introduction in Non Destructive Testing

introduction in Non Destructive Testing

Non-destructive testing refers to a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component, or system without causing damage to it. methods are employed to detect defects, flaws, discontinuities, or irregularities in materials or structures, ensuring their reliability, safety, and performance. These tests are often used during manufacturing, construction, maintenance, and quality control processes. Here are some common non-destructive testing methods:

1. Visual Inspection: Visual inspection involves visually examining the surface of a material or component to detect any visible defects, such as cracks, corrosion, or surface irregularities. It is the simplest and most widely used form of.
2. Ultrasonic Testing: Ultrasonic testing utilizes high-frequency sound waves (ultrasound) to detect internal flaws or discontinuities in materials. A transducer emits ultrasonic waves into the material, and the reflected waves are analyzed to identify defects such as cracks, voids, or inclusions.
3. Radiographic Testing: Radiographic testing uses X-rays or gamma rays to inspect the internal structure of materials. X-ray or gamma-ray radiation passes through the material, and the resulting image (radiograph) reveals internal defects, such as cracks, voids, or porosity.
4. Magnetic Particle Testing: Magnetic particle testing is used to detect surface and near-surface defects in ferromagnetic materials. A magnetic field is applied to the material, and magnetic particles (usually iron filings or fluorescent particles) are applied to the surface. The particles accumulate at areas of magnetic flux leakage caused by defects, making the flaws visible.
5. Liquid Penetrant Testing: Liquid penetrant testing is used to detect surface-breaking defects in non-porous materials, such as metals, ceramics, and plastics. A liquid penetrant (dye) is applied to the surface of the material, and after a certain dwell time, excess penetrant is removed. A developer is then applied to draw out the penetrant from any defects, making them visible.
6. Eddy Current Testing: Eddy current testing utilizes electromagnetic induction to detect surface and subsurface defects in conductive materials. An alternating current is passed through a coil, creating eddy currents in the material. Changes in the eddy currents caused by defects are detected and analyzed to identify flaws.
7. Acoustic Emission Testing: Acoustic emission testing monitors the release of transient stress waves (acoustic emissions) from materials under stress. It is used to detect and locate active defects, such as crack growth or material degradation, in real-time.
8. Thermographic Testing: Thermographic testing uses infrared imaging to detect variations in temperature on the surface of materials. Temperature differences can indicate defects such as delamination, voids, or inclusions.

These non-destructive testing methods offer valuable insights into the integrity, quality, and performance of materials and components without causing damage, making them essential tools for ensuring safety and reliability in various industries, including aerospace, automotive, construction, energy, manufacturing, and infrastructure.

 

 

Water Pipeline Construction

 

Water Pipeline Construction

Construction steel pipelines, are an integral component of many large-scale infrastructure projects,

Serving as the lifelines for transporting fluids, gases, and other materials, over long distances.

These pipelines are constructed using high-quality steel materials, known for their durability, strength, and resistance to corrosion.

They play a crucial role in industries such as oil and gas, water supply, and transportation, facilitating the efficient and safe movement of resources, from production sites to distribution centers, or end-users.

The construction process involves meticulous planning, precise engineering, and stringent quality control, measures to ensure structural integrity, and compliance with safety standards.

 

The installation of an overhead crane, is a meticulous process that requires careful planning and skilled execution.

It begins with a thorough assessment of the site, and consideration of factors, such as building layout, load requirements, and safety regulations.

Engineers design the crane system to meet the specific needs, of the facility, including selecting the appropriate crane type, capacity, and lifting mechanism.

Once the design is finalized, the installation team begins by assembling the structural components, including the runway beams, support columns, and crane bridge.

Precision is paramount during assembly, to ensure proper alignment and structural integrity.

Electrical wiring and control systems, are then integrated, followed by rigorous testing, to ensure smooth operation and adherence to safety standards.

Throughout the installation process, safety measures are implemented to protect both workers and equipment.

Once installation is complete, thorough inspections and final adjustments, are made to ensure the crane is ready for operation.

Effective installation of an overhead crane is essential, for optimizing workflow efficiency, and enhancing workplace safety in industrial environments.

 

Building steel storage tanks requires careful planning, and precise work.

First, experts decide where to put the tank, and how big it should be, making sure it follows all the rules, for safety and the environment.

Skilled workers then put together the tank's parts, like the shell, roof, and base, using strong steel materials.

They pay close attention to welding, making sure everything is strong and won't leak.

Sometimes, they use special machines to help them weld faster and better.

After the tank is built, it's tested to make sure it can hold up under pressure and won't break.

Then, coatings are added inside and outside to stop rust and make the tank last longer.

Making steel storage tanks, takes a lot of care and hard work, but it gives us sturdy containers to store things safely.

 

Constructing steel spherical tanks, is a bit like building giant balls to hold things.

First, experts decide where to put the tank and how big it should be.

Then, they start by making a strong frame, like the skeleton of the tank.

They use steel plates to cover the frame, and make a round shape, just like a ball.

Workers weld the plates together carefully so they don't come apart.

Once the tank's shape is complete, they check to make sure it's strong and won't leak.

Finally, they add special paint to protect the tank from rust.

Building steel spherical tanks needs careful planning and strong teamwork, but when it's done right, it gives us safe places to store important things.

 

surge vessel, also known as a surge tank or surge drum, lies in its ability to regulate water pressure and mitigate the damaging effects, of water hammering in a piping system.

By absorbing excess pressure caused by rapid changes in flow rate, the surge vessel helps prevent pipe bursts, equipment damage, and system failures.

Additionally, surge vessels contribute to the overall stability and efficiency of water distribution networks, ensuring consistent and reliable performance.

Their value extends beyond monetary terms, as they play a crucial role in safeguarding infrastructure integrity and maintaining uninterrupted water supply, thereby supporting public safety and community well-being.

 

Transporting and installing a steel surge vessel for water hammering requires careful planning and precise execution.

Firstly, logistics experts coordinate the transportation, selecting suitable trucks or trailers equipped to handle the weight and size of the vessel.

Upon arrival at the installation site, crane operators carefully lift the vessel and maneuver it into position, ensuring that safety protocols are strictly followed throughout the process.

Site engineers oversee the foundation preparation, ensuring it's level and stable to support the weight of the vessel.

Skilled technicians then meticulously connect the vessel, to the water system, integrating pipes, valves, and fittings with precision to prevent any leaks or disruptions.

Each connection is thoroughly inspected and tested to guarantee its integrity.

Once the vessel is securely installed, comprehensive tests are conducted, to assess its performance under various operating conditions, including water pressure simulations.

Any necessary adjustments are made to optimize its efficiency in mitigating water hammering effects.

Throughout the entire transportation and installation process, a keen focus on safety measures, and quality control is maintained to ensure the successful integration, of the surge vessel into the water system.

 

 

 

Pipe Fittings

 

Pipe Fittings:

Pipe fittings are components used to connect, terminate, or redirect pipes or tubing sections in a piping system. They come in various shapes, sizes, and materials to accommodate different piping requirements and applications. Here are some common types of pipe fittings:

1. Elbows: Elbows are fittings used to change the direction of piping systems, typically at 90 degrees or 45 degrees angles. They allow pipes to bend and navigate around obstacles or corners while maintaining the flow of fluids.
2. Tees: Tees are fittings with a T-shaped design used to branch off a main pipeline into two or more directions. They are commonly used in piping systems where multiple branches are required.
3. Reducers: Reducers are fittings used to connect pipes of different sizes, diameters, or types. They help transition between pipes with different dimensions while maintaining a smooth and continuous flow.
4. Couplings: Couplings are fittings used to join two pipes or tubing sections together in a straight line. They provide a secure and leak-proof connection between pipes and are available in various types, including threaded couplings, socket weld couplings, and compression couplings.
5. Flanges: Flanges are flat, circular fittings with bolt holes used to connect pipes, valves, or equipment to each other or to a structure. They provide a strong and reliable connection that can be easily assembled and disassembled for maintenance or repair purposes.
6. Unions: Unions are fittings that allow for easy disassembly and reassembly of pipes without the need for cutting or threading. They consist of two parts that can be quickly connected or disconnected using nuts and bolts or threaded connections.
7. Caps and Plugs: Caps and plugs are fittings used to seal the ends of pipes to prevent the escape of fluids or contaminants. Caps are typically used to seal the ends of open pipes, while plugs are used to seal threaded or unthreaded openings.

Valves:

Valves are mechanical devices used to control the flow, pressure, and direction of fluids within piping systems. They open, close, or regulate the flow of fluids by means of a movable element, such as a gate, ball, globe, butterfly, or plug. Here are some common types of valves:

1. Gate Valves: Gate valves are linear-motion valves with a sliding gate or wedge-shaped disc that controls the flow of fluids by moving perpendicular to the direction of flow. They provide a tight seal and are commonly used in on/off applications.
2. Ball Valves: Ball valves are quarter-turn valves with a rotating ball-shaped disc that controls the flow of fluids by opening or closing a passageway. They offer quick and reliable operation and are suitable for both on/off and throttling applications.
3. Globe Valves: Globe valves are linear-motion valves with a disc or plug that moves up and down to regulate the flow of fluids. They provide precise control of flow rates and are commonly used in applications requiring throttling or regulation.
4. Butterfly Valves: Butterfly valves are quarter-turn valves with a rotating disc or vane that controls the flow of fluids by turning perpendicular to the direction of flow. They offer low pressure drop and are commonly used in large-diameter piping systems.
5. Check Valves: Check valves are one-way valves that allow fluids to flow in one direction and prevent backflow in the opposite direction. They are used to prevent reverse flow and protect equipment from damage.
6. Pressure Relief Valves: Pressure relief valves are safety devices used to control and limit the pressure within a piping system by releasing excess pressure to the atmosphere or a low-pressure system. They are essential for protecting equipment and preventing overpressurization.
7. Control Valves: Control valves are specialized valves used to regulate the flow, pressure, or temperature of fluids within a piping system. They incorporate an actuator to adjust the position of the valve's closure element based on control signals from an external source.

Pipe fittings and valves are critical components in piping systems that ensure the efficient, safe, and reliable operation of fluid transportation and control processes across various industries and applications. Proper selection, installation, and maintenance of pipe fittings and valves are essential for optimizing system performance, minimizing leaks, and ensuring compliance with regulatory standards and safety requirements.

 

Stainless Steel

Stainless Steel

Stainless steel is a versatile and widely used alloy characterized by its excellent corrosion resistance, high strength, and aesthetic appeal. It is composed primarily of iron (Fe), chromium (Cr), and varying amounts of other alloying elements such as nickel (Ni), molybdenum (Mo), and manganese (Mn). Here are more details about stainless steel:

Types of Stainless Steel:

Austenitic Stainless Steel: The most common type of stainless steel, characterized by its high corrosion resistance, excellent formability, and weldability. Austenitic stainless steels contain nickel and/or manganese to stabilize the austenitic microstructure at room temperature. Examples include grades such as 304 (18-8 stainless), 316 (marine grade), and 321 (stabilized with titanium).

Ferritic Stainless Steel: Contains chromium but no nickel, making it less expensive and magnetic. Ferritic stainless steels offer good corrosion resistance in mildly corrosive environments and are often used in automotive exhaust systems, appliances, and architectural applications.

Martensitic Stainless Steel: Contains higher levels of carbon compared to austenitic and ferritic stainless steels, resulting in increased hardness and strength through heat treatment. Martensitic stainless steels are used in applications requiring high wear resistance, such as cutlery, surgical instruments, and industrial blades.

Duplex Stainless Steel: Combines the benefits of austenitic and ferritic stainless steels, offering high strength, corrosion resistance, and resistance to stress corrosion cracking. Duplex stainless steels are used in marine environments, oil and gas processing, and chemical processing industries.

Properties:

Corrosion Resistance: Stainless steel exhibits excellent resistance to corrosion, oxidation, and staining in a wide range of environments, including acidic, alkaline, and chloride-containing environments.

Strength and Toughness: Depending on the alloy composition and heat treatment, stainless steel can offer a combination of high strength, ductility, and toughness, making it suitable for structural and load-bearing applications.

Hygienic and Easy to Clean: Stainless steel surfaces are smooth, non-porous, and resistant to bacterial growth, making them ideal for applications in food processing, pharmaceuticals, and medical devices.

Aesthetic Appeal: Stainless steel has a shiny, reflective surface finish that is aesthetically pleasing and can be polished or brushed to achieve different appearances.

High Temperature Performance: Certain grades of stainless steel exhibit good oxidation resistance and retain strength at elevated temperatures, making them suitable for high-temperature applications such as exhaust systems and furnace components.

Applications:

Stainless steel finds applications in a wide range of industries and sectors, including: Architecture and construction. Food processing and catering equipment. Medical and healthcare. Automotive and transportation. Aerospace and defense. Chemical and petrochemical processing. Consumer goods and appliances.

Stainless steel's unique combination of properties, durability, and aesthetic appeal make it a preferred material for a wide range of applications across various industries, contributing to its widespread use and popularity. Ongoing research and development continue to improve stainless steel grades and expand their capabilities to meet evolving needs and challenges.

 

 

Introduction to the Fundamentals of Corrosion

 Introduction to the Fundamentals of Corrosion

THE SIGNIFICANT TECHNICAL CHALLENGES and the high cost directly related to corrosion provide
strong incentives for engineers and other technical personnel to develop a firm grasp on the fundamental bases
of corrosion. Understanding the fundamentals of corrosion is necessary not only for identifying corrosion
mechanisms (a significant achievement by itself), but also for preventing corrosion by appropriate corrosion
protection means and for predicting the corrosion behavior of metallic materials in service conditions.
Understanding the mechanisms of corrosion is the key to the development of a knowledge-based design of
corrosion resistant alloys and to the prediction of the long-term behavior of metallic materials in corrosive
environments.
Two major areas are usually distinguished in the corrosion of metals and alloys. The first area is where the
metal or alloy is exposed to a liquid electrolyte, usually water, and thus typically called aqueous corrosion. The
second area is where corrosion takes place in a gaseous environment, often called oxidation, high- temperature
oxidation, or high-temperature corrosion, and called gaseous corrosion here. These two areas have been (and
still are sometimes) referred to as wet corrosion and dry corrosion. This distinction finds its origin (and its
justification) in some fundamental differences in the mechanisms, in particular the electrochemical nature of
reactions occurring in aqueous solution (or in a nonaqueous electrolyte), as compared to the formation of thick
oxide layers in air or other oxidizing atmospheres, at high temperature with fast transport processes by solidstate
diffusion through a growing oxide. The separation between the two areas, however, should not be
overemphasized, because there are also similarities and analogies, for example:
· The initial stages of reaction involve the adsorption of chemical species on the metal surface that can be
described by the Gibbs equation for both liquid and gaseous environments.
· The nucleation and growth phenomena of oxide layers and other compounds
· The use of surface analytical techniques
The fundamental aspects of aqueous and gaseous corrosion are addressed in this first Section of the Handbook.
Corrosion of metallic materials is generally detrimental and must be prevented, but if it is well understood and
controlled, it can also be used in a powerful and constructive manner for electrochemical production of fine
patterns on metal as well as on semiconductor surfaces. These constructive purposes also include
electrochemical machining (down to the micro- or even the nanoscale), electrochemical and chemicalmechanical
polishing, and anodes for batteries and fuel cells. These topics are also addressed in this Section.

THE DRIVING FORCE of corrosion is the lowering of energy associated with the oxidation of a metal.

Thermodynamics examines and quantifies this driving force. It predicts if reactions can or cannot occur (i.e., if

the metal will corrode or be stable). It does not predict at what rate these changes can or will occur: this is the

area of kinetics. However, knowing from thermodynamics what reactions are possible is a necessary step in the

attempt to understand, predict, and control corrosion.

Ions that are present in the solution are charged because of the loss or gain of electrons. The positive charged

ions (cations) and negative charged ions (anions) also have an electric field associated with them. The solvent

(water) molecules act as small dipoles; therefore, they are also attracted to the charged ions and align

themselves in the electric field established by the charge of the ion. Because the electric field is strongest close

to the ion, some water molecules reside very close to an ionic species in solution. The attraction is great enough

that these water molecules travel with the ion as it moves through the solvent. The tightly bound water

molecules are referred to as the primary water sheath of the ion. The electric field is weaker at distances outside

the primary water sheath, but it still disturbs the polar water molecules as the ion passes through the solution.

The water molecules that are disturbed as the ion passes, but do not move with the ion, are usually referred to as

the secondary water sheath. Figure 1 shows a representation of the primary and secondary solvent molecules for

a cation in water. Because of their smaller size relative to anions, cations have a stronger electric field close to

the ion and more water molecules are associated in their primary water sheath. Anions have few, if any,

primary water molecules. A detailed description of the hydration of ions in solution is given in Ref 1

ONE OF THE IMPORTANT FEATURES of the electrified interface between the electrode and the electrolyte

in the aqueous corrosion of metals is the existence of a potential difference across the double layer, which leads

to the definition of the electrode potential. The electrode potential is one of the most important parameters in

both the thermodynamics and the kinetics of corrosion. The fundamentals of electrode potentials are discussed

in this article. Examples of the calculations of the potential at equilibrium are given in the article “Potential

versus pH (Pourbaix) Diagrams” in this Section of the Volume.

electrodes are left unchanged. Hence, the magnitude and the sign of the cell voltage at equilibrium (emf)

depend only on the couple of half-reactions involved. From the thermodynamic convention, the free energy

change of a spontaneous cell reaction, which liberates energy, is negative. If the emf is the potential of

electrode 2 minus the potential of electrode 1 (ΔE = E2 - E1), and if ΔrG designates the free energy of the cell

reaction written in the sense of Eq 13, that is, reduction at electrode 2 (Eq 12) and oxidation at electrode 1 (Eq

11), the relation between ΔrG and ΔE is:

ΔrG = -nFΔE (Eq 14)

where n is the number of electrons exchanged in both half-reactions, and F is the Faraday constant (96,487

coulombs) equal to the charge of 1 mole of electrons.

Using Eq 3 and 4, Eq 14 may be rewritten as:

(Eq 15)

Compared to a chemical equilibrium where, at a given temperature, the ratio is equal to a

constant Keq (Eq 6), the electrochemical cell at equilibrium is a system with one more degree of freedom,

because either the ratio of activities or the cell emf can impose on it.

If the cell reaction occurs under conditions in which the reactants and products are in their standard states, the

equation becomes:

(Eq 16)

where ΔE0 is the standard cell emf.

The Hydrogen Potential Scale. The absolute potential of an electrode, or even the potential difference between a

metal electrode and the surrounding solution, cannot be determined experimentally. It is only possible to

measure the voltage across an electrochemical cell, that is, the difference of potential between two identical

wires connected to two electrodes. A potential scale may be defined by measuring all electrode potentials with

respect to an electrode of constant potential, called the reference electrode. The reference electrode arbitrarily

chosen to establish a universal potential scale is the standard hydrogen electrode (SHE). It consists of a

platinized platinum electrode (wire or sheet) immersed in an aqueous solution of unit activity of protons,

saturated with hydrogen gas at a fugacity of 1 bar. The half-cell reaction is the equilibrium:

H+(aq) + e- H2(g) (Eq 17)

The SHE possesses the advantages of achieving its equilibrium potential quickly and reproducibly and

maintaining it very stable with time (see comparison with other reference electrodes in the article “Potential

Measurements with Reference Electrodes” in this Volume). From the convention, the SHE potential is taken as

zero. The potential of any electrode can then be determined with respect to this zero reference and is called the

potential of the electrode on the standard hydrogen scale, denoted E(SHE).

The Potential Sign Convention (Reduction Convention). Before establishing tables of standard potentials for

various electrodes on the standard hydrogen scale, it is necessary to fix the convention for the sign of a standard

potential value (E0). The potential of any electrode is expressed with respect to the SHE by building (really or

virtually) a cell in which the other electrode is a SHE. Consider a typical half-cell reaction with an oxidationreduction

(O/R) couple, where O represents the oxidized species and R the reduced species. The cell is

represented as:

Pt, H2(g)(f = 1 bar)|H+(aq)(a = 1) || O/R (Eq 18)

Depending on the position of the O/R electrode on the hydrogen scale, the spontaneous single electrode or halfcell

reaction will proceed in one direction or the other: R oxidation: cR → bO + ne- combined with proton

reduction: H+(aq) + e- → H2(g); or, O reduction: bO + ne- → cR combined with hydrogen oxidation: H2(g)

→ H+(aq) + e-.

For example, the coupling of the oxidation/ reduction couple Fe2+/Fe with the H+/H2 couple brings about the

spontaneous oxidation of iron (the free energy of the cell reaction is negative, the Fe2+/Fe electrode is

negative). The situation is entirely different with a Cu2+/Cu system. If coupled with the H+/H2 couple, the

Potential Measurements with Reference Electrodes

ELECTRODE POTENTIAL MEASUREMENT is an important aspect of corrosion studies and corrosion

prevention. It is included in any determination of the corrosion rate of metals and alloys in various

environments and in the control of the potential in cathodic and anodic protection. The potential of an electrode

can be determined only by measuring the voltage in an electrochemical cell between this electrode and an

electrode of constant potential, called the reference electrode. Many errors and problems can be avoided by

careful selection of the best reference electrode for a specific case and by knowledge of the electrochemical

principles that control the potential measurements in order to obtain meaningful measurements. A reference

electrode, once selected, must be properly used, taking into account the stability of its potential and the problem

of ohmic (IR) drop. Many different reference electrodes are available, and others can be designed by the users

themselves for particular situations. Each electrode has its characteristic rest potential value, which is used to

convert potential values measured with respect to this reference into values expressed with respect to other

references. In particular, the conversion of the potentials from or to the hydrogen scale is frequently required

for use of potential- pH diagrams, which are discussed in the article “Potential versus pH (Pourbaix) Diagrams”

in this Section of the volume.

The Three-Electrode Device

When a system is at rest and no significant current is flowing, the use of only one other electrode as a reference

is sufficient to measure the test (or working) electrode potential versus the reference potential. When a current

is flowing spontaneously in a galvanic cell or is imposed on an electrolytic cell, reactions at both electrodes are

not at equilibrium, and there is consequently an overpotential on each of them. The potential difference

measured between the two electrodes then includes the value of the two overpotentials, and it is not possible to

determine the potential of the test electrode. To obtain this value, a third electrode, the auxiliary or

counterelectrode, must be used (Fig. 1). In this arrangement, current flows only between the test and the

auxiliary electrodes. A high-impedance voltmeter placed between the test and the reference electrodes prevents

any significant current flow through the reference electrode, which then shows a negligible overpotential and

remains very close to its rest potential. Most reference electrodes can be damaged by current flow. The test

electrode potential and its changes under electric current flow can then be measured with respect to a fixed

reference potential. The three- electrode system is widely used in laboratory and field potential measurement.

Operating Conditions for Reference Electrodes

When a reference electrode has been selected for a particular application, its proper use requires caution and

specific measurement conditions. When measuring the potential of a polarized test electrode versus a reference

electrode, it is important not to polarize or damage the latter by applying a significant current density. Also, the

ohmic (IR) drop must be minimized.

Very Low Current Density. It is important to use a reference electrode that operates at its known open-circuit

potential and thus avoid applying any significant overpotential to it. This is achieved by using a highimpedance

voltmeter that has a negligible input current and, for test electrode polarization measurements, by

using an auxiliary electrode in a three-electrode system (Fig. 1).

The value tolerated for the maximum overpotential on the reference electrode, at the condition that it stays

under the limit over which the electrode suffers irreversible damages (like the calomel electrode), is a matter of

judgment that depends on the accepted magnitude of error in the particular case under investigation. The use of

an electrometer or a high-impedance voltmeter (1012 ohms) fulfills the usual requirements. When a lower

impedance instrument is used, an unacceptable overpotential could result if the electrode is too polarizable.

The IR Drop and Its Mitigation. The IR drop is an ohmic voltage that results from electric current flow in ionic

solutions. Electrolytes have an ohmic resistance; when a current passes through them, an IR voltage can be

observed between two distinct points. When the reference electrode is immersed at some distance from a

working or test electrode, it is in the electric field somewhere along the current path. An electrolyte resistance

exists along the path between the test and the reference electrodes. As current flows through that path, an IR

voltage appears in the potential measurement according to:

V = VT - VR + IR (Eq 13)

where VT is the test potential to be measured, VR is the reference electrode potential, and IR is the ohmic drop.

In this case, the liquid junction potential has been neglected. The IR drop constitutes a second unknown value in

a single equation. It must be eliminated or minimized.

The Luggin capillary is a tube, usually made of glass, that has been narrowed by elongation at one end. The

narrow end is placed as close as possible to the test electrode surface (Fig. 1), and the other end of the tube goes

to the reference electrode compartment. The Luggin capillary is filled with cell electrolyte, which provides an

electric link between the reference and the test electrode. The use of a high-impedance voltmeter prevents

significant current flow into the reference electrode and into the capillary tube between the test electrode and

the reference electrode compartment (Fig. 1). This absence of current eliminates the IR drop, and the

measurement of VT is then possible. A residual IR drop may, however, exist between the tip of the Luggin

capillary and the test electrode. This is usually negligible, however, especially in high-conductivity media.

The remote electrode technique can be used only for measurement in an electrolyte with very low resistivity,

usually in the laboratory. It is applicable, for example, in a molten salt solution, in which the ohmic resistance R

is very small. In such a case, the reference electrode can be placed a few centimeters away from the test

electrode, because the IR drop remains negligible. In other electrolytes (for example, in measurements in soils),

the ohmic resistance is rather large, and the IR drop cannot be eliminated in this manner.

The Current Interruption Technique. In this case, when the current is flowing, the IR drop is included in the

measurement. A recording of the potential is shown in Fig. 5. At time t1, the current is interrupted so that I = 0

and IR = 0.

At the moment of the interruption, however, the electrode is still polarized, as can be seen at point P in Fig. 5.

The progressive capacitance discharge and depolarization of the test electrode take some time. The potential

measured at the instant of interruption then represents the test electrode potential corrected for the IR drop.

Precise measurements of this potential are obtained with an oscilloscope.

Potential Conversion Between Reference Electrodes. Due to the number of different reference electrodes used,

each potential measurement must be accompanied by a clear statement of the reference used. It is often needed

to express electrode potentials versus a particular reference, regardless of the actual reference used in the

measurement. The procedure is illustrated in the following example. The electrode potential of a buried steel

pipe is measured with respect to a CuSO4/Cu electrode, and the value is -650 mV for a pH 4 environment. If

that value is mistakenly placed in the iron E-pH (Pourbaix) diagram (Fig. 1 in the article “Potential versus pH

(Pourbaix) Diagrams” in this Volume), it could be concluded that corrosion will not occur. This conclusion,

however, would be incorrect, because the E-pH (Pourbaix) diagrams are always computed with respect to the

SHE. It is then necessary to express the measured electrode potential with respect to the SHE before consulting

the E-pH (Pourbaix) diagram. The CuSO4/Cu electrode potential is +310 mV versus SHE, so this value must be

added to the measured potential: ESHE = -650 + 310 = -340 mV.

Calculation and Construction of E-pH Diagrams

Potential-pH diagrams are based on thermodynamic calculations. The equilibrium lines that set the limits

between the various stability domains are calculated for the various electrochemical or chemical equilibria

between the chemical species considered. There are three types of reactions to be considered:

· Electrochemical reactions of pure charge (electron) transfer

· Electrochemical reactions involving both electron and solvated proton (H+) transfer

· Acid-base reactions of pure H+ transfer (no electrons involved)

Pure Charge Transfer Reactions. These electrochemical reactions involve only a reduced species on one side

and an oxidized species and electrons on the other side. They have no solvated protons (H+) as reacting

particles; consequently, they are not influenced by pH. An example of a reaction of this type is the oxidation/

reduction of Ni/Ni2+: Ni Ni2+ + 2e-. From the Nernst equation (Eq 1), the equilibrium potential for the

couple Ni2+/Ni can be written

Molten Salt Corrosion Thermodynamics

MOLTEN SALTS—in contrast to aqueous solutions in which an electrolyte (acid, base, salt) is dissolved in a

molecular solvent—are essentially completely ionic. Thus the terms solute and solvent can be defined only in

quantitative terms. For example, the terms lose their meaning in a NaCl-AgCl melt where composition can vary

continuously from pure NaCl to pure AgCl. This is true even when the electrodes immersed in the melt are

reversible only to some of the ions in the melt. For example, in the cell:

Ag|AgCl|1NaCl|Cl2 (Eq 1)

the chlorine electrode is reversible to Cl- and the silver electrode is reversible to Ag+. When this cell is used to

obtain thermodynamic data, it is assumed that the cell is stable; that is, its composition does not change with

time. However, when the concentration of AgCl is very low, this will not be the case, since the silver electrode

will react spontaneously with the melt:

Ag + NaCl = AgCl + Na (Eq 2)

Thus the concentration of AgCl will spontaneously increase in the melt, and the electromotive forces (emf)

measured for the preceding cell will not be stable but will change in the direction indicating increasing Ag

concentration. The point at which this happens depends on the system.

Thermodynamics of Cells

One major use of electrochemical cells is to obtain thermodynamic data for salts. The basic thermodynamics

applicable to galvanic cells for aqueous solutions is discussed elsewhere in this Volume. Only those aspects that

are different for molten salts are emphasized in this article. Thus, for the cell given in Eq 1, the cell reactions

Methods for Determining Aqueous Corrosion
Reaction Rates.

CORROSION OF MATERIALS IN AQUEOUS SOLUTIONS is often thermodynamically possible but

kinetically limited. Therefore, it is important to determine the rates of corrosion processes. Corrosion rate

determination can serve many engineering and scientific purposes. For example, it can be used to:

· Screen available materials to find the most resistant material for a given application.

· Determine operating conditions where corrosion rates are low versus those where rates are high, by

varying conditions.

· Determine probable service lifetimes of materials forming components, equipment, and processes.

· Evaluate new alloys or treatments or existing alloys in new environments.

· Evaluate lots, heats, or treatments of materials to ensure that specified quality is achieved before release,

shipment, or acceptance.

· Evaluate environmental conditions such as new chemical species, inhibitors, or plant- operation

conditions such as temperature excursions.

· Determine the most economical means of reducing corrosion through use of inhibitors, pretreatments,

coatings, or cathodic protection.

· Determine the relative corrosivity of one environment compared to another.

· Study corrosion mechanisms.

Methods for determination of corrosion rates can be differentiated between those that measure the cumulative

results of corrosion over some period of time and those that provide instantaneous rate information. Corrosion

rates do not often increase monotonically with environmental conditions but exhibit sharp thresholds that

distinguish regions of low corrosion rates from other regions where corrosion rates are dangerously high. It is

sometimes of greater interest to define these thresholds than it is to determine the rates in the regions where

corrosion rates are high. Examples of the latter are pitting or crevice corrosion where passive films are broken

down and local corrosion rates can be extremely high. This article addresses electrochemical methods for

instantaneous rate determination and threshold determination as well as nonelectrochemical methods that can

determine incremental or cumulative rates of corrosion.

Fundamentals of Corrosion in Gases..

ENGINEERING MATERIALS are subject to deterioration when exposed to high-temperature environments.

Whether they survive or not in technological applications depends on how fast they react. The rate of corrosion

varies widely; some intermetallics (β-NiAl) react extremely slowly. Some metals (Fe) oxidize very rapidly,

whereas other metals (Cr, Co) react relatively slowly. From the chemical point of view, the gas- metal reactions

represent a broad class of heterogeneous reactions. The composition and structure of the scales produced on

metals are a key factor in their behavior in technical applications.

Historically, corrosion in gases has been primarily a problem in combustion systems. Thus, the gas-metal

reactions are usually referred to as oxidation in its broad chemical sense, whether the reaction is with pure

oxygen, water, sulfur dioxide (SO2), or whatever the gas might be. The corrosion product (oxide layer) is

termed scale. Being the corrosion product, the protective properties of scale decrease the reaction rate. The

concepts and methods developed to understand gas-metal reactions can be used to describe any arbitrary gas solid reaction at high temperature; for example, oxidation of the silicone carbide. The high temperature

corrosion is a highly technical challenge; the reason for this is that the efficiency of thermal processes and

engines increases with operating temperature. Such high- temperature service is especially damaging to most

metals because of the exponential increase of reaction rate with temperature. In most cases, corrosion resistance

at high temperatures does not accompany the good mechanical properties of structural materials; therefore,

protective coatings must be applied.

Electrochemical principles are insufficient to understand the mechanism of oxidation. For gaseous reactions, a

basic understanding of the diffusion processes is much more profitable. The first results of a high-temperature

corrosion study (not yet defined as corrosion and even diffusion) were published in 1684 by Boyle in

Experiments and Considerations about the Porosity of Bodies in Two Essays. In studying reactive diffusion in

the Cu-S system, Boyle reported the observation of interaction between copper and sulfur through examination

of metallographic cross sections. Electrochemistry and aqueous corrosion principles were developed at the

beginning of the 19th century. In 1855 Fick formulated the basic principles of diffusion in solids. The

systematic study of high-temperature oxidation began in the 1920s. In 1933 Wagner published his pioneering

paper on gas corrosion of metals. The first journal devoted to corrosion in gases, Oxidation of Metals, was

published in the 1960s.

The following articles introduce the subject of gas corrosion to professional engineers and students. A brief

summary of thermodynamic concepts is followed by an explanation of the defect structure of solid oxides and

the effect of these defects on the rate of mass transport. Commonly observed kinetics of oxidation are described

and related to the observed corrosion mechanisms, as illustrated in Fig. 1 of the next article, “Thermodynamics

of Gaseous Corrosion.”

In high temperature gaseous corrosion, the oxidant first adsorbs on the metal surface in molecular (physical

adsorption) and ionic form (chemical adsorption), and it may also dissolve in metal. Oxide nucleates at

favorable sites and most commonly grows laterally, due to surface diffusion, to form a complete thin film

(scale). As the scale thickens, it provides a protective barrier to shield the metal from the gas. For scale growth,

electrons must move through the film to reach the oxidant atoms adsorbed on the surface, and oxidant ions

and/or metal ions must move through the scale barrier. Diffusion of the oxidant into the metal may result in

internal oxidation.

Growth and thermal stresses in the oxide scale may create microcracks and/or delaminate scale from the

underlying metal. Stresses affect the diffusion process and modify the oxidation mechanism and very often

cause scale spallation. Improved oxidation resistance can be achieved by developing better alloys, by applying

protective coatings, and by altering the composition of the gas phase.

Thermodynamics of Gaseous Corrosion

METALS can react chemically when exposed to air or to other more aggressive gases. The reaction rate of

some metals is so slow that they are virtually unattacked, but for others, the reaction can be violent. As with

most chemical processes, elevated-temperature service is more severe because of the exponential increase in

reaction rate with temperature.

The most common reactant is oxygen in the air; therefore, all gas-metal reactions are usually referred to as

oxidation, using the term in its broad chemical sense whether the reaction is with oxygen, water vapor,

hydrogen sulfide (H2S), or whatever the gas might be. Throughout this article, the process is called oxidation,

and the corrosion product is termed an oxide.

Corrosion in gases differs from aqueous corrosion in that electrochemical principles do not help greatly in

understanding the mechanism of oxidation. For gaseous reactions, a fundamental knowledge of the diffusion

processes involved is much more useful. The principles of high-temperature oxidation began to be understood

in the 1920s, whereas electrochemistry and aqueous corrosion principles were developed approximately 100

years earlier. The first journal devoted to corrosion in gases, Oxidation of Metals, began publication in 1970.

This article addresses thermodynamic concepts; the commonly observed kinetics of oxidation are described in

the article “Kinetics of Gaseous Corrosion Processes” in this Volume.

The mechanisms of oxidation are shown schematically in Fig. 1. The gas is first adsorbed on the metal surface

as atomic oxygen. Oxide nucleates at favorable sites and most commonly grows laterally to form a complete

thin film. As the layer thickens, it provides a protective scale barrier to shield the metal from the gas. For scale

growth, electrons must move through the oxide to reach the oxygen atoms adsorbed on the surface, and oxygen

ions, metal ions, or both must move through the oxide barrier. Oxygen may also diffuse into the metal.

Growth stresses in the scale may create cavities and microcracks in the scale, modifying the oxidation

mechanism or even causing the oxide to fail to protect the metal from the gas. Improved oxidation resistance

can be achieved by selection of suitable alloys for the given environment and by application of protective

coatings.

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