SPECIFIC LEARNING SPECIFIC LEARNING OBJECTIVES:OBJECTIVES:

At the end of this topic you are expected At the end of this topic you are expected to:to:– Define how metallic hydroxide is formed when Define how metallic hydroxide is formed when

an iron immersed in an acidic solutionan iron immersed in an acidic solution– Define the effect of dissolved oxygen and high Define the effect of dissolved oxygen and high

acidity on polarizationacidity on polarization– State that boiler water should be alkaline and State that boiler water should be alkaline and

contain little or no dissolved oxygencontain little or no dissolved oxygen– Explain the fundamental process of corrosionExplain the fundamental process of corrosion– Name common engineering materials which Name common engineering materials which

produce passive oxide filmsproduce passive oxide films

SPECIFIC LEARNING SPECIFIC LEARNING OBJECTIVES:OBJECTIVES:

– State the main cause of corrosionState the main cause of corrosion– Name the components of galvanic cell Name the components of galvanic cell

and applies these to the corrosion of a and applies these to the corrosion of a metalmetal

– Define that seawater is an electrolyteDefine that seawater is an electrolyte– Define an anodeDefine an anode– From a list of common metals, selects From a list of common metals, selects

relative anodesrelative anodes– Define metals as being noble or base Define metals as being noble or base

relative to each otherrelative to each other– Define the use of sacrificial anodeDefine the use of sacrificial anode

SPECIFIC LEARNING SPECIFIC LEARNING OBJECTIVES:OBJECTIVES:

– Recognize the problems of graphitization Recognize the problems of graphitization of cast ironof cast iron

– Define the reason why corrosion Define the reason why corrosion increases when seawater velocity increases when seawater velocity increasesincreases

– Define the terms and what is meant by Define the terms and what is meant by stress corrosion and names the metals in stress corrosion and names the metals in which its commonly occurswhich its commonly occurs

– Explain what is meant by dezincification Explain what is meant by dezincification and de-aluminificationand de-aluminification

– Define how the process in the above Define how the process in the above objective can be preventedobjective can be prevented

SPECIFIC LEARNING SPECIFIC LEARNING OBJECTIVES:OBJECTIVES:

– Explain what is meant by fretting corrosionExplain what is meant by fretting corrosion– Define the factors which increases the rate Define the factors which increases the rate

of frettingof fretting– Define what is meant by corrosion fatigueDefine what is meant by corrosion fatigue– The ff. major factors affecting the corrosion The ff. major factors affecting the corrosion

process are identified:process are identified:– Differential temperaturesDifferential temperatures– Stresses within the metal structureStresses within the metal structure– Variation in crystal structure of the metalVariation in crystal structure of the metal– Distribution/concentration of impurities in the Distribution/concentration of impurities in the

metal crystalsmetal crystals– Flow of oxygen to the cathodeFlow of oxygen to the cathode– Hydroxyl ion concentration of the aqueous Hydroxyl ion concentration of the aqueous

solutionsolution

SPECIFIC LEARNING SPECIFIC LEARNING OBJECTIVES:OBJECTIVES:

– Recognize that some films and coatings Recognize that some films and coatings on metal surfaces can provided on metal surfaces can provided protection so long as they remain intactprotection so long as they remain intact

– Recognize that surface preparation prior Recognize that surface preparation prior to the application of protective coatings to the application of protective coatings is very importantis very important

– Identify the important methods of Identify the important methods of surface protection as:surface protection as:– PaintsPaints– Chemical filmsChemical films– Metallic coatingsMetallic coatings– AnodizingAnodizing

What is corrosion?What is corrosion?• Corrosion is the chemical or

electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties.

• Corrosion is a state of deterioration in metals caused by oxidation or chemical action

• Corrosion is the deterioration of a materials useful properties due to reactions with its environment. e.g. Weakening of steel due to oxidation of the iron atoms. Corrosion also includes the discolouration and weakening of non-metals by the sun's UV light.

What is corrosion?What is corrosion?• Corrosion is the gradual destruction of material,

usually metals, by chemical reaction with its environment. In the most common use of the word, this means electro-chemical oxidation of metals in reaction with an oxidant such as oxygen.

• Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. This type of damage typically produces oxide(s) or salt(s) of the original metal.

• Corrosion can also occur in materials other than metals, such as ceramics or polymers, although in this context, the term degradation is more common. Corrosion degrades the useful properties of materials and structures including strength, appearance and ability to contain a vessel's contents.

What is corrosion?What is corrosion?• Ulick R. Evans, the British scientist who is

considered the "Father of Corrosion Science", has given the definition of corrosion:

• "Corrosion is largely an electrochemical phenomenon, may be defined as destruction by electrochemical or chemical agencies".

Causes of CorrosionCauses of CorrosionIt is a common misconception that metal-to-metal contact coupled with water entrapment is the major cause of corrosion at these points. This is not the case; the sequence of events is as follows:1. Water is trapped - The very nature of the supports allows water to be held in contact with the painted pipe surface as well as the paint on the support element.2. The paint system fails - Even if the paint on the pipe and support beam are perfect, the paint system is designed for atmospheric exposure and not immersion service. The longer the paint surface is continuously exposed to water, the more it softens. As the pipe softens it is inevitable that the steel substrate will be directly exposed to the water.3. Corrosion is initiated - The small area of steel now exposed to oxygenated water (often with high chlorides) starts to corrode.4. Corrosion undercuts paint film - The initial corrosion soon undercuts and spreads. Soon the whole support area is bare steel.

Causes of CorrosionCauses of Corrosion5. Crevice corrosion starts - From this point on the crevice corrosion driven by differential aeration takes over from the general corrosion mechanism that initiated the corrosion. As corrosion products build they further restrict oxygen diffusion and the oxygen concentration gradient gets steeper. Pitting now becomes the main problem with corrosion rates acceleration by an order of magnitude.

6. Pipe fails - If the inspection program is not set up to detect this mostly concealed wall loss. The pipe will fail.

Sources of CorrosionSources of CorrosionThere are three primary sources of corrosion:• Oxidation: Oxygen in the atmosphere combines with a metallic receptor to

generate an oxide film. Since the volume of the oxide film is greater than the original metal receptor, this process causes paint coatings to uplift and fail. Electrolysis: If two dissimilar metals are connected by a conducting film, an electrical potential is created between the two metals. The metal with the higher electrical potential is the cathode, and it loses electrons to the other metal, which is an anode.

• There are more exotic forms of corrosion, such as stress corrosion cracking (will be discussed later on the part of this slide). These types of corrosion are very important to engineers who build turbine blades or wing pivot points, not the average consumer or producer.

Oxygen Attack• Without proper mechanical and chemical deaeration, oxygen in the

feedwater will enter the boiler. Much is flashed off with the steam; the remainder can attack boiler metal. The point of attack varies with boiler design and feedwater distribution. Pitting is frequently visible in the feedwater distribution holes, at the steam drum waterline, and in downcomer tubes.

• Oxygen is highly corrosive when present in hot water. Even small concentrations can cause serious problems. Because pits can penetrate deep into the metal, oxygen corrosion can result in rapid failure of feedwater lines, economizers, boiler tubes, and condensate lines. Additionally, iron oxide generated by the corrosion can produce iron deposits in the boiler.

• Oxygen corrosion may be highly localized or may cover an extensive area. It is identified by well defined pits or a very pockmarked surface. The pits vary in shape, but are characterized by sharp edges at the surface. Active oxygen pits are distinguished by a reddish brown oxide cap (tubercle). Removal of this cap exposes black iron oxide within the pit

Oxygen Attack

The influence of temperature is particularly important in feedwater heaters and economizers. A temperature rise provides enough additional energy to accelerate reactions at the metal surfaces, resulting in rapid and severe corrosion.At 60°F and atmospheric pressure, the solubility of oxygen in water is approximately 8 ppm. Efficient mechanical deaeration reduces dissolved oxygen to 7 ppb or less. For complete protection from oxygen corrosion, a chemical scavenger is required following mechanical deaeration.Major sources of oxygen in an operating system include poor deaerator operation, in-leakage of air on the suction side of pumps, the breathing action of receiving tanks, and leakage of undeaerated water used for pump seals.

Oxygen Attack• The acceptable dissolved oxygen level for any system depends on

many factors, such as feedwater temperature, pH, flow rate, dissolved solids content, and the metallurgy and physical condition of the system. Based on experience in thousands of systems, 3-10 ppb of feedwater oxygen is not significantly damaging to economizers. This is reflected in industry guidelines.

• the ASME consensus is less than 7 ppb (ASME recommends chemical scavenging to "essentially zero" ppb)

• TAPPI engineering guidelines are less than 7 ppb• EPRI fossil plant guidelines are less than 5 ppb dissolved oxygen

Different Different CorroCorro

Kinds of Kinds of sionsion

Uniform CorrosionUniform Corrosion• This is also called general corrosion.

The surface effect produced by most direct chemical attacks (e.g., as by an acid) is a uniform etching of the metal. On a polished surface, this type of corrosion is first seen as a general dulling of the surface and, if allowed to continue, the surface becomes rough and possibly frosted in appearance. The discoloration or general dulling of metal created by its exposure to elevated temperatures is not to be considered as uniform etch corrosion. The use of chemical-resistant protective coatings or more resistant materials will control these problems.

Uniform CorrosionUniform Corrosion• While this is the most common form of

corrosion, it is generally of little engineering significance, because structures will normally become unsightly and attract maintenance long before they become structurally affected. The facilities shown in the picuture below show how this corrosion can progress if control measures are not taken.

Galvanic CorrosionGalvanic Corrosion

• Galvanic corrosion is an electrochemical action of two dissimilar metals in the presence of an electrolyte and an electron conductive path. It occurs when dissimilar metals are in contact.

• It is recognizable by the presence of a buildup of corrosion at the joint between the dissimilar metals. For example, when aluminum alloys or magnesium alloys are in contact with steel (carbon steel or stainless steel), galvanic corrosion can occur and accelerate the corrosion of the aluminum or magnesium. This can be seen on the photo above where the aluminum helicopter blade has corroded near where it was in contact with a steel counterbalance.

Galvanic CorrosionGalvanic Corrosion• The most common type of galvanic corrosion in a boiler system is The most common type of galvanic corrosion in a boiler system is

caused by the contact of dissimilar metals, such as iron and copper. caused by the contact of dissimilar metals, such as iron and copper. These differential cells can also be formed when deposits are These differential cells can also be formed when deposits are present. Galvanic corrosion can occur at welds due to stresses in present. Galvanic corrosion can occur at welds due to stresses in heat-affected zones or the use of different alloys in the welds. heat-affected zones or the use of different alloys in the welds. Anything that results in a difference in electrical potential at discrete Anything that results in a difference in electrical potential at discrete surface locations can cause a galvanic reaction. Causes include:surface locations can cause a galvanic reaction. Causes include:1.1. scratches in a metal surfacescratches in a metal surface2.2. differential stresses in a metaldifferential stresses in a metal3.3. differences in temperaturedifferences in temperature4.4. conductive depositsconductive deposits

Cathode and Anode

Galvanic Corrosion PotentialsFigure 1 illustrates the idea of an electro-chemical reaction. If a Figure 1 illustrates the idea of an electro-chemical reaction. If a metal is placed in a conducting solution like salt water, it metal is placed in a conducting solution like salt water, it dissociates into ions, releasing electrons, as the iron is shown doing dissociates into ions, releasing electrons, as the iron is shown doing in the figure, via the in the figure, via the ionization reactionionization reaction

Fe Fe Fe Fe++++ + 2e + 2e--

The electrons accumulate on the iron giving it a negative charge The electrons accumulate on the iron giving it a negative charge that grows until the electrostatic attraction starts to pull the Fethat grows until the electrostatic attraction starts to pull the Fe++++ ions back onto the metal surface, stifling further dissociation. At ions back onto the metal surface, stifling further dissociation. At this point the iron has a potential (relative to a standard, the this point the iron has a potential (relative to a standard, the hydrogen standardhydrogen standard) of –0.44 volts. Each metal has its own ) of –0.44 volts. Each metal has its own characteristic corrosion potential (called the characteristic corrosion potential (called the standard reduction standard reduction potentialpotential), as plotted in Figure 2.), as plotted in Figure 2.

If two metals are connected together in a cell, like the iron and If two metals are connected together in a cell, like the iron and copper samples in Figure 1, a potential difference equal to their copper samples in Figure 1, a potential difference equal to their separation on Figure 2 appears between them. The corrosion separation on Figure 2 appears between them. The corrosion potential of iron, -0.44, differs from that of copper, +0.34 , by 0.78 potential of iron, -0.44, differs from that of copper, +0.34 , by 0.78 volts, so if no current flows in the connection the voltmeter will volts, so if no current flows in the connection the voltmeter will register this register this

Figure 1. A bi-metal corrosion cell. The corrosion potential is the potential to which the metal falls relative to a hydrogen standard

Figure 2. Standard reduction potentials of metals.

The natural differences in metal potentials The natural differences in metal potentials produce galvanic differences, such as the produce galvanic differences, such as the galvanic series in sea water. If electrical galvanic series in sea water. If electrical contact is made between any two of these contact is made between any two of these materials in the presence of an electrolyte, materials in the presence of an electrolyte, current must flow between them. The farther current must flow between them. The farther apart the metals are in the galvanic series, the apart the metals are in the galvanic series, the greater the galvanic corrosion effect or rate greater the galvanic corrosion effect or rate will be. Metals or alloys at the upper end are will be. Metals or alloys at the upper end are noble while those at the lower end are active. noble while those at the lower end are active. The more active metal is the anode or the one The more active metal is the anode or the one that will corrode.that will corrode.Control of galvanic corrosion is achieved by Control of galvanic corrosion is achieved by

using metals closer to each other in the using metals closer to each other in the galvanic series or by electrically isolating galvanic series or by electrically isolating metals from each other. Cathodic protection metals from each other. Cathodic protection can also be used to control galvanic corrosion can also be used to control galvanic corrosion effects.effects.

The scuba tank above suffered galvanic The scuba tank above suffered galvanic corrosion when the brass valve and the steel corrosion when the brass valve and the steel tank were wetted by condensation. Electrical tank were wetted by condensation. Electrical isolation flanges like those shown on the right isolation flanges like those shown on the right are used to prevent galvanic corrosion. are used to prevent galvanic corrosion. Insulating gaskets, usually polymers, are Insulating gaskets, usually polymers, are inserted between the flanges, and insulatinginserted between the flanges, and insulatingsleeves and washers isolate the bolted sleeves and washers isolate the bolted

connections.connections.

Concentration Cell Corrosion

• Concentration cell corrosion occurs when two or more areas of a metal surface are in contact with different concentrations of the same solution. There are three general types of concentration cell corrosion:1. metal ion concentration cells2. oxygen concentration cells, and3. active-passive cells.

Metal Ion Concentration Cells• In the presence of water, a high concentration of metal ions will

exist under faying surfaces and a low concentration of metal ions will exist adjacent to the crevice created by the faying surfaces. An electrical potential will exist between the two points. The area of the metal in contact with the low concentration of metal ions will be cathodic and will be protected, and the area of metal in contact with the high metal ion concentration will be anodic and corroded. This condition can be eliminated by sealing the faying surfaces in a manner to exclude moisture. Proper protective coating application with inorganic zinc primers is also effective in reducing faying surface corrosion.

Oxygen Concentration Cells• A water solution in contact with the metal surface will normally contain

dissolved oxygen. An oxygen cell can develop at any point where the oxygen in the air is not allowed to diffuse uniformly into the solution, thereby creating a difference in oxygen concentration between two points. Typical locations of oxygen concentration cells are under either metallic or nonmetallic deposits (dirt) on the metal surface and under faying surfaces such as riveted lap joints. Oxygen cells can also develop under gaskets, wood, rubber, plastic tape, and other materials in contact with the metal surface. Corrosion will occur at the area of low-oxygen concentration (anode). The severity of corrosion due to these conditions can be minimized by sealing, maintaining surfaces clean, and avoiding the use of material that permits wicking of moisture between faying surfaces.

Active-Passive Cells• Metals that depend on a tightly adhering passive film (usually an

oxide) for corrosion protection; e.g., austenitic corrosion-resistant steel, can be corroded by active-passive cells. The corrosive action usually starts as an oxygen concentration cell; e.g., salt deposits on the metal surface in the presence of water containing oxygen can create the oxygen cell. If the passive film is broken beneath the salt deposit, the active metal beneath the film will be exposed to corrosive attack. An electrical potential will develop between the large area of the cathode (passive film) and the small area of the anode (active metal). Rapid pitting of the active metal will result. This type of corrosion can be avoided by frequent cleaning and by application of protective coatings

Sacrificial Anode

• Sacrificial Anodes are highly active metals that are used to prevent a less active material surface from corroding. Sacrificial Anodes are created from a metal alloy with a more negative electrochemical potential than the other metal it will be used to protect. The sacrificial anode will be consumed in place of the metal it is protecting, which is why it is referred to as a "sacrificial" anode.

Sacrificial Anode

• Sacrificial Anodes are highly active metals that are used to prevent a less active material surface from corroding. Sacrificial Anodes are created from a metal alloy with a more negative electrochemical potential than the other metal it will be used to protect. The sacrificial anode will be consumed in place of the metal it is protecting, which is why it is referred to as a "sacrificial" anode.

• The hulls of large sea vessels use small zinc blocks placed at regular intervals to prevent rust. Periodically these zinc blocks have to be replaced as they corrode away.

Sacrificial Anode• The earliest experiments on cathodic protection were performed with zinc anodes The earliest experiments on cathodic protection were performed with zinc anodes

that were electrically connected to copper plates immersed in seawater. As can be that were electrically connected to copper plates immersed in seawater. As can be seen on the galvanic series, such an arrangement would produce a cathode seen on the galvanic series, such an arrangement would produce a cathode (copper) and an anode (zinc). In the (copper) and an anode (zinc). In the large galvanic cell so formedlarge galvanic cell so formed, the zinc cylinder , the zinc cylinder corroded away in a manner to protect the copper substrate. This method of corroded away in a manner to protect the copper substrate. This method of cathodic protection can be used with other combination of metals providing the cathodic protection can be used with other combination of metals providing the necessary current to the metal to be protected, as necessary current to the metal to be protected, as Sir Humphry DavySir Humphry Davy and and Michael Michael FaradayFaraday illustrated almost two centuries ago. illustrated almost two centuries ago.

• When two metals are electrically connected to each other in a electrolyte e.g. When two metals are electrically connected to each other in a electrolyte e.g. seawater, electrons will flow from the more active metal to the other, due to the seawater, electrons will flow from the more active metal to the other, due to the difference in the electrical potential, the so called difference in the electrical potential, the so called 'driving force'driving force'. When the most '. When the most active metal (active metal (anodeanode) supplies current, it will gradually dissolve into ions in the ) supplies current, it will gradually dissolve into ions in the electrolyte, and at the same time produce electrons, which the least active electrolyte, and at the same time produce electrons, which the least active ((cathodecathode) will receive through the metallic connection with the anode. The result is ) will receive through the metallic connection with the anode. The result is that the cathode will be negatively polarized and hence be protected against that the cathode will be negatively polarized and hence be protected against corrosion. To calculate the rates at which these processes occur, one has to corrosion. To calculate the rates at which these processes occur, one has to understand the electrochemical kinetics associated with the complex sets of understand the electrochemical kinetics associated with the complex sets of reactions that can all happen simultaneously on these metals.reactions that can all happen simultaneously on these metals.

Pitting Corrosion

• Passive metals, such as stainless steel, resist corrosive media and can perform well over long periods of time. However, if corrosion does occur, it forms at random in pits. Pitting is most likely to occur in the presence of chloride ions, combined with such depolarizers as oxygen or oxidizing salts. Methods that can be used to control pitting include maintaining clean surfaces, application of a protective coating, and use of inhibitors or cathodic protection for immersion service. Molybdenum additions to stainless steel (e.g. in 316 stainless) are intended to reduce pitting corrosion.

Pitting Corrosion

• The rust bubbles or tubercules on the cast iron above indicate that pitting is occurring. Researchers have found that the environment inside the rust bubbles is almost always higher in chlorides and lower in pH (more acidic) than the overall external environment. This leads to concentrated attack inside the pits.

Pitting Corrosion

• Similar changes in environment occur inside crevices, stress corrosion cracks, and corrosion fatigue cracks. All of these forms of corrosion are sometimes included in the term "occluded cell corrosion.“

Pitting Corrosion

• Pitting corrosion can lead to unexpected catastrophic system failure. The split tubing above left was caused by pitting corrosion of stainless steel. A typical pit on this tubing is shown above right.

• Sometimes pitting corrosion can be quite small on the surface and very large below the surface. The figure below left shows this effect, which is common on stainless steels and other film-protected metals. The pitting shown below right (white arrow) led to the stress corrosion fracture shown by the black arrows

Crevice Corrosion

• Crevice or contact corrosion is the corrosion produced at the region of contact of metals with metals or metals with nonmetals. It may occur at washers, under barnacles, at sand grains, under applied protective films, and at pockets formed by threaded joints. Whether or not stainless steels are free of pit nuclei, they are always susceptible to this kind of corrosion because a nucleus is not necessary.

• Cleanliness, the proper use of sealants, and protective coatings are effective means of controlling this problem. Molybdenum-containing grades of stainless steel (e.g. 316 and 316L) have increased crevice corrosion resistance.

Crevice Corrosion

• The crevice corrosion shown above happened when an aerospace alloy (titanium - 6 aluminum - 4 vanadium) was used instead of a more corrosion-resistant grade of titanium. Special alloying additions are added to titanium to make alloys which are crevice corrosion resistant even at elevated temperatures.

• Screws and fasteners have are common sources of crevice corrosion problems. The stainless steel screws shown below corroded in the moist atmosphere of a pleasure boat hull.

Crevice Corrosion

• Another example of crevice corrosion is rubber pads under pipes. These do a wonderful job of reducing the life of the pipe. The crevice that was formed without the rubber pad is mild in comparison to the new crevice, which now has the ability to actually suck water in (by capillary action). Not only does the pad invite water in, it is better at holding it trapped against the pipe surface, since air circulation and natural evaporation is eliminated. The situation is further worsened by the length of the crevice which allows an oxygen concentration gradient to go from full natural concentration to anaerobic in a few centimeters.

Crevice Corrosion

Filiform Corrosion

• This type of corrosion occurs under painted or plated surfaces when moisture permeates the coating. Lacquers and "quick-dry" paints are most susceptible to the problem. Their use should be avoided unless absence of an adverse effect has been proven by field experience. Where a coating is required, it should exhibit low water vapor transmission characteristics and excellent adhesion. Zinc-rich coatings should also be considered for coating carbon steel because of their cathodic protection quality.

• Filiform corrosion normally starts at small, sometimes microscopic, defects in the coating.

Filiform Corrosion

• The picture on the left shows filiform corrosion causing bleed-through on a welded tank. The picture on the right shows "worm-like" filiform corrosion tunnels forming under a coating.

• Filiform corrosion is minimized by careful surface preparation prior to coating, by the use of coatings that are resistant to this form of corrosion (see above), and by careful inspection of coatings to insure that holidays, or holes, in the coating are minimized.

43

Filiform Corrosion on PVC-Coated Al Foil

Advancing head and cracked tail of a filiform cell.

Scale: 0.125 mm

Gelatinous corrosion products oozing out of porous tail section.

Scale: 1.25 m

Filiform Corrosion

Intergranular Corrosion

• Intergranular corrosion is an attack on or adjacent to the grain boundaries of a metal or alloy. A highly magnified cross section of most commercial alloys will show its granular structure. This structure consists of quantities of individual grains, and each of these tiny grains has a clearly defined boundary that chemically differs from the metal within the grain center. Heat treatment of stainless steels and aluminum alloys accentuates this problem.

• The picture left above shows a stainless steel which corroded in the heat affected zone a short distance from the weld. This is typical of intergranular corrosion in austenitic stainless steels. This corrosion can be eliminated by using stabilized stainless steels (321 or 347) or by using low-carbon stainless grades (304L or 3I6L).

• Heat-treatable aluminum alloys (2000, 6000, and 7000 series alloys) can also have this problem. The picture right above shows an aircraft aluminum alloy in an intergranular corrosion.

Exfoliation Corrosion

• Exfoliation is a form of intergranular corrosion. It manifests itself by lifting up the surface grains of a metal by the force of expanding corrosion products occurring at the grain boundaries just below the surface. It is visible evidence of intergranular corrosion and most often seen on extruded sections where grain thickness is less than in rolled forms. This form of corrosion is common on aluminum, and it may occur on carbon steel.

• The picture on the left shows exfoliation of aluminum. Exfoliation of carbon steel is apparent in the channel on the coating exposure panel on the right. The expansion of the metal caused by exfoliation corrosion can create stresses that bend or break connections and lead to structural failure.

46

Figure 22.14 (a) Intergranular corrosion takes place in austenitic stainless steel. (b) Slow cooling permits chromium carbides to precipitate at grain boundaries. (c) A quench anneal to dissolve the carbides may prevent intergranular corrosion.

Stress Corrosion Cracking

• Stress corrosion cracking (SCC) is caused by the simultaneous effects of tensile stress and a specific corrosive environment. Stresses may be due to applied loads, residual stresses from the manufacturing process, or a combination of both.

• Cross sections of SCC frequently show branched cracks. This river branching pattern is unique to SCC and is used in failure analysis to identify when this form of corrosion has occurred.

Stress Corrosion Cracking

• The photo above shows SCC of an insulated stainless-steel condensate line. Water wetted the insulation and caused chlorides to leach from the insulation onto the hot metal surface. This is a common problem on steam and condensate lines. Control is by maintaining the jackets around the lines so that moisture doesn't enter the insulation or is quickly drained off.

Stress Corrosion Cracking

• The next two photos show intergranular SCC of an aluminum aerospace part. The intergranular nature of the corrosion can be seen in the scanning electron microscope image on the left and in the microscopic cross section on the right. The arrows indicate the primary crack shown in both pictures. Note that secondary cracks are also apparent. These secondary cracks are common in stress corrosion cracking.

• The failure above occurred on an aluminum alloy subjected to residual stresses and salt water. Changes in alloy heat treatment recommended by materials lab eliminated this problem.

Corrosion Fatigue

• Corrosion fatigue is a special case of stress corrosion caused by the combined effects of cyclic stress and corrosion. No metal is immune from some reduction of its resistance to cyclic stressing if the metal is in a corrosive environment. Damage from corrosion fatigue is greater than the sum of the damage from both cyclic stresses and corrosion. Control of corrosion fatigue can be accomplished by either lowering the cyclic stresses or by corrosion control.

• The "beach marks" on the propeller shown above mark the progression of fatigue on this surface.

Corrosion Fatigue

• Similar beach marks are shown on the aerospace part above left. The high magnification scanning electron microscope image on the middle shows striations (individual crack progression marks).

• An infamous example of corrosion fatigue occured in 1988 on an airliner flying between the Hawaiian islands. This disaster, which cost one life, prompted the airlines to look at their airplanes and inspect for corrosion fatigue.

Fretting Corrosion

• The rapid corrosion that occurs at the interface between contacting, highly loaded metal surfaces when subjected to slight vibratory motions is known as fretting corrosion.

• The photo above shows fretting corrosion of a fence post and wires which swing in the wind and wear against the post. Both the fence post and the connecting wires are experiencing fretting corrosion.

Fretting Corrosion• Fretting corrosion refers to corrosion damage at the

asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration. Pits or grooves and oxide debris characterize this damage, typically found in machinery, bolted assemblies and ball or roller bearings. Contact surfaces exposed to vibration during transportation are exposed to the risk of fretting corrosion.

• Damage can occur at the interface of two highly loaded surfaces which are not designed to move against each other. The most common type of fretting is caused by vibration. The protective film on the metal surfaces is removed by the rubbing action and exposes fresh, active metal to the corrosive action of the atmosphere.

Fretting Corrosion

• Fretting corrosion is greatly retarded when the contacting surfaces can be well lubricated as in machinery-bearing surfaces so as to exclude direct contact with air.

• The bearing race above is a classic example of fretting corrosion. This is greatly retarded when the contacting surfaces can be well lubricated as in machinery-bearing surfaces so as to exclude direct contact with air.

• The fretting on a large aluminum part (above left) led to deposits of debris (shown in the cross sections on the right).

Erosion Corrosion

• Erosion corrosion is the result of a combination of an aggressive chemical environment and high fluid-surface velocities. This can be the result of fast fluid flow past a stationary object, such as the case with the oil-field check valve shown on the left below, or it can result from the quick motion of an object in a stationary fluid, such as happens when a ship's propeller churns the ocean.

• Surfaces which have undergone erosion corrosion are generally fairly clean, unlike the surfaces from many other forms of corrosion.

Erosion Corrosion

• Erosion corrosion can be controlled by the use of harder alloys (including flame-sprayed or welded hard facings) or by using a more corrosion resistant alloy. Alterations in fluid velocity and changes in flow patterns can also reduce the effects of erosion corrosion.

• Erosion corrosion is often the result of the wearing away of a protective scale or coating on the metal surface. The oil field production tubing shown above on the right corroded when the pressure on the well became low enough to cause multiphase fluid flow. The impact of collapsing gas bubbles caused the damage at joints where the tubing was connected and turbulence was greater.

Erosion Corrosion

• Many people assume that erosion corrosion is associated with turbulent flow. This is true, because all practical piping systems require turbulent flow-the fluid would not flow fast enough if lamellar (nonturbulent) flow were maintained. Most, if not all, erosion corrosion can be attributed to multiphase fluid flow. The check valve on the left above failed due to sand and other particles in an otherwise noncorrosive fluid. The tubing on the right failed due to the pressure differences caused when gas bubbles collapsed against the pipe wall and destroyed the protective mineral scale that was limiting corrosion.

Dealloying• Dealloying is a rare form of corrosion found

in copper alloys, gray cast iron, and some other alloys. Dealloying occurs when the alloy loses the active component of the metal and retains the more corrosion resistant component in a porous "sponge" on the metal surface. It can also occur by redeposition of the noble component of the alloy on the metal surface.Control is by the use of more resistant alloys-inhibited brasses and malleable or nodular cast iron.

• Types of dealloying:– Dezincification– Dealuminification– Graphitization

Dealloying

• The brass on the left dezincified leaving a porous copper plug on the surface. The gray cast iron water pipe shown on the right photo has graphitized and left graphitic surface plugs which can be seen on the cut surface. The rust tubercules or bubbles are also an indication of pitting corrosion.

• The middle photo shows a layer of copper on the surface of a dealloyed 70% copper-30% nickel cupronickel heat exchanger tube removed from a ship. Stagnant seawater is so corrosive that even this normally corrosion-resistant alloy has corroded. Virtually all copper alloys are subject to dealloying in some environments.

Dealumification

• This type of attack, similar to the dezincification of duplex brasses, results in selective dissolution of the principal alloying element (in this case aluminium) from one phase of the alloy leaving a residue of porous copper which retains the original shape and dimensions of the component but has little strength. By controlling the composition and, for the alloys of high aluminium content, the cooling rate from casting or working temperature, metallurgical structures are ensured that will not suffer dealuminification to any significant extent under any normal conditions of use.

Al

Cu

Dealumification

• The middle photo shows the microstructure of a nickel aluminum bronze impeller that has experienced dealuminification in filtered water. ~170X, ammonium hydroxide etch.

Dezincification

• Dezincification removes zinc from copper-zinc solid solution alloys (brasses), resulting in localized regions of copper on the surface.

• Copper is a relatively soft, very easily worked metal, and is used extensively for small-bore pipes and tubes. Also, this residue has a porous structure and very low strength but it retains the shape and approximate dimensions of the original alloy.

• This is a form of corrosion affecting some copper alloys results in selective removal of the principal alloying element leaving a residue of copper. Consequently the depth to which the attack has penetrated is very difficult to assess except by destructive methods such as the preparation of metallographic sections.

63

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

Figure 22.2 Photomicrograph of a copper deposit in brass, showing the effect of dezincification (x50). Dezincification is a special chemical corrosion process by which both zinc and copper atoms are removed from brass, but the copper is replated back onto the metal.

Graphitization

• Cast iron contains carbon, in the form of graphite, in its molecular structure. It is composed of a crystalline structure as are all metals; i.e. it is a heterogeneous mass of crystals of its major elements (Iron, Manganese, Carbon, Sulphur and Silicon). One condition which can occur in the presence of acid rain and/or sea water is "graphitization." The stable graphite crystals remain in place, but the less stable iron becomes converted to insoluble iron oxide (rust). The result is that the cast iron piece retains its shape and appearance but becomes weaker mechanically because of the loss of iron. Graphitization is not, however, a common problem. It generally will occur only after bare metal is left exposed for extended periods, or where failed joints allow the penetration of acidic rainwater to interior surfaces.

Graphitization

• This corrosion process is galvanic, with the carbon present acting as the most noble (least corrosive) element and the iron acting as the least noble (most corrosive) element. The composition or microstructure of the iron affects the durability of the object because the rate of corrosion is dependent upon the amount and structure of the graphite present in the iron.

• Graphitization is a special chemical corrosion process by which iron is leached from cast iron, leaving behind a weak, spongy mass of graphite.

Graphitization

• Graphitization occurs when cast iron is left unpainted for long periods, acidic rainwater or seawater is present, or where caulked joints have failed

• A porous graphite residue is impregnated with insoluble products as cast iron corrodes

• Cast iron retains its appearance and shape when it corrodes so graphitization only becomes apparent when the surface is scraped to reveal any crumbling iron underneath

• Often the only solution is to replace the damaged element

Dealloying Prevention• How to prevent dealloying? Dealloying, selective leaching and How to prevent dealloying? Dealloying, selective leaching and

graphitic corrosion can be prevented through the following methods:graphitic corrosion can be prevented through the following methods:1.1. Select metals/alloys that are more resistant to dealloying. For Select metals/alloys that are more resistant to dealloying. For

example, inhibited brass is more resistant to dezincification than example, inhibited brass is more resistant to dezincification than alpha brass, ductile iron is more resistant to graphitic corrosion alpha brass, ductile iron is more resistant to graphitic corrosion than gray cast iron.than gray cast iron.

2.2. Control the environment to minimize the selective leachingControl the environment to minimize the selective leaching3.3. Use sacrificial anode cathodic protection or impressed current Use sacrificial anode cathodic protection or impressed current

cathodic protection cathodic protection

Hydrogen Damage• Hydrogen can cause a number of corrosion problems. Hydrogen

embrittlement is a problem with high-strength steels, titanium, and some other metals. Control is by eliminating hydrogen from the environment or by the use of resistant alloys.

• Hydrogen blistering can occur when hydrogen enters steel as a result of the reduction reaction on a metal cathode. Single-atom nascent hydrogen atoms then diffuse through the metal until they meet with another atom, usually at inclusions or defects in the metal. The resultant diatomic hydrogen molecules are then too big to migrate and become trapped. Eventually a gas blister builds up and may split the metal as shown in the picture below.

•

Hydrogen Damage

• Hydrogen blistering is controlled by minimizing corrosion in acidic environments. It is not a problem in neutral or caustic environments or with high-quality steels that have low impurity and inclusion levels.

• The broken spring above on the left was brought to the KSC Materials Laboratory for failure analysis. Examination at high magnification in the scanning electron microscope (above right) revealed intergranular cleavage characteristic of hydrogen assisted cracking (hydrogen embrittlement). The part was zinc plated during refurbishment, and the hydrogen which entered the metal during the plating process had not been baked out. A postplating bakeout procedure should be standard for high strength steels.

Corrosion in Concrete

• The picture on the left shows cracking and staining of a seawall near the Kennedy Space Center. The pitting corrosion in the right photo occured on an aluminum railing on a concrete causeway over an inlet to the Atlantic Ocean.

• Concrete is a widely-used structural material that is frequently reinforced with carbon steel reinforcing rods, post-tensioning cable or prestressing wires. The steel is necessary to maintain the strength of the structure, but it is subject to corrosion. The cracking associated with corrosion in concrete is a major concern in areas with marine environments (like KSC) and in areas which use deicing salts.

Corrosion in ConcreteThere are two theories on how corrosion in

concrete occurs:1. Salts and other chemicals enter the

concrete and cause corrosion. Corrosion of the metal leads to expansive forces that cause cracking of the concrete structure.

2. Cracks in the concrete allow moisture and salts to reach the metal surface and cause corrosion.

Corrosion in Concrete• Both possibilities have their advocates, and it is also possible that corrosion in

concrete can occur either way. The mechanism isn't truly important, the corrosion leads to damage, and the damage must be controlled.

• In new construction, corrosion in concrete is usually controlled by embedding the steel deep enough so that chemicals from the surface don't reach the steel (adequate depth of cover). Other controls include keeping the water/cement ratio below 0.4, having a high cement factor, proper detailing to prevent cracking and ponding, and the use of chemical admixtures. These methods are very effective, and most concrete structures, even in marine environments, do not corrode.

• Unfortunately, some concrete structures do corrode. When this happens, remedial action can include repairing the cracked and spalled concrete, coating the surface to prevent further entry of corrosive chemicals into the structure, and cathodic protection, an electrical means of corrosion control. KSC has experience with all of these methods of controlling corrosion on existing concrete structures.

Microbial Corrosion

• Microbial corrosion (also called microbiologically-influenced corrosion or MIC) is corrosion that Microbial corrosion (also called microbiologically-influenced corrosion or MIC) is corrosion that is caused by the presence and activities of microbes. This corrosion can take many forms and is caused by the presence and activities of microbes. This corrosion can take many forms and can be controlled by biocides or by conventional corrosion control methods.can be controlled by biocides or by conventional corrosion control methods.

• There are a number of mechanisms associated with this form of corrosion, and detailed There are a number of mechanisms associated with this form of corrosion, and detailed explanations are available at the web sites listed at the bottom of this section. Most MIC takes explanations are available at the web sites listed at the bottom of this section. Most MIC takes the form of pits that form underneath colonies of living organic matter and mineral and the form of pits that form underneath colonies of living organic matter and mineral and biodeposits. This biofilm creates a protective environment where conditions can become quite biodeposits. This biofilm creates a protective environment where conditions can become quite corrosive and corrosion is accelerated.corrosive and corrosion is accelerated.

• The picture below shows a biofilm on a metallic condenser surface. These biofilms can allow The picture below shows a biofilm on a metallic condenser surface. These biofilms can allow corrosive chemicals to collect within and under the films. Thus the corrosive conditions under a corrosive chemicals to collect within and under the films. Thus the corrosive conditions under a biofilm can be very aggressive, even in locations where the bulk environment is noncorrosive.biofilm can be very aggressive, even in locations where the bulk environment is noncorrosive.

Microbial Corrosion• MIC can be a serious problem in stagnant water systems such as the fire-protection MIC can be a serious problem in stagnant water systems such as the fire-protection

system that produced the pits. Use of biocides and mechanical cleaning methods can system that produced the pits. Use of biocides and mechanical cleaning methods can reduce MIC, but anywhere where stagnant water is likely to collect is a location where reduce MIC, but anywhere where stagnant water is likely to collect is a location where MIC can occur.MIC can occur.

• Corrosion (oxidation of metal) can only occur if some other chemical is present to be Corrosion (oxidation of metal) can only occur if some other chemical is present to be reduced. In most environments, the chemical that is reduced is either dissolved oxygen reduced. In most environments, the chemical that is reduced is either dissolved oxygen or hydrogen ions in acids. In anaerobic conditions (no oxygen or air present), some or hydrogen ions in acids. In anaerobic conditions (no oxygen or air present), some bacteria (anaerobic bacteria) can thrive. These bacteria can provide the reducible bacteria (anaerobic bacteria) can thrive. These bacteria can provide the reducible chemicals that allow corrosion to occur. That's how the limited corrosion that was chemicals that allow corrosion to occur. That's how the limited corrosion that was found on the hull of the Titanic occurred. The picture below shows a "rusticle" removed found on the hull of the Titanic occurred. The picture below shows a "rusticle" removed from the hull of Titanic. This combination of rust and organic debris clearly shows the from the hull of Titanic. This combination of rust and organic debris clearly shows the location of rivet holes and where two steel plates overlapped.location of rivet holes and where two steel plates overlapped.

Microbial Corrosion

• Much microbial corrosion involves anaerobic or stagnant conditions, but it can also be found on structures exposed to air. The pictures below show a spillway gate from a hydroelectric dam on the Columbia River. The stress corrosion cracks were caused by pigeon droppings which produced ammonia-a chemical that causes stress corrosion cracking on copper alloys like the washers used on this structure. Since it's impossible to potty train pigeons, a new alloy resistant to ammonia was necessary.

• In addition to the use of corrosion resistant alloys, control of MIC involves the use of biocides and cleaning methods that remove deposits from metal surfaces. Bacteria are very small, and it is often very difficult to get a metal system smooth enough and clean enough to prevent MIC

Microbial Corrosion

• Much microbial corrosion involves anaerobic or stagnant conditions, but it can also Much microbial corrosion involves anaerobic or stagnant conditions, but it can also be found on structures exposed to air. The pictures below show a spillway gate be found on structures exposed to air. The pictures below show a spillway gate from a hydroelectric dam on the Columbia River. The stress corrosion cracks were from a hydroelectric dam on the Columbia River. The stress corrosion cracks were caused by pigeon droppings which produced ammonia-a chemical that causes caused by pigeon droppings which produced ammonia-a chemical that causes stress corrosion cracking on copper alloys like the washers used on this structure. stress corrosion cracking on copper alloys like the washers used on this structure. Since it's impossible to potty train pigeons, a new alloy resistant to ammonia was Since it's impossible to potty train pigeons, a new alloy resistant to ammonia was necessary.necessary.

• In addition to the use of corrosion resistant alloys, control of MIC involves the use In addition to the use of corrosion resistant alloys, control of MIC involves the use of biocides and cleaning methods that remove deposits from metal surfaces. of biocides and cleaning methods that remove deposits from metal surfaces. Bacteria are very small, and it is often very difficult to get a metal system smooth Bacteria are very small, and it is often very difficult to get a metal system smooth enough and clean enough to prevent MICenough and clean enough to prevent MIC

77

©20

03 B

rook

s/C

ole,

a d

ivis

ion

of T

hom

son

Lea

rnin

g, I

nc.

Tho

mso

n L

earn

ing ™

is a

tr

adem

ark

used

her

ein

unde

r lic

ense

. Figure 22.10 (a) Bacterial cells growing in a colony (x2700). (b) Formation of a tubercule and a pit under a biological colony.

Microbial Corrosion

CORROSION preventionCORROSION prevention

Corrosion Prevention

• Coatings– Barrier films– Inhibitive Pigments– Sacrificial treatments– Paint

• Heat Treatment• Mechanical Working

• Active Cathodic Protection

• Thermochemical diffusion treatments

• Chemical Treatments

• Cathodic protection is an electrical means of corrosion control. Cathodic protection can be applied using sacrificial (galvanic) anodes or by means of more complicated impressed current systems.

• This Louisiana fishing boat has sacrificial zinc anodes welded to the hull to slow down corrosion. No pattern is apparent to how the anodes were attached-the design philosophy seems to be that if one anode is good, more is better.

Cathodic ProtectionCathodic Protection

Cathodic ProtectionCathodic Protection

• This can be achieved in two ways:This can be achieved in two ways:1.) Sacrificial Anode (discussed before)1.) Sacrificial Anode (discussed before)2.) Impressed current2.) Impressed current• It has been shown that a combination of It has been shown that a combination of

cathodic protection & coating is the most cathodic protection & coating is the most economical means of protecting steel economical means of protecting steel structuresstructures

Impressed Current (ICCP)

• Involves the use of an external power source – metal to be protected is made cathodic to its surroundings – inert anodes used which are virtually non-consumable – insulated from structure

• Early anodes made from scrap steel but most modern ICCP systems use lead silver alloy, titanium or niobium

Transformer Rectifier Unit

Cont’d

• Has been used in the protection of steel reinforcement in concrete

• The use of modern electronics makes the system self regulating

• Very costly to run – mainly used in marine applications – oil rigs – large anodes placed on sea bed approximately 100m away

Impressed Current Cathodic Protection System

The system depicted above shows one way by which cathodic protection may be applied.  In this system, power is drawn from the national grid and converted into a dc current by means of a transformer-rectifier.  This is not the only method by which the dc current which is required may be supplied.  In remote areas, or parts of the world where a mains supply is not available, the driving force for the current is often provided by a diesel generator, solar cell, ..

Impressed Current System

Cathodic Protection

Typical Alternatives for a Buried PipelineMg

Magnesium Anode

Impressed current

Inhibitors and Other Means of Environmental Alteration

• Corrosion inhibitors are chemicals that are added to controlled environments to reduce the corrosivity of these environments. Examples of corrosion inhibitors include the chemicals added to automobile antifreezes to make them less corrosive.

Corrosion Allowance• Engineering designers must consider how much metal is necessary to withstand the anticipated load

for a given application. Since they can make mistakes, the use of the structure can change, or the structure can be misused, they usually are required to over design the structure by a safety factor that can vary from 20% to over 300%. Once the necessary mechanical load safety factor has been considered, it becomes necessary to consider whether or not a corrosion allowance is necessary to keep the structure safe if it does corrode. The picture below shows extra steel added to the bottom of an offshore oil production platform. The one inch of extra steel was added as a corrosion allowance.

COATINGSCOATINGS The general protective process consists of The general protective process consists of

four phases: four phases:

1.1. Preparation and conditioning of the metal surfacePreparation and conditioning of the metal surface2.2. Determination of the best coating needed for the material Determination of the best coating needed for the material

and its use (environment)and its use (environment)3.3. Control of coating processControl of coating process4.4. Maintenance of protective coatingMaintenance of protective coating

Protective Coatings / Wrapping

• Provide barrier between metal and environment. • Coatings may act as sacrificial anode or release

substance that inhibit corrosive attack on substrate.

• Metal coatings : – Noble – silver, copper, nickel, Cr, Sn, Pb on steel.

Should be free of pores/discontinuity coz creates small anode-large cathode leading to rapid attack at the damaged areas.

– Sacrificial – Zn, Al, Cd on steel. Exposed substrate will be cathodic & will be protected.

– Application – hot dipping, flame spraying, cladding, electroplating, vapor deposition, etc.

• Surface modification – to structure or composition by use of directed energy or particle beams. E.g ion implantation and laser processing.

• Inorganic coating : cement coatings, glass coatings, ceramic coatings, chemical conversion coatings.

• Chemical conversion – anodizing, phosphatizing, oxide coating, chromate.

• Organic coating : paints, lacquers, varnishes. Coating liquid generally consists of solvent, resin and pigment. The resin provides chemical and corrosion resistance, and pigments may also have corrosion inhibition functions.

Categories of Corrosion Prevention• Category 1 - Category 1 - Modifying the surface without altering the substrate‘s Modifying the surface without altering the substrate‘s

chemical constitution. In this case, the existing metallurgychemical constitution. In this case, the existing metallurgy of the of the component is changed within the surface regions, either by thermal component is changed within the surface regions, either by thermal or mechanical means, to increase its hardness.or mechanical means, to increase its hardness.

• Category 2 - Category 2 - Changing the surface layers by altering the alloy Changing the surface layers by altering the alloy chemistry. New elements are diffusedchemistry. New elements are diffused into the surface, usually at into the surface, usually at elevated temperatures, so that the outer layers are changed in elevated temperatures, so that the outer layers are changed in composition and properties compared to those of the bulk.composition and properties compared to those of the bulk.

• Category 3 - Adding layers of material to the surface .This group Category 3 - Adding layers of material to the surface .This group incorporates a wide group of coating processes, where a material incorporates a wide group of coating processes, where a material different from the bulk is laid upon the surface. Unlike the first two different from the bulk is laid upon the surface. Unlike the first two categories, there will be a clear boundary at the coating/substrate categories, there will be a clear boundary at the coating/substrate interface and the adhesion of the coating is a primary issue.interface and the adhesion of the coating is a primary issue.

Modifying the Surface Without Altering the Modifying the Surface Without Altering the Substrate's Chemical ConstitutionSubstrate's Chemical Constitution

ii) By heating ) By heating

• When dealing with transformation hardenable alloys, in particular carbon steels, low alloy steels and cast irons, the option to harden using flame, induction , laser or electron beam techniques may be the most attractive.

• In this case, instead of heating the whole component (as in through hardening), only the surface is affected, so that the bulk properties, specifically the toughness, remain unaffected, and component distortion , is minimised.

• These processes can be fully automated and precisely controlled.

By heating

• The desired core properties can be developed by standard heat treatment practices and the surfaces hardened by rapidly heating them to approx 850 and then quenching . ℃

• When dealing with transformation hardenable alloys, in particular carbon steels, low alloy steels and cast irons, the option to harden using flame, induction , laser or electron beam techniques may be the most attractive.

By heating

Induction Hardening

• is achieved via surface heating from a purpose designed water-cooled induction coil.

• Hardening depths of several mm are usual and the process is amenable to accurate control and automation, ideally for large numbers of identical components.

Flame hardening

• is achieved through the local application of an oxyacetylene flame (usually by hand) so that the process is less well controlled.

• However, it is ideal for treating specific areas (those needing wear resistance) of complex-shaped components.

Laser hardening

• can compete in high volume production with other low cost processes such as induction hardening.

• Using self-quenching techniques it is possible to obtain case depths of 0.75mm.

• Lasers are particularly useful for hardening relatively small or inaccessible areas.

Electron beam techniques

• both the capital and operating costs are lower. • The beam operates in a vacuum but the workpiece need only

be at 60 mbar pressure. • Area hardening is obtained by scanning the area on a raster .

By mechanical working

• Cold working the surface by peening , shot blasting or other specialized machining processes to produce deformed layers increases the stored energy and compressive stress,

• increasing the hardness, fatigue and stress corrosion resistance.

By mechanical working

• In particular, shot peening has developed with automation, computerized control, and highly reproducible properties.

• It is often used to other surface engineering techniques which might otherwise impair the fatigue or mechanical performance of a component.

i) Thermochemical diffusion treatments

• introduce interstitial elements, such as carbon, nitrogen and boron, or combinations of carbon and nitrogen, into a ferrous metal surface at elevated temperatures.

Altering the Chemistry of the Surface Regions

Thermochemical diffusion treatments

• interstitial diffusion;

• metallic substitutional elements or metalloids are used in processes such as chromising , aluminizing and siliconising.

• Interstitial element diffusion into steels falls into two categories:

i) those carried out at low temperatures, i.e. within the ferritic range, or

ii) high temperature treatments in the austenitic range.

• Ferritic processes include

1. gas nitriding (typically 525 )℃ ； 2. plasma nitriding (400 to 600 ) ℃ ； 3. Nitrocarburising processes (approx 500 ). ℃

Thermochemical diffusion treatments

• For ferritic nitrocarburising processes, many different treatment media may be employed, including

• salt baths (cyanides or non toxic cyanate mixtures), • endothermic ammonia gas mixtures, and • methane or propane /ammonia/oxygen mixtures.

Thermochemical diffusion treatments

• Typically, such processes produce case-depths of around 250 microns on alloy steels, but they can also be applied to a much wider variety of ferrous alloys.

• On low carbon mild steel they can produce a thin 'compound layer' (of the order of 10 microns thick) which can improve both wear and corrosion resistance.

Thermochemical diffusion treatments

• The austenitic treatments broadly include carburising employing solid (pack), liquid (salt bath) or gaseous media, carbo-nitriding and boronising .

• They are performed at temperatures near 900 and ℃produce much greater case depths (up to several mm) than the ferritic treatments.

• However, they also produce greater surface growth and distortion.

Thermochemical diffusion treatments

• Thermochemical treatments involving diffusion of substitutional elements, chromium (chromising) or aluminium (aluminising), which may be pack, salt bath or vapour processes are often used for elevated temperature service.

• The substrates are often nickel-based super-alloys or nickel/chromium gas turbine materials.

Thermochemical diffusion treatments

ii) Electroplating and thermal diffusion treatments

• One process involves the electrolytic deposition of tin on to ferrous materials.

• This is followed by a diffusion treatment at 400 to 600 to ℃form Fe/Sn compounds which resist scuffing and corrosion resistance.

• Bronze coatings may be developed in a similar way to add a bearing surface to a steel substrate.

iii) Oxide coatings

• When oil is present they prevent scuffing, adhesive wear and metal transfer.

• On ferrous substrates, chemical conversion layers may be produced by immersion in caustic nitrate solutions.

• This type of process is applied to needle or roller bearings, gears and piston rings.

• Similar coatings can be developed by thermal exposure at 300 to 600 to produce an oxide film.℃

• Steam tempering or autoclaving , is applied to high-speed steel drills and zirconium alloy components for this purpose.

Oxide coatings

iv) Anodising treatments

• for aluminium alloys produce oxide layers which reduce adhesive wear and are significantly harder than the substrate (up to 500Hv).

• In this case, the process of hard anodising is carried out in an oxidizing acid at around 0 , so that a layer of oxide up to 500 ℃microns thick is produced.

• Surface growth is exactly half of that layer thickness.

• Thinner layers, for decorative or corrosion protection purposes, are produced at room temperature.

• Anodising may be followed by treatments to seal the surface and improve the corrosion resistance or by incorporation solid lubricants into the surface to lower friction and reduce wear rates.

• In this respect, the cellular structure of the layer readily lends itself as a key and a reservoir for low friction polymers.

Anodising treatments

v) Sulphur treatments

• incorporate sulphur into the surface of ferrous components.

• Sulphur, because of its low melting point, and some sulphides because of their crystal structures, have good lubricating properties.

• • The processing temperature is generally below 200 . ℃

vi) Phosphating

• The process is based on dilute phosphoric acid solutions of iron, zinc and manganese phosphates .

• Accelerators are added to shorten the process times to just a few minutes at approx 40 to 70 . ℃

• The simplest phosphate coatings consist of grey or black crystals of Fe3(PO4)2 and some FePO4.

• Zinc and manganese produce more complex layers which absorb lubricant more readily.

• They are effective in reducing galling, and scuffing.

• All phosphate coatings absorb oil and grease, thereby assisting 'running-in' by preventing adhesive wear and fretting.

Phosphating

vii) Ion Implantation

• In this process, atoms of gaseous or metallic elements are ionised and pass to a high vacuum chamber, where they are accelerated through a mass separator.

• Selected ions are then further accelerated and implanted into the target component.

• The implanted species occupy interstitial sites and distort the lattice .

• It is a low temperature process, typically 150 for small ℃items and less for larger components.

• The depth of effect is very shallow, 0.2 microns, but the surface properties such as wear resistance, friction and oxidation/corrosion resistance can be enhanced.

• This process has been used to improve the performance of forming tools for plastics , press tools and some surgical implants.

Ion Implantation

• There are numerous processes which involve coating with a layer of material, not necessarily metallic, to meet the requirements of specific service environments.

Adding a Layer of Material to the Surface

i) Weld or roll cladding

• usually involves relatively thick layers (1mm to several cm).

• Weld cladding can be used to good effect where abrasive wear is a problem, such as coating digger teeth, tank tracks and mineral handling equipment.

• Roll cladding is usually associated with corrosive or mild erosive wear problems, typically those encountered in the chemical, wood pulp, paper and food process industries.

ii) Laser Alloying

• In addition to laser glazing and laser transformation, the power of the laser can be used to alloy a mixture of metal or cermet powders on a component surface.

• The process is normally concurrent, with the laser spot following the spray nozzle, so that the coating is fused into an alloy and mixed with the outer regions of the substrate material.

iii) Thermal Spraying

• involves heating metal, ceramic or mixtures of metal and ceramic powders to a semi-molten state and depositing them at high velocities on to components.

• These ‘line of sight’ processes can be divided into flame, electric arc, plasma arc and detonation gun techniques.

• In spray fusing , the coating is heated after deposition (usually by a torch) to fuse the material into a dense alloyed structure and produce a diffusion bond to the substrate.

• The thermal spray process is very versatile and the coating material and application method can be tailored to produce specific surface properties.

Thermal Spraying

• These can range from

1. extreme abrasion resistance with cermets (e.g. WC/Co) and ceramics (e.g. chromium oxide),

2. adhesive wear and corrosion resistance (e.g. Ni/Cr with carbide additions),

3. anti-scuffing(e.g. molybdenum), 4. Abradables (e.g. ceramic/graphite coatings for gas turbine

stators), 5. thermal barriers (e.g. zirconia ) and 6. corrosion resistant coatings (e.g. zinc).

Thermal Spraying

• The process can be automated and accurately controlled, with robot manipulation of the gun, rotation of the component being sprayed, and computer control of the spray parameters.

• For high integrity coatings the application of hot isostatic pressing (HIPing) after coating has been found to seal the porosity and further improve the bond to substrate quality.

Thermal Spraying

• Thermochemically formed comprise a slurry of ceramic particles in an aqueous chromium-based chemical.

• Through a sequence of applications (spraying, painting or dipping ) and heat curing cycles, the composite (which is free form porosity) can be built to a thickness over 100 microns.

• The coatings are hard (so effective against low stress abrasion) but tend to be brittle under high loading.

Thermal Spraying

iv) Electroplating

• Over 30 metals can readily be deposited from aqueous solutions.

• There is a tendency to think that electrolytic deposits are mainly for corrosion resistance, decorative(e.g. gold, rhodium and platinum or electronic/electrical usage, but there are many engineering and tribological applications for electroplates.

• Hard or soft deposits are used, depending on the particular function required.

• Hard chromium plates (typically 1000Hv and up to 1mm thick) are ideal for resisting abrasive wear, pick-up and corrosion/abrasion.

• Porous or intentionally cracked chromium deposits are used for oil retention as in automotive cylinder liners, precision bearing sleeves and piston rings.

• Softer (600Hv), crack-free versions of Cr plate (maximum 30 microns) are also available.

Electroplating

• Nickel and copper deposits are applied mainly as corrosion barriers, often as an undercoat for hard chrome, so that the combination provides both wear and corrosion protection.

• Nickel deposits are now available with the addition of a dispersion of file ceramic particles; such layers provide excellent oil retention and wear properties for cylinder liners in high revving engines.

Electroplating

• Cadmium and zinc (usually around 10 microns thick) are used to provide sacrificial corrosion protection.

• Because of their position relative to iron in the galvanic scale, such coatings will continue to protect the substrate even if they are scratched or worn.

• In the case of cadmium, the environmental pressure is towards its replacement, with zinc/nickel coatings currently providing some of the best alternatives.

Electroplating

• Soft deposits, such as tin , are used to facilitate ‘running in’, prevent fretting and confer corrosion resistance, whereas silver is used for anti-fretting.

• Cobalt is used for high temperature oxidation resistance and electrolytically deposited cobalt incorporating chromium carbide has been successfully used in both dry and lubricated conditions at 800 .℃

Electroplating

v) Electroless plating

• The autocatalytic deposition of nickel/ phosphorous and nickel/ boron has many useful corrosion and tribo / corrosion applications.

• Unlike the electrolytic processes, they produce a deposit with completely uniform coverage.

• In the case of Ni P, deposits around 25 to 50 microns thick with a hardness of about 500Hv is obtained, but thermal ageing at temperatures around 400 can develop hardness ℃values in excess of 1000Hv.

• Composite Electroless Plated Deposits involve the production of plated metals into which micron sized dispersions of non-metallic particles are incorporated via co-deposition.

• Composite coatings of electroless nickel containing silicon carbide exhibits superior abrasive wear resistance to hard chromium plate in some applications.

• Incorporation of 1 to 5 micron sized particles of PTFE as a solid lubricant in nickel coatings produces low friction, self-lubricating surfaces.

Electroless plating

vi) Galvanizingand bath aluminizing

• are widely used for sacrificial corrosion protection of steels, for instance in the construction industry and automotive exhausts.

• They are both based on submersion in liquid metal (zinc, in the case of galvanizing), usually with a strip steel product being continuously fed through the bath.

vii) Chemical Vapour Deposition (CVD)

• involves the dissociation of metal compound vapors at temperatures in excess of 850 to produce thin, diffusion-℃bonded, adherent coatings of metal carbides, nitrides, carbo nitrides and oxides; typically TiN, TiC, Ti(CN) and Al2O3.

• CVD coatings are used on carbide tool tips (indexable inserts) and on selected tribological items.

• Only selected ferrous items can be treated, e.g. carbides with cobalt binders or high speed steel items of simple shapes (the latter permitting them to be re-heat treated after deposition).

• However, techniques for plasma assisted chemical vapor deposition (PACVD) have developed which permit coatings to be deposited at temperatures well below the tempering temperatures for high speed steel, i.e. <550 . ℃

• In particular, this technique allows the deposition of ultra-hard carbon based coatings, called Diamond-Like Carbon which confers unique properties of low friction, wear resistance and 'kindness' to the sliding counterface.

Chemical Vapour Deposition (CVD)

viii) Physical Vapor Deposition (PVD)

• is becoming increasingly important for small engineering components.

• PVD embraces evaporative deposition, sputtering and ion plating in reactive or inert environments.

• Process temperatures are relatively low, up to 400 , thus ℃minimizing distortion and preserving the heat-treated state of the substrate.

• Reactive plating takes place in an inert gas.

• A partial pressure of reactive gas supplies the carbon or nitrogen, and the metallic species is added to the system by resistance heating, arc or electron beam evaporation, or sputtering from a solid target.

• Nitrides of titanium, zirconium, hafnium or chromium and other metals have been deposited onto metallic components to provide thin (3 ～ 5µm), hard (>3000Hv) layers of inert, low friction coefficient compounds.

Physical Vapor Deposition (PVD)

• These ceramic layers enhance the performance of cutting tools and have considerable potential for many other small components.

• Sputter ion plating techniques are also used to deposit solid lubricants like MoS2, PTFE and lead onto bearing surfaces, for instance for service in vacuum and space satellites.

• MoS2, is particularly attractive, with the resulting layers producing the lowest dry sliding friction coefficients so far obtained with any coating, even under normal atmospheric conditions.

• Corrosion protection layers and coating compounds for high temperature tribological service in gas turbines are also effectively deposited by PVD techniques.

Physical Vapor Deposition (PVD)

ix) Painting

• to protect a substrate against corrosion and improve its aesthetic appearance is probably the best known surface modification process, with coatings based on acrylics , polyester , polyurethane etc.

• There have been considerable advances in paints, application techniques and pre-coating / painting treatments.

• Surface preparation and corrosion protection methods such as phosphating have brought painting into the range of engineering coatings.

• Organic coatings deposited on metal parts by spraying, brush application or dipping are replacing electroplated deposits on some automotive parts.

Painting

• Painting, dipping or spraying with organic resins and polymeric materials, to which metallic, ceramic or solid lubricant compounds are added is providing for both the corrosion and tribological requirements.

• One process consists of zinc flakes bonded with zinc chromate and a proprietary organic material.

• This process provides excellent surface protection and is widely used in the automotive industry for fasteners, springs, clips, sintered parts and items for steering gears.

Painting

• Powder coating techniques are now increasingly used for application of organics and polymers.

• The process of air-spraying and electro-ferritic deposition without the need for solvents or carriers provides obvious environmental benefits.

• It is a rapidly growing area of surface engineering.

Painting

Comparative features of ion implantation, case hardening and hard coatings

Surface TreatmentsApplied coatingsApplied coatings• Plating, painting, and the application of enamel are the most common anti-Plating, painting, and the application of enamel are the most common anti-

corrosion treatments. They work by providing a barrier of corrosion-corrosion treatments. They work by providing a barrier of corrosion-resistant material between the damaging environment and the structural resistant material between the damaging environment and the structural material. Aside from cosmetic and manufacturing issues, there are tradeoffs material. Aside from cosmetic and manufacturing issues, there are tradeoffs in mechanical flexibility versus resistance to abrasion and high temperature. in mechanical flexibility versus resistance to abrasion and high temperature. Platings usually fail only in small sections, and if the plating is more noble Platings usually fail only in small sections, and if the plating is more noble than the substrate (for example, chromium on steel), a galvanic couple will than the substrate (for example, chromium on steel), a galvanic couple will cause any exposed area to corrode much more rapidly than an unplated cause any exposed area to corrode much more rapidly than an unplated surface would. For this reason, it is often wise to plate with active metal surface would. For this reason, it is often wise to plate with active metal such as zinc or cadmium. Painting either by roller or brush is more desirable such as zinc or cadmium. Painting either by roller or brush is more desirable for tight spaces; spray would be better for larger coating areas such as steel for tight spaces; spray would be better for larger coating areas such as steel decks and waterfront applications. Flexible polyurethane coatings, like decks and waterfront applications. Flexible polyurethane coatings, like Durabak-M26 for example, can provide an anti-corrosive seal with a highly Durabak-M26 for example, can provide an anti-corrosive seal with a highly durable slip resistant membrane. Painted coatings are relatively easy to durable slip resistant membrane. Painted coatings are relatively easy to apply and have fast drying times although temperature and humidity may apply and have fast drying times although temperature and humidity may cause dry times to vary.cause dry times to vary.

Surface TreatmentsReactive coatingsReactive coatings• If the environment is controlled (especially in recirculating systems), If the environment is controlled (especially in recirculating systems),

corrosion inhibitors can often be added to it. These form an corrosion inhibitors can often be added to it. These form an electrically insulating or chemically impermeable coating on exposed electrically insulating or chemically impermeable coating on exposed metal surfaces, to suppress electrochemical reactions. Such methods metal surfaces, to suppress electrochemical reactions. Such methods obviously make the system less sensitive to scratches or defects in obviously make the system less sensitive to scratches or defects in the coating, since extra inhibitors can be made available wherever the coating, since extra inhibitors can be made available wherever metal becomes exposed. Chemicals that inhibit corrosion include metal becomes exposed. Chemicals that inhibit corrosion include some of the salts in hard water (Roman water systems are famous some of the salts in hard water (Roman water systems are famous for their mineral deposits), chromates, phosphates, polyaniline, for their mineral deposits), chromates, phosphates, polyaniline, other conducting polymers and a wide range of specially-designed other conducting polymers and a wide range of specially-designed chemicals that resemble surfactants (i.e. long-chain organic chemicals that resemble surfactants (i.e. long-chain organic molecules with ionic end groups).molecules with ionic end groups).

Surface TreatmentsAnodizationAnodization• Aluminium alloys often undergo a surface treatment. Electrochemical Aluminium alloys often undergo a surface treatment. Electrochemical

conditions in the bath are carefully adjusted so that uniform pores conditions in the bath are carefully adjusted so that uniform pores several nanometers wide appear in the metal's oxide film. These several nanometers wide appear in the metal's oxide film. These pores allow the oxide to grow much thicker than passivating pores allow the oxide to grow much thicker than passivating conditions would allow. At the end of the treatment, the pores are conditions would allow. At the end of the treatment, the pores are allowed to seal, forming a harder-than-usual surface layer. If this allowed to seal, forming a harder-than-usual surface layer. If this coating is scratched, normal passivation processes take over to coating is scratched, normal passivation processes take over to protect the damaged area.protect the damaged area.

• Anodizing is very resilient to weathering and corrosion, so it is Anodizing is very resilient to weathering and corrosion, so it is commonly used for building facades and other areas that the surface commonly used for building facades and other areas that the surface will come into regular contact with the elements. Whilst being will come into regular contact with the elements. Whilst being resilient, it must be cleaned frequently. If left without cleaning, panel resilient, it must be cleaned frequently. If left without cleaning, panel edge staining will naturally occur.edge staining will naturally occur.

Surface TreatmentsBiofilm coatings• A new form of protection has been developed by applying certain

species of bacterial films to the surface of metals in highly corrosive environments. This process increases the corrosion resistance substantially. Alternatively, antimicrobial-producing biofilms can be used to inhibit mild steel corrosion from sulfate-reducing bacteria.

Surface TreatmentsControlled permeability formwork• Controlled permeability formwork (CPF) is a method of preventing

the corrosion of reinforcement by naturally enhancing the durability of the cover during concrete placement. CPF has been used in environments to combat the effects of carbonation, chlorides, frost and abrasion.

Surface TreatmentsCathodic protection• Cathodic protection (CP) is a technique to control the corrosion of a

metal surface by making that surface the cathode of an electrochemical cell. Cathodic protection systems are most commonly used to protect steel, water, and fuel pipelines and tanks; steel pier piles, ships, and offshore oil platforms.

Surface TreatmentsSacrificial anode protection• For effective CP, the potential of the steel surface is polarized

(pushed) more negative until the metal surface has a uniform potential. With a uniform potential, the driving force for the corrosion reaction is halted. For galvanic CP systems, the anode material corrodes under the influence of the steel, and eventually it must be replaced. The polarization is caused by the current flow from the anode to the cathode, driven by the difference in electrochemical potential between the anode and the cathode.

Surface TreatmentsImpressed current cathodic protection• For larger structures, galvanic anodes cannot economically deliver

enough current to provide complete protection. Impressed Current Cathodic Protection (ICCP) systems use anodes connected to a DC power source (such as a cathodic protection rectifier). Anodes for ICCP systems are tubular and solid rod shapes of various specialized materials. These include high silicon cast iron, RUST, mixed metal oxide or platinum coated titanium or niobium coated rod and wires.

Surface TreatmentsAnodic protection• Anodic protection impresses anodic current on the structure to be

protected (opposite to the cathodic protection). It is appropriate for metals that exhibit passivity (e.g., stainless steel) and suitably small passive current over a wide range of potentials. It is used in aggressive environments, e.g., solutions of sulfuric acid.

Surface TreatmentsAnodic protection• Anodic protection impresses anodic current on the structure to be

protected (opposite to the cathodic protection). It is appropriate for metals that exhibit passivity (e.g., stainless steel) and suitably small passive current over a wide range of potentials. It is used in aggressive environments, e.g., solutions of sulfuric acid.

Polarization• Hydrogen can cause a number of corrosion problems. Hydrogen

embrittlement is a problem with high-strength steels, titanium, and some other metals. Control is by eliminating hydrogen from the environment or by the use of resistant alloys.

• Hydrogen blistering can occur when hydrogen enters steel as a result of the reduction reaction on a metal cathode. Single-atom nascent hydrogen atoms then diffuse through the metal until they meet with another atom, usually at inclusions or defects in the metal. The resultant diatomic hydrogen molecules are then too big to migrate and become trapped. Eventually a gas blister builds up and may split the metal as shown in the picture below.

Electrolysis• The splitting (lysing) of a substance or decomposing by forcing a The splitting (lysing) of a substance or decomposing by forcing a

current through a cell to produce a chemical change for which the current through a cell to produce a chemical change for which the cell potential is negative.cell potential is negative.

• The electrolysis of water produces hydrogen gas at the cathode (on The electrolysis of water produces hydrogen gas at the cathode (on the right) and oxygen gas at the anode the right) and oxygen gas at the anode (on the left).(on the left).

Houghton Mifflin Company and G. Hall. All rights reserved. 160

A voltaic (Galvanic) cell can power an electrolytic cell

Houghton Mifflin Company and G. Hall. All rights reserved. 161

Fig. 21.17

162

Electrochemical corrosion Electrochemical corrosion - Corrosion produced by the - Corrosion produced by the development of a current in an electrochemical cell that development of a current in an electrochemical cell that removes ions from the material.removes ions from the material.

Electrochemical cell Electrochemical cell - A cell in which electrons and ions can - A cell in which electrons and ions can flow by separate paths between two materials, producing a flow by separate paths between two materials, producing a current which, in turn, leads to corrosion or plating.current which, in turn, leads to corrosion or plating.

Oxidation reaction Oxidation reaction - The anode reaction by which electrons are - The anode reaction by which electrons are given up to the electrochemical cell.given up to the electrochemical cell.

Reduction reaction Reduction reaction - The cathode reaction by which electrons - The cathode reaction by which electrons are accepted from the electrochemical cell.are accepted from the electrochemical cell.

Electrochemical Corrosion

163

Effect of dissolved oxygen and high acidity on polarization

The accumulation of negative charge on the metal due to the residual electrons The accumulation of negative charge on the metal due to the residual electrons leads to an increase in the potential difference between the metal and the solution. leads to an increase in the potential difference between the metal and the solution. This potential difference is called the electrode potential or, simply, the potential of This potential difference is called the electrode potential or, simply, the potential of the metal, which thus becomes more negative. This change in the potential tends to the metal, which thus becomes more negative. This change in the potential tends to retard the dissolution of metal ions but to encourage the deposition of dissolved retard the dissolution of metal ions but to encourage the deposition of dissolved metal ions from the solution onto the metal, i.e. the reverse of reaction. Continuation metal ions from the solution onto the metal, i.e. the reverse of reaction. Continuation of the dissolution and deposition of metal ions would result in the metal reaching a of the dissolution and deposition of metal ions would result in the metal reaching a stable potential such that the rate of dissolution becomes equal to the rate of stable potential such that the rate of dissolution becomes equal to the rate of deposition. This potential is termed the reversible potential Er and its value depends deposition. This potential is termed the reversible potential Er and its value depends on the concentration of dissolved metal ions and the standard reversible potential on the concentration of dissolved metal ions and the standard reversible potential Eo for unit activity of dissolved metal ions, aMEo for unit activity of dissolved metal ions, aMn+, i.e.n+, i.e.

164

Effect of dissolved oxygen and high acidity on polarization

where R is the gas constant, T the absolute temperature, F the Faraday and n the number of electrons transferred per ion. Once the potential reaches the reversible potential, no further net dissolution of metal occurs. The net amount of metal which dissolves during this process is generally very small.The potential of a metal in a solution does not usually reach the reversible potential but remains more positive because electrons can be removed from the metal by alternative reactions. In acid solutions, electrons can react with hydrogen ions, adsorbed on the metal surface from the solution, to produce hydrogen gas.

165

Effect of dissolved oxygen and high acidity on polarization

The occurrence of reaction (4) permits the continued passage of an equivalent quantity of metal ions into solution, leading to corrosion of the metal. Reaction (4) is also reversible and has a reversible potential given by

where pH2 is the partial pressure (fugacity) of hydrogen gas. If the partial pressure of hydrogen is allowed to build up, then the reversible potential of reaction (4) could be attained. No further net reaction of hydrogen ions would occur and so the net dissolution of metal ions would effectively cease. Normally hydrogen escapes from the system, so that the potential remains more negative than the reversible potential and corrosion continues. In neutral solutions, the concentration of hydrogen ions is too low to allow reaction (4) to proceed at a significant rate, but electrons in the metal can react with oxygen molecules,adsorbed on the metal surface from air dissolved in the solution, to produce hydroxyl ions

166

Effect of dissolved oxygen and high acidity on polarization

167

Effect of dissolved oxygen and high acidity on polarization

Factors Affecting Corrosion• 1. Presence of impurities in metals: Speed of corrosion increases

with the presence of impurities in the metals because these impurities help in setting up the voltaic cells.

• 2. Presence of electrolyte: Electrolytes present in water also increases the rate of corrosion e.g. corrosion of iron in sea water takes place in large extent than in distilled water because sea water contains salts i.e. electrolytes.

• 3. Position of metals in e.m.f. series: Highly reactive metals undergo corrosion faster than least reactive metals. Reactivity of metals can be found from the electrochemical series.

Factors Affecting Corrosion• 4. Presence of carbon dioxide in water: Presence of carbon dioxide

in natural water also increases the rusting of iron because it acts as an electrolyte and increases the flow of electron from one place to another.

• 5. Presence of protective coating: When the iron surface is coated with the metal, which is more reactive than the iron, then the rate of corrosion is retarded e.g. when iron is coated with zinc, iron is protected from rusting.

Stress in Steel• The major effect of tensile stress in soil nails is that of producing cracks in The major effect of tensile stress in soil nails is that of producing cracks in

the grout body due to tensile strains in the steel. As noted earlier, flexural the grout body due to tensile strains in the steel. As noted earlier, flexural stresses can also contribute to the development of cracks particularly near stresses can also contribute to the development of cracks particularly near the failure surface within the soil nailed mass. Therefore, the level of steel the failure surface within the soil nailed mass. Therefore, the level of steel stress at which cracks are produced is of importance with regard to stress at which cracks are produced is of importance with regard to corrosion in view of the crack-corrosion correlation discussed earlier. The corrosion in view of the crack-corrosion correlation discussed earlier. The development of cracks has the effect of setting up corrosion cells that tend development of cracks has the effect of setting up corrosion cells that tend to produce pitting (Houston, et al., 1972). Pitting corrosion can rapidly to produce pitting (Houston, et al., 1972). Pitting corrosion can rapidly decrease the cross-sectional area of steel in a localized area thus increasing decrease the cross-sectional area of steel in a localized area thus increasing the stress levels in the steel leading to potentially unsafe structural the stress levels in the steel leading to potentially unsafe structural conditions.conditions.

• On the other hand, the use of sacrificial steel reduces the resulting stress in On the other hand, the use of sacrificial steel reduces the resulting stress in an element and the likelihood of cracking grout as well as providing added an element and the likelihood of cracking grout as well as providing added resistance in the steel element. It should be noted that these observations resistance in the steel element. It should be noted that these observations about the stress in soil nail steel apply to both HBSNs and SBSNs.about the stress in soil nail steel apply to both HBSNs and SBSNs.

Metal Crystal Structure• There are two main forms of solid substance, characterizing

different atoms arrangement in their microstructures:– Amorphous solid– Crystalline solid

Metal Crystal StructureAmorphous solid• Amorphous solid substance does not possess long-range

order of atoms positions. Some liquids when cooled become more and more viscous and then rigid, retaining random atom characteristic distribution.

• This state is called undercooled liquid or amorphous solid. Common glass, most of Polymers, glues and some of Ceramics are amorphous solids. Some of the Metals may be prepared in amorphous solid form by rapid cooling from molten state.

Metal Crystal StructureCrystalline solid• Crystalline solid substance is characterized by atoms

arranged in a regular pattern, extending in all three dimensions. The crystalline structure is described in terms of crystal lattice, which is a lattice with atoms or ions attached to the lattice points. The smallest possible part of crystal lattice, determining the structure, is called primitive unit cell.

Metal Crystal StructureCrystal lattice examples:

Metal Crystal Structure• Metal crystal structureMetal crystal structure and specific metal properties are determined and specific metal properties are determined

by by metallic bonding metallic bonding – force, holding together the atoms of a metal. – force, holding together the atoms of a metal. Each of the atoms of the metal contributes its Each of the atoms of the metal contributes its valence electronsvalence electrons to to the crystal lattice, forming an electron cloud or electron “gas”, the crystal lattice, forming an electron cloud or electron “gas”, surrounding positive metal ions. These surrounding positive metal ions. These free electronsfree electrons belong to the belong to the whole metal crystal.whole metal crystal.

• Ability of the valence free electrons to travel throughout the solid Ability of the valence free electrons to travel throughout the solid explains both the high electrical conductivity and thermal explains both the high electrical conductivity and thermal conductivity of metals. conductivity of metals. Other specific metal features are: luster or shine of their surface Other specific metal features are: luster or shine of their surface (when polished), their malleability (ability to be hammered) (when polished), their malleability (ability to be hammered) and ductility(ability to be drawn).and ductility(ability to be drawn).These properties are also associated with the metallic bonding and These properties are also associated with the metallic bonding and presence of free electrons in the crystal lattice.presence of free electrons in the crystal lattice.

END OF END OF PRESENTATIONPRESENTATION