH

•Buffering capacity

•Solubility of gases, of which oxygen and carbon dioxide in particular are important in the context of corrosion

•Organic compounds exist

•Biological life is represented as microfouling, such as bacteria and slime, and macrofouling, such as seaweed, mussels, barnacles, and many kinds of fish. All these biological effects depend on depth, temperature, intensity of light, and the availability of nutrients. The salinity of seawater is its only main specification.

In 1902, salinity was defined as the total amount of solid material, weighed by grams, contained in 1 kg of seawater. This was expressed by Eq. (1.6), where S is the salinity and Cl is chlorinity, which are expressed as parts per thousand (‰).

si6_e    (1.6)

Based on this equation, at zero chlorinity the salinity is 0.03%, which required more investigation. In 1969, the United Nations Scientific, Education and Cultural Organization (UNESCO) defined a more accurate relationship between chlorinity and salinity. The proposed equation is as follows:

si7_e    (1.7)

It can be noted that there are no major differences between these two equations and their results are the same at salinity equal to 35%.

In 1978, another salinity definition was raised in terms of conductivity ratio, which is given by this equation:

si8_e

   (1.8)

where S is the salinity of a sample of seawater, K is the seawater sample electrical conductivity at 15°C and 1 standard atmosphere pressure, to that of a potassium chloride (KCl) solution in which the mass fraction of KCl is 0.0324356, at the same temperature and pressure.

Note that K is equal to a practical salinity of 35.

It is worth mentioning that this definition (‰) is not used; however, an old value of 35‰ corresponds to a new value of 35. In general, the electrical conductivity falls within the range 32‰ to 35‰.

It is found that when corrosion occurs, the anodic reaction rate is exactly equal to the cathodic reaction rate. In an environment having good conductivity, as in seawater or seabed mud, the corroding metal displays a single potential that lies between Ec and Ea. In Fig. 1.2A, this condition is met where the anodic and cathodic curves cross. The potential at the crossover point is referred to as the corrosion potential, Ecor. The current, Icor, is referred to as the corrosion current, and it is an electrical current that represents the corrosion rate. In practice, a corroding metal does not take up potential Ea or Ec, but spontaneously moves to Ecor.

Fig. 1.2

Fig. 1.2 Evan's diagram.

While the shape of the individual E − log I curves may vary, as it depends on environmental conditions, the manner in which the diagrams, so-called polarization diagrams, are interpreted in terms of Ecor and Icor remains the same.

Fig. 1.2A represents the Evans diagram, showing the polarization curves for separate cathodic and anodic reactions. When the cathodic and anodic current densities are equal, the two curves shall intersect at the point that defines the corrosion rate in terms of a mean corrosion current density (icor). The electrode potential of the couple at this point is termed the corrosion potential (Ecor).

In some time at the metal surface, there is a difference between the electrode potentials developed at the anodic and cathodic sites. It may be the amount is significant in ohmic drop under conditions where corrosion macrocells are formed. This happens if the anodic and cathodic areas are separated by a medium of high electrolytic resistance. Therefore the Evans diagram will be modified as shown in Fig. 1.2D. Thus the mean corrosion rate Icor is now reduced and the corrosion potential varies with the location between the limits anode Ecor and cathode Ecor, the positions of local anodes being indicated by the region of low corrosion potential. If the anodic reaction is steeply polarized, such as owing to the presence of a passive film, then icor is small and Ecor assumes a value close to the reversible potential of the cathodic reaction, as shown in Fig. 1.2C. On the other hand, if the cathodic reaction is steeply polarized, such as owing to limited oxygen availability, the situation is as shown in Fig. 1.2B, with icor again small but the corrosion potential close to the reversible potential of the anodic reaction.

1.2.2: Steel corrosion in seawater

The corrosion of steel in seawater and also in seabed mud can be adequately represented by Eq. (1.1), although the process normally proceeds to the precipitation of ferric hydroxide. In the case of clean steel in seawater, the anodic process occurs with greater facility than the cathodic. In consequence, the corrosion reaction can go no faster than the rate of cathodic oxygen reduction. The latter usually proves to be controlled by the rate of arrival of the oxygen at the metal surface, which in turn is controlled by the linear water flow rate and the dissolved oxygen concentration in the bulk seawater.

Fig. 1.3 shows the corrosion in the bracing of an offshore structure.

Fig. 1.3

Fig. 1.3 Corrosion on a jacket structure.

This may be represented on a polarization diagram as shown in Fig. 1.4. At first, the cathodic kinetics get faster as the potential becomes more negative from Ec. This has the effect of depleting the oxygen immediately adjacent to the metal surface, thus causing the reaction to be more difficult. Ultimately, a point is reached where the surface concentration of oxygen has dropped to zero and oxygen can then only be reduced as and when it reaches the surface. Further lowering of the potential cannot increase the cathodic reaction rate, because the kinetics are now governed by potential-independent diffusion processes. A plateau, or limiting, current is observed.

Fig. 1.4

Fig. 1.4 Polarization diagram with increasing oxygen concentration.

Fig. 1.4 shows that the corrosion rate is then equal to this limiting current. It is worth mentioning that the limiting current can be increased by increasing the oxygen flow, either by raising the bulk oxygen concentration or increasing the flow rate; both cause the corrosion rate to increase.

To a first approximation, it can be stated that the corrosion rate of clean steel in aerated seawater under turbulent flow conditions is directly proportional to the bulk oxygen concentration and the linear velocity.

In 1994 Ashworth used a technique to estimate the maximum corrosion rates of clean steel in North Sea water at 7°C, as illustrated in Table 1.1.

Table 1.1

In practice, corrosion products and marine fouling build up on steel as it corrodes in seawater. These generally produce lower corrosion rates.

In 1994 Rowlands suggested that the corrosion rate of fully immersed steel is fairly rapid in the first few months of exposure, but it falls progressively with time. A value of 0.13 mm/annual-year is considered a good estimation in any part of the world. However, pits may grow at 3 to 10 times more than that rate.

In the case of offshore structures in shallow waters, they are simultaneously exposed to a number of discrete corrosive environments such as the marine atmosphere, the splash zone, the tidal zone, the fully submerged zone, and the mud zone. There are different results for those discussed in the corrosion rate data presented in 1949 in the work of Humble. The peak corrosion rates are found immediately below the mean low tide zone and in the splash zone. The typical mean corrosion rate in the splash zone, given quiet sea conditions, is estimated as 0.25 to 0.75 mm/a.

Fig. 1.4 shows the polarization diagram representing control of the corrosion rate by controlling the rate of oxygen arrival at the surface and the effect of increasing oxygen availability.

The peak corrosion rate is often attributed to galvanic action between steel in contact with the oxygen-rich surface waters, which is the cathodic area, and the steel at somewhat greater depth exposed to waters of lesser oxygen content, which is the anodic area. Really, it is difficult to conceive that the change in oxygen concentration with depth is sufficiently great enough to cause the effect. The corrosion effect in the topside of offshore structures is presented in Fig. 1.5.

Fig. 1.5

Fig. 1.5 Corrosion on the platform topside.

The corrosion in an FPSO vessel of one of its tanks is shown in Fig. 1.6.

Fig. 1.6

Fig. 1.6 Corrosion in FPSO tank.

1.3: Pipelines

As per a study done by the National Association of Corrosion Engineers (NACE), the cost due to corrosion in a pipeline is in the range of $5.4 billion and $8.6 billion in the United States alone. Therefore corrosion in pipelines is one of the major challenges for any pipeline integrity engineer, as it is the main reason for pipeline failure in the oil and gas industry.

The corrosion in the pipeline reduces the thickness at the same time that the pipeline is under pressure, so if the pressure is the same with time, the strength will be reduced dramatically due to the reduction in wall thickness. This reduction can cause a leak, which affects the reliability of the pipeline, as any leak will directly affect the production.

Note that low-carbon steel has been associated with susceptibility to oxidation in the presence of electrolytes, water, and carbon dioxide.

Pipeline corrosion will occur due to internal or external factors; internal corrosion is due to the fluid inside it, which is a function of the material composition of the fluid and also its velocity is considered one of the main factors. The external corrosion is due to its contact with the soil; Baker (2008) mentioned that controlling the external corrosion is mainly by coating and utilizing a cathodic protection system.

The concept of cathodic protection is producing an electric current in the pipeline to compensate for the electron movement from the anode to the cathode as per the corrosion fundamentals, as discussed in the previous section. The cathodic protection system function is to create a cathodic field over the pipeline, which implies that the anodes in the exposed surface are nonreactive. The pipe will act like a cathode, as there is a lack of movement of electrons. In addition, cathodic protection leads to the development of deposits that protect the steel since they are alkaline in nature.

Baker (2008) suggested two main types of cathodic protection, which are sacrificial anode protection by connecting the pipeline with an external metal that has a relatively higher activity than steel. This metal will be located away from the pipeline but within the electrolyte, which is the soil. As a result, there will be an electric current that transfers electrons from this metal to the pipeline and it is depilated with time. Therefore the sacrificial metal undergoes corrosion, thereby protecting the oil and gas pipeline from corrosion.

The second method is the impressed-current anode type, by producing a direct current between the pipeline and anode from an external electrical source.

Choosing between these two types depends on many factors, such as the cost and its operability and maintenance.

Another scenario involving pipelines is that the corrosion will happen inside, due to a chemical reaction between the carbon steel and the hydrocarbon inside it (Fig. 1.7).

•In the oil and gas industry, the corrosion usually occurs due to the result of chemical reactions between CO2 or H2S and steel, with water usually acting as a catalyst for the reaction. In general, the most common types of corrosion that cause a pitting corrosion in the oil and gas industry are sweet, sour, oxygen, crevice, and microbiologically induced corrosion.

Fig. 1.7

Fig. 1.7 Internal pipeline corrosion.

1.4: Onshore structure

In the past, it was stated that concrete is a more durable material than steel structures; around 1980–1990 the statement changed to concrete is a durable material but needs some precautions. It was shown by researchers Cady and Weyers (1984); Kilareski (1980); Mori and Ellingwood (1994a,b); Takewaka and Matsumoto (1988); and Thoft-Christensen (1995) that in general any reinforced concrete structure will be exposed during its lifetime to environmental stress such as corrosion or expansive aggregate reactions or others, which attacks the concrete or steel reinforcement. Based on this, it has been shown by statistics worldwide that the corrosion of reinforced steel bars is the most common problem facing structural engineers in the last 30 years.

To prevent corrosion of the steel reinforcement bars in a concrete structure, the main step is to understand the corrosion phenomenon, so that we can suggest the best method of protection and repair methods as well.

During the winter the ice accumulates over bridges in cold-weather countries like the United States, Canada, and others, and salt is used to melt the ice as a normal practice. The salt is mainly sodium chloride, which is a main source of steel corrosion. Therefore chloride attack is the main challenge in these countries and the cost is high every year for maintenance and repairs, especially for the bridge decks and piers.

Some countries like the Middle East, especially in the Gulf area as these countries are the main producers of oil and gas worldwide, have very hot weather and high humidity. Their ground water table also has a high content of salt. Thus the main challenge is the corrosion in the steel bars. Therefore in the Middle East a great deal of money is spent due to corrosion of steel bars.

The corrosion process occurs slowly and propagates with time, and the deterioration rate varies from one structure to another, depending on many factors. Corrosion of steel reinforcing bars directly affects the structure's safety. The corrosion depends mainly on the concrete quality and the environmental conditions surrounding the structure. Therefore the main factors affecting the structure safety over time are the corrosion rate, the location of the member in the structure, the type of structure member, and its redundancy.

The main challenge for an integrity management system is to predict the time period in which to do inspection or minor repairs, considering that minimizing the time interval between inspections will cost but will maintain structural safety. On the other hand, increasing the time interval between inspections can decrease the cost of inspections during the structure lifetime, but it will affect structure safety and can lead to a major repair, which is expensive. So, the main task of the integrity engineer is to define the optimum inspection time intervals.

Many accidents worldwide have occurred due to a lack of maintenance strategy. For example, a large part of a bridge in New York fell, causing a motorcycle rider to lose control of his vehicle and die. It is obvious that bad maintenance strategy, lack of understanding of the corrosion process, and not considering risk during maintenance planning, which is the main core of the integrity management system, caused a death.

In 1960 to 1970, there was worldwide acceptance of using seawater in concrete mixtures as it increases the concrete strength, which really is true. In this time period calcium chloride additives were also used as an accelerator for concrete setting; these additives were used in 1970. For example, in the Middle East in the past pure water was very expensive, and they were just starting in their building growth, so they used seawater extensively; after that, they found severe deterioration in reinforced concrete structures, causing many problems and requiring expensive rehabilitation and rebuilding.

The effects of corrosion not only create a serious problem for structural engineers but also affect national economies. Therefore engineers must have an in-depth understanding of the nature of the corrosion process and the reasons for its occurrence. Understanding the problem is the first step in solving the problem.

Many textbooks examine the corrosion process in concrete. There are two main factors: chemical reactions and electrical currents. The mix between them to match with the structural engineering requirements was illustrated by El-Reedy (2007), to describe corrosion in an easy-to-understand way so that structure engineers can take action in a timely manner. On the other hand, it is required to select the best protection method based on the environmental conditions, type of corrosion, and nature of the project.

As a rule of thumb, if the piece of steel is in dry air or is submerged in water, the piece of steel will not corrode. However, if it is submerged in water and removed and then exposed to air, and this is repeated over a cyclic period, this piece of steel will corrode.

In the case of steel bars embedded in concrete, the concrete is a porous material containing water in the voids during the pouring and after that, during the curing process or due to rainy weather or any weather with high relative humidity. Thus the concrete will contain humidity, which is a main cause of corrosion. This is contrary to the old statement from textbooks, articles, and lectures that a concrete structure is durable and has no need for inspection like steel structures; nowadays, this statement has been changed to concrete is a durable material but ….

In reality, the reinforced concrete structure is durable. The reason why the steel bars in concrete do not corrode is that concrete is alkaline in nature and alkalinity counteracts acidity. As a result of its alkalinity, concrete can protect the steel bar from corrosion. The concrete is alkaline because inside its microvoids there is a high concentration of the oxides calcium, sodium, and magnesium. After mixing water with Portland cement, a dehydration process occurs and produces very alkaline oxides and hydroxides. The measure of alkalinity and acidity is pH, which is hydrogen ion concentration. A strong acid has pH equal to 1 (or less), a strong alkali has pH equal to 14 (or more), and a neutral solution has pH equal to 7. Concrete has a pH of 12–13; steel starts to rust at a pH of 8–9.

These oxides produce hydroxides from the cement reaction with water that have a high alkalinity, with pH of 12–13. This alkalinity will produce a passive layer around the steel reinforcement surface; this layer is dense and prevents corrosion, and if the alkalinity is maintained around the steel bars corrosion will not occur during its lifetime. In this case, the old statement is valid, as the reinforced concrete structure is durable and does not need inspections.

But, unfortunately, this concrete alkalinity around the steel bars will not remain for a long time. Two main factors always have an effect in breaking the alkalinity, which are the carbonation process and the chlorides attached to the concrete from the inside or outside. The corrosion process in this case is the same as that discussed in Section 1.1 and the same chemical reactions are valid. After the passive layer is broken down, the pH changes until it reaches 8–9, which means it has reached acidity; rust will appear instantly on the steel bar's surface. The chemical reaction processes are almost similar in carbonation, sulfate, and chloride attacks. When corrosion of the reinforced steel bars in concrete occurs, they melt in the water-containing voids.

The chemical reaction in brief is the transformation of steel from ferrous hydroxide (Fe(OH)2), that will react with oxygen (O2) and water (H2O) to produce ferric hydroxide (Fe(OH)3). The ferric hydroxide is the hydrated ferric oxide, which is rust, and its chemical term is Fe2O3·H2O.

It has been found that ferric hydroxide is a major factor in concrete deterioration and spalling of concrete cover, because its volume can increase from two times to ten times or more that of its original volume. Because the steel bars are confined by concrete, if the steel bar volume increases, it will produce high stresses and the weak points will start to crack. These cracks propagate over time until the concrete cover breaks down. Then during a visual inspection rust can be seen, with its brown color, on the steel