STEEL (IRON), Fe

WATERH2O

WATERH2O

OXYGENO2

OXYGENO2

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)]

CORROSION is the degradation of materials by reaction with surrounding media through chemical or electrochemical process

1

Cathode (+)Cathode (+)

Anode (-)

Acid Solution Droplet

Iron (Fe+++) Ions

e -e -

Hydrogen Gas

2 H+ + 2 e- H2 [ Acid Solution + Iron Ion = Hydrogen Gas ]

2

1. UNIFORM (General) CORROSIONCorrosion that develops at approximately the samerate over the entire metal surfaces.

Steel[Ignoble]

Brass[Noble]

Corrosion on Steel 2. GALVANIC (Bimetallic) CORROSIONOccurs when there is metallic contact between two dis-similar metals in a corrosive environment.

Water more rich in Oxygenbecomes the Cathodicregion

Crevice becomes the Oxygen depleted area, i.e. Anodic region

Corroding Area

3. CREVICE CORROSIONNarrow 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.

3

Erosion Corrosion on Copper

5. EROSION CORROSIONOccurs 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) CORROSIONUsually 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

4. PITTING CORROSIONInvolves localised attack on metals in the form of localised pits, often found on metals with passivating oxide film such as Aluminum & Stainless Steel.

4

Stress Cracking

7. STRESS CORROSION CRACKINGSSC 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 (NO3

-)- COPPER Alloys in AMMONIA Solutions (NH3)- STAINLESS Steels in CHLORIDE Solutions (Cl-)

8. FATIGUE CORROSIONWhen 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

ALGAE ANIMALSMobilespores

Mobilelarvae

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

4,000 - 5,000 different species involved in fouling

ALGAE ZONE - depth of 2 meters

(most heavily fouled)

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

Fouling is the settlement and growth of marine plants and animals on man-made structures in the

sea.

5 micron

6

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)

7

Animal Fouling: Barnacles as we know them!!!

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)

Animal Fouling:Barnacles: Larvae (cypris) early

stage of barnacle life cycle.

8

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

9

0.0

7.0

pH Value

B. - Oxygen Formation

A. - Hydrogen Formation

Corrosion

CorrosionImmune

Passivation

+2.0

+1.0

-1.6

-0.8

-0.4

-1.2

0.0 14.0

Po

ten

tia

l, E

- V

olt

s(S

tan

da

rd H

ydro

ge

n E

lec

tro

de

, S

HE

)

10

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 CPotentials are measured

Versus Silver-Silver ChlorideReference 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

11

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.

12

# 1 - SUBSTRATE

# 2 - ENVIRONMENT

# 3 - SURFACE PREPARATION

METHOD

# 4 - COATING SYSTEM 1.) Primer Coat

2.) Intermediate Coat3.) Finish Coat

# 5 - GENERIC TYPE

SELECTION

# 6 - PAINT APPLICATION

METHOD

# 7 - TOTAL DRY FILM THICKNESS

DFT

13

Surface of earth or water

e-

Electron flow inexternal circuit

Electric current flowingthrough electrolyte

Metal Ions into solution CATHODE

A pile or other metal structure

being protected

+

e-

e-e-

ANODEMagnesium/ Zinc or Aluminum w/ higher potential than metal

being protected

-

Insulated wire to allowcurrent to complete

circuit

e-

Zn+Zn+

Zn+

14

Surface of earth or water

Insulated wire to allowCurrent to complete

circuit

Electron flow inexternal circuit

Electric current flowingthrough electrolyte

e- GaseousAnodeReactionProducts

CATHODEA pile or other structure being protected from

corrosion

RECTIFIERDC 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

15

STEEL

Dew pointDew pointcalculatorcalculator

AIR / AMBIENT TEMPERATURE35 oC (20 - 25 oC)

RELATIVE HUMIDITY85% max (40-70%)

STEEL SURFACE TEMPERATURE3 - 5 oC above DEW POINT

DEW POINT TEMPERATURE3 - 5 oC below STEEL TEMP

16

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 -5oCbelow Steel

Surface

- 3oC belowMetal ( 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).

17

5 - Barometer/ Thermometer (Wall Mounted)

1a – Sling Psychrometer

1b – Sling Psychrometer (Bacharach Type)

1c - Dew Point Calculator

2 – Surface Thermometer with Probe (Digital)

3 – RH & Surface Thermometer with Probe (Digital)

4– RH & Surface Thermometer with Probe (Digital)

1d – Dial Surface Thermometer (Magnetic Backing)

18

Whirling / Sling PSYCHROMETER or HYGROMETER :Dry Bulb Thermometer – measures air or ambient temperature (from 10 – 50oC) liquidMercury filled.Wet Bulb Thermometer – measures wet temperature. Mercury filled thermometer withFabric wick cover & tube water container.

Dry Bulb

Wet Bulb

Dial Gage

Digital Probe

Electronic Digital Probe & Magnetic Dial METAL SURFACETHERMOMETER : Measures metal surface temperature (from 10 – 100oC).

19

1.) LOCATION – As close as possible to work area externally andinternally such as inside the tank.2.) CHECK THE INSTRUMENT – Thermometers and Mercury columns are not broken. Container for wet bulb wicking is wet andsecured 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 DewPoint Temperature using the following:-Mollier’s Diagram-Dew Point CalculatorReport the following: Date & Time of Monitoring, Air & Steel Temperature, Dew Point Temperature, % Relative Humidity (RH).4.) FREQUENCY – Check microclimate every 2 hours interval.

20