Corrosion in Metals and Protections

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

2. 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.

3. 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. 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

5. 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

6. Isolation of dissimilar metals by electrical
insulation.

7. 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.

8. 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**

9. Big Cathode, Small Anode = Big Trouble

10. 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:

11. 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

12. 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

13. 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

14. • 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

15. Steel bolt (less noble) is
isolated from copper
plates.
See handout! – Read
Payer video HO

16. 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.

17.  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.

18. 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.

19. 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

20. 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.

21. 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).

22. 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).

23. 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.

24. 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.

25. Erosion Corrosion of Condenser Tube Wall

26. 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

27. 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.

28. 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.

29. Tube-tube-sheet & Tube-baffle Fretting

30. Theories of Fretting Corrosion

31. 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.

32. 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

33. 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.

34. 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

35. Stress Corrosion Cracking:
See handout, review HO
hydron!

36. 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.

37. 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

38. 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.

39. 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.

40. 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.

41. 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!!

42. 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.

43. Intergranular Attack:

44. Factors affecting corrosion rates
Temperature
As a rule of thumb for each 10'C rise in temperature doubles the rate of
corrosion.

45. 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

46. 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

47. Methods to Control Corrosion
There are five methods to control corrosion:
 material selection
 coatings
 changing the environment
 changing the potential
 design

48. 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!!!

49. 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.

50. 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..

51. Corrosion Control:

52. 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.

53. 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”.

54. • 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)

55. 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.

56. 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

57. Corrosion Control for Iron
-2
2
0

58. Often several approaches to control corrosion
Often several “system” constraints pertain

59. 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.

60. 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.

61. 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.

64. 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

65. 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.

66. image Credit: marineinsight.com
Fig: MGPS

67. 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.

68. 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!