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*Name: Engr. Dr. Khursheed Mahmood

Qualification: B.E.(Mechanical),NED,1974 M.Sc.(Materials), UK,1978, Ph.D.(Matellurgical Engg.) UK,1988

Experience: More than 34 years of teaching & research at N.E.D. University of Engg. & Technology, Karachi. Present Status: Professor, Department of Materials EngineeringNED University of Engineering & Technology,Karachi

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*Introduction to corrosionCORRODED

*CORRSION IN PIPE LINES31 March,2009

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* Corrosion is a universal and all time problem. It causes deterioration in structures, machines, metallic equipment, vessels, pipelines etc etc. Once developed it grows very rapidly to capture all equipment in its vicinity under its influence. If due care and proper protection is not taken it destroys every thing very rapidly.

*COST OF CORROSION 20 % of the world s steel production goes to replace corroded steel.

In March 1969 the committee on corrosion and protection formed by the UK Ministry of Technology estimated the cost of corrosion at 1.356 per annum which amounted to 3.5% of their GNP.

It is generally accepted that cost of corrosion amounts to similar proportion of the GNP in all industrialized countries.

Here are some current facts and figures:-

*USA: 1.25% of GNP., 2002, an study estimated the cost of corrosion in USA to be $ 276 billion(3.1 % of GDP)

UK: 3.5% of GNP ( 136m,1969)

India: estimated to be around $ 364 billion as of 2004.

Sweden: 125% of GNP.

West Germany: 3% (DM 6 billion, 1969)

Any estimation of cost of corrosion for Pakistancan only be conjecture since no survey exists.

*Pakistans GNP for 2005 was estimated as $ 107.28 billion.3.5% of GNP, the cost of corrosion well exceeds$3.5 billion .

The recommendations that have arisen from suchsurveys in general were:

1.A need for better disemination of information On corrosion and protection

2. A need for greater educationin corrosion and Protection, and

3. An increased awareness of the hazards of corrosion.

*The above is especially pertinent to Pakistan where a lack of awareness about Corrosion and a low level of concern about its occurrence exists.The cost of corrosion includes:-

That part of the maintenance cost arising from corrosion damage.

The cost of inhibitors and other protective systems.Consequential losses known to be caused by Corrosion.

The cost of conservatism in design due to uncertainties in operating conditions ,properties of materials, etc.

The cost of technical effort devoted to corrosion

* Pakistan has been incurring a loss of about $ 3 billion per annum on account of infra structure and industrial corrosion and it needs a comprehensive national strategy for corrosion control. This was stated by engineers and experts at a local Conference on Corrosion, Prevention and Management held in 2009.

It was told in the same conference that industrial sector faces huge losses due to corrosion as the replacement of costly machinery is an expensive business. It was also mentioned by a Sindh Minister in the same conference that corrosion was a neglected subject in the government quarters & there was no plan for its prevention.

*The losses of corrosion can be minimized by creating awareness among the society about its damage and by using new technologies for corrosion prevention.

Billions of rupees of public money are lost due to corrosion in infrastructure facilities like water supply and sewarage pipe line, bridges, etc.

An assessment report mentioned that about 3% of GNP is lost every year in Pakistan due to corrosion in industrial, infrastructure and house hold sectors.

In the year 2004-2005 the calculated extent of loss was about $ 105.28 billion.

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Corrosion control is,therefore, necessary as it enhances the cost of the projects.

Therefore, there is a need for th adoption of anti- corrosion measures at the time of designing of infrastructure projects.

Some of the measures adopted to protect metals from corrosion may be through adequate concrete cover and admixture , coatings and water roofing, cathodic and anodic protection.

* In the limited time available it may not be possible to cover a very vast area of Corrosion control and Protection.

However, an under standing of the fundamental principle s of corrosion damage and its various types are now presented as follows.

*Definition of Corrosion:Corrosion is basically deterioration and degradation of Materials due to the interaction of variety of Environments.

Specifically speaking about metals it isdefined as:

"AN ELECTRO-CHEMICAL REACTION BETWEEN A METAL AND ITS ENVIRONMENT THAT IS ELECTROLYTE ,IN WHICH METAL DETERIORATESAND LOSES ITS PROPERTIES".

*A Corrosion Cell, shown in figure 1, is develped. Causing deterioration of metals

.ANODE IS THE DEPLETING

*ANODE IS THE DEPLETING METAL.

Anode the metal that corrodes. Cathode the metallic part that is protected. Electrolyte the cell substance in which electrons flow. Metallic Path path for flow of current out side the cell.

* CATIONS: +vely charged ions of electrolyte. ANIONS: - vely charged ions of electrolyte. CONVENTIONAL CURRENT FLOW:- Outside Cell : +ve to ve (cathode to anode)Inside Cell : -ve to +ve (anode to cathode) ELECTRON FLOW is just opposite to conventional current flow. EXTERNAL CIRCUIT: cathode, anode, and metallic path.

* POTENTIAL DIFFERNCE - electromotive force between anode and cathode measured in volts RESISTIVITY OF ELECTROLYTE hindrance to flow of current by the electrolyte measured as ohm-cm. CONTACT RESISTANCE - between Anode & Electrolyte to flow of current. - between Cathode & Electrolyte to current flow.

* It comprises of only two reactions: - Oxidation at Anode.- Reduction at Cathode.

OXIDATION REACTION- removal of one or more electrons from anode metal. REDUCTION REACTION- reaction of electron with hydrogen in the absence of oxygen or reaction of electrons with dissolved oxygen and breakdown of water into hydroxyle ions.

*M M n+ + n e-

Where,M = Metal involved n = Valence of corroding metal e = electronsExample:Fe Fe++ + 2e-

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*Cathodic ReactionO2 + 2H2O + 2 e- 4(OH)-

*Because there is no net gain or loss of electron, two atoms must dissolve to provide the four electrons required at the cathode. Therefore, the anodic and the cathodic reactions would be 2Fe 2Fe++ + 4e- (ANODIC) O2 + 2H2O + 4e- 4(OH-) (CATHODIC)

*2Fe + O2 + 2H2O 2Fe++ + 4(OH-)

After dissolution, ferrous ions (Fe++) generally oxidize to ferric ions (Fe+++); these will combine with hydroxide ions (OH)- formed at cathode to give a corrosion product called Rust {Fe(OH)3 or Fe203+ H2O}.The important point to remember is that anodic dissolution of metal occurs electrochemically, the insoluble corrosion products are formed by a secondary chemical reaction.

*COMPARISON OF A TYPICAL CORROSION CELL WITH THEPIPELINE CORROSION CELL

* A typical corrosion cell is similar to a pipeline corrosion cell. The anodes and the cathodes are the portions, which are actually developed on the pipeline due to varying potential difference at different locations and due to varying environment. A potential difference is present in pipeline portions due to electrolyte (soil) conditions. The electrical path is in the form of pipeline itself. The electrolyte is the soil containing the moisture in it.

* The anode and cathode are the localized areas on pipe and due to driving potential at that point electrons from anodic area of pipe go through the wet soil and gather at cathode area of pipe. The free iron atoms, combine with OH ions in soil (containing moisture) to form Ferric Hydroxide in the form of rust. The rust remains on the pipe and deteriorates it. While the electron at the cathode are taken by hydrogen ions to evolve the hydrogen gas. So anodic areas in the pipe corrode while the cathode remains at its shape because it only gains electrons.

* Types of corrosionAqueous CorrosionDissimilar Metal CorrosionGrain Boundary CorrosionInter-granular CorrosionSelective LeachingMicrobiological Induced CorrosionCrevice and Pitting CorrosionFlow Induced CorrosionEnvironmental Sensitive Cracking

*The aqueous or wet corrosion:-

This is a very common form of corrosion, also generally known as uniform corrosion.

Corrosion takes place in the presence of moisture or wet environments.

This form of corrosion is not of great concern and can be prevented or reduced by: Proper materials, including coatings, inhibitors or cathodic protection.

Most of the other forms of corrosion insidious in nature and are considerably more difficult to predict.

They are also localized as the attack is limited to specific areas or parts of a structure.

*Two metal corrosionThis form of corrosion is also known as galvanic corrosion.

The potential difference existing between two different metals in contact (galvanic coupling) is responsible for the electrons to flow from one to the other, thereby, causing metal loss or corrosion.

The less resistant metal becomes anodic and more resistant metal becomes cathodic.

*As an example, both steel and zinc corrode by themselves, but when they are coupled, Zn corrodes and the steel is protected.

The severity of the galvanic corrosion largely depends on the type and amount of moisture present e.g near the sea shore and inland location.

No galvanic corrosion when the metals are completely dry.

Severity of attack is more near the junction.

Unfavorable area ratio: Large Ac Aa

*PREVENTIONAvoide unfavorable area ratioInsulate dissimilar metals (possibly completely)Apply coatingsAdd inhibitorsselect combination closer together from galvanic seriesAvoide threaded jointsDesign for the use of readily replaceable anodic parts or make them thickerInstall a third metal which is anodic to both metals in the galvanic series.

*SACRIFICIAL ANODES

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*Several Types of Corrosion Cells are Developed Dissimilar Metal Corrosion Cells. Corrosion Cells due to Dissimilar Soils. Differential Aeration Corrosion Cells. New and Old Pipe Corrosion. Mill Scale Corrosion.

*INTER GRANULAR CORROSION

*A cell is produced if there is an electrical contact between two dissimilar metals on the same pipeline and there is a common contact between electrolyte (soil or water) and both metals.

* As a consequence, any two metals have an electric potential between them. The magnitude of the potential and which metal should be anodic (corroded) and which should be cathodic, depends on the position of metals in the EMF series. Some of the EMF series of the metals were illustrated in the previous table:

*A dissimilar metal corrosion: A buried pipeline is an example of two dissimilar metals.i.e a plain steel section and a section of copper pipe. Steel will be anodic and would corrode due to upper position in EMF series.Yet another Example, (Figure on next slide) Plain steel pipe and galvanized steel pipe with no electrical insulation between them. In the table of emf series zinc is more active metal and would act as an anode and, therefore, it would corrode.

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* A corrosion cell has been established.

The potential of electrolyte (soil "A) is slightly different from the potential of the electrolyte (soil "B).

*Even partially concrete buried pipe can develop a corrosion cell

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* Another special case in figure1(f) results in many special corrosion cells at the pipe surface which is not detected by potential measurements at the surface of the ground as shown in the figure. These cells occur when the soil in the small area is totally different nature and each portion is of different composition as illustrated in the figure. The local corrosion cells will develop at many small portions of pipe.Small Pockets of Dissimilar Soils

*Crevice & Pitting Corrosion Crevice Corrosion: The attack occurs because a part of metal surface is in shielded or restricted environment, compared to the rest of the metal which is exposed to a large of electrolyte.Crevice corrosion is very much associated with the geometry of structures, such as in riveted plates, In welded structures and In threaded components

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*Corrosion in Universal Joint

*Case Study- Renovation of a BuildingOn the outside of each apartment, the panels of S.S were mounted, through neat circular holes stamped into them, vertically between the concrete and balcony floors. Even during installation, the panels shown trails of rust, emanating from the edges,holes and running vertically downward across the sheet.Panels were not so badly affected where they had been mounted in exposed to more severe weather.

*Some more observations:The crevice corrosion depends the geometry of structures (riveted plates, welded structures and threaded components)Contact of metal with non-metals such as plastics, rubber and glass; or deposits of sand, dirt or corrosion products also influences. The way of assembly of components or through the surface accumulation of solid debris is important. This leads to a heterogeneous distribution of species dissolved in the electrolyte, which in turn provides the conditions for local attack.Considerable research has been devoted to the electrochemistry within crevices, within the extremely small confines of a crevice (widths 25-100 mills).

* corrosion in crevices

*Promotion of corrosion

*Pitting CorrosionPitting is localized corrosion which selectively attacks areas of a metal surface where there is a surface scratch or mechanically induced break in an otherwise protective film An emerging dislocation or slip step caused by applied or residual tensile stressesA compositional heterogeneity such as an inclusion, segregate or precipitateThe observation of corrosion pits as a result of crevice corrosion can sometimes cause confusion about the difference between the two forms of corrosion.

*Pitting in Pipes

*Pitting CorrosionMuch research has now been carried out into the detailed mechanisms of pitting and its initiation. According to Burstein, pit nucleation in stainless steel is viewed as a microscopically violent process which is unstable because pit propagation may not be achieved. Nucleation current dies continuously and most pit initiation events terminate. If the pit survives nucleation then the pit growth is called metastable because continued survival depends upon maintenance of an effective barrier to diffusion provided by a perforated cover of corrosion product over the pit mouth.

*Pitting in Stainless Steels.If the cover is lost, but the current density is not sufficiently great, this stage dies too. If the pit survives then stable pitting occurs. This process is also diffusion-controlled; the diffusion barrier depends upon the pit depth. These are the three identifiable phases to pitting of stainless steel.

*CeriumMolybdinum ProcessThere has also been much recent research into surface modification leading to improved resistance to pitting corrosion, particularly for aluminium alloys and stainless steels. The procedure, known as the Ce-Mo process, consists of immersion boiling in Cerium and Molibdinum salys.

*Distinction- Crevice/Pitting Pitting is distinguishable from crevice corrosion in the initiation phase.Whereas crevice corrosion is initiated by differential concentration of oxygen or ions in the electrolyte, Pitting corrosion is initiated (on plane surfaces) by metallurgical factors alone.Once initiated, the pit takes on very similar geometrical characteristics to those of a crevice and the propagation electro-chemistries of pitting and crevice corrosion converge.

*Selective LeachingA Specific Type of Corrsion It is the net removal of one element from an alloy.It is also called dealloying or demetallification.Dezincification Destannification DenickelificationDe-alloying of BrassesGraphitization

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An important source of corrosion damage. A pipeline completely of uniform steel but buried in a soil where some of the areas of soil are well aerated (free oxygen supply) whereas other portions are poorly aerated. The pipeline portion which is well aerated will behave cathode while the portion of pipe which is poorly aerated will be anodic and corroded as shown in figure1 (g).

* concreted

* Aerobic bacteria form oxygen and chemical concentration cells. The bacteria capable of oxidizing ferrous ions, further accelerate corrosion. Many produce mineral or organic acids which may also breakdown structure coatings. The breakdown products are then sometimes usable as food for bacteria, leading to accelerated corrosion.

* Bacteria stick and grow on the surface of a pipe/struc. Temperature of 150-450C are suitable for their growth. These bacteria are aerobic or anaerobic (need oxygen or no oxygen). Their metabolism products influence the electro-chem. reaction by forming materials or films which act as a diffusion barrier or change ion concentrations and pH. Some bacteria are capable of being directly involved in the oxidation or reduction of metal ions and can shift the chemical equilibrium which influences the corrosion rate.

* Old and New Pipelines behave like dissimilar metals pipeline. Figure 1 (h) shows the process of this typical corrosion cell. This often happens when old pipelines from some areas are replaced by new pipelines.

*New & Old Pipes Junction(Corrosion Cell)

* The galvanic potential of old and new pipeline is totally different. This causes development of a corrosion cell on the pipeline. Table (2) shows the galvanic series of metals, which can show the two different potentials of new and old pipelines. The new steel is anodic and is corroded. Similarly, if during cutting scraping of old pipeline, exposed areas of bright steel are produced, which also acts as anode and result in corrosion.

*Galvanic Potentials of Metals

* New pipes have mill scale. It is not metal but scale on hot rolled steel. It acts as a dissimilar metal. Table (2) shows that steel pipe will be anodic to mill scale and will cause severe corrosion in a low resistivity environment.

* Each of these previously discussed types of electrochemical corrosion cells may cause significant corrosion. In many cases there are combinations of many different types of corrosion simultaneously at work to make corrosive situations even worse on the metal surface. Understanding the actual cause of corrosion is of utmost importance in maintaining a submerged or buried metallic structure, such as a pipeline or storage tank.

* This type of electrochemical corrosion cell is caused by an electromotive force from an external source affecting the structure by developing a potential gradient in the electrolyte. Inducing a current in the metal which forces a part of the structure to become an anode and another part a cathode. This pickup and discharge of current occurs when a metallic structure offers a path of lower resistance for current flowing in the electrolyte.

*Stray Current CorrosionStructure under stray current attack.

* This type of corrosion can be extremely severe as very high voltages are forced into the earth by various sources. The potential gradient, in the electrolyte forces one part of the structure to pick up current (becoming a cathode) and another part of the structure to discharge current (becoming an anode). The figure 16 shows the phenomenon of stray current problems.

* Stray current corrosion occurs where the current from the external source leaves the metal structure and enters back into the electrolyte. The external power source is the driving force for the voltage of the cell. Stray current corrosion is different from natural corrosion because it is caused by an externally induced electrical current. It is independent of such environmental factors as concentration cells, resistivity, pH and galvanic cells.

* The amount of current (corrosion) depends on the external power source. The resistance of the path through the metallic structure is low. The resistance of the path between the external source, anode and cathode is high. It may be caused by ICCP systems if a foreign electrically continuous structure is built near the protected structures anodes. The corrosion is usually found after failures in the foreign structure occur.

* Stray current corrosion is the most severe form of corrosion. The metallic structure is forced to become an anode and the amount of current translates directly into metal loss. Different metals have specific amounts of weight loss when exposed to current discharge. The following table is a good indicator. Basic unit is one amp-year. A Big Foe

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* For example, if a stray current of just two amps was present on a steel pipeline, the result would be a loss of 18.2 kilograms (40.2 pounds) of steel in one year. For a coated pipeline, this could result due to a penetration at a defect in the coating in an extremely short period of time, sometimes only a few days.

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