Lecture 1 Corrosion: Introduction Definitions and Types · valves in petroleum refinery at high...

Lecture 1: Corrosion: Introduction – Definitions and Types NPTEL Web Course

1 Course Title: Advances in Corrosion Engineering

Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore

Lecture 1

Corrosion: Introduction – Definitions and Types

Keywords: Definition of Corrosion, Corrosion Types, Environment.

Corrosion can be viewed as a universal phenomenon, omnipresent and omnipotent.

It is there everywhere, air, water, soil and in every environment, we encounter.

There is no single figure for loss to the nation due to corrosion. It can be a minimum

of 3.5% of the nation‟s GDP. Losses due to corrosion could be around Rs. 2.0 lakh

crores per annum in India. Corrosion costs manifest in the form of premature

deterioration or failure necessitating maintenance, repairs and replacement of

damaged parts.

In the US, total direct cost of corrosion is estimated at about 300 billion dollars per

year; which is about 3.2% of domestic product. Corrosion has a huge economic and

environmental impact on all facets of national infrastructure; from highways,

bridges, buildings, oil and gas, chemical processing, water and waste water treatment

and virtually on all metallic objects in use. Other than material loss, corrosion

interferes with human safety, disrupts industrial operations and poses danger to

environment. Awareness to corrosion and adaptation of timely and appropriate

control measures hold the key in the abatement of corrosion failures.

Definitions:

Corrosion is the deterioration or destruction of metals and alloys in the presence of

an environment by chemical or electrochemical means.

In simple terminology, corrosion processes involve reaction of metals with

environmental species.




As per IUPAC,

“Corrosion is an irreversible interfacial reaction of a material (metal, ceramic,

polymer) with its environment which results in its consumption or dissolution into

the material of a component of the environment. Often, but not necessarily,

corrosion results in effects detrimental to the usage of the material considered.

Exclusively physical or mechanical processes such as melting and evaporation,

abrasion or mechanical fracture are not included in the term corrosion”

With the knowledge of the role of various microorganisms present in soil and water

bodies, the definition for corrosion need be further widened to include microbially-

influenced factors.

Corrosion can be classified in different ways, such as

Chemical and electrochemical

High temperature and low temperature

Wet corrosion and dry corrosion.

Dry corrosion occurs in the absence of aqueous environment, usually in the presence

of gases and vapours, mainly at high temperatures.

Electrochemical nature of corrosion can be understood by examining zinc dissolution

in dilute hydrochloric acid.

Zn + 2HCl = ZnCl2 + H2

Anodic reaction is Zn = Zn++

+ 2e with the reduction of 2H+ + 2e = H2 at cathodic

areas on the surface of zinc metal. There are two half reactions constituting the net

cell reaction.

Environmental effects such as those of presence of oxygen and other oxidizers,

changes in flow rates (velocity), temperature, reactant concentrations and pH would

influence rates of anodic and cathodic reactions.




Even though the fundamental mechanism of corrosion involves creation or existence

of corrosion cells, there are several types or forms of corrosion that can occur. It

should however be borne in mind that for corrosion to occur, there is no need for

discrete (physically independent) anodes and cathodes. Innumerable micro level

anodic and cathodic areas can be generated at the same (single) surface on which

anodic (corrosion) and cathodic (reduction) reactions occur.

Each form of corrosion has a specific arrangement of anodes and cathodes and

specific patterns and locations depending on the type can exist.

The most important types are

Uniform corrosion.

Galvanic corrosion, concentration cells, water line attack

Pitting.

Dezincification, Dealloying (selective leaching)

Atmospheric corrosion.

Erosion corrosion

Fretting

Crevice corrosion; cavitation

Stress corrosion, intergranular and transgranular corrosion, hydrogen

cracking and embrittlement

Corrosion fatigue.




Table 1.1 ASM classifications of corrosion types

General Corrosion: Localized Corrosion:

Metallurgically

Influenced

Corrosion:

Mechanically

Assisted

Degradation:

Environmentally

Induced Cracking:

Corrosive attack

dominated by

uniform thinning

Atmospheric

corrosion

Galvanic

corrosion

Stray-current

corrosion

General

biological

corrosion

Molten salt

corrosion

Corrosion in

liquid metals

High –

temperature

corrosion

High rates of metal

penetration at

specific sites

Crevice

corrosion

Filiform

corrosion

Pitting corrosion

Localized

biological

corrosion

Affected by alloy

chemistry & heat

treatment

Intergranular

corrosion

Dealloying

corrosion

Corrosion with a

mechanical

component

Erosion

corrosion

Fretting

corrosion

Cavitation

and water

drop

impingement

Corrosion

fatigue

Cracking

produced by

corrosion, in the

presence of stress.

Stress –

Corrosion

Cracking

(SCC)

Hydrogen

Damage

Liquid metal

embrittlement

Solid metal

induced

embrittlement

(Ref: Sully J R, Taylor D. W, Electrochemical Methods of Corrosion Testing, Metals

Hand Book. Vol 13, 1987.)




Crevice corrosion is a localized attack on a metal adjacent to the crevice between

two joining surfaces (two metals or metal-nonmetal crevices). The corrosion is

generally confined to one localized area to one metal. This type of corrosion can be

initiated by concentration gradients (due to ions or oxygen). Accumulation of

chlorides inside crevice will aggravate damage. Various factors influence crevice

corrosion, such as.

Materials: alloy composition, metallographic structure.

Environmental conditions such as pH, oxygen concentration, halide

concentrations, temperature.

Geometrical features of crevices, surface roughness.

Metal to metal or metal to nonmetal type.

Filiform corrosion is a special type of crevice corrosion.

Pitting corrosion is a localized phenomenon confined to smaller areas. Formation of

micro-pits can be very damaging. Pitting factor (ratio of deepest pit to average

penetration) can be used to evaluate severity of pitting corrosion which is usually

observed in passive metals and alloys. Concentration cells involving oxygen

gradients or ion gradients can initiate pitting through generation of anodic and

cathodic areas. Chloride ions are damaging to the passive films and can make pit

formation auto-catalytic. Pitting tendency can be predicted through measurement of

pitting potentials. Similarly critical pitting temperature is also a useful parameter.

Uniform corrosion is a very common form found in ferrous metals and alloys that are

not protected by surface coating or inhibitors. A uniform layer of „rust‟ on the

surface is formed when exposed to corrosive environments Atmospheric corrosion is

a typical example of this type.




Galvanic corrosion often referred to as dissimilar metal corrosion occurs in galvanic

couples where the active one corrodes. EMF series (thermodynamic) and galvanic

series (kinetic) could be used for prediction of this type of corrosion. Galvanic

corrosion can occur in multiphase alloys.

Eg: - Copper containing precipitates in aluminium alloys.

Impurities such as iron and copper in metallic zinc.

Differential aeration (oxygen concentration cell) and ion concentration (salt

concentration) cells create dissimilar polarities (anodic and cathodic areas)

Eg:-Pitting of metals. Rusting of iron (Fig. 1.1).

Fig. 1.1 Differential oxygen cells in rusting of iron

Selective leaching (Dealloying) refers to selective dissolution of active metal phase

from an alloy in a corrosive environment.




Examples:

a) Brass containing copper and zinc. Since zinc is anodic to copper, selective

dezincification occurs in a corrosive medium, enriching the cathodic copper

in the matrix (colour of brass turns red from yellow).

b) Graphitization of grey cast iron-graphite being cathodic enhances dissolution

of iron in the matrix, leaving behind a graphite network.

There are several other examples of dealloying besides the above.

Tin Bronzes in hot brine or steam-Destannification.

Precious metal alloys such as gold containing copper or silver –

strong acids, sulfide environment - preferential dissolution of copper

or silver.

Cupro-nickel alloys in condenser tubes-denickelisation.

Localised attack at or nearer to grain boundaries in a metal or alloy can be termed as

intergranular corrosion. Generally the following factors contribute to intergranular

corrosion.

Impurities and precipitation at grain boundaries.

Depletion of an alloying element (added to resist corrosion) in the grain-

boundary area.

A typical example is sensitized 18-8 stainless steels when chromium carbide is

precipitated along grain boundaries. Lowered chromium content in the area adjacent

to grain boundaries, leads to formation of anodic and cathodic areas.

Such intergranular corrosion is common in stainless steel welded structures and is

referred to as weld decay. Intergranular attack can occur in other alloys as well.

For example, Duralumin-type alloys (Al – Cu) due to precipitation of CuAl2.




Erosion corrosion is the deterioration of metals and alloys due to relative movement

between surfaces and corrosive fluids. Depending on the rate of this movement,

abrasion takes place. This type of corrosion is characterized by grooves and surface

patterns having directionality. Typical examples are

Stainless alloy pump impeller,

Condenser tube walls.

All equipment types exposed to moving fluids are prone to erosion corrosion.

Many failures can be attributed to impingement (impingement attack). Erosion

corrosion due to high velocity impingement occurs in steam condenser tubes, slide

valves in petroleum refinery at high temperature, inlet pipes, cyclones and steam

turbine blades.

Cavitation damage can be classified as a special form of erosion corrosion. This is

usually caused by formation and collapse of vapour bubbles in liquids closer to a

metal surface. Typical examples include ship‟s propellers, pump impellers and

hydraulic turbines. Surface damage similar to that of pitting can occur and both

corrosion and mechanical factors are involved.

Corrosion occurring at contact regions between materials under load subjected to

slip and vibration can be termed Fretting. Such friction oxidation can occur in

engine and automotive parts. Fretting is known to occur at bolted tie plates on rails.

Parameters promoting fretting include:

Relative motion between two surfaces.

Interface under load.

Both the above produce slip and deformation of surfaces. Wear-oxidation and

oxidation-wear theories are proposed to explain fretting corrosion.




Stress corrosion cracking (SCC) refers to failure under simultaneous presence of a

corrosive medium and tensile stress. Two classic examples of SCC are caustic

embrittlement of steels occurring in riveted boilers of steam-driven locomotives and

season cracking of brasses observed in brass cartridge cases due to ammonia in

environment. Stress cracking of different alloys does occur depending on the type of

corrosive environment. Stainless steels crack in chloride atmosphere. Major

variables influencing SCC include solution composition, metal/alloy composition

and structure, stress and temperature. Crack morphology for SCC failures consists

of brittle fracture and inter - or trans-granular cracking could be observed. Higher

stresses decrease time before crack initiation. Tensile stresses of sufficient threshold

levels are involved (applied, residual or thermal stresses).

Hydrogen embrittlement although many a time classified under stress corrosion,

need be considered separately since the two types respond very differently to

environmental factors.

Fracture of metals and alloys under repeated cyclic stresses is termed fatigue and

corrosion under such circumstances is corrosion fatigue (reduction of fatigue

resistance).

Electrochemical factors come into play in many of the above corrosion forms. Both

thermodynamic and kinetic aspects of electrochemistry of corrosion are discussed in

the following lectures with respect to both corrosion mechanisms and corrosion

protection.

Lecture 14: Prevention Strategies – Design and Coatings NPTEL Web Course



Lecture 14

Prevention Strategies – Design and Coatings

Keywords: Corrosion Prevention, Designs, Protective Coatings.

There are a number of methods to control corrosion. The choice of any one control

technique depends on economics, safety aspects and other technical considerations.

Design

Materials selection

Protective coatings

Inhibitors and environmental alterations

Corrosion allowances

Engineering design with a view to corrosion abatement is important. For example, a

simple aspect such as providing drainage, as for an automobile side panel. Choice of

appropriate materials keeping in mind the probability of corrosion in the existing

environmental conditions is very critical. Among the materials available for

selection; titanium, copper – alloys, stainless steels, carbon steels and aluminium and

its alloys are often chosen.

Proper design of equipment

In the design of equipment, fittings such as baffles, valves and pumps to be

considered

Elimination of crevices

Complete drainage of liquids

Easy to clean

Facilitate easy access to inspection and maintenance

Avoid bimetal contacts – Insulation of Joints.




General choice of metals and alloys for corrosive applications is given in Table 14.1.

Table 14.1 Choice of materials for corrosive environments

Material Environment

Nickel and alloys

Caustic solutions

Monel Hydrofluoric acid

Hastelloys Hot hydrochloric acid

Stainless steels Nitric acid

Lead Dil. sulfuric acid

Tin Water

Titanium Hot strong oxidizing

acids/liquids

Carbon steels are readily available cheaply and can easily be formed and worked into

different shapes. Carbon steels can undergo different types of corrosion, such as

rusting, hydrogen embrittlement and galvanic corrosion. Galvanization is commonly

used to protect structural steels. Protective coatings, cathodic protection and

inhibitor are extensively used to improve the structural life of carbon steels.

Stainless steels are generally immune to corrosion in mild environments. However,

they may experience pitting, crevice and stress corrosion cracking in aggressive

environments such as sea water, chemical processing etc. Ferritic and austenitic

stainless steels are used in thin wall tubing in heat exchangers and also in many

industrial and marine applications. Type 304 stainless steel is used in valve parts,

pump shafts and fasteners. Duplex stainless steels (Cr – Mo alloys of iron) are used

in chloride and high temperature environments. Martensitic stainless steels possess

good mechanical strength.




Nickel and alloys are used in chemical process industries. Nickel – copper alloys as

monel possess resistance to nonoxidizing acids. Nickel-chromium-iron alloys

passivate in presence of oxidizers. Addition of molybdenum increases chloride

resistance.

Copper and its alloys are quite resistant to non-oxidizing aqueous and many

atmospheric environments. Brass undergoes dezincification. Aluminium and naval

brasses are more resistant. Bronzes and aluminium bronzes are resistant to

impingement. Copper-nickel alloys exhibit good resistance to impingement and

stress corrosion.

Corrosion resistance of aluminium alloys vary widely depending on type of alloy

addition and environments.

Titanium and alloys show stable, protective oxide film (passivation). Very good

corrosion resistance in hot acids and many other corrosive environments.




Some general approaches for corrosion prevention are detailed in Tables – 14.2 and

14.3.

Table 14.2 Corrosion protection methods and processes

Approach Process

Removal of oxidizers

Boiler water

Corrosion inhibition Inhibitors & pH control

General corrosion prevention

Anodic and Cathodic protection

Coatings:

Metallic

Organic

Nonmetallic

Electroplating, galvanizing, metal spray or immersion.

Claddings and paints.

Anodizing, Conversion coatings.

Metal modification Alloying

Change in surface /

environment conditions

Removal of corrosives (maintenance)

Proper designs • Avoid crevices

Provide drainage

Avoid bimetallic joints

Since general corrosion is predictable, design considerations can include preventive

measures whenever and wherever possible.

Some examples: Wall thickness control

Control of process stream composition

(Elimination of chlorides)

Prevention of acid contacts - neutralization.

Minimization of vapor condensations and collection.

Prevention of leakage of corrosives.

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For carbon steels: Cathodic protection combined with coatings.

Channel and angle sections positioned to collect and drain water,

liquids and debris.

Table 14.3 Corrosion types with prevention strategies

Type of corrosion Prevention Strategies

Stress corrosion cracking

More resistant alloys. Remove tensile stress,

control of environment (elimination of

chlorides)

Corrosion fatigue Eliminate cyclic stress and corrosive

environment.

More rigid design to reduce stresses due to

vibrations.

Avoid stress concentration in design.

Hydrogen embrittlement Choice of less – susceptible alloy / coatings.

Avoid cathodic protection (steels in acid

Environments)

Galvanic Corrosion Selection of metals / alloys closer in galvanic

series. Favorable cathode to anode ratio.

Coating taking care not to create smaller

anodes with larger cathodes, insulation of

dissimilar joints.

Crevice corrosion Proper design of junctions and joints to

minimize crevices. Welded joints preferable to

rivets and bolts. Pitting and crevice corrosion

are enhanced in stagnant / slow flowing

solution. Provide drainages.

Erosion corrosion and cavitation Design to reduce velocity and turbulence,

avoid abrupt changes in flow directions.




Coatings

Barrier between corrosive environment and metal.

Coatings may serve as sacrificial anodes (zinc on steels ) or release

substances that resist corrosion.

Metal coatings - Noble coat - Silver, copper, nickel, chromium, tin, lead on

steels (ensure pore - free, uniform, adherent coating; favorable anode /

cathode ratio to minimise galvanic attacks).

Sacrificial coatings – Zinc, aluminium, cadmium on steels. (steel is cathodic

to plated metal).

Coatings can be applied through hot dipping, hot spraying, electroplating, electro-

less plating, vapour deposition and metal cladding.

Aluminium, stainless steel, titanium, platinum etc can be cladded on various metallic

substrate for enhanced corrosion protection (physical or chemical).

Other types of surface treatments.

Modification of substrates through ion implantation and laser processing.

Inorganic coatings: glass, cement, ceramic and chemical conversion coatings.

Chemical conversion: Anodizing, oxide, chromate, phosphatizing.

Organic coatings: Paints, lacquers, varnishes (Resin, solvent + pigment in the coating

liquid). High performance organic coatings used in petroleum industries.

Development of corrosion – resistant synthetic resins.




Types of paint coatings

Good adhesion, flexibility, impact resistance and protection from chemicals,

moisture, and atmospheric conditions.

Lacquer – synthetic resins (vinyl chloride, acrylic, rubber).

Latex (Acrylics and Vinyls)

Oil-based and Epoxy coatings (good bending, hard and flexible)

Coal – tar – epoxy.

Poly – urethanes, polyester and vinyl ester (hard, brittle or elastomeric).

Organic zinc rich coatings (organic barrier + galvanic Zn protection)

Co-polymeric protective coatings.(thermoplastic – copolymer - aromatic

coatings).

Anti - corrosion paints – various types additives to improve corrosion resistance,

durability and impermeability.

Lecture 15: Prevention Strategies – Inhibitors and Surface Engineering NPTEL Web Course



Lecture 15

Prevention Strategies - Inhibitors and Surface Engineering

Keyword: Inhibitors, Passivators, Surface Engineering.

Inhibitors are chemicals which adsorb on metal surfaces. A corrosion inhibitor can

act in several ways:

Arrest or slow down anodic or cathodic reactions by blocking active sites on

metal surfaces.

Eg: Amines, thiourea, benzoate, antimony trichloride.

Promote surface passivation (active-passive metals and alloys).

Eg: Chromate, nitrite, red lead, calcium plumbate.

Formation of a surface layer blocking exposure of the bare metal to corrosive

medium. Eg: Phosphate, silicate, bicarbonate, hexametaphosphate.

Hexylamines or sodium benzoate – radiator fluids in cooling circuits of engines.

Antimony trichloride – De-scaling of steels in sulfuric acid.

Volatile (vapour phase) inhibitors (Amines) -Metal (steel) articles or equipment

during transport.

A classification of inhibitors based on their functionality is given below:

Passivating inhibitors

Cathodic inhibitors

Organic inhibitors

Precipitation inhibitors

Volatile corrosion inhibitors.




Two types of passivating inhibitors.

Oxidizing anions – Chromate, nitrite and nitrate that can passivate steel in

absence of oxygen.

Nonoxidizing ions – phosphate, tungstate and molybdate that require oxygen

to passivate steel.

Inhibitors to be used in just the required concentration.

Higher concentration – Over protection? or corrosion?

Lower concentration do not protect!

Inhibitors generally used in quantities less than 0.1% by weight.

Cathodic inhibitors:

• Slow down cathodic reaction or selectively precipitate on cathodic areas.

Act as poisons, precipitates or as oxygen scavenger.

Compounds of As and Sb make combination of and discharge of hydrogen

difficult. Ions of Ca, Zn or Mg precipitate as oxides to form protective layers.

Oxygen scavengers prevent cathodic depolarization due to O2 (Na2 SO3).

Organic inhibitors – Both anodic and cathodic effects.

Adsorption depending on charge of inhibitor.

Precipitation inhibitors: Film forming compounds – block anodic and

cathodic sites. (E.g.: calcium, magnesium precipitation of silicates and

phosphates).

Vapour phase inhibitors – used during transport in closed environment.

Morpholine, Hydrazine.

Vapor condenses and hydrolyzed by moisture to liberate protective ions.




Corrosion inhibitors used in:

Chemicals processing

Petroleum refining

Cement and concrete

Pulp and paper

Oil and gas production

Metals

Utilities

Effect of addition of cathodic, anodic and mixed inhibitors on the corrosion rate of a

metal is illustrated in Fig. 15.1, 15.2 and 15.3. Influence of the inhibitors on the

anodic and cathode reactions, respectively could be seen. As can be seen, cathodic

inhibitors selectively influence the cathodic polarization, bringing down corrosion

rates. Similarly, anodic inhibitors specifically interfere with the anodic oxidation

reactions, decreasing icorr values. On the otherhand, mixed inhibitors influence both

anodic and cathodic reaction rates.

Fig 15.1 Role of cathodic inhibitor on corrosion rate of a metal.




Fig 15.2 Role of anodic inhibitors on the corrosion rate of a metal

Mixed inhibitors (amines, selenides)

Fig 15.3 Role of mixed inhibitors on the corrosion rate of a metal




General inhibitors used in some industrial operations are listed in Table 15.1

Table 15.1 Industrial uses of inhibitors

Recirculation cooling water -

Silicates, chromate, nitrate, polyphosphates

Automotives coolants - Benzoate, borax, phosphate, nitrite

Mercaptobenzothiazole.

Steam condensates - Ammonia, amines (benzylcyclohexamine).

Octadecylamine (long chain aliphatic)

Sea Water and brines - Chromates, nitrite etc.

Pickling acids - Phenylthiourea, mercaptans, quinoline,

Pyridine, various long chain amines.

Oil refining and production - Primary, amido-, quaternary amines

Imidazoline.




Surface modification approaches for corrosion protection of steels.

Modification of surface region of engineering alloys through diffusion of different

elements and formation of a layer having desirable chemical composition,

microstructure and properties.

Thermo-chemical treatments – Physical and chemical

Vapour deposition.

Coatings by plasma spraying

Electrospark deposition

Ion implantation

Sputter deposition of selected elements and compounds.

Surface layers developed by such materials, can be classified as:

Overlay coatings

Diffusion coatings

Recast layers

Thermo-chemical treatment for surface modification of steels – nonmetals or metals

introduced into metal surfaces by thermo - diffusion after chemical reaction and

adsorption.

Caburizing, nitriding, carbonitriding, boronizing, chromising and aluminizing are

some popular methods. Other examples include surface modification by Electrical

Discharge Machining to remove surface material-Melted zones are transformed to

recast layers with specific structures.

Surface modification by electrical discharge treatment in electrolyte where a high

energy thermal process is involved at surfaces leading to melting, vaporization,

activation and alloying in an electrolyte.




Laser surface engineering for corrosion protection

a) Microstructure modification

Laser surface melting

Laser shock peening.

b) Chemical composition and microstructure modification.

Laser cladding

Laser surface alloying

Pulsed laser deposition

Laser – based thermal spray

Types of surface engineering

Coatings – sputtering, CVD, spin coat,

Passivation

Chemical treatment

Plasma treatment

Surface derivitization

Laser treatment

Plasma deposition

Polymerized coatings

Fluropolymers and siloxanes

Scratch - resistant coats

Paint adhesion

Electropolishing – 316 stainless steel, Nitinol (oxide enrichment).

Conversion coatings

Oxidation, passivation

Chromate, phosphate, black oxide

Pore surface engineering.

Lecture 16: Cathodic Protection – Principles and Classification NPTEL Web Course



Lecture 16

Cathodic Protection – Principles and Classification

Keywords: Cathodic Protection, Equipotential Surface, Impressed Current, Sacrificial Anode.

Sri Humphrey Davy ‘s pioneering work (1824) on protecting the copper sheathing on wooden

hulls in the British Navy by sacrificial zinc and iron anodes is considered to be the earliest

example of application of cathodic protection.

Copper-sheathed ship hulls protected by sacrificial blocks of iron.

Zinc alloy as sacrificial anode. Galvanising – Typical example of sacrificial anode to protect

steels.

Various definitions

Reducing or eliminating altogether corrosion by making the metal a cathode by application of

either an impressed DC current or attaching the metal to a sacrificial anode.

Corrosion occurs at anodic areas – if all anodic areas can be converted to cathodic areas, the

entire structure will become cathode and corrosion is stopped.

Corrosion occurs at the regions where current discharges from metal to environment (soil, water)

(anodic areas). There is no corrosion at regions where current enters from the environment to

metal (cathodic areas).

Objective should then be to force the entire structure to collect current from the environment

(making it cathodic entirely).




Current flow depends on factors such as:

a. Resistivity of environment and

b. Degree of polarization of anode and cathodic areas.

Cathodic protection is achieved by supplying electrons to the structure being protected.

Driving force for corrosion is the potential difference. Equipotential surface - No driving force

(no current flows). In Fig. 16.1, the above principles underlying cathodic protection are

illustrated diagrammatically.

Fig 16.1 Basic concept of cathodic protection.

Reactions

M = M++

+ 2e (anodic, corrosion)

2H+ + 2e = H2 (cathodic – Acid Solutions)

O2 + 2H2O + 4e = 4OH- (cathodic-neutral to mild alkaline)




Principles governing cathodic protection are illustrated in Fig. 16.2 below. As per mixed

potential theory, the zero current criterion is shown. An equilibrium is established on metal (M)

in which anodic oxidation rate is equal to cathodic reduction rate [Ecorr and icorr(A)]. By cathodic

polarization of the metal with an applied DC current (iapp), initial corrosion potential is seen

shifted to a lower value [icorr(B)]. Complete stoppage of corrosion, requires polarization of the

metal to the reversible potential of the metal (EoM).

Fig 16.2 Electrochemical principles governing cathodic protection

Principles of cathodic protection of a metal (steel, for example) in neutral aerated water or sea

water are shown in Fig. 16.3. Diffusion controlled cathodic oxygen reduction is the cathodic

reaction marked by a limiting current. Applied current and corrosion rate are limited by the

limiting (diffusion) current density. Current requirements can be further reduced by surface

coatings.




Fig 16.3 Electrochemical aspects of cathodic protection in neutral sea water.

Two methods of cathodic protection

a) Use of sacrificial anodes.

b) Impressed current method.

Fig. 16.4 and Fig. 16.5 illustrate the two types of cathodic protection, namely, sacrificial anode

and impressed current methods.

Fig 16.4 Sacrificial anode method




Fig 16.5 Impressed current method

Factors to be considered in the design and execution of cathodic protection installations.

Impressed current system

a) How much current necessary for complete protection?

b) Source of DC Current.

c) Installation, Design, erection and maintenance.

d) Auxiliary anodes – choice, size, number, installation.

e) How to assess elimination of corrosion through entire structure?

There are a few limitations based on current flow reaching all through protected conducting

structure. For example, in a pile-up of pipes, current may not efficiently reach pipe surfaces

placed in between. Internal pipe surface may not receive protection. Similarly, portions of pipe

lines above ground, valves etc, cannot receive complete protection.

The above conditions are generally referred to as ‘electrical shielding’.




Current necessary for protection need be just sufficient; neither less nor excess.

Excess current may do harm!

Lower current do not protect!

Requirements of galvanic sacrificial anodes

a. Potential between the anode and the corroding metal structure should be large enough to

overcome the anode-cathode cells.

b. Sacrificial anode to have sufficient Electrical Energy Content (EEC) which predicts its

life.

c. Good current efficiency relevant to anodic corrosion.

EEC can be estimated and expressed as ampere hours/weight (kg or lb)

Eg: Pure Zinc that possesses high EEC of 372 ampere hour / pound.

This means if the zinc sacrificial anode has to discharge continuously one ampere, on pound of

its weight would be consumed in 372 hours. Lower current discharge will prolong its life

further.

Lecture 17: Cathodic Protection – Influencing Factors and Monitoring NPTEL Web Course



Lecture 17

Cathodic Protection – Influencing Factors and Monitoring

Keywords: Coated Surfaces, Protection Criterion, Anode Materials, Pipeline Protection.

For large structures such as underground pipe lines, impressed current cathodic

protection is used, while for smaller structures such as house-hold water tanks, ship’s

hull etc, sacrificial anodes can be effectively used. Painting of steel pipe lines and

tubes can significantly reduce protection current requirements and thus save cost.

Approximate current requirements for cathodic protection of steel pipes are given

below:

Uncoated in flowing sea water 10-15 mA/ft2

Well-coated in water 0.01-0.003 mA/ft2

Excellently coated and exposed to water

or under soil 0.0003 or less mA/ft2

As can be seen above, good surface coating significantly reduces protection current

requirements.

Electrochemical basis for protection criterion can be assessed:

Protection of steel is taken as example:

Fe = Fe++

+ 2e E0 = - 0.44 V

When polarized to half –cell potential of above reaction, corrosion rate reduces to 0.

Rate of forward and reverse reaction are same when net reaction rate is zero.

Eh = - 0.44 + log [Fe++

]




Fe++

+ 2OH- = Fe (OH)2

Calculated potential (based on solubility product) is -0.59V (SHE) which

corresponds to about -0.90V (vs Cu/CuSO4).

Accepted criterion for protection of steel in water is -0.85V (vs Cu/CuSO4).

Potential of structure to environment is generally measured using Cu/CuSO4

reference electrode. Test coupons made of same metal and previously weighed can

be electrically connected to protected structures. These coupons are also exposed to

same cathodic current in the corrosive environment. Estimation of weight losses of

such coupons is a better proof of cathodic protection.

Table 17.1 Potentials for Cathodic protection (Cu/CuSO4 electrode)

Iron and Steel

-0.85 to -0.95 V

Lead -0.6 V

Copper and alloys -0.5 to -0.66 V

Aluminium -0.95 to -1.2 V




Anode materials that can be used as ground-beds in impressed current cathodic

protection are given Table 17.2

Table 17.2 Anode materials for impressed current cathodic protection

Magnesium, zinc and aluminium and their alloys can be used as sacrificial anodes.

Design considerations for both impressed current and sacrificial anode systems have

some common steps.

a) Area to be protected –

Exposed areas of the structure – in coated system, exposed area at breaks and

deteriorated coatings.

b) Polarised potential – Current density based on area need be estimated.

c) Current demand – Current – density demands depend on the environment and

nature of surface coating.

d) Anode consumption – Required number and weights of anode materials

determined from known consumption rates for the desired current demand.

Anode number and distribution for the protected structure can be thus

estimated.

Anode resistance and design output current can then be estimated.

Material Average

consumption rate

kg/A-year

Cast Iron 5 – 7

Steel scrap 5 - 8

Aluminium 4 – 5

Graphite 0.6 – 1.0

Lead -----

Platinum -----




Monitoring of effectiveness of pipeline protection

Most widespread method is based on potential measurements of a cathodically

polarized structure with reference to a standard electrode. A potential of -0.85V (Cu

/ CuSO4) is sufficient for protection of steel in soil and natural water environments.

It may however be borne in mind that the above criterion is not optimum and

situations may arise when more negative (upto – 1.0V) may be required or even

lower (-0.7V) potential may suffice for protection. Interference from IR components

can introduce errors in pipeline potential measurements. Elimination of IR drop can

be achieved using ‘switch – off’ method. Potential measurements in chosen control

points in a pipeline are frequently insufficient to ensure effective protection. Close

Interval Potential Survey (CIPS) is an intensive monitoring technique based on

connecting a thin cable to a pipeline to monitor frequent potential readings all the

way. Special computer software together with appropriate instrumentation can be

used for gathering and processing the data. Another technique called Direct Current

Voltage Gradient (DCVG) method enables protection evaluation and also detection

of defects in insulation. Potential gradient is monitored in the soil with a sensitive

potential measurement meter using two reference electrodes kept at both sides of the

pipeline at shorter distances.

Corrosion coupons (probes) are generally used for monitoring of cathodic protection.

A schematic representation of a coupon probe connected to a cathodically protected

pipeline is illustrated in Fig. 17.1 . The arrangement allows measurement of switch-

off potential without any interruption of pipeline protection.




Fig 17.1 Circuit for monitoring cathodic protection.

Different types of simulation probes are available for determination of :

a) Level of protection in sections in casing pipes.

b) Polarization resistance and depolarization rate.

c) Insulation coating resistance.

d) Any interference on neighbouring underground installations.

e) Corrosion rate of protected structures.

Such probes need be located in various geological locations through a running

pipeline. Recently kinetic cathodic protection criterion has been proposed to allow

maintenance of metal corrosion rate at a desired level. There are several pipeline

corrosion rate control methods including both physical and electrochemical

techniques, which allow determination of effective protection in chosen regions of

structures.




Table 17.3 Corrosion rate control in pipelines.

Electrochemical Physical

Impedance

spectroscopy

Electrochemical noise

Harmonic synthesis

Polarization curves

Polarization

resistance

Electrical resistance

Radiography

Ultrasonic

Weight loss determination

There are several developments in cathodic protection instrumentation. Use of

thyristor – controlled rectifiers will enable automatic control of current output

depending on corrosive environment requirements.

There is also a possibility of controlled potential cathodic protection to suit specific

structures. For example, in sea-going vessels, the hull is subjected to variations in

flow velocities leading to alteration in limiting current density (with respect to

oxygen reduction). Such limiting current fluctuations significantly influence

cathodic protection current requirements from time to time. In such environments,

controlling the potential (rather than current) would be more beneficial. Controlled

potential protection is extensively used for ship hulls incorporating anode –

reference electrode attachment along with automatically – controlled power supply

unit.

Lecture 18: Design Aspects of Cathodic Protection NPTEL Web Course



Lecture 18

Design Aspects of Cathodic Protection

Keywords: Cathodic Protection Design, Choice of Protection, Engineering Aspects.

Advantages and uses of cathodic protection:

Compared to alternative protection methods, cathodic protection is applied by

simply maintaining a DC power circuit and its effectiveness can be

continuously monitored.

Generally applied to coated structures to protect areas where coatings are

damaged-enable longer life span for existing structures.

Can avoid other design considerations for corrosion resistance (such as

corrosion allowance) if cathodic protection is pre-specified.

Can be applied to all metallic structures / including concrete).

Application for protection of exterior surfaces of

Ship hulls

Pipelines

Storage tank bases

Seashore structures

Off shore platforms

and internal surfaces of • Large diameter pipelines.

Storage tanks (water and oil)

Water circulation systems

• Can be applied to copper – base alloys (water systems), lead – sheathed

cables, aluminium alloys and reinforced concrete structures (buildings, bridges,

sea shore, and marine structures).




Basic requirements:

For galvanic protection (sacrificial anode)

Sacrificial anodes

Direct connection to the structure.

Minimum resistance between anodes – connection.

For impressed current protection

Inert anodes (backfill – ground-bed)

DC power supply.

Well insulated, minimum resistance and secure conducting connections

Background information for choice of cathodic protection type and design

considerations:

Structure’s physical dimensions (surface area).

Size, shape, material – type and locations.

Electrical isolation and elimination of short circuits.

Corrosion history in the area with respect to environment.

Resistivity survey information.

Information on pH, potential between structure and environment, current

requirements per unit area.

For ensuring reliable and cost-effective protection, the following aspects need be

ascertained.

Electrical continuity – minimize iR drop.

Coatings to minimize current requirements.

Structure isolation – introduction of isolation joints (insulating flanges).

Availability of test stations with facilities for monitoring and data

aquisition.




Current requirements for complete protection can be assessed through.

Actual tests on existing structure using a temporarily – organized cathodic

protection setup.

Based on prior experience and theoretical calculations based on coating

efficiency.

Suggested formula

Total protective current = (Area in ft2) (required current density) x (1.0 – coating

efficiency)

Table 18.1 Current requirements for cathodic protection of uncoated steels

Approximate current requirements (mA/ft2) for uncoated steel

Soil at natural pH 0.4 – 1.5

Highly acidic soil 3 – 15

Fresh water (static) 1 – 6

Flowing water with oxygen 5 – 15

Seawater 5 - 10

Total current requirements can be estimated by multiplying current density

requirements with surface area

Choice between the two methods of cathodic protection depends on

Conditions at site

Current density requirements

Soil resistivity

If the soil resistivity is lower and current requirements are less than about

1mA/ft2, galvanic anodes can be used. For larger resistivity and current

requirements, impressed current protection may be opted for.




Design aspects for galvanic anode cathodic protection

Soil resistivity assessment – Site of lowest resistivity to be chosen for

location of anode.

Choice of anode material – Data from commercially available anodes to

be carefully assessed.

Table 18.2 Properties of some sacrificial anodes.

Metal Potential (Cu / CuSO4) Density, g/cm3

EEC (amp – h / Kg)

Aluminium - 1.15 V 2.7 2700

Magnesium - 1.55 V 1.7 1230

Zinc - 1.10 V 7.1 780

Aluminium and magnesium – alloy anodes can also be chosen:

Open circuit potentials for various anodes to be known to facilitate

selection. Similarly, for protection of steel, its potential in soil or water

need be known. Net driving potential between the metal to be protected

and the sacrificial anode in the environment to be the criterion. This will

involve the polarized potential of the steel (protected) when contacted

with the anode such as magnesium.

Estimate number of anodes required for desired protection and to

compensate resistance limitations (anode to electrolyte and lead – wire

resistance as well as structure to electrolyte resistance).

Based on the knowledge of ground-bed resistance and life expectancy of anodes,

requirement of number of anodes is calculated.

Design aspects for impressed current cathodic protection

Soil resistivity

Estimation of required current density. Actual current requirements can be

assessed using a provisional test setup, where battery-power supply can be

used. Effectiveness of insulating joints (as in a pipeline) can be tested.




Selection of appropriate ground-bed anode (high silicon, chromium

bearing cast iron commonly used). Backfill materials such as coal-coke

breeze, calcined petroleum coke or graphite can be chosen for ground-bed

anodes for protection of subsoil steel structures such as pipelines.

Number of anodes to meet current density and design requirement.

Selection of anode sites and calculation of total circuit resistance.

Selection of suitable DC power system.

Table 18.3 Comparison between the two cathodic protection systems.

Galvanic Impressed current

No external power External power supply required

Driving potential fixed Adjustable applied potential current

Used in low resistivity environment Can be used even in high resistivity

environment

Lower maintenance High maintenance

Cannot originate stray currents Can cause stray current problems

Used for small and well - coated

structures

Suitable for larger structures (coated or

uncoated)

REFERENCES

1. Cathodic protection – Guide. www.npl.co.uk (from web)

2. J. P. Guyer, Introduction to cathodic protection, 2009, CED

enginerring.com (from web)

3. J. B. Bushman, Impressed current cathodic protection system design,

Bushman and Associates. Ohio (from web)

4. NACE literature on cathodic protection criteria: NACE, Houston (1989).

5. J. H. Morgan, cathodic protection, NACE, Houston, 1987.

6. D. A. Jones, Principles and prevention of corrosion, Prentice – Hall, N. J.

(1996).

7. A.W.Peabody, Principles of Cathodic Protection, Chapter 5, NACE Basic

Corrosion Course, NACE, Houston (1970)

http://www.npl.co.uk/

Lecture 19: Stray Current Corrosion NPTEL Web Course



Lecture 19

Stray Current Corrosion

Keywords: Stray Current, Electrical Bonding, Insulated Couplings

Stray currents are currents flowing from external sources. Any metallic structure,

such as a buried pipeline represents a low resistant current path and is thus

vulnerable to the effect of stray currents.

Stray-current effects are encountered in several impressed current cathodic

protection systems. This is very common in industrial protected systems, such as

oil production industries having innumerable buried pipe lines. Current leakage

from auxiliary anodes associated with cathodic protection systems can enter

unintentionally to a near-by unprotected structure and leave from the surfaces

creating severe corrosion (see Fig. 19.1).

Fig 19.1 Stray current leakage from a cathodic protection system to a nearby pipeline.




Other sources of stray currents include DC electric power traction, welders,

electroplating units and ground electric DC power.

If there is a current path due to a low resistance metallic object (for example, a

pipe line or another metallic structure), current leakage from an impressed current

protected system will enter such unprotected structure before returning to the

protected object. Regions from where current leaves are susceptible to stray-

current corrosion.

A solution to such a problem is through electrical bonding of the near-by

structure. Simultaneously additional anodes and increasing DC power capacity

can accord full protection to all structures in the vicinity. Properly insulated

couplings can help reduce the problem (see Fig. 19.2).

Fig 19.2 Proper design through additional anodes to prevent stray current corrosion.

When impressed current protection systems are installed, anode ground beds

should be so located that stray current from them cannot make entry into other

near-by structures.




Direct stray currents can cause anodic, cathodic or a combined interference.

Anodic interference is generally found in close proximity to a buried anode. The

pipeline will pick up current and will be discharged at a distance farther away

from the anode. In the current pickup site, the potential of the pipe will shift in

negative direction and is thus beneficial as cathodic protection. Sometimes,

overprotection could be created by such potential shifts. On the other hand,

cathodic interference is produced in close proximity to a polarized cathode; the

potential shifting in a positive direction where current leaves the structure

(causing corrosion damage). In combined interference, current pickup occurs

close to anode and discharge occurs closer to cathodically polarized areas. The

damage could be higher in this case since current pickup (overprotection) and

discharge (corrosion) are both detrimental.

Stray current corrosion control in DC rail transit systems.

Fig 19.3 Stray current corrosion of a pipeline from a DC rail transit system.

Consequences of transit stray current from DC traction are illustrated in Fig. 19.3.

Stray current from the rails enters part of a water pipeline through soil and after

traversing through the pipeline path, leaves at another end. Regions in the




pipeline where current enters are protected, while those from where current

leaves to reenter the rail line suffer unintentional corrosion.

The question arises?

How much corrosive stray current is harmful?

It is estimated that for one ampere of stray current discharged from the transit

system to earth, complete perforation of one square inch (0.25 inch wall

thickness) steel pipe can occur within about a week.

Besides unintentional severe corrosion of nearby structures such as pipelines. DC

stray currents can result in

‘Free’ cathodic protection to regions where stray current enters a

structure.

Reduction in effectiveness and life of cathodic protection systems.

Breakdown of reinforced concrete structures.

Electrical shocks and loss of electrical grounding.

As a control measure, track-to-earth potentials under multi-loads can be

monitored through computer simulations and predictive modeling. On the other

hand, controlling transit stray current at the source itself will be preferable. The

following suggestion in this regard is noteworthy.

Reasonable spacing of traction substations.

Continuously welded rails.

Stray current collector – Track slab rebar - epoxy coated.

Use of high resistivity concrete for track slab.

High track to earth resistance.

Insulated track designs are available.

Apart from this, pipeline designers and engineers can also keep in mind the

potential for stray current and monitor pipeline currents and potentials frequently.

Routine surveillance is required.

Lecture 20: Passivity – Definitions and Influencing Parameters NPTEL Web Course

1

Course Title: Advances in Corrosion Engineering


Lecture 20

Passivity – Definitions and Influencing Parameters

Keywords: Definition of Passivity, Flade Potential, Anodic Polarization, Critical

Anodic Current Density.

In the Eh – pH diagrams, resistance to metallic corrosion is indicated at stability

regions where either the metal remains thermodynamically stable (immunity) or

the metal surface is covered with an oxide / hydroxide layer (passivity).

Passivity is due to the formation of thin, impermeable and adherent surface films

under oxidizing conditions often associated with anodic polarization. Only

certain metals and alloys exhibit active-passive behavior, which is essentially an

acquired property.

Faraday in the 1840’s showed that iron reacted rapidly in dilute nitric acid, but

was visibly unattacked in concentrated (fuming) HNO3. An invisible surface

oxide film formed in concentrated acid was found to be unstable in dilute acid

and through scratching, the surface oxide could be removed.

Definitions of passivity as proposed by Uhlig are given below:

1. A metal active in the EMF series or an alloy composed of such metals is

considered passive when its electrochemical behavior becomes that of an

appreciably less active or noble metal.

2. A metal or alloy is passive if it substantially resists corrosion in an

environment where thermodynamically there is a large free energy

change associated with its passage from the metallic state to appropriate

corrosion products.


2



Examples for definition 1 are Cr, Ni, Ti, Zr and stainless steels.

Examples for definition 2 are lead in sulfuric acid, magnesium in water and iron in

inhibited pickling acid.

Two types of passivity thus exist.

a) A metal is passive if it resists corrosion under anodic polarization (noble

potential, low corrosion rate).

b) A metal is passive if it resists corrosion in spite of thermodynamic

amenability to react (active potential, low corrosion rate).

The Eh – pH diagram for the Fe – H2O – O2 system can be superimposed on that for

chromium to understand the role of chromium as an alloying addition in steel for

enhanced corrosion resistance (Fig. 20.1). Chromium forms very stable, thin and

resistant surface films in less oxidizing conditions. Chromium addition is the basis

for stainless steels and other corrosion resistant alloys.

Fig 20.1 Eh – pH diagram for iron superimposed on the chromium diagram (enhanced passivity range due to stable

Cr2O3)


3



Since chromium is capable of forming a very stable oxide at much lower potentials,

alloying with chromium (minimum 12%) leads to development of corrosion resistant

stainless steels and cast irons. Other metals that can form passive surface films

include aluminium, silicon, titanium, tantalum and niobium.

Electrochemical basis of active-passive behavior is illustrated in Fig. 20.2

Fig 20.2 Potentiostatic Anodic polarization curve

Epp – Primary passive potential, above which passive film becomes stable.

icrit = Critical passivating anodic current density, at which passivity is induced.

ipass – Passive current density.


4



On increasing the potential beyond the passive region, the passive film breaks down

and anodic corrosion current further increases in the transpassive state. Oxygen

evolution at the anode occurs at higher potentials.

Based on the above, it is possible to establish

a) Passive potential region.

b) Passive corrosion rate and

c) Necessary conditions to achieve and maintain passivity.

Decay of passivity on interruption of anodic current is characterized by Flade

potential.

If the potential as a function of time is monitored after interrupting the applied

current, the potential value first changes to a value more noble on the hydrogen

scale, then slowly changes and finally rapidly decays towards the normal active

value. The noble potential reached just before rapid decay was found by Flade to be

more noble, the more acid the solution in which passivity decayed (Fig. 20.3).

Fig 20.3 Decay of passivity showing Flade potential


5



EF = E0

F – 0.059 pH (for Fe, Ni, Cr and alloys of Fe).

Stability of passivity is related to EF. The lower the E0

F, the easier it becomes for

passivation and higher film stability. For Cr – Fe alloys, the value ranges from 0.63

V to -0.10V with 25% chromium addition.

Lecture 21: Passivity – Application of Mixed Potential Theory NPTEL Web Course

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Lecture 21

Passivity-Application of Mixed Potential Theory

Keywords: Activation Controlled Reduction Process, Diffusion Control, Spontaneous

Passivation

Increasing temperature and hydrogen ion concentration (high acidity) tend to

increase the critical current density for passivation. Similarly, chlorides are

detrimental to passivity.

To understand, mixed potential behavior for active – passive metals and alloys, it is

essential to introduce cathodic reduction processes superimposed on the anodic

polarization curve. Three different activation controlled reduction processes with

different exchange current densities are superimposed on the passivity curve as

shown in Fig. 21.1.

Fig 21.1 Effect of activation – controlled cathodic processes on stability of passivity.


2



Three different cases are apparent:

1) Only one stable potential at M where the mixed potential theory is satisfied.

High Corrosion rate at M.

Eg:- Fe in dil H2SO4, Ti in dil H2SO4/ HCl.

2) Three points of intersection R, P and N where rate of oxidation is equal to

rate of reduction. Point P is not in stable state. Only N and R are stable.

N in active region (high corrosion rate) and R in passive state (lowest

corrosion rate).

This system may exist in either active or passive state.

Eg:- Cr in dil HCl or H2SO4.

Stainless steel in H2SO4 (containing oxidizers).

3) The most desirable condition-spontaneous passivation - Only stable potential

S in the passive region.

Eg:- Cr – noble metal alloys in H2SO4 or HCl.

Ti – noble metal alloys in dil H2SO4.

18 – 8 stainless steel in acid (containing Fe+++

, O2)

Achievement of condition (3) is essential for the development of corrosion resistant

alloys.

The position of the current maximum or ‘nose’ of the anodic curve is important.

Spontaneous passivation occurs only if the cathodic process clears the tip of the

‘nose’ of the anodic curve.

For a stated reduction-curve, values of Epp and icrit will then decide whether a metal

or alloy will spontaneously passivate or not.


3



Total cathodic current density at Epp should be equal to or greater than icrit to achieve

spontaneous passivation.

Such a criterion can be stated in terms of a passivity index (PI) defined as

PI = crit

ppc

i

atEi )(

For PI ≥ 1, Spontaneous passivation occurs and

for PI < 1, no spontaneous passivation occurs, even though as in condition (2), a

stable passive region may exist.

A comparison of the behavior of two active-passive alloys under an activation

controlled cathodic system is depicted in Fig. 21.2.

Fig 21.2 Active – passive alloys under activation controlled cathodic process.

Alloy A corrodes readily at potential X, while alloy B spontaneously passivates at Y.


4



The above two alloys are exposed to a cathodic process under complete diffusion

control as shown in Fig. 21.3.

Fig. 21.3 Active – passive alloy behavior under diffusion controlled cathodic reaction

Alloy A spontaneously passivate at potential X, while alloy B exhibits two stable

states, namely, active at Q and passive at Y.

Two significant factors emerge out of the above observations.

a) To achieve passive behavior where cathodic reduction is activation

controlled, a metal or alloy with an active Epp is superior.

b) If the reduction process is diffusion controlled, a metal or alloy having a

small icrit will passivate faster.

(Ref: N. D. Greene, Predicting behavior of corrosion resistant alloys by

potentiostatic polarization methods, Corrosion (NACE), 18, pp 136 – 1432 (1962).

Lecture 22:Passivity – Design of Corrosions Resistant Alloys NPTEL Web Course

1



Lecture 22

Passivity – Design of Corrosion Resistant Alloys

Keywords: Alloy Design, Pitting Potential, Oxidizers.

For the development of corrosion-resistant alloys through passivity criterion, two

approaches then become possible.

a) Increase ease of passivation by reducing icrit or making Epp more active.

Anodic dissolution behavior can be changed by alloying (to decrease icrit)

Examples are titanium, chromium – alloying additions, molybdenum, nickel

tantalum and columbium.

b) Increase cathodic reduction rates.

Alloying with noble metals having high exchange currents for the reduction reaction.

Metals with active Epp such as titanium and chromium and alloys containing these

metals which possess high exchange current densities for hydrogen reduction can

undergo spontaneous passivation.

Effect of alloy additions on the corrosion resistance of titanium is given in Table

22.1.

Table 22.1 Average corrosion rate of titanium after alloying addition.

Alloying addition Corrosion rate (mpy) in 15%

boiling HCl

Ti (not alloyed)

Addition of 0.5% Au

Addition of 0.5% Pt

Addition of 0.6% Ir

4400

135

110

85


2



Effect of oxidizer concentration and solution velocity on the corrosion rate of a

normal metal has already been discussed (lecture 13). It will be interesting to

understand the role of oxidizer and solution velocity on the behavior of an active-

passive metal or alloy.

For an active-passive metal exposed to a diffusion controlled cathodic reaction, the

corrosion rate will increase upto certain velocity levels, beyond which the corrosion

rate decreases rapidly to a very low value on the onset of passivity and would

remain at passive state for still higher velocities.

Effect of oxidizer concentrations (ferric, chromate etc) on the electrochemical

behavior of active-passive alloys can also be compared with those of normal metals

under similar conditions. Corrosion rate of an active-passive alloy initially increases

with oxidizer concentration (while in its active state). As soon as passive state is

reached, the corrosion rate steeply decreases to a very low value and remains at this

low corrosion passive level. With still further increase in oxidizer concentration,

corrosion rate further increases due to transpassive behavior.

It is however, interesting to note that, once the passive film has been formed, it is

retained at oxidizer concentrations even lower than that needed for passive film

formation.

It may however be kept in mind that to maintain passivity, oxidizer concentration

should be same or higher than the required minimum to induce spontaneous

passivation. There is also a region of ‘borderline passivity’ in which any surface

disturbance (scratching) will destabilize passivity, leading to increase in corrosion

rate. The following conditions need to be kept in mind to judge passive behavior of

an alloy.


3



Corrosion rate is proportional to anodic current density in the active state

irrespective of whether the alloy is passive type or not.

Rate of cathodic reduction must exceed icrit to ensure lower corrosion rates.

Border line passivity to be avoided.

Avoid breakdown of passive films in oxidizing environments due to

transpassivity.

Stable passive state in oxidizing conditions is essential.

Detrimental role of chloride concentrations and temperature on the passive region

and critical anodic current density is illustrated in Fig. 22.1.

Fig. 22.1 Effect of increasing chloride and temperature on passive behavior.

Chloride ions breakdown passivity or even at times prevent passivation of Fe, Cr, Ni,

Co and stainless steels. They can penetrate oxide films through pores and influence

exchange current density (overvoltage). Breakdown of passivity by chloride ions is

local and leads to pitting corrosion. However, chloride ions have no significant

effect on the polarization curve of titanium, unlike that of stainless steels.


4



Anodic polarization of active-passive metals and alloys can be established either

potentiostatically or galvanostatically. The differences in the nature of the

polarization curves in either case are illustrated in Fig. 22.2. Only potentiostatic

approach allows a detailed study of the important parameters influencing passivity.

Galvanostatic methods are not adequate for establishing the active-passive behavior.

Above icrit, the curve no longer follows the anodic curve in the passive region;

suddenly jumping into the transpassive region with oxygen evolution.

Feg 22.2 comparison of potentiostatic and galvanostatic anodic polarization curves.

Theories of passivation

Major theories that have been proposed are the

Oxide film theory and

Adsorption theory

The oxide theory attributes corrosion resistance of passive metals and alloys to the

formation of a protective film on the metal surface; the film can be as a monolayer.

There are different opinions expressed about the potential at which the oxide film is


5



formed, mechanisms of formation, causes of passivity and film thickness. Early

theories proposed formation of a primary layer of lower conductivity and high

porosity. As the current increases in the pores, passive layer is formed at a potential

closer to the Flade potential. A stable passive film is free from porosity and presents

a protective barrier between the metal and the corrosive environment. There are

similar hypotheses regarding monolayer oxide formation.

The adsorption theory is based on chemisorbed films. Oxygen adsorption on

surfaces can reduce corrosion activity. Uhlig proposed in 1946 that an adsorbed

oxygen film is the primary source of passivity. The observed Flade potential of

passive iron is too noble by about 0.6V to be explained by any known oxides of iron

at equilibrium. It is consistent with a chemisorbed film of oxygen, which is formed

preferentially on transition metals due to interaction of oxygen with uncoupled

electrons to form a stable bond. Adsorbed oxygen atoms significantly decrease the

exchange current density, thus increasing anodic polarization, favorable for

passivation.

An alternative passivity mechanism could be direct film formation, dissolution and

precipitation and anodic deposition.

Several models also have been proposed to explain growth kinetics of surface oxide

films. Anodic polarization curve is time-dependent in both active and passive

regions. Passive current density (ipass) should be proportional to the rate of passive

film formation and rate of its growth in thickness.

ipass = K

Logarithmic law of passive film formation has been derived taking into consideration

continuous adsorption.

Film thickness ( X) = A + B log t

Inverse logarithmic rate law, = A – B log t

Lecture 23: Anodic Protection NPTEL Web Course

1



Lecture 23

Anodic Protection

Keywords: Anodic Protection Range, Protection Design, Aggressive Environment.

Anodic protection refers to prevention of corrosion through impressed anodic

current. This method of protection tested and demonstrated by Edeleanu in 1954

however can be applied only to metals and alloys that exhibit active-passive

behavior. The interface potential of the structure is increased to passive domain.

If an active-passive alloy such as stainless steel is maintained in the passive region

through an applied potential (or current) from a potentiostat, its initial corrosion rate

(icorr) can be shifted to a low value at ipass as shown in Fig. 23.1.

Fig 23.1 Polarization curves depicting principles of anodic protection

As per mixed-potential theory,

Applied anodic current density = oxidation current density – reduction current

density.


2



Anodic protection is more effective in acid solutions than cathodic protection.

Current requirements for cathodic protection in acid solutions are several orders of

magnitude higher than that necessary for complete anodic protection. Cathodic

protection currents in acid solution can also lead to hydrogen liberation and

embrittlement of steels.

Anodic protection unlike cathodic protection is ideally suited for protection of

active-passive alloys in aggressive environments such as high acidity and corrosive

chemicals. Hence anodic protection is the most preferred choice for protection of

chemical process equipment.

Anodic protection parameters include.

a) Protection range – range of potentials in which the metal/alloy exhibits stable

passivity.

b) Critical anodic current density.

c) Flade potential.

Potential corresponding to middle of the passive region can be taken as optimum for

anodic protection. While choosing the desirable protection potential, an assessment

of the aggressiveness of the environment need be made. Since chloride ions are

detrimental to passivity, higher chloride concentrations can decrease the protection

range. Metals and alloys having relatively larger pitting and protection potentials

can only be chosen for very aggressive chemical environments. Higher temperatures

can deleteriously influence the protection potential.


3



Anodic protection of inner surface of a steel acid storage tank is shown in Fig. 23.2.

Fig. 23.2 Anodic protection of inner surface of a steel acid storage tank

A. Auxiliary cathode

B. Reference electrode

C. Anode connection to the tank

Inert cathode materials having large surface area preferred-Recommended cathode

materials for acid and corrosive industrial liquids include platinum-clad brass,

chromium-nickel steel, silicon cast iron, copper, Hastelloy C and nickel-plated steel.

Various types of reference electrodes such as Calomel, Ag/AgCl, Hg/HgSO4 and

platinum are used depending on the chemical environment.


4



The DC power supply used in anodic protection is more or less similar to the one

used in cathodic protection. There should be provisions for varying applied currents

and also to reduce the minimum current output. Electronic controls to maintain and

adjust current (or potential) in continuous (uninterrupted) mode could be very

advantageous.

Anodic protection can substantially reduce corrosion rate of active-passive alloys in

very aggressive environments. For example, anodic protection of 304 stainless steels

exposed to aerated sulfuric acid (5M) containing about 0.1 M chlorides could reduce

corrosion rate from an unprotected value of about 2000 µm/year, to about 5 µm/year.

It has been widely applied to protect chemical storage tanks, reactors, heat

exchangers and even transportation vessels.

A comparison between anodic and cathodic protection is given in Table 23.1:

Table. 23.1 Comparison of cathodic and anodic protection methods

Factors Cathodic protection Anodic protection

Suitability

To all metals in general.

Only to those exhibiting active-

passive behavior

Environment

Only for moderate corrosion

environment.

Even aggressive chemical

corrosives.

Cost benefit

Low investment, but higher

operative costs..

Higher investment, but low

operative costs.

Operation

Protective currents to be

established through initial

design and field trials

More precise electrochemical

estimation of protection range

possible.

It has been mentioned in earlier discussions on passivity that the magnitude of anodic

current density required for maintaining passivity is much lower than that required to


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passivate the metal or alloy. Such a beneficial aspect can be used with advantage of

low cost in anodic protection systems.

Approximate current density requirements for anodic protection in some aggressive

environments are given in Table 23.2:

Table. 23.2 Current density for passivation and maintenance in different corrosive environments (Alloy S30400,

room temperature)

Environment Average current density

for passivation mA/cm2

for maintaining µA/cm2

30-40% H2SO4

0.5

22

70% H2SO4 4.9 4.2

Strong H3PO4 at high temp 2 x 10-5

1.2 x 10-4

20-25% NaOH 4.3 9

Ref: Anodic protection – (Web-PowerPoint and PDF).

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