Corrosion Process and Control

2. CORROSION is the degradation of materials by reaction with surrounding
media through chemical or electrochemical process
STEEL (IRON), Fe
WATER
H2O
WATER
H2O
OXYGEN
OXYGEN
O2
O2
RUST, Fe (OH)
Cathode Site : 2 H2O + O2 + 4 e -> 4 OH-
[Water + Oxygen + Electron (Iron) = Hydroxyl Ions]
Cathode Site : 2 Fe++ + 4 OH- -> 2 Fe (OH)2
[Iron Ions + Hydroxyl Ions = Ferrous Hydroxide]
Anode Site :
2 Fe (OH)2 + H2O + ½ O2 -> 2 Fe (OH)3
[Iron Hydroxide + Water + Oxygen = Iron
Hydroxide (RUST)]
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3. Cathode (+) Cathode (+)
e- e-
Anode (-)
Acid Solution Droplet
Iron (Fe+++) Ions
Hydrogen Gas
2 H+ + 2 e-  H2
[ Acid Solution + Iron Ion = Hydrogen Gas ]
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4. 1. UNIFORM (General)
CORROSION
Corrosion that develops at approximately the same
rate over the entire metal surfaces.
Steel
[Ignoble]
Corrosion on Steel 2. GALVANIC (Bimetallic)
Brass
[Noble]
CORROSION
Occurs when there is metallic contact between two dis-similar
metals in a corrosive environment.
Water more rich in Oxygen
becomes the Cathodic
region
Crevice becomes the
Oxygen depleted area, i.e.
Anodic region
Corroding Area
3. CREVICE CORROSION
Narrow crevices exposed to a liquid, typically water
containing solutions, may be open enough to allow the
liquid to penetrate, but still narrow that liquid becomes
stagnant & crevice corrosion occurs. The driving force
is the difference in Oxygen content inside & outside the
crevice.
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5. 4. PITTING CORROSION
Involves localised attack on metals in the form of localised pits,
often found on metals with passivating oxide film such as
Aluminum & Stainless Steel.
Erosion Corrosion on Copper
5. EROSION CORROSION
Occurs when a metal is exposed to mechanical abrasion and a
corrosive environment e.g., liquid or gas flowing at a high
velocity in pipes may cause erosion corrosion.
6. SELECTIVE (De-Alloying) CORROSION
Usually appears on brass and cast iron if these are exposed in sea water. It causes one of the alloying
elements to be preferentially attacked, it leaves a porous material with little or no mechanical strength
examples such as:
- DEZINCIFICATION of BRASS and GRAPHITIZATION of CAST IRON
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6. Stress Cracking
7. STRESS CORROSION CRACKING
SSC is term given to INTER- or TRANSGRANULAR cracking of
metals by joint action of static tensile stress & a specific
environment. Such metals that are affected are as follows:
- CARBON Steels in NITRATE Solutions (NO-)
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- COPPER Alloys in AMMONIA Solutions (NH3)
- STAINLESS Steels in CHLORIDE Solutions (Cl-)
8. FATIGUE CORROSION
When metal is subjected to either temporary or continuous
stresses, cracking may suddenly occur above a certain stress
level.
Dynamic Stress
9. MICROBIOLOGICALLY INFLUENCED CORROSION (MIC)
Usually occurs in buried oil pipelines where varied soil elements (e.g. subkha areas) including
microorganisms or bacteria are present & also found in sewage treatment pipe internals & other related
biological/ petrochemical storage and transfer facilities. Most common corrosion influencing bacteria
are identified as : APB - Acid Producing Bacteria and SRB - Sulfate Reducing Bacteria.
5

7. Fouling is the settlement and growth of marine
plants and animals on man-made structures in the
ALGAE ANIMALS
Mobile
spores
Mobile
larvae
Attack Attack
Animal Fouling:
Barnacles: When they reach cypris stage of life cycle,
they can attach themselves to man made structures –
even at fairly high speed!
Plant Fouling:
Bio-film is formed by diatoms amphor, which are spores from the
various seawater plants and grasses. Experts at attaching
themselves to man made structures
ALGAE ZONE -
depth of 2 meters
(most heavily fouled)
4,000 - 5,000 different species involved in fouling
VERTICAL ZONE (below the Algae
Zone) (barnacles, encrusting
bryzoans, tubeworms &
goosenecks)
FLAT BOTTOM is dominated
by hydroids, barnacles,
mussels, tunicates, bryzoans
& goosenecks.
BACTERIA, Diatoms & other MICRO
ORGANISMS- NO specific zones or areas on
the ship’s bottom of settling
sea.
5 micron
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8. 5 micron
"Slime“(Heavy magnification)
Plant Fouling:
Bio-film consisting of Slime-forming
diatom amphor
Micro-organisms are the
first to settle; they form the
primary biofilm, the so
called SLIME layer. The
most important ones are:
- BACTERIA
- DIATOMS (unicellular
algae)
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9. Animal Fouling: Barnacles as
we know them!!!
Animal Fouling:
Barnacles: Larvae (cypris) early
stage of barnacle life cycle.
Macro organisms are big
enough to be seen
without the aid of a
microscope. They are:
- ALGAE (seaweed or
“grass” in red, green or
brown)
- ANIMALS (hard or soft
shelled)
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10. 1. PAINTING & COATING (ANTI-FOULING) APPLICATION
2. CATHODIC PROTECTION SYSTEM
 Sacrificial Anode System (Galvanic Anode)
 Impressed Current Cathodic Protection System (ICCP –
Inert Anode)
3. COMBINED PROTECTION
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11. 0.0
7.0
B. - Oxygen Formation
pH Value
Corrosion
A. - Hydrogen Formation
Passivation
Immune Corrosion
+2.0
+1.0
-0.4
-0.8
-1.2
-1.6
0.0 14.0
(Standard Hydrogen Electrode, SHE)
Potential, E - Volts
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12. METAL / ALLOY
(Normally Used in Offshore/ Marine Structures)
Potential in VOLTS
(Ag/AgCl ref.)
MAGNESIUM / Mg-6Al-3Zn/ ALUMINUM Anode -1.15 to -1.64
Al 5257-H25/ ZINC (MIL-A-18001G) -1.03 to -1.13
ALUMINUM Alloys (5083-0; X7005-T63; 5456-H321) -0.96 to -0.98
ALUMINUM Anode (5Zn)/ ALUMINUM Alloys -0.65 to -0.95
2% Ni CAST IRON/ Cast IRON / Carbon Steel A1010 -0.61 to -0.68
Hi-Strength, Low-Alloy STEEL/ 430 SS (Active) -0.57 to -0.61
304 STAINLESS STEEL (Active) / 410 SS (Active) -0.52 to -0.53
Ni Resist Type 1/ Tobin BRONZE -0.40 to -0.47
Yellow BRASS/ COPPER / Admiralty BRASS (24.6 C) -0.36
Red BRASS/ G BRONZE/ Admiralty BRASS (11.9 C) -0.30 to -0.33
Aluminum BRASS/ 90-10 CUPRONickel (0.82 & 1.4 Fe) -0.28 to -0.29
70-30 & 90-10 CUPRONICKEL (0.45, 0.51, 1.4 & 1.5 Fe) -0.22 to -0.25
430 SS (Passive)/ 70-30 CuproNICKEL (0.51Fe) -0.20 to -0.26
NICKEL 200/ 316 SS (Active)/ INCONEL 600 -0.17 to -0.20
410 SS (Passive)/ PDA TITANIUM/ SILVER -0.13 to -0.15
BI TITANIUM/ 304 SS (Passive)/ HASTELLOY C -0.08 to 0.10
MONEL 400/ 316 SS (Passive) -0.06 to -0.08
PLATINUM +0.26
GRAPHITE +0.25
Note:
Seawater Velocity = 7.8 to 13 ft/sec
Temperature = 11 to 30 deg C
Potentials are measured
Versus Silver-Silver Chloride
Reference Electrode (SSC)
Saturated Calomel Electrode (SCE) =
+0.245 Volt
Silver/ Silver Chloride (SSC) =
+0.25 Volt
Copper/ Copper Sulfate (CSE) =
+0.32 Volt
Zinc Electrode = -0.78 Volt
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13. METAL TWO (2 ) YEARS* FIVE (5 ) YEARS* TEN (10) YEARS*
STEEL 51.1 32.8 20.7
ALUMINIUM 0.48 0.76 0.35
COPPER 1.8 1.1 0.71
ZINC 3.6 2.6 1.7
Note* = Coastal Marine Environment exposure at a testing station along west coast of Sweden.
TYPE OF STEEL Moderate Marine Atmosphere 7.5
Years Exposure*
Severe Marine Atmosphere
Exposure
Structural Carbon Steel 18.8 (0.74 mpy) 414 (16.3 mpy) - 3.5 years
Structural Copper Steel 15.2 (0.6 mpy) 274 (10.8 mpy) - 3.5 years
ASTM A517 Grade F 9.9 (0.39 mpy) 25.4 (1.0 mpy) – 5 years
ASTM A242 Type 1
(Cr-Si-Cu-Ni-P)
7.9 (0.31 mpy) 99.1 (3.9 mpy) – 5 years
Note** = MPY– mils per year (25.4 microns = 1 mil). Corrosion rate of steel immersed in sea water =127 microns per year or
5 mils per year. Steel piling at Wrightsville, North Carolina, USA.
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14. # 5 - GENERIC
TYPE
SELECTION
# 1 - SUBSTRATE
# 3 - SURFACE
PREPARATION
METHOD
# 2 - ENVIRONMENT
# 4 - COATING SYSTEM
1.) Primer Coat
2.) Intermediate Coat
3.) Finish Coat
# 6 - PAINT
APPLICATION
METHOD
# 7 - TOTAL DRY
FILM THICKNESS
DFT
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15. Surface of earth or water
Metal Ions into
solution CATHODE
e-
Electron flow in
external circuit
Electric current flowing
through electrolyte
A pile or
other metal
structure
being
protected
+
e-e-
e-
ANODE
Magnesium/ Zinc or
Aluminum w/ higher
potential than metal
being protected
-
Insulated wire to allow
current to complete
circuit
e-
Zn+
Zn+
Zn+
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16. Surface of earth or water
Insulated wire to allow
Current to complete
circuit
Electron flow in
external circuit
e- Gaseous
Electric current flowing
through electrolyte
Anode
Reaction
Products
CATHODE
A pile or other
structure being
protected from
corrosion
RECTIFIER
DC Current Source e- e-e-
+
-
-
e-
*Cathode reactions are usually oxygen reduction of Hydrogen to Water,
Formation of Hydrogen films, or discharge of Hydrogen Gas.
ANODE (INERT)
Graphite, Lead Alloy or
other suitable material
w/c will best discharge
the impressed current
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17. STEEL
DDeeww ppooiinntt
ccaallccuullaattoorr
AIR / AMBIENT TEMPERATURE
35 oC (20 - 25 oC)
RELATIVE HUMIDITY
85% max (40-70%)
STEEL SURFACE TEMPERATURE
3 - 5 oC above DEW POINT
DEW POINT TEMPERATURE
3 - 5 oC below STEEL TEMP
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18. ITEM DEFINITION PARAMETER MAX. LIMIT*
1 – AIR / AMBIENT Temperature
in oC
Prevailing temperature of the
air/ atmosphere.
20 - 30 35oC
2 – METAL or STEEL SURFACE
Temperature in oC
Actual skin temperature of the
metal or steel, Usually 10-20oC
higher than air when exposed
directly under the sunlight.
30 - 60 + 3oC above
Dew Point
3 – PAINT Temperature inoC Ideal temperature for paint for
application & proper film
formation.
15 - 25 20oC
4 – DEW POINT Temperature in
oC
Maximum temperature at
which moisture / water vapour
condenses.
- 3 to -5oC
below Steel
Surface
- 3oC below
Metal ( Steel)
Surface
5 – RELATIVE HUMIDITY in % Relative quantity of moisture/
water vapor in the air.
40 - 70 85 %
* Acceptable limits for blast cleaning and painting (Hempel).
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19. 5 - Barometer/ Thermometer
(Wall Mounted)
1a – Sling Psychrometer
1b – Sling Psychrometer
(Bacharach Type)
1c - Dew Point Calculator
4– RH & Surface Thermometer
with Probe (Digital)
3 – RH & Surface Thermometer
with Probe (Digital)
2 – Surface Thermometer
with Probe (Digital)
1d – Dial Surface Thermometer
(Magnetic Backing)
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20. Dry Bulb
Wet Bulb
Whirling / Sling PSYCHROMETER or
HYGROMETER :
Dry Bulb Thermometer – measures air or
ambient temperature (from 10 – 50oC) liquid
Mercury filled.
Wet Bulb Thermometer – measures wet
temperature. Mercury filled thermometer with
Fabric wick cover & tube water container.
Dial Gage
Digital Probe
Electronic Digital Probe & Magnetic Dial METAL SURFACE
THERMOMETER :
Measures metal surface temperature (from 10 – 100oC).
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21. 1.) LOCATION – As close as possible to work area externally and
internally such as inside the tank.
2.) CHECK THE INSTRUMENT – Thermometers and Mercury
columns are not broken. Container for wet bulb wicking is wet and
secured at both ends. Filled with distilled water.
3.) TAKE THE MEASUREMENT –
Whirl or spin carefully the hygrometer slightly faster at 180 spins/
revolutions per minute (3 revolution per second) for 1 minute.
Read both thermometers, wet bulb temperature first.
Make/ perform at least two (2) spins/ whirlings.
Record dry & wet bulb temperatures of both thermometers.
Determine/ calculate the Relative Humidity (%RH) and the Dew
Point Temperature using the following:
-Mollier’s Diagram
-Dew Point Calculator
Report the following: Date & Time of Monitoring, Air & Steel
Temperature, Dew Point Temperature, % Relative Humidity (RH).
4.) FREQUENCY – Check microclimate every 2 hours interval.
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