Defects in Metal Forgings

Inspection is an important aspect of metal forging manufacture. All parts should be checked for defects after the manufacturing process is complete. Defects of metal forged product include exterior cracking, interior cracking, laps, cold shuts, warping of the part, improperly formed sections and dead zones.

Cracking

 Cracking both interior and exterior is caused by excessive stress, or improper stress distribution as the part is being formed. Cracking of a forging can be the result of poorly designed forging die or excess material in the work piece. Cracks can also be caused by disproportionate temperature distributions during the manufacturing operation. High thermal gradients can cause cracks in a forged part.

Laps or folds

Laps or folds in a metal forging are caused by a buckling of the part, laps can be a result of too little material in the work piece.

Cold shuts

Cold shuts occur when metal flows of different temperatures meet, they do not combine smoothly, a boundary layer, (cold shut), forms at their intersection. Cold shuts indicate that there is a problem with metal flow in the mold as the part is being formed.

Warping

Warping of a forged part can happen when thinner sections cool faster than the rest of the forging.

Dead Zones

Improperly formed sections and dead zones can be a result of too little metal in the work piece or flawed forging die design resulting in incorrect material distribution during the process.


In general, defects in parts manufactured by metal forging can be controlled first by careful consideration of work stock volume, and by good design of both the forging die, (mold), and the process. The main principle is to enact the right material distributions, and the right material flow to accomplish these distributions. Die cavity geometry and corner radius play a large roll in the action of the metal. Forging die design, and forging process design will be discussed in later sections.

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Hot Twist Test

 In a hot twist test, a round bar is twisted in one direction until material failure occurs. The amount of rotation is taken as a quantitative measurement of metal forgeability. This test is often conducted on a material at several different temperatures. Other tests are also used in industrial metal forging manufacture. Impact testing is sometimes used to gauge the forgeability of a material.

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Upsetting Test

In an upsetting test, the work stock is compressed by flat open die, reducing the work in height until cracks form. The amount of reduction can be considered a measurement of forgeability. Upsetting tests can be performed at different temperatures and different compression speeds. Testing various temperatures and strain rates will help determine the best conditions for the forging of a particular metal.

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Metal Forgeability

Metal selection must be considered carefully in forging manufacture. The ability of a metal to experience deformation without failure or cracking is an important characteristic to consider in its selection as a material for a forging process. In metal forging industry, several tests have been developed to try and quantify this ability. The amount of deformation a particular metal can tolerate without failure is directly related to that metal's forgeability. The higher the amount of deformation, the higher the forgeability.
Following two tests are used to measure Forgeability

1. Upsetting Test
2. Hot Twist Test

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Classification of metal forging process

Metal forging processes can be classified by the degree to which the flow of material is constrained during the process. There are three major classifications in metal forging manufacture. First, open die forging, in which the work is compressed between two die that do not constrain the metal during the process. Secondly, Impression Die Forging, in which cavities within the die restrict metal flow during the compression of the part, causing the material to deform into a desired geometric shape. Some material in impression die forging is not constrained by the cavities and flows outward from the die, this metal is called flash. In industrial metal forging, a subsequent trimming operation will be performed to remove the flash. The third type of metal forging is Precision/Flashless Forging. In flashless forging manufacture the entire work piece is contained within the die in such a way that no metal can flow out of the die cavity during the compression of the part, hence no flash is produced.

Following is the classification of Metal Forging

1. Open die forging
• Cogging
  Fullering
  Edging
2. Impression Die Forging
3. Precision/Flashless Forging

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Precision Forging / Flashless Forging

Modern technological advances in the metal forging process and in the design of die, have allowed for the development of precision forging. Precision forging may produce some or no flash and the forged metal part will be at or near its final dimensions, requiring little or no finishing. The number of manufacturing operations is reduced as well as the material wasted. In addition, precision forging can manufacture more complex parts with thinner sections, reduced draft angles, and closer tolerances. The disadvantages of these advanced forging methods are that special machinery and die are needed, also more careful control of the manufacturing process is required. In precision forging, the amount of material in the work, as well as the flow of that material through the mold must be accurately determined. Other factors in the process such as the positioning of the work piece in the cavity must also be performed precisely.

Flashless Forging

Flashless forging is a type of precision forging process in which the entire volume of the work metal is contained within the die and no material is allowed to escape during the operation. Since no material can leave the mold as the part is forged, no flash is formed. Like other precision forging processes, flashless forging has rigorous process control demands, particularly in the amount of material to be used in the work piece. Too little material and the die will not fill completely, too much material will cause a dangerous build up of forces.

Back To Forging Classification of Forging 

Open Die Forging

The manufacturing process of metal forging has been performed for at least 7,000 years, perhaps even 10,000 years. The most basic type of forging would have been shaping some metal by striking it with a rock. Latter the employment of different materials, such as bronze then iron and steel, and the need for forged metal products such as swords and armor, led way to the art of blacksmithing or blacksmith forging. Blacksmithing is an open die forging process where the hammer and anvil surfaces serve as opposing flat die. Bronze forgings, followed by iron and steel forgings, mark some of man's earlier manufacturing prowess.

Following phenomenon occur during Open Die Forging Process
Upsetting
Barreling

Types of Open Die Forging
Cogging
Fullering
Edging

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Classification of Forging

Impression Die Forging

Impression die forging manufacture involves compression of a work piece by the use of impression die, (a mold), that contain cavities that act to restrict the flow of metal within the die during the deformation of the work. The metal will fill the space within the die cavity as it is plastically compressed into the mold. Closing of the mold completes the deformation, hence impression die forging is also referred to as closed die forging. The forged metal part will now have the geometric dimensions of the mold, provided a complete filling of the die cavity occurred during the process. The operation of forcing metal to flow into and fill the impressions in the die will also alter the grain structure of the metal. The creation of favorable grain structure through controlled material deformation should always be a consideration in the design of an impression die forging process.
One characteristic of impression die forging manufacture is the formation of flash or fin around the forged part. During the design of the metal forging operation, the volume of the starting work piece is made slightly higher than that of the closed die cavity. As the die close, and the work metal flows into and fills the contours of the impression, some excess material will flow out of the die and into the area between the two die. This will form a thin plane of metal all around the work at the parting line, (where the two die meet when they close), of the forged product. Flash is trimmed from the forging in a latter process.
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Classification of Forging

Edging (Open Die Forging)

Edging is also an open die forging process often used in manufacturing practice, to prepare a work for sequential metal forging processes. In edging, open die with concave surfaces plastically deform the work material. Edging acts to cause metal to flow into an area from both sides. Edging and fullering both are used to redistribute bulk quantities of the metal forging's material.

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Fullering (Open Die Forging)

A typical open die forging process performed in metal forging manufacture is fullering. Fullering is mostly used as an earlier step to help distribute the material of the work in preparation for further metal forging operations. This often occurs when a manufacturing process requires several forging operations to complete. The metal forging process design section will discuss this concept later. In fullering, open die with convex surfaces are used to deform the work piece. The result is to cause metal to flow out of one area and to both sides.
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Cogging

Cogging, or drawing out, is often used in manufacturing industry. Cogging is an open die forging process in which flat or slightly contoured die are employed to compress a work piece, reducing its thickness and increasing its length. In a cogging operation, the forging is large relative to the size of the die. The part is forged in a series of steps. After each compression of the material, the open die advance along the length of the work piece and perform another forging compression. The distance the die travel forward on the work piece between each forging step is called the bite, and is usually about 40 to 75 percent of the width of the die, in industrial practice. A greater reduction in the thickness of the forged part can be accomplished by decreasing the width of the bite. Cogging allows for smaller machinery with less power and forces to form work of great length. Often in commercial manufacture of metal products, cogging may be just one metal forging process in a series of metal forging processes required to form a desired part. Sometimes formed products such as metal fences may be produced directly from cogging.

 

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Barreling (Open Die Forging)

During forging, friction forces at the die-work interface oppose the spreading of the material near the surfaces, while the material in the center can expand more easily. The result is to create a barrel shape to the part. This effect is called barreling in in metal forging terms. Barreling is generally undesirable and can be controlled by the use of effective lubrication. Another consideration, during hot forging manufacture, that would act to increase the barreling effect would be the heat transfer between the hot metal and the cooler die. The metal nearer to the die surfaces will cool faster than the metal towards the center of the part. The cooler material is more resistant to deformation and will expand less than the hotter material in the center, also causing a barreling effect.

 

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Upsetting (Open Die Forging)

In an upsetting process the work is placed between two flat die and its height is decreased by compressive forces exerted between the two die. Since the volume of a metal will remain constant throughout its deformation, a reduction in height will be accompanied by an increase in width. Figure shows a flat die upsetting process, under ideal conditions.

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Hot Vs Cold Die Forging

Most metal forging operations are carried out hot, due to the need to produce large amounts of plastic deformation in the part, and the advantage of an increased ductility and reduced strength of the work material. Hot die forging also eliminates the problem of strain hardening the metal. In cases where it is desirable to create a favorable strain hardening of the part, cold die forging may be employed. Cold die forging manufacture, while requiring higher forces, will also produce greater surface finish and dimensional accuracy than hot die forging. Some specific metal forging processes are always performed cold, such as coining.

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Classification of Metal Forming Processes

Metal forming processes can be classified under two major groups.

• Bulk Deformation
• Rolling
• Forging
• Extrusion
• Drawing
• Sheet Metal Working 
• Sheering
• Bending
• Deep Drawing

Forging

Metal forging is a metal forming process that involves applying compressive forces to a work piece to deform it, and create a desired geometric change to the material.

 
The forging process is very important in industrial metal manufacture, particularly in the extensive iron and steel manufacturing industry. A steel forge is often a source of great output and productivity. Work stock is input to the forge, it may be rolled, it may also come directly from cast ingots or continuous castings. The forge will then manufacture steel forgings of desired geometry and specific material properties. These material properties are often greatly improved.
Metal forging is known to produce some of the strongest manufactured parts compared to other metal manufacturing processes, and obviously, is not just limited to iron and steel forging but to other metals as well. Different types of metals will have a different factors involved when forging them, some will be easier to forge than others. Various tests are described latter to determine forging process factors for different materials. Aluminum, magnesium, copper, titanium, and nickel alloys are also commonly forged metals. It is important to understand the principles of manufacturing forged products, including different techniques and basic metal forging design. The following will provide a comprehensive overview of the metal forging process.
Metal forging, specifically, can strengthen the material by sealing cracks and closing empty spaces within the metal. The hot forging process will highly reduce or eliminate inclusions in the forged part by breaking up impurities and redistributing their material throughout the metal work. However, controlling the bulk of impurities in the metal should be a consideration of the earlier casting process. Inclusions can cause stress points in the manufactured product, something to be avoided. Forging a metal will also alter the metal's grain structure with respect to the flow of the material during its deformation, and like other forming processes, can be used to create favorable grain structure in a material greatly increasing the strength of forged parts. For these reasons, metal forging manufacture gives distinct advantages in the mechanical properties of work produced, over that of parts manufactured by other processes such as only casting or machining.
Metal forgings can be small parts, or weigh as much as 700,000 lbs. Products manufactured by forging in modern industry include critical aircraft parts such as landing gear, shafts for jet engines and turbines, structural components for transportation equipment such as automobiles and railroads, crankshafts, levers, gears, connecting rods, hand tools such as chisels, rivets, screws, and bolts to name a few. The manufacture of forging die and the other high costs of setting up an operation make the production of small quantities of forged parts expensive on a price per unit basis. Once set up, however, operation costs for forging manufacture can be relatively low, and many parts of the process may be automated. These factors make manufacturing large quantities of metal forgings economically beneficial.

Hot Vs Cold Die Forging

Classification of Metal Forging Process

1. Open die forging
• Cogging
• Fullering
• Edging
2. Impression Die Forging
3. Precision/Flashless Forging

Metal Forgeability
Defects in Metal Forging

 

 

 

Metal Forming

Metal forming is a general term for a large group, that includes a wide variety of manufacturing processes. Metal forming processes are characteristic in that the metal being processed is plastically deformed to shape it into a desired geometry. In order to plastically deform a metal, a force must be applied that will exceed the yield strength of the material.

Flow Stress

During a metal forming operation, it is important to know the force and power that will be needed to accomplish the necessary deformation. The stress-strain graph shows us that the more a work piece is deformed plastically, the more stress is needed. The flow stress is the instantaneous value of the force necessary to continue the yielding and flow of the work material at any point during the process. Flow stress can be considered as a function of strain. The flow stress value can be used to analyze what is going on at any particular point in the metal forming process. The maximum flow stress may be a critical measurement in some metal forming operations, since it will specify the force and power requirements for the machinery to perform the process. The force needed at the maximum strain of the material would have to be calculated in order to determine maximum flow stress.

Strain Rate

The strain rate for any particular manufacturing metal forming process is directly related to the speed at which deformation is occurring. A greater rate of deformation of the work piece will mean a higher strain rate. The specific process and the physical action of the equipment being used has a lot to do with strain rate. Strain rate will affect the amount of flow stress. The effect strain rate has on flow stress is dependent upon the metal and the temperature at which the metal is formed. The strain rate with relation to flow stress of a typical metal at different temperatures is shown in figure.

Effect of Temperature in Metal Forming

Properties of a metal change with an increase in temperature. Therefore, the metal will react differently to the same manufacturing operation if it is performed under different temperatures and the manufactured part may posses different properties. For these reasons, it is very important to understand the materials that we use in our manufacturing process. This involves knowing their behavior at various temperature ranges.

There are three basic temperature ranges at which the metal can be formed

• Cold Working
• Warm Working
• Hot Working

Classification of Metal Forming Process

 

 

 





Hot Working Process

Hot working, (or hot forming), is a metal forming process that is carried out at a temperature range that is higher than the recrystallization temperature of the metal being formed. The behavior of the metal is significantly altered, due to the fact that it is above its recrystallization temperature. Utilization of different qualities of the metal at this temperature is the characteristic of hot working.
Although many of these qualities continue to increase with increasing temperature, there are limiting factors that make overly high temperatures undesirable. During most metal forming processes the die is often cold or slightly heated. However, the metal stock for hot working will usually be at a higher temperature relative to the die. In the design of metal forming process, it is critical to consider the flow of metal during the forming of the work. For metal forming manufacturing, in general, the temperature gradient between the die and the work has a large effect on metal flow during the process. The metal nearer to the die surfaces will be cooler than the metal closer to the inside of the part, and cooler metal does not flow as easily. High temperature gradients, within the work, will cause greater differences in flow characteristics of different sections of the metal, these could be problematic. For example, metal flowing significantly faster at the center of the work compared to cooler metal near the die surfaces that is flowing slower, can cause part defects. Higher temperatures are harder to maintain throughout the metal forming process. Work cooling during the process can also result in more metal flow variations. Another consideration with hot forming manufacture, with regard to the temperature at which to form the part, is that the higher the temperature the more reactive the metal is likely to be. Also if a part for a hot working process is too hot then friction, caused during the process, may further increase heat to certain areas causing melting, (not good), in localized sections of the work. In an industrial hot metal working operation, the optimum temperature should be determined according to the material and the specific manufacturing process.
When above its recrystallization temperature a metal has a reduced yield strength, also no strain hardening will occur as the material is plastically deformed. Shaping a metal at the hot working temperature range requires much less force and power than in cold working. Above its recrystallization temperature, a metal also possesses far greater ductility than at its cold worked temperature. The much greater ductility allows for massive shape changes that would not be possible in cold worked parts. The ability to perform these massive shape changes is a very important characteristic of these high temperature metal forming processes.
The work metal will recrystallize, after the process, as the part cools. In general, hot metal forming will close up vacancies and porosity in the metal, break up inclusions and eliminate them by distributing their material throughout the work piece, destroy old weaker cast grain structures and produce a wrought isotropic grain structure in the part. These high temperature forming processes do not strain harden or reduce the ductility of the formed material. Strain hardening of a part may or may not be wanted, depending upon the application. Qualities of hot forming that are considered disadvantageous are poorer surface finish, increased scale and oxides, decarburization, (steels), lower dimensional accuracy, and the need to heat parts. The heating of parts reduces tool life, results in a lower productivity, and a higher energy requirement than in cold working.

Cold Working Process
Selection of Temperature Range for Forming
Friction and Lubrication in Metal Forming
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Classification of Metal Forming Processes

Cold Working Process

Cold working, (or cold forming), is a metal forming process that is carried out at room temperature or a little above it. In cold working, plastic deformation of the work causes strain hardening as discussed earlier. The yield point of a metal is also higher at the lower temperature range of cold forming. Hence, the force required to shape a part is greater in cold working than for warm working or hot working. At cold working temperatures, the ductility of a metal is limited, and only a certain amount of shape change may be produced. Surface preparation is important in cold forming. Fracture of the material can be a problem, limiting the amount of deformation possible. In fact, some metals will fracture from a small amount of cold forming and must be hot formed.

Advantages

1. The part will be stronger and harder due to strain hardening.
2. Cold forming causes directional grain orientation, which can be controlled to produce desired directional strength properties. 
3. Work manufactured by cold forming can be created with more accurate geometric tolerances and a better surface finish.
4. Since low temperature metal forming processes do not require the heating of the material, a large amount of energy can be saved and faster production is possible.
5. Despite the higher force requirements, the total amount of energy expended is much lower in cold working than in hot working.

 

Disadvantage

1. One main disadvantage of this type of process is a decrease in the ductility of the part's material

Hot Working
Selection of Temperature Range for Forming
Friction and Lubrication in Metal Forming
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Classification of Metal Forming Processes

Friction and Lubrication in Metal Forming

Metal forming processes are characteristic of high pressures between two contacting surfaces. In hot forming operations, these high pressures are accompanied by extreme temperatures. Friction and die wear are a serious consideration in metal forming manufacturing. A certain amount of friction will be necessary for some metal forming processes, but excessive friction is always undesirable. Friction increases the amount of force required to perform an operation, causes wear on tooling, and can affect metal flow, creating defects in the work.
Where friction is involved, lubricants can usually help. For some metal forming processes and materials no lubrication is used, but for many lubrication is applied to contacting surfaces to reduce friction forces. Lubricants used in industry are different depending upon the type of metal forming process, the temperature at which the operation occurs, and the type of material formed. Lubricants should be effective and not produce any toxic fumes. Lubricants used in manufacturing industry for metal forming processes include, vegetable and mineral oils, soaps, graphite dispensed in grease, water based solutions, solid polymers, wax, and molten glass

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Classification of Metal Forming Processes
 

Selection of Temperature Range for Metal Forming

Production at each of these temperature ranges has a different set of advantages and disadvantages. Sometimes, qualities that may be undesirable to one process may be desirable to another. Also, many times work will go through several processes. The goal is to design the manufacture of a part in such a way as to best utilize the different qualities to meet or enhance the specifications of the part. To produce a strong part with excellent surface finish, then a cold forming process could be a good choice. However, to produce a part with a high ductility a hot forming process may be best. Sometimes the advantages of both hot forming and cold forming are utilized when a part is manufactured by a series of processes. For example, hot working operations may first be performed on a work piece to achieve large amounts of shape change that would not be possible with cold forming due to strain hardening and limited ductility. Then the last process that completes the manufacture of the part is a cold working operation. This process does not require a significant shape change, since most of the deformation was accomplished by the hot forming process. Having a cold forming process last will finish the shape change, while strengthening the part, giving a good surface finish and highly accurate tolerances

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Classification of Metal Forming Processes

What is hydrogen attack

A surprising fact is that hydrogen atoms are very small and hydrogen ions even smaller and can penetrate most metals. Hydrogen, by various mechanisms, embrittles a metal especially in areas of high hardness causing blistering or cracking especially in the presence of tensile stresses.

Prevent

This problem can be prevented by

• Using a resistant or hydrogen free material
• Avoiding sources of hydrogen such as cathodic protection, pickling processes and certain welding processes
• Removal of hydrogen in the metal by baking.

Corrosion Main

What is Localized Corrosion

Localized corrosion is responsible for 70 % failures from corrosion. 
The consequences of localized corrosion can be a great deal more severe than uniform corrosion generally because the failure occurs without warning and after a surprisingly short period of use or exposure.
Following are the different forms of Localized Corrosion.

1. Galvanic Corrosion
2. Pitting Corrosion
3. Selective Attack
4. Stray current corrosion
5. Microbial Corrosion
6. Inter Granular Corrosion
7. Concentration Cell Corrosion
8. Thermogalvanic Corrosion
9. Corrosion Caused by combined action
10. Corrosion Fatigue
11. Fretting Corrosion
12. Stress Corrosion Cracking
13. Hydrogen Attack

 

Corrosion Main

What is stress corrosion cracking

The combined action of a static tensile stress and corrosion which forms cracks and eventually catastrophic failure of the component. This is specific to a metal material paired with a specific environment.

Prevention

Prevention can be achieved by:

• Reducing the overall stress level and designing out stress concentrations
• Selection of a suitable material not susceptible to the environment
• Design to minimise thermal and residual stresses
• Developing compressive stresses in the surface the material
• Use of a suitable protective coating

Corrosion main

What is Fretting Corrosion

Relative motion between two surfaces in contact by a stick-slip action causing breakdown of protective films or welding of the contact areas allowing other corrosion mechanisms to operate.

Prevention

Prevention is possible by:

• Designing out vibrations
• Lubrication of metal surfaces
• Increasing the load between the surfaces to stop the motion
• Surface treatments to reduce wear and increase friction coefficient.

Corrosion Main

What is Corrosion Fatigue

The combined action of cyclic stresses and a corrosive environment reduce the life of components below that expected by the action of fatigue alone. This can be reduced or prevented by;

• Coating the material
• Good design that reduces stress concentration
• Avoiding sudden changes of section
• Removing or isolating sources of cyclic stress

Corrosion Main

What is corrosion caused by combined action

This is corrosion accelerated by the action of fluid flow sometimes with the added pressure of abrasive particles in the stream. The protective layers and corrosion products of the metal are continually removed exposing fresh metal to corrosion.

Prevention

Prevention can be achieved by:

• Reducing the flow rate and turbulence
• Use of replaceable or robust linings in susceptible areas
• Avoiding sudden changes of direction
• Streamlining or avoiding obstructions to the flow

Corrosion Main

What is Thermogalvanic Corrosion

Temperature changes can alter the corrosion rate of a material and a good rule of thumb is

that 10 oC rise doubles the corrosion rate. If one part of component is hotter than another the

difference in the corrosion rate is accentuated by the thermal gradient and local attack occurs in a zone between the maximum and minimum temperatures. The best method of prevention is to design out the thermal gradient or supply a coolant to even out the difference.

Corrosion main

Concentration Cell Corrosion (Crevice)

If two areas of a component in close proximity differ in the amount of reactive constituent available the reaction in one of the areas is speeded up. An example of this is crevice corrosion which occurs when oxygen cannot penetrate a crevice and a differential aeration cell is set up. Corrosion occurs rapidly in the area with less oxygen.

 

 

Prevention

The potential for crevice corrosion can be reduced by:

• Avoiding sharp corners and designing out stagnant areas
• Use of sealants
• Use welds instead of bolts or rivets
• Selection of resistant materials

Corrosion Main

What is intergranular corrosion

This is preferential attack of the grain boundaries of the crystals that form the metal. It is caused by the physical and chemical differences between the centres and edges of the grain.

 

Prevention

It can be avoided by:

• Selection of stabilised materials

• Control of heat treatments and processing to avoid susceptible temperature range

Corrosion Main

What is Microbial Corrosion

This general class covers the degradation of materials by bacteria, molds and fungi or their by-products. It can occur by a range of actions such as:

• Attack of the metal or protective coating by acid by-products, sulphur, hydrogen sulphide or ammonia

• Direct interaction between the microbes and metal which sustains attack.

Prevention

Prevention can be achieved by:

• Selection of resistant materials
• Frequent cleaning
• Control of chemistry of surrounding media and removal of nutrients
• Use of biocides
• Cathodic protection.

Corrosion Main

What is stray current corrosion

When a direct current flows through an unintended path and the flow of electrons supports corrosion. This can occur in soils and flowing or stationary fluids.

 

 

Prevention

The most effective remedies involve controlling the current by:

• Insulating the structure to be protected or the source of current
• Earthing sources and/or the structure to be protected.
• Applying cathodic protection
• Using sacrificial targets.

Corrosion main

What is Selective Attack

This occurs in alloys such as brass when one component or phase is more susceptible to attacke than another and corrodes preferentially leaving a porous material that crumbles. It is best avoided by selection of a resistant material but other means can be effective such as:

• Coating the material
• Reducing the aggressiveness of the environment
• Use of cathodic protection

Corrosion Main

What is Pitting Corrosion

Pitting corrosion occurs in materials that have a protective film such as a corrosion product or when a coating breaks down. The exposed metal gives up electrons easily and the reaction initiates tiny pits with localised chemistry supporting rapid attack.

Note: Pits can be crack initiators in stressed components or those with residual stresses resulting from forming operations. This can lead to stress corrosion cracking.

 

 

Prevention

Control can be ensured by:

• Selecting a resistant material
• Ensuring a high enough flow velocity of fluids in contact with the material or frequent washing
• Control of the chemistry of fluids and use of inhibitors

• Use of a protective coating
• Maintaining the material’s own protective film.

Corrosion Main

What is Galvanic Corrosion

This can occur when two different metals are placed in contact with each other and is caused by the greater willingness of one to give up electrons than the other. Three special features of this mechanism need to operate for corrosion to occur:

• The metals need to be in contact electrically
• One metal needs to be significantly better at giving up electrons than the other
• An additional path for ion and electron movement is necessary.

 

Metals have to be significantly different in terms of their nobility in the specific environment that they are exposed to. This can be looked up in a table.

Ranking in sea water (top of table is most noble)

Noble

Platinum

Gold

Titanium

Stainless Steel 316 (passive state)

Brasses

Tin

Lead

Stainless steel 316 (active state

Carbon steel

Aluminium alloys

Zinc

Magnesium

 

Prevention

Prevention of this problem is based on ensuring that one or more of the three features do not

exist.

• Break the electrical contact using plastic insulators or coatings between the metals.
• Select metals close together in the galvanic series.
• Prevent ion movement by coating the junction with an impermeable material, or ensure environment is dry and liquids cannot be trapped.

Corrosion main

What is Uniform Corrosion

It is responsible for 30% of failures due to corrosion.
Uniform corrosion, as the name suggests, occurs over the majority of the surface of a metal at a steady and often predictable rate. Although it is unsightly its predictability facilitates easy control, the most basic method being to make the material thick enough to function for the lifetime of the component. Uniform corrosion can be slowed or stopped by using the five basic facts;

Corrosion Main 

What is Corrosion

Corrosion is the deterioration of materials by chemical interaction with their environment. The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals. The most widely used metal is iron (usually as steel) and the following discussion is mainly related to its corrosion.

 

It results from oxidation of metal.

• First the metal releases electrons to form positive ions.
• Atmospheric water, condenses over the metallic surface dissolves oxygen.
• Oxygen + Water results in formation of OH ion. 
• OH (-) and Fe(+) combine to for iron hydroxide.
• Iron hydroxide reacts with oxygen to give iron oxide (rust)

We infer from the above steps that 

• Ions are involved and need a medium to move in (usually water)
• Oxygen is involved and needs to be supplied
• The metal has to be willing to give up electrons to start the process
• A new material is formed and this may react again or could be protective of the original metal
• A series of simple steps are involved and a driving force is needed to achieve them

To avoid corrosion read How to avoid corrosion

 

Types of Corrosion

1. Uniform Corrosion
2. Localized Corrosion 
• Galvanic Corrosion 
• Pitting Corrosion
• Selective Attack
• Stray Current Corrosion
• Microbial Corrosion
• Intergranular corrosion
• Concentration Cell Corrosion
• Thermogalvanic Corrosion
• Corrosion caused by combined action
• Corrosion Fatigue
• Fretting Corrosion
• Stress Corrosion Cracking
• Hydrogen Attack

 

 



How to avoid corrosion

In order to avoid corrosion

1. Slow down or stop the movement of electrons

• Coat the surface with a non-conducting medium such as paint, lacquer or oil
• Reduce the conductivity of the solution in contact with the metal an extreme case being to keep it dry. Wash away conductive pollutants regularly.
• Apply a current to the material (see cathodic protection).

2. Slow down or stop oxygen from reaching the surface. Difficult to do completely but coatings can help.
3.Prevent the metal from giving up electrons by using a more corrosion resistant metal higher in the electrochemical series. Use a sacrificial coating which gives up its electrons more easily than the metal being protected. Apply cathodic protection. Use inhibitors.
4.Select a metal that forms an oxide that is protective and stops the reaction.

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