Mechanism Corrosion

"Mechanism Corrosion"

After you know the definition corrosion and localized corrostion,you will get the information about mechanism corrosion, just read the article below.

The picture above shows us why the metal to be corroded, just see the reaction process below. This series of steps tells us a lot about the mechanism corrosion.

(1) Ions are involved and need a medium to move in (usually water)

(2) Oxygen is involved and needs to be supplied

(3) The metal has to be willing to give up electrons to start the process

For those step happen the major component of steel, iron (Fe) at the surface of a component undergoes a number of simple changes. Firstly,

<Fe> → <Feⁿ⁺> + n electrons

The iron atom can lose some electrons and become a positively charged ion. This allows it to bond to other groups of atoms that are negatively charged. We know that wet steel rusts to give a variant of iron oxide so the other half of the reaction must involve water (H₂O) and oxygen (O₂) something like this

{O₂}+ 2 [H₂O] + 4e^- → 4[OH]

This makes sense as we have a negatively charged material that can combine with the iron and electrons, which are produced in the first reaction are used up.

2<Fe> + [O₂]+ 2[H₂O] → 2[Fe (OH)₂]

Iron + Water with oxygen → Iron Hydroxide dissolved in it.

Oxygen dissolves quite readily in water and because there is usually an excess of it, reacts with the iron hydroxide.

4[Fe(OH)₂]+ {O₂}→ 2[H₂O] + 2<Fe₂O₃.H₂O>

Iron hydroxide + oxygen → water + Hydrated iron oxide (brown rust)

 

(4) A new material is formed and this may react again or could be protective of the original metal

(5) A series of simple steps are involved and a driving force is needed to achieve them, you can see the schematic process from the picture below.

The most important fact is that interfering with the steps allows the corrosion reaction to be stopped or slowed to a manageable rate in order to get persistence of the right life time of those material.

After you know the mechanism corrosion, so you will be ready to be the corrosion scientist or corrosion engineer, see the next writing how to differ the corrosion scientist and corrosion engineer. 

CORROSION ENGINEER

"Corrosion Engineer"

After you read Definition Corrosion, Mechanism Corrosion, and Localized Corrosion, it must be the basic knowledge in order to be Corrosion Scientist and Corrosion Engineer, but actually both are different. We must know what is the differences between them, just read this article.

 It has been said that the aim of science is “knowing why”, while technology deals with “knowing how”. One of the famous corrosion scientists, T.P. Hoar, considered the technologist as a person who utilizes his scientific knowledge to solve practical problems.

However, it is the situation and the technological problem that one is facing that decides the extent to which the practical solution can be based on scientific knowledge. 

In corrosion technology as Corrosion Engineering many problems are solved more or less by pure experience because the conditions are too complex to be described and explained theoretically. On the other hand, it is evident that great progress in corrosion technology can be obtained by the application of corrosion theory to practical problems to a much higher extent than what has been done traditionally.

This can be done i) in order to explain corrosion cases, to find the reasons and prevent new attacks, ii) in corrosion testing with the aim of materials selection, materials development etc., and in monitoring by electrochemical methods, iii) for the prediction of corrosion rates and localization, and iv) to improve methods for corrosion prevention in general, and to select and apply the methods more properly in specific situations.

This point of view is considered useful as a basis for corrosion education and it has been a guideline for teaching corrosion to mechanical and marine engineering. It starts with an article of the most important corrosion theory. Then we apply this theory as much as possible to practical corrosion problems.

Corrosion and corrosion protection is more interdisciplinary than most subjects in engineering. Consequently, mastery of corrosion means that it is necessary to have insight into physical chemistry and electrochemistry, electronics/electrical techniques, physical metallurgy, mechanical properties of materials, fluid dynamics, selective design, joining technology, and the materials market situation.

These areas of knowledge constitute the foundation upon which corrosion technology is built. Since corrosion involves chemical change, the student must be familiar with principles of chemistry in order to understand corrosion reactions. Because corrosion processes are mostly electrochemical, an understanding of electrochemistry is also important. Furthermore, since structure and composition of a metal often determine corrosion behavior, the student should be familiar with the fundamentals of physical metallurgy as well.

The corrosion scientist studies corrosion mechanisms to improve (a) the understanding of the causes of corrosion and (b) the ways to prevent or at least minimize damage caused by corrosion.

The Corrosion Engineer, on the other hand, applies scientific knowledge to control corrosion. For example, the corrosion engineer uses cathodic protection on a large scale to prevent corrosion of buried pipelines, tests and develops new and better paints, prescribes proper dosage of corrosion inhibitors, or recommends the correct coating.

The corrosion scientist, in turn, develops better criteria of cathodic protection, outlines the molecular structure of chemical compounds that behave best as inhibitors, synthesizes corrosion - resistant alloys, and recommends heat treatment and compositional variations of alloys that will improve their performance. Both the scientific and engineering viewpoints supplement each other in the diagnosis of corrosion damage and in the prescription of remedies.

Those all the explanation of differences between corrosion scientist and Corrosion Engineer, I hope those make it clear in our mind set. If you have another opinion, just send comment in this blog, and let's discuss.

LOCALIZED CORROSION

"LOCALIZED CORROSION"

After read Definition Corrosion and Main Groups of Corrosion which one of them is localized corrosion, here I explain about the type of localized corrosion and how to prevent it.

1. 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:

Ø  The metals need to be in contact electrically

Ø  One metal needs to be significantly better at giving up electrons than the other

Ø  An additional path for ion and electron movement is necessary.

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.

2. 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 localized chemistry supporting rapid attack. Control can be ensured by:

Ø  Selecting a resistant material

Ø Ensuring a high enough flow velocity of fluids in contact with the material or frequent washing

Ø    Control of the chemistry of fluids and use of inhibitors

Ø  Use of a protective coating

Ø  Maintaining the material’s own protective film.

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, just see the Scheme of pitting corrosion below.

3. SELECTIVE ATTACK

This occurs in alloys such as brass when one component or phase is more susceptible to attack 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:

Ø  Coating the material

Ø  Reducing the aggressiveness of the environment

Ø  Use of cathodic protection

4. 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. The most effective remedies involve controlling the current by:

Ø  Insulating the structure to be protected or the source of current

Ø  Earthing sources and/or the structure to be protected.

Ø  Applying cathodic protection

Ø  Using sacrificial targets.

5. MICROBIAL CORROSION

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

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

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

The microbiological corrosion is shown below with the schematic also.

Prevention can be achieved by:

Ø  Selection of resistant materials

Ø  Frequent cleaning

Ø  Control of chemistry of surrounding media and removal of nutrients

Ø  Use of biocides

Ø  Cathodic protection.

6. 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 centers and edges of the grain. It can be avoided by:

Ø  Selection of stabilized materials

Ø  Control of heat treatments and processing to avoid susceptible temperature range.

Here the schematic of intergranular corrosion, just see below

7. 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. Just see the example of crevice corrosion of gasket below and the schematic phase of it.

The potential for crevice corrosion can be reduced by:

Ø  Avoiding sharp corners and designing out stagnant areas

Ø  Use of sealants

Ø  Use welds instead of bolts or rivets

Ø  Selection of resistant materials

8. 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.

9. 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 can be achieved by:

Ø  Reducing the flow rate and turbulence

Ø  Use of replaceable or robust linings in susceptible areas

Ø  Avoiding sudden changes of direction

Ø  Streamlining or avoiding obstructions to the flow

10. 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;

Ø  Coating the material

Ø  Good design that reduces stress concentration

Ø  Avoiding sudden changes of section

Ø  Removing or isolating sources of cyclic stress

11. 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 is possible by:

Ø  Designing out vibrations

Ø  Lubrication of metal surfaces

Ø  Increasing the load between the surfaces to stop the motion

Ø  Surface treatments to reduce wear and increase friction coefficient.

12. 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 can be achieved by:

Ø  Reducing the overall stress level and designing out stress concentrations

Ø  Selection of a suitable material not susceptible to the environment

Ø  Design to minimise thermal and residual stresses

Ø  Developing compressive stresses in the surface the material

Ø  Use of a suitable protective coating

13 HYDROGEN DAMAGE

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. 

This problem can be prevented by:

Ø  Using a resistant or hydrogen free material

Ø  Avoiding sources of hydrogen such as cathodic protection, pickling processes and certain welding 

     processes

Ø  Removal of hydrogen in the metal by baking.

After we know about localized corrosion, then we will go to the profession of corrosion namely corrosion engineer that you will see in the next article.

Definition Corrosion

Definition Corrosion

While we want to know about why the material will be degraded or corroded, first we must know the definition corrosion in order to know it.

1. Definition:

Many ways talks about definition corrosion, but the usual interpretation of the term is “an attack on a metallic material by reaction with its environment”. The concept of corrosion can also be used in a broader sense, where this includes attack on nonmetallic materials, but such attacks are outside the scope of this writing. 

There are four requirement in presenting corrosion as you can see in the picture below namely:
1. Anode
2. Cathode
3. Electrolyte
4. Electrolyte Media

2. Main Group of Corrosion

Corrosion of metallic materials, based on electrolyte media and temperature, can be divided into three main groups:

1. Wet corrosion, where the corrosive environment is water with dissolved species. The liquid is an electrolyte and the process is typically electrochemical and best – known as low temperature corrosion.

2. Corrosion in other fluids such as fused salts and molten metals.

3. Dry corrosion, where the corrosive environment is a dry gas. Dry corrosion is also frequently called chemical corrosion and the best-known example is high temperature corrosion.

Based on the attacking location, corrosion divided into:

a. Uniform 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;

(1) Slow down or stop the movement of electrons

(a) Coat the surface with a non-conducting medium such as paint, lacquer or oil

(b) Reduce the conductivity of the solution in contact with the metal an extreme case being to keep it dry. Wash away conductive pollutants regularly.

(c) 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.

a. Use a sacrificial coating which gives up its electrons more easily than the metal being protected. 

b. Apply cathodic protection.

c. Use inhibitors.

(4) Select a metal that forms an oxide that is protective and stops the reaction. Control and consideration of environmental and thermal factors is also essential.

b. Localized 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. Application of the five basic facts needs greater thought and insight. 

Surface Preparation

"SURFACE PREPARATION"

In the rubber lining section, the next step after substrate inspection, the next step is surface preparation. All surfaces to be lined shall be grit blasted to blasting quality SA 2.5 Swedish Standard, to be checked using "Tested" system once this is achieved. The temperature of the substrate shall be a minimum of 30^oC above the dew point during surface preparation and rubber lining application. The substrate temperature during rubber lining application shall be between 100^oC and 420^oC. The relative humidity shall not exceed 85%. Steel surfaces which are to receive rubber lining shall be smooth and free of protrusions. 

Getting the most out of surface preparation

Preparing a substrate properly is the first step toward a successful coating and lining. Proper preparation can vary, depending on the substrate, the end use of the coating and the cost. Reputable coating manufacturers outline all details of the recommended surface preparation on the product data sheet included with the coating shipment. Here's a brief summary of some of the basic steps to achieve ideal preparation:

Alkaline Wash

A properly maintained alkaline wash system will effectively clean most organic and inorganic surface contaminants. By varying the strength of the alkaline solution, these washers can apply an effective alkaline etch to certain metal substrates, like aluminum, and in some cases eliminate the need for further substrate pretreatments like grit-blasting. An alkaline wash is an effective pretreatment prior to grit- blasting as it will reduce contamination of the blast media, and thus extend the media’s usable life.

Vapor Degreasing

This is the easiest and most efficient way to remove petroleum-based oils and greases, but could have difficulty effectively removing waterborne, or emulsified oils and some inorganic contaminants, like salts. A degreaser is a chamber into which a solvent is placed, which is then heated at the bottom and cooled at the top. The solvent vaporizes, rises, and condenses on parts that have been placed in the vapor zone of the chamber. It then falls back to the bottom, carrying any impurities with it. The solvent is vaporized repeatedly, and since it vaporizes more readily than oil, any oil and impurities are left behind on the bottom of the chamber.

Mechanical Cleaning (Grit-Blasting)

This involves blasting the substrate with a sharp medium. The substrate and application will determine the blast profile required. It is crucial that the parts to be blasted be as clean as possible before blasting, since contamination with dirt or oil not only shortens the life of the blast media, but can also interfere with adhesion. Note: because of labor, this is the most costly method of preparation.

Zinc Phosphate Pretreatment

This process deposits fine crystals of zinc phosphate with minimum porosity on the substrate. These improve adhesion, flexibility and corrosion resistance. The 5 steps typically recommended here include: an alkaline clean, water rinse, deposition of zinc phosphate, a second water rinse and chromate sealer. It's important to monitor temperature, immersion time, pH and concentration of solution, and rinse water contamination to insure consistent results.

Chemical Cleaning /Acid Etching:

This method, also called "pickling," is a common shortcut that involves dipping parts into a solution of heated hydrochloric or sulfuric acid; however, this does not effectively clean corrosion from the porous folds and cavities of metal. There are two other problems:

• Acid etching may cause hydrogen embritlement (the loss of ductility in metal caused by the absorption of hydrogen gas).
• The acid attacks the metal, leaving residues of "smut" that are deposits of carbon and metal oxide. "Smut" adheres loosely to the metal underneath, thereby reducing adhesion.

After we do surface preparation the next step is blasting process, just see in the next article.Whichever method is used, careful and complete surface preparation is the best way to guarantee maximum results from a coating and rubber lining. 

Blasting Process

"Blasting Process"

After doing surface preparation, now we carry on to blasting process. Here I provide the article of  in order to make surface treatment completed in the right procedure. Here type of blasting process.

Shot Blasting - A rapid, dust free process that leaves the substrate clean and dry. Shot blast machines hurl shot toward the intended surface at a high speed, removing debris, paint and buildup from the surface.

Sand Blasting - Procedure for cleaning of metal surfaces, for which fine silica sand is blasted through a nozzle onto the surface by means of compressed air to remove scale as well as other coverings.

Grit Blasting - A technique of abrasive cleaning or surface preparation using sharp particles (e.g. cast iron shot, aluminum oxide). It covers such processes as removal of scale, corrosion, paint and other surface films. Use of free silica presents a health threat and should be avoided. Increasing of coating adhesion on surfaces can increase the life time of coating, safety factor of transmitting line pipe and decreasing the rate of corrosion and costs. Preparation of steel pipe surfaces before doing the coating process is done by shot and grit blasting. This is a mechanical way to do it.

Some effective parameters on blasting process:
1. Particle size of abrasives, 

2. Distance to surface, 

3. Rate of abrasive flow, 

4. Abrasive physical properties, shapes, selection of abrasive, kind of machine and its power,
5. Standard of surface cleanness degree, 

6. Roughness,
7. Time of blasting and 

8. Weather humidity.

Enviro-Grit sandblasting abrasives are a reusable recycled glass product that is 100% free of crystalline silica. Excellent for use on steel, aluminium, concrete, and wood, we stock general purpose (12-50 and 20-50 grit), and fine (30-60 grit) to make sure we have the best product for your project. Put simply, Enviro-Grit abrasives are setting the industry standard for worker safety, environmental protection, and superior cutting power, because Enviro-Grit is made from recycled glass (and contains no ingredients which are harmful to the environment), the abrasives are inert, non-flammable, non-corrosive, and non-fibrogenic.

After you read the blasting process for the next article will talk about surface finish standard. It will be glad if you put you question in comment box, and also put your constructive critic and advise.