Corrosion in Metals

Mohd. Hanif Dewan, IEng, IMarEng, MIMarEST (UK), MRINA(UK), Maritime Lecturer and Consultant

Corrosion

WHAT IS CORROSION

• Corrosion is the deterioration of materials by

chemical interaction with their environment.

• The term corrosion is sometimes also applied to the

degradation of plastics, concrete and wood, but

generally refers to metals. The most widely used

metal is iron (usually as steel) and the following

discussion is mainly related to its corrosion.

RESULTS OF CORROSION

The consequences of corrosion are many and varied and the effects of

these on the safe, reliable and efficient operation of equipment or

structures are often more serious than the simple loss of a mass of metal. Some of the major harmful effects of corrosion can be

summarised as follows:

1. Reduction of metal thickness leading to loss of mechanical strength

and structural failure or breakdown. When the metal is lost in localised zones so as to give a crack like structure, very

considerable weakening may result from quite a small amount of

metal loss.

2. Hazards or injuries to people arising from structural failure or breakdown

3. Loss of time in availability of profile-making industrial equipment.

4. Reduced value of goods due to deterioration of appearance.

5. Contamination of fluids in vessels and pipes. 6. Perforation of tanks and pipes allowing escape of their contents and

possible harm to the surroundings. For example corrosive sea water

may enter the boilers of a power station if the condenser tubes

perforate.

7. Loss of technically important surface properties of a metallic

component. These could include frictional and bearing properties,

ease of fluid flow over a pipe surface, electrical conductivity of

contacts, surface reflectivity or heat transfer across a surface.

8. Mechanical damage to valves, pumps, etc, or blockage of pipes by

solid corrosion products.

9. Added complexity and expense of equipment which needs to be

designed to withstand a certain amount of corrosion, and to allow

corroded components to be conveniently replaced.

RESULTS OF CORROSION

Galvanic Corrosion • Noble or Cathodic

– Platinum

– Gold, Titanium

– Silver

– Stainless steel

– Bronze/Copper/Brass

– Cast Iron

– Steel

– Aluminium

– Zinc

– Magnesium

• Active or Anodic

Electrolyte

Copper Zinc

Isolation of dissimilar metals by electrical

insulation.

Galvanic Corrosion:

• Possibility when two dissimilar metals are electrically

connected in an electrolyte*

• Results from a difference in oxidation potentials of metallic

ions between two or more metals. The greater the difference

in oxidation potential, the greater the galvanic corrosion.

• Refer to Galvanic Series (Figure 13-1)

• The less noble metal will corrode (i.e. will act as the anode)

and the more noble metal will not corrode (acts as cathode).

• Perhaps the best known of all corrosion types is galvanic

corrosion, which occurs at the contact point of two metals or

alloys with different electrode potentials.

GALVANIC SERIES Galvanic Series in Seawater ( supplements Faraq Table 3.1 , page 65), EIT Review Manual, page 38-2

Tendency to be protected from corrosion, cathodic, more noble end

Mercury

Platinum

Gold

Zirconium Graphite

Titanium

Hastelloy C Monel

Stainless Steel (316-passive)

Stainless Steel (304-passive)

Stainless Steel (400-passive)

Nickel (passive oxide)

Silver

Hastelloy 62Ni, 17Cr

Silver solder

Inconel 61Ni, 17Cr

Aluminum (passive AI203)

70/30 copper-nickel

90/10 copper-nickel

Bronze (copper/tin)

Copper

Brass (copper/zinc)

Alum Bronze Admiralty Brass

Nickel

Naval Brass Tin

Lead-tin

Lead

Hastelloy A

Stainless Steel (active)

316 404 430 410

Lead Tin Solder

Cast iron

Low-carbon steel (mild steel)

Manganese Uranium

Aluminum Alloys

Cadmium

Aluminum Zinc

Beryllium

Magnesium

Note, positions of

ss and al**

Big Cathode, Small Anode = Big Trouble

Figure 1 illustrates the idea of an electro-chemical

reaction. If a metal is placed in a conducting

solution like salt water, it dissociates into ions,

releasing electrons, as the iron is shown doing in

the figure, via the ionization reaction

Fe Fe++ + 2e- The electrons accumulate on the iron giving it a

negative charge that grows until the electrostatic

attraction starts to pull the Fe++ ions back onto the

metal surface, stifling further dissociation. At this

point the iron has a potential (relative to a standard,

the hydrogen standard) of –0.44 volts. Each metal

has its own characteristic corrosion potential (called

the standard reduction potential), as plotted in

Figure 2.

If two metals are connected together in a cell,

like the iron and copper samples in Figure 1, a

potential difference equal to their separation on

Figure 2 appears between them. The corrosion

potential of iron, -0.44, differs from that of copper,

+0.34 , by 0.78 volts, so if no current flows in the

connection the voltmeter will register this

Figure 1. A bi-metal corrosion cell. The

corrosion potential is the potential to

which the metal falls relative to a

hydrogen standard.

Figure 2. Standard reduction

potentials of metals.

Galvanic Corrosion Potentials:

Liquid Cell Battery:

dry cell is a galvanic electrochemical cell with a pasty low-

moisture electrolyte. A wet cell, on the other hand, is a cell with a

liquid electrolyte, such as the lead-acid batteries in most cars

Zn(s) → Zn2+(aq) + 2 e- - oxidation reaction that happens at zinc = anode

Dry Cell - Zinc-carbon battery

2MnO2(s) + 2 H+(aq) + 2 e- → Mn2O3(s) + H2O(l) - reduction reaction at

carbon rod = cathode

How to avoid Galvanic Corrosion

• Material Selection:

Do not connect dissimilar metals!

Or if you can’t avoid it: – Try to electrically isolate one from the other

(rubber gasket).

– Make the anode large and the cathode small

• Bad situation: Steel siding with aluminum fasteners

• Better: Aluminum siding with steel fasteners

• Eliminate electrolyte

• Galvanic of anodic protection

• Galvanic severity depends on:

– NOT

• Not amount of contact

• Not volume

• Not mass

– Amount of separation in the galvanic series

– Relative surface areas of the two. Severe corrosion if anode area (area eaten away) is smaller than the cathode area. Example: dry cell battery

Steel bolt (less noble) is

isolated from copper

plates.

See handout! – Read

Payer video HO

Prevention of Galvanic Corrosion

Use a single material or a combination of materials that are

close in the galvanic series.

Avoid the use of a small ratio of anode area to cathode area.

Use equal areas or a large ratio of anode to cathode area.

Electrically insulate dissimilar metals where possible. This

recommendation is illustrated in figure. A flanged joint is

equipped with bolts contained in insulating sleeves with

insulating washers under the head and nut. Paint, tape, or

asbestos gasket material are alternative insulations.

Local failure of the protective coating, particularly at the anode,

can result in the small anode-to-cathode area syndrome marked

by accelerated galvanic corrosion. Maintain all coatings in good

condition, especially at the anode.

Decrease the corrosion characteristics of the fluid where

possible by removing the corrosive agents or adding

inhibitors.

Avoid the use of threaded or riveted joints in favor of

welded or brazed joints. Liquids or spilled moisture can

accumulate in thread grooves or lap interstices and form a

galvanic cell.

Design for readily replaceable anodic parts or, for long life,

make the anodic parts more substantial than necessary for

the given stress conditions.

Install a sacrificial anode lower in the galvanic series than

both the metals involved in the process equipment.

Pitting Corrosion

• Pitting corrosion is the phenomenon whereby an extremely localized attack results in the formation of holes in the metal surface that eventually perforate the wall. The holes or pits are of various sizes and may be isolated or grouped very closely together.

Preventive measures

There are several preventive approah to avoid pitting. There are :

1. Proper material selection e.g. SS316 with molydenum having

higher pitting resistance compare to SS304

2. Use higher alloys for increased resistance to pitting corrosion

3. Control O2 level by injecting O2 scavenger in boiler water system

4. Control pH, chloride concentration and temperature

5. Cathodic protection and/or Anodic Protection

6. Proper monitoring of O2 & chloride contents by routine

sampling

7. Agitation of stagnant fluid

Selective Leeching Corrosion

• Selective leaching is the term used to describe a

corrosion process wherein one element is removed

from a solid alloy. The phenomenon occurs

principally in brasses with a high zinc content

(dezincification) and in other alloys from which

aluminum, iron, cobalt, chromium, and other

elements are removed.

• Grey cast iron is subject to leeching known as

graphitization, whereby the iron is dissolved leaving

behind a weak porous graphite network.

Dealloying:

• When one element in an alloy is anodic to the other

element.

• Example: Removal of zinc from brass (called

dezincification) leaves spongy, weak brass.

• Brass alloy of zinc and copper and zinc is anodic to

copper (see galvanic series).

Dealloying:

Two common types:

– Dezincification – preferential removal of zinc in brass

• Try to limit Zinc to 15% or less and add 1% tin.

• Cathodic protection

– Graphitization – preferential removal of Fe in Cast

Iron leaving graphite (C).

Prevention of Selective

Leeching/dealloying

The only effective method of preventing corrosion by

selective leaching is to avoid the use of materials

known to be subject to it in association with the

fluids concerned. Brasses with high zinc content (>

35 percent) in acid environments are particularly

susceptible.

Erosion Corrosion

• Erosion corrosion is the term used to describe

corrosion that is accelerated as a result of an

increase in the relative motion between the

corrosive fluid and a metal wall. The process is

usually a combination of chemical or electrochemical

decomposition or dissolution and mechanical wear

action.

Erosion Corrosion of Condenser Tube Wall

Prevention of Erosion Corrosion

Use materials with superior resistance to erosion

corrosion.

Design for minimal erosion corrosion.

Change the environment.

Use protective coatings.

Provide cathodic protection

Cavitation Erosion Cavitation erosion is a special class of erosion

corrosion that is associated with the periodic

growth and collapse of vapour bubbles in liquids.

Normally it occurs to the diesel engine (wet

cylinder liner), cooling systems and pumps.

• Prevention of Cavitation Erosion:

- Using of cavitation inhibitor chemicals or

supplemental coolant additives, that form a

film on surfaces, in cooling system can reduce

the cavitation erosion.

- Using of pressurized cooling system can

reduce the cavitation erosion.

- pH control to avoid corrosion.

Fretting Corrosion

• Fretting corrosion occurs at the contact points of

stressed metallic joints that are subject to vibration

and slight movement. It is also called friction

oxidation, wear oxidation, chafing, and false

brinelling. Fretting corrosion to be a special case of

erosion corrosion occurring in air rather than

aqueous conditions.

Tube-tube-sheet & Tube-baffle Fretting

Theories of Fretting Corrosion

Essential Elements for Fretting Corrosion

A loaded interface. Tube-tube-sheet joints are

heavily loaded by the strains induced in rolling the

tubes in the tube-sheet.

Vibration or repeated relative motion between the

two surfaces.

The load and relative motion of the interface must

be sufficient to produce slip or deformation on the

surfaces.

Prevention of Fretting Corrosion

Eliminate vibration

Eliminate high-stress interface

Lubricate the joint

Use hard surface

Increase friction at the interface

Use soft metallic or non-metallic interface

gaskets

Stress Corrosion Cracking

• Stress corrosion is the name given to the process

whereby cracks appear in metals subject

simultaneously to a tensile stress and specific

corrosive media. The metal is generally not subject to

appreciable uniform corrosion attack but is

penetrated by fine cracks that progress by expanding

over more of the surface and proceeding further into

the wall.

Factors:

• Must consider metals and environment. What to observe for:

– Stainless steels at elevated temperature in chloride solutions.

– Steels in caustic solutions

– Aluminum in chloride solutions

• 3 Requirements for SCC:

1. Susceptible alloy

2. Corrosive environment

3. High tensile stress or residual stress

Stress Corrosion Cracking:

See handout, review HO

hydron!

Prevention of Stress Corrosion Cracking

Reducing the fluid pressure or increasing the wall thickness.

Relieve residual stress by annealing.

Change the metal alloy to one that is less subject to stress-corrosion cracking. E.g. carbon steel is more resistant than stainless steel to corrosion cracking in a chloride-containing environment, but less resistant to uniform corrosion. Replacing stainless steel with an alloy of higher nickel content is often effective.

Prevention of Stress Corrosion cracking

Modify the corrosive fluid by process treatment or

the addition of corrosion inhibitors such as

phosphates.

Apply cathodic protection with sacrificial anodes or

external power supply.

Use shot peening method to induce surface stress.

Use venting air pockets to avoid concentration of

chloride in the cooling water

Intergranular Attack:

• Corrosion which occurs preferentially at grain

boundries.

• Why at grain boundries?

– Higher energy areas which may be more anodic

than the grains.

– The alloy chemistry might make the grain

boundries dissimilar to the grains. The grain can

act as the cathode and material surrounding it the

anode.

Intergranular Attack:

• How to recognize it?

Near surface

Corrosion only at grain boundries (note if only a

few gb are attacked probably pitting)

Corrosion normally at uniform depth for all grains.

Example 1: Intergranular Attack

Sensitization of stainless steels:

– Heating up of austenitic stainless steel (750 to

1600 F) causes chromuim carbide to form in the

grains. Chromuim is therefore depleted near the

grain boundries causing the material in this area

to essentially act like a low-alloy steel which is

anodic to the chromium rich grains.

Example 2: Intergranular Attack

Sensitization of stainless steels:

– Heating up of austenitic stainless steel (750 to 1600 F) causes chromuim carbide to form in the grains. Chromuim is therefore depleted near the grain boundries causing the material in this area to essentially act like a low-alloy steel which is anodic to the chromium rich grains.

– Preferential Intergranular Corrosion will occur parallel to the grain boundary – eventually grain boundary will simply fall out!!

How to avoid Intergranular Attack:

• Watch welding of stainless steels (causes

sensitization). Always anneal at 1900 – 2000 F after

welding to redistribute Cr.

• Use low carbon grade stainless to eliminate

sensitization (304L or 316L).

• Add alloy stabilizers like titanium which ties up the

carbon atoms and prevents chromium depletion.

Intergranular Attack:

Factors affecting corrosion rates

Temperature

As a rule of thumb for each 10'C rise in temperature doubles the rate of

corrosion.

Corrosion in Close system and Open system

• The rate of oxygen diffusion increases in an open system with temperature up to around 80'C. A rapid tailing off (to reduce in amount) then occurs due to the solubility of oxygen. For this reason

open system feed tanks seen on many vessels have heating coils which maintain the temperature at 85'C or higher.. In a closed system there is no such tail off as the oxygen cannot escape

pH/Alkalinity The electrochemical nature of the metal will determine its

corrosion rate with respect to pH. The corrosion rate of iron

reduces as the pH increases to about 13 due to the

reduced solubility of the Fe ions. Aluminium and zinc, being

ampoteric, have rates of corrosion that increases with pH

higher or lower than neutral

Methods to Control Corrosion

There are five methods to control corrosion:

material selection

coatings

changing the environment

changing the potential

design

How to avoid Corrosion?

1. Material Selection.

2. Eliminate any one of the 4 requirements for corrosion.

3. Galvanic - Avoid using dissimilar metals.

– Or close together as possible

– Or electrically isolate one from the other

– Or MAKE ANODE BIG!!!

How to avoid Corrosion?

4. Pitting/Crevice: Watch for stagnate water/

electrolyte.

– Use gaskets

– Use good welding practices

5. Intergranular – watch grain size,

environment, temperature, etc.. Careful with

Stainless Steels and AL.

How to avoid Corrosion?

6. Consider organic coating (paint, ceramic, chrome,

etc.) – But DANGER IF IT GETS SCRACTHED!!

7. BETTER to consider cathodic protection (CP):

– such as zinc (or galvanized) plating on steel

– Mg sacrificial anode on steel boat hull

– Impressed current (ICCP) etc..

Corrosion Control:

Surface Treatment (Coatings) Organic paints

Chromating and phosphating:

The Process - chromating and phosphating are surface-coating processes that

enhance the corrosion resistance of metals. Both involve soaking the component

in a heated bath based on chromic or phosphoric acids. The acid reacts with the

surface, dissolving some of the surface metal and depositing a thin protective

layer of complex chromium or phosphorous compounds

Anodizing (aluminum, titanium)

– The Process - Aluminum is a reactive metal, yet in everyday objects it does not

corrode or discolor. That is because of a thin oxide film - Al2O3 - that forms

spontaneously on its surface. This oxide formed by anodizing is hard, abrasion

resistant and resists corrosion well. The film-surface is micro-porous, allowing it

to absorb dyes, giving metallic reflectivity with an attractive gold, viridian, azure

or rose-colored sheen; and it can be patterned. The process is cheap, an imparts

both corrosion and wear resistance to the surface.

Surface Treatment (Coatings) • Electro-plating

– The Process -Metal coating process wherein a thin metallic coat is

deposited on the workpiece by means of an ionized electrolytic solution.

The workpiece (cathode) and the metallizing source material (anode) are

submerged in the solution where a direct electrical current causes the

metallic ions to migrate from the source material to the workpiece. The

workpiece and source metal are suspended in the ionized electrolytic

solution by insulated rods. Thorough surface cleaning precedes the plating

operation. Plating is carried out for many reasons: corrosion resistance,

improved appearance, wear resistance, higher electrical conductivity,

better electrical contact, greater surface smoothness and better light

reflectance.

• Bluing

– Bluing is a passivation process in which steel is partially protected against rust, and is named after the blue-black appearance of the resulting

protective finish. True gun bluing is an electrochemical conversion coating

resulting from an oxidizing chemical reaction with iron on the surface

selectively forming magnetite (Fe3O4), the black oxide of iron, which

occupies the same volume as normal iron. Done for bolts called “blackening”.

• Hot-dip Coating (i.e. galvanizing)

– Hot dipping is a process for coating a metal, mainly ferrous metals, with

low melting point metals usually zinc and its alloys. The component is first degreased in a caustic bath, then pickled (to remove rust and scale)

in a sulfuric acid bath, immersed (dipped) in the liquid metal and, after

lifting out, it is cooled in a cold air stream. The molten metal alloys with

the surface of the component, forming a continuous thin coating. When

the coating is zinc and the component is steel, the process is known as galvanizing.

– The process is very versatile and can be applied to components of any

shape, and sizes up to 30 m x 2 m x 4 m. The cost is comparable with

that of painting, but the protection offered by galvanizing is much

greater, because if the coating is scratched it is the zinc not the

underlying steel that corrodes ("galvanic protection"). Properly

galvanized steel will survive outdoors for 30-40 years without further

treatment.

Surface Treatment (Coatings)

Material Selection:

Importance of Oxide films

• The fundamental resistance of stainless steel to corrosion

occurs because of its ability to form an oxide protective

coating on its surface. This thin coating is invisible, but

generally protects the steel in oxidizing environments (air and

nitric acid). However, this film loses its protectiveness in

environments such as hydrochloric acid and chlorides. In

stainless steels, lack of oxygen also ruins the corrosion

protective oxide film, therefore these debris ridden or

stagnant regions are susceptible to corrosion.

The “Right” material

depends on the

environment.

Polarization can

have a major

effect on metal

stability.

Recall CES Rankings: strong acid, weak acid, water, weak alkali, strong alkali

Corrosion Control for Iron

-2

2

0

Often several approaches to control corrosion

Often several “system” constraints pertain

Cathodic Protection (CP)

• Cathodic protection (CP)

- It is a technique to control the corrosion of a metal surface by

making it work as a cathode of an electrochemical cell.

- This is achieved by placing in contact with the metal to be

protected another more easily corroded metal to act as the

anode of the electrochemical cell.

Uses:

Cathodic protection systems are most commonly used to protect

steel, water or fuel pipelines and storage tanks, steel pier piles,

ships, offshore oil platforms and onshore oil well casings.

Cathodic Protections:

1. sacrificial anodes

– zinc, magnesium or aluminum. The sacrificial anodes are

more active (more negative potential) than the metal of the

structure they’re designed to protect. The anode pushes the potential of the steel structure more negative and therefore the

driving force for corrosion halts. The anode continues to corrode

until it requires replacement,

2. Galvanized steel (see above slide)

– again, steel is coated with zinc and if the zinc coating is

scratched and steel exposed, the surrounding areas of zinc

coating form a galvanic cell with the exposed steel and protects

in from corroding. The zinc coating acts as a sacrificial anode.

3. Impressed current Cathodic Protection (ICCP)

ICCP is Using an arrangement of hull mounted anodes and

reference cells connected to a control panel(s), the system

produces a more powerful external current to suppress the

natural electro-chemical activity on the wetted surface of the

hull.

This eliminates the formation of aggressive corrosion cells

on the surface of plates and avoids the problems which can

exist where dissimilar metals are introduced through welding

or brought into proximity by other components such as

propellers.

An essential feature of ICCP system is that they constantly

monitor the electrical potential at the seawater/hull interface

and carefully adjust the output to the anodes in relation to

this.

Therefore, the system is much more effective and reliable.

ICCP System advantages: 1. Increased life of rudders, shafts, struts and propellers as well as

any other underwater parts affected by electrolysis

2. Anodes are light, sturdy and compact for easy shipping, storage

and installation

3. Anodes, reference cells and automatic control systems maintain just the right amount of protection for underwater hulls and fittings, unlike standard zinc anodes, which can't adjust to changes in salinity or compensate for extreme paint loss

4. Automatic control equipment ensures reliable, simple operation

5. Optimum documented corrosion protection at minimum overall cost

6. Only one installation required for the life of the vessel or structure

7. Increased dry-dock interval

8. Approved by all classification societies for all types of vessels

9. Designed to provide a 20 plus year service life

MGPS Working Principle:

• Basic principle on which MGPS runs is electrolysis. The

process involves usage of copper, aluminum and ferrous

anodes. The anodes are normally fixed in pairs in the main

sea chest or in such place where they are in the direction of

the flow of water.

• The system consists of a control unit which supplies

impressed current to anodes and monitors the same. While in

operation, the copper anode produces ions, which are carried

away by water into the piping and machinery system.

Concentration of copper in the solution is less then 2 parts per

billion but enough to prevent marine life from settling.

• Due to the impressed current, the aluminum/ferrous anode

produces ions, which spread over the system and produce a

anti corrosive film over the sea water system’s pipes, heat

exchanger and valves etc, internally.

image Credit: marineinsight.com

Fig: MGPS

Marine Growth: Sea water contains both macro and micro marine organisms such as

sea worm, molluscs, barnacles, algae, hard shells like acorn barnades etc. These organisms stick to the surface of the ship and flourish over there, resulting in marine growth. Effects of Marine Growth As the marine organisms flourish they block and narrow the passage of cooling water in the ship’s system resulting in the following factors: – Impairing the heat transfer system. – Overheating of several water-cooled machineries.

– Increase in the rate of corrosion and thinning of pipes. – Reduced efficiency which can lead to loss of vessel speed and loss of time. Fighting Marine Growth: To avoid formation of marine growth MGPS or marine growth preventive system (MGPS) is used onboard ship.

References & Websites: 1. http://www.corrosionsource.com/ 2. http://www.westcoastcorrosion.com/Papers/Why%20Metals%20Corrode.

pdf

3. http://www.corrosioncost.com/home.html

4. http://www.intercorr.com/failures.html

5. http://www.3ninc.com/Cast_Magnesium_Anodes.htm 6. http://en.wikipedia.org/wiki/1992_explosion_in_Guadalajara

7. http://www.marineinsight.com

8. http://www.marineplantsystems.com/resources/Cathelco-Cathodic-

Protection/cathiccp.pdf

Thank you!