Lecture notes on Chapt. 13 and 14

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Lecture notes on Chapt. 13 and 14 Corrosion
Engineering  Corrosion Engineering is the
engineering design of corrosion control methods
and the solution of in-service problems,
including Corrosion prevention Material
selection Paints and coatings Chemical
treatments Corrosion results in billions of
dollars. The problem is largely avoidable by
proper maintenance and protection methods.
Corrosion prevention and control programs can
help reduce the high cost of corrosion.
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Economics are a key consideration in corrosion
engineering. Develop and implementation of
corrosion prevention and control maintenance
procedures Implement a monitoring program for
early detection of incipient corrosion
problems. Reduce the impact of corrosion and
develop cost-effective corrosion control programs
A corrosion Engineer has to provide technical
services in four areas Prediction, Assessment
and Diagnosis, Testing Remediation/Mitigation.
A Corrosion engineer should be able to Set
up test programs, Analyzes information acquired
from different sources Construct profiles of
corrosion problems. Suggest operating or
maintenance schemes, create test programs for
selecting new materials or altering operating
conditions, and devise remedial action plans for
corrosion problems.
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Classification of corrosion protection methods
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• Active corrosion protection
• The aim of active corrosion protection is to
  influence the reactions which proceed during
  corrosion, it being possible to control not only
  the package contents and the corrosive agent but
  also the reaction itself in such a manner that
  corrosion is avoided. Examples of such an
  approach are the development of
  corrosion-resistant alloys and the addition of
  inhibitors to the aggressive medium.
• Passive corrosion protection
• In passive corrosion protection, damage is
  prevented by mechanically isolating the package
  contents from the aggressive corrosive agents,
  for example by using protective layers, films or
  other coatings. However, this type of corrosion
  protection changes neither the general ability of
  the package contents to corrode, nor the
  aggressiveness of the corrosive agent and this is
  why this approach is known as passive corrosion
  protection. If the protective layer, film etc. is
  destroyed at any point, corrosion may occur
  within a very short time.

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• Permanent corrosion protection
• The purpose of permanent corrosion protection
  methods is mainly to provide protection at the
  place of use. The stresses presented by climatic,
  biotic and chemical factors are relatively slight
  in this situation. Machines are located, for
  example, in factory sheds and are thus protected
  from extreme variations in temperature, which are
  frequently the cause of condensation. Examples of
  passive corrosion protection methods are
• Tin plating
• Galvanization
• Coating
• Enameling
• Copper plating
• Temporary corrosion protection
• The stresses occurring during transport,
  handling and storage are much greater than those
  occurring at the place of use. Such stresses may
  be manifested, for example, as extreme variations
  in temperature, which result in a risk of
  condensation. Especially in maritime transport,
  the elevated salt content of the water and air in
  so-called salt aerosols (salt spray) may cause
  damage, as salts have a strongly
  corrosion-promoting action. The following are the
  main temporary corrosion protection methods

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• Protective coating method, Desiccant method and
  VCI method
• 1. Protective coating method
• The protective coating method is a passive
  corrosion protection method. The protective
  coating isolates the metallic surfaces from the
  aggressive media, such as moisture, salts, acids
  etc..
• The following corrosion protection agents are
  used
• Solvent-based anticorrosion agents
• Very high quality protective films are obtained.
  
• Once the anticorrosion agent has been applied,
  the solvent must vaporize so that the necessary
  protective film is formed.
• Depending upon the nature of the solvent and
  film thickness, this drying process may take as
  long as several hours. The thicker the film, the
  longer the drying time. If the drying process is
  artificially accelerated, there may be problems
  with adhesion between the protective film and the
  metal surface.
• Since protective films are very thin and soft,
  attention must always be paid to the dropping
  point as there is a risk at elevated temperatures
  that the protective film will run off, especially
  from vertical surfaces.
• Since solvent-based corrosion protection agents
  are often highly flammable, they may only be used
  in closed systems for reasons of occupational
  safety.
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• Water-based anticorrosion agents
• Water-based anticorrosion agents contain no
  solvents and thus do not require closed systems.
• Drying times are shorter than for solvent-based
  anticorrosion agents.
• Due to their elevated water content, water-based
  anticorrosion agents are highly
  temperature-dependent (risk of freezing or
  increased viscosity).

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• 2. Desiccant method
• Introduction
• Desiccant bags are intended to protect the
  package contents from humidity during transport
  and storage in order to prevent corrosion, mold
  growth and the like".
• The desiccant bags contain desiccants which
  absorb water vapor, are insoluble in water and
  are chemically inert, such as silica gel,
  aluminum silicate, alumina, blue gel, bentonite,
  molecular sieves etc. Due to the absorbency of
  the desiccants, humidity in the atmosphere of the
  package may be reduced, so eliminating the risk
  of corrosion. Since absorbency is finite, this
  method is only possible if the package contents
  are enclosed in a heat sealed barrier layer which
  is impermeable to water vapor. This is known as a
  climate-controlled or sealed package. If the
  barrier layer is not impermeable to water vapor,
  further water vapor may enter from outside such
  that the desiccant bags are relatively quickly
  saturated, without the relative humidity in the
  package being reduced. 
• Desiccants are commercially available in
  desiccant units.
• "A desiccant unit is the quantity of desiccant
  which, at equilibrium with air at 23 2C,
  adsorbs the following quantities of water vapor
• min. 3.0 g at 20 relative humidity
• min. 6.0 g at 40 relative humidity
• The number of desiccant units is a measure of
  the adsorption capacity of the desiccant bag."
• Desiccants are supplied in bags of 1/6, 1/3,
  1/2, 1, 2, 4, 8, 16, 32 or 80 units. They are
  available in low-dusting and dust-tight forms.
  The latter are used if the package contents have
  particular requirements in this respect.

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• Barrier films
• Barrier films are available in various forms,
  for example as a polyethylene film or as a
  composite films with two outer polyethylene
  layers and an aluminum core. The composite film
  performs far better with regard to water vapor
  permeability (WVP), achieving WVP values of below
  0.1 (g/m²d). In the composite film, the barrier
  layers are arranged so as to bring about a
  considerable reduction in permeability in
  comparison with a single layer.
• In accordance with current standards, water
  vapor permeability is always stated for both 20C
  and 40C. According to information from the
  manufacturer, it may be concluded that water
  vapor permeability rises with increasing
  temperature and falls with increasing thickness.
  This problem occurs most particularly with
  polyethylene films, while aluminum composite
  films are largely insensitive to rises in
  temperature.
• Placement of desiccant bags
• The desiccants should be suspended from strings
  in the upper part of the climate-controlled
  package to ensure good air circulation around
  them.
• It is essential to avoid direct contact between
  the desiccant bag and the package contents as the
  moist desiccant would promote corrosion.
• It is advisable to use numerous small bags
  rather than fewer large ones, as this increases
  the available surface area of the desiccant and
  so improves adsorption of the water.
• In order to ensure the longest possible duration
  of protection, the barrier film must be heat
  sealed immediately once the desiccant bags have
  been inserted.
• Desiccant bags are always supplied in certain
  basic package sizes which, depending upon the
  desiccant unit size, may contain a single bag (of
  80 units) or up to 100 bags (of 1/6 unit). The
  basic outer package should only be opened
  directly before removal of a bag and must
  immediately be heat sealed again.

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• Comparison of advantages and disadvantages of the
  desiccant method
• Advantages
• Desiccants provide excellent corrosion
  protection to both metallic and nonmetallic items
  
• Removal of the desiccant on delivery to the
  receiver is straightforward, unlike the removal
  of protective films in the protective coating
  method. The package contents are immediately
  available.
• No particular occupational hygiene requirements
  apply as the desiccant is non-hazardous.
• Disadvantages
• Placement of the desiccant bags and heat sealing
  of the barrier films are relatively
  labor-intensive.
• The slightest damage to the barrier layer may
  negate the effectiveness of corrosion protection.
  
• Calculating the required number of desiccant
  units is not entirely simple and it is easy to
  over-calculate. However, too much protection is
  better than too little.
• Humidity indicators inside the package are not
  very reliable as they are only valid for certain
  temperature ranges.

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• 3. VCI (Volatile Corrosion Inhibitor) method
• Mode of action and use
• Inhibitors are substances capable of inhibiting
  or suppressing chemical reactions. They may be
  considered the opposite to catalysts, which
  enable or accelerate certain reactions.
• Unlike the protective coating method, the VCI
  method is an active corrosion protection method,
  as chemical corrosion processes are actively
  influenced by inhibitors.
• In simple terms, the mode of action is as
  follows the substance (applied onto paper
  supports or in a powder or spray formulation)
  passes relatively continuously into the gas phase
  and is deposited as a film onto the item to be
  protected. This change of state proceeds largely
  independently of ordinary temperatures or
  humidity levels. The inhibitor inhibits corrosion
  in the aggressive, corrosive medium, suppressing
  either the anodic or cathodic half-reactions.
  Under certain circumstances, the period of action
  may extend to two years.
• The mode of action dictates how VCI materials
  are used. At item to be protected is, for
  example, wrapped in VCI paper. The metallic
  surfaces of the item should be as clean as
  possible to ensure the effectiveness of the
  method. The VCI material should be no further
  than 30 cm away from the item to be protected.
  Approximately 40 g of active substances should be
  allowed per 1 m³ of air volume. It is advisable
  to secure this volume in such a manner that the
  gas is not continuously removed from the package
  due to air movement. This can be achieved by
  ensuring that the container is as well sealed as
  possible, but airtight heat sealing, as in the
  desiccant method, is not required.
• The VCI method is primarily used for items made
  from steel, iron, nickel, chromium, aluminum and
  copper, for which it provides good protection.
  The protective action or compatibility of
  inhibitors with specific alloys must be clarified
  with the manufacturer. Ordinary commercial VCI
  materials provide no protection to zinc, cadmium,
  tin, tungsten or lead.

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• Comparison of advantages and disadvantages of the
  VCI method
• Advantages
• Since the gas also penetrates holes and cavities,
  these areas also receive adequate protection.
• The period of action may extend to two years.
• The wrapping need not be provided with an
  airtight heat seal.
• On completion of transport, the packaged item
  need not be cleaned, but is immediately
  available.
• Disadvantages
• The VCI method is not suitable for all metals. It
  may cause considerable damage to nonmetallic
  articles (plastics etc.).
• Most VCI active substances may present a hazard
  to health, so it is advisable to have their
  harmlessness confirmed by the manufacturer and to
  obtain instructions for use.

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Different Corrosion protection methods Anodizing
Advantages A tough surface layer which has very
good corrosion protection properties and very
good adhesion to the surface. Disadvantages
Must be applied after welding or brazing if the
joining areas are to be protected. This can be
complicated for large structures. Can not be done
on site. When to use For protection against
weathering and scratching/abrasion.
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Conversion coating Advantages A non costly and
simple protection of the aluminum surface, which
in addition increases the adhesion of lacquers
and adhesives. Disadvantages Has limited
resistance to mechanical and thermal influence.
When to use Primarily used as a pre-treatment
before lacquering or adhesive bonding.
Lacquering Advantages A high quality lacquering
system has very good corrosion protection
properties. Disadvantages The performance of
the lacquering system is very dependent on the
quality of the pre-treatment and application
work. Relatively expensive. When to use
lacquering Where appearance and/or corrosion
performance is very important
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Inhibitors Advantages Can be tailored to give
excellent protection in specific environments.
Disadvantages Expensive to use with large
amounts of liquid. May cause increased corrosion
if incorrectly used. When to use inhibitors For
protection against internal corrosion in closed
systems, circulating or non-circulating
Protective adhesive tapes Advantages Prevents
galvanic contact. Grease filled tapes will seal
crevices. Disadvantages Costly to apply. May
need to be supported in place. When to use
Buried pipelines
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• Application of CP
• Engineering and Design, Cathodic Protection,
  requires both cathodic protection (CP) and
  coatings, regardless of soil or water
  resistivity, for the following buried or
  submerged ferrous metallic structures
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• Natural gas and propane piping
• Liquid fuel piping
• Oxygen piping
• Underground storage tank (UST) systems
• Fire protection piping
• Steel water tank interiors
• Ductile or cast iron pressurized piping under
  floor (slab on grade) in soil
• Underground heat distribution chilled water
  piping in ferrous metallic conduit in soils
• Other structures with hazardous products

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• Schematic of an Impressed Current CP System
• In the impressed current CP, the large
  electrochemical is formed between an anode and
  the structure to be protected by a power supply
  that is controlled by reading a reference
  electrode close to the structure.  

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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, ..
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Protection of underground tanks
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• The basic principle of cathodic protection (CP)
  is simple. A metal dissolution is reduced through
  the application of a cathodic current. Cathodic
  protection is often applied to coated structures,
  with the coating providing the primary form of
  corrosion protection. The CP current requirements
  tend to be excessive for uncoated systems. The
  first application of CP dates back to 1824, long
  before its theoretical foundation was
  established. Cathodic protection has probably
  become the most widely used method for preventing
  the corrosion deterioration of metallic
  structures in contact with any forms of
  electrolytically conducting environments, i.e.
  environments containing enough ions to conduct
  electricity such as soils, seawater and basically
  all natural waters. 
• Cathodic protection basically reduces the
  corrosion rate of a metallic structure by
  reducing its corrosion potential, bringing the
  metal closer to an immune state. The two main
  methods of achieving this goal are by either
• Using sacrificial anodes with a corrosion
  potential lower than the metal to be protected
  (see the seawater galvanic series)
• Using an impressed current provided by an
  external current source      

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Design Procedures

• 1. Area to be protected
• 2. Polarized Potential
• 3. Current Demand
• 4. Anode Consumption
• 5. Anode and distribution
• 6. Anode resistance
• 7. design output current
• Use FEM and Laplace Eqn d2V/dx20

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Criteria for effective cathodic protection
To design, operate and monitor a cathodic
protection system it is important to measure its
effectiveness against well established protection
criteria. Since CP involves the depression of the
structure potential and the supply of electrons,
invariably most of the protection criteria are
based on either potential or current.
The potential criterion From the basic
electrochemical theory absolute protection (zero
corrosion rate) is achieved if the structure is
polarized to the reversible electrode potential
of the anodic reaction. However, the
determination of the reversible electrode
potential by either calculation or experiment is
impossible in environments of practical interest,
since for most environments the ferrous ion
concentration is not known and thermodynamics
(the Nernst equation) cannot apply. Field
experience has shown that in aerated soils mild
steel was fully protected at a potential of -850
mV vs. Cu/CuSO4 (-800 mV vs. Ag/AgCl/seawater,
250 mV vs. Zn/seawater and -780 mV vs. SCE). It
is also widely accepted that this potential value
ensures complete cathodic protection of steel in
aerated seawater at ambient temperatures. Under
anaerobic conditions (for example in mud), it is
recommended that the protection potential should
be -950 mV vs. Cu/CuSO4 in order to combat the
increase in corrosion rate caused by microbial
activities. It is important to note that the
values quoted for the protection potential refer
to the potential difference between the structure
and the reference electrode without extraneous
effects such as IR drop or field interference.
100 mV Shift criterion This criterion requires
that when the current is switched off, the
instant potential shall be at least 100 mV more
negative than the free corrosion potential. This
criterion has seen some acceptance with the CP of
rebar steel in concrete. Potential 'swing'
criterion Another criterion based on field
experience, is that a negative potential change
of 200 - 300 mV from the free corrosion potential
is a good measure of adequate protection.
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• Impressed current systems
• An alternative method of providing the current
  to protect a system is to use some sort of
  external power supply. As with the sacrificial
  system, the structure to be protected is made the
  cathode the difference being that the driving
  force behind the current is not the difference in
  potential between the anode and cathode of the
  system but from the power supply.
• As the anode need not be less noble than the
  structure, the choice of materials is wider.
  Examples of different anode materials are
• Platinum
• Titanium
• Graphite
• High Silicon Cast Iron

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Stray Current Corrosion Whenever a metallic
structure is placed in the electric field between
the structure and the anode, it provides an
alternative route for the electron current path.
Thus current can enter a foreign structure at one
point and leave it at another location. At the
interface on the foreign structure where the
electrons move away from, corrosion is enhanced.
This is known as stray current corrosion. This
may be easily demonstrated in the laboratory and
may be explained using a modified galvanic
corrosion polarization diagram. Stray currents in
soils could originate as well from DC electrified
rail tracks. A pipeline buried nearby could
suffer stray current corrosion. The influence of
high AC voltage overhead power lines on the
corrosion of nearby structures is subject to
substantial investigation. Stray current
corrosion refers to corrosion damage resulting
from current flow other than in the intended
circuit(s). For larger structures this term
usually alludes to corrosion damage caused by
extraneous current(s) flowing through soil and /
or water.
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Consequences of Overprotection It is possible
during cathodic protection to supply excess
direct current to polarize a structure below the
recommended protection potential. This state of
affairs is termed 'overprotection'. There are two
main consequences of overprotection, namely,
waste of current and more seriously the violation
of the structural integrity of the metal. The
waste of current is due to the polarization of
the metal below its equilibrium potential with
the excess current being used to evolve hydrogen.
The gas produced could cause the detachment of
organic coatings and the removal of calcareous
deposits in offshore structures. Hydrogen
production has also adverse effects on both the
corrosion fatigue life and hydrogen embrittlement
properties of structures especially those made of
high strength materials. During overprotection
large amounts of hydroxyl ions are also produced.
On bare surfaces immersed in seawater, these
could have a beneficial effect since the hydroxyl
species may passivate and /or enhance the
formation of calcareous deposits which in turn
will reduce the current demand. However for
organically coated surfaces the strong alkali
condition at the metal surface may result in loss
of adhesion for the paint. This phenomenon is
known as cathodic disbonding.
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• Introduction to Stray Current Corrosion
• Stray currents which cause corrosion may
  originate from direct-current distribution lines,
  substations, or street railway systems, etc., and
  flow into a pipe system or other steel structure.
  Alternating currents very rarely cause corrosion.
  The corrosion resulting from stray currents
  (external sources) is similar to that from
  galvanic cells (which generate their own current)
  but different remedial measures may be indicated.
  In the electrolyte and at the metal-electrolyte
  interfaces, chemical and electrical reactions
  occur and are the same as those in the galvanic
  cell specifically, the corroding metal is again
  considered to be the anode from which current
  leaves to flow to the cathode. Soil and water
  characteristics affect the corrosion rate in the
  same manner as with galvanic-type corrosion.
• However, stray current strengths may be much
  higher than those produced by galvanic cells and,
  as a consequence, corrosion may be much more
  rapid. Another difference between galvanic-type
  currents and stray currents is that the latter
  are more likely to operate over long distances
  since the anode and cathode are more likely to be
  remotely separated from one another. Seeking the
  path of least resistance, the stray current from
  a foreign installation may travel along a
  pipeline causing severe corrosion where it leaves
  the line. Knowing when stray currents are present
  becomes highly important when remedial measures
  are undertaken since a simple sacrificial anode
  system is likely to be ineffectual in preventing
  corrosion under such circumstances.

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• Consider how to protect a steel structure
• It has been empirically determined that the
  corrosion protection for mild steel is -840mV
  with reference to a copper/copper sulfate
  reference electrode.
• What is needed is a metal that is less noble than
  steel to afford this form of protection.
  Different applications favor different materials,
  for example
• Submersed Marine Structures         Zinc or
  Aluminum
• Buried Pipelines                                 
  Magnesium
• The amount of anode material used and the
  positioning of the anodes is determined by the
  individual application.