Corrosion Control of Marine Structures

Contents １．Steel Corrosion In Marine Environment 2. Mechanism of Corrosion 3. TYPICAL CORROSION CONTROL METHOD

3.1 Cathodic Protection 3.2 Coating & Covering Method 4. Maintenance 5. Repair -１-

-２-

Environment

Atmospheric zone

Characteristics of Environment

Characteristic of Corrosion

Wind brings fine sea-salt particle （Corrosively environment varies depend on distance from sea. Velocity and direction of wind, rainfall, temperature, sunshine, dust, seasonal conditions, pollution affect to corrosion)）

Corrosion rate of the part of shade where suffered by the wind and rain is larger than part of no suffering by wind and rain.

Splash zone

Steel surface is wetted with oxygen rich water film and no fouling by marine growth.

Largest corrosion rate zone

Tidal zone

Repeated wetting and drying of structure’s surface due to tide of seawater surface

Structure around tidal zone or MSL acts as cathode of oxidation concentration cell. Corrosion rate at coating damaged part is normally large.

Marine growth on the structure and seawater current affect as corrosion factor.

Structure just below MLWL part acts as an anode of oxidation concentration cell and corrosion late is very large.

Seabed

Possibility of sulfate reduction bacteria presence

Sulfides suffer corrosion of structure and effect of cathodic protection.

Backside

・Above the residual water level: Sane as environment as soil ・Below the residual water level: Same as environment as sea bed

・Similar to the soil environment

Underwater

-３-

・Similar to seabed environment

Atmospheric Zone

Splash Zone

H．W．L

Occur the most severe corrosion

Tidal Zone

Occur severe corrosion at from

L．W．L

M.L.W.L to Just below L.W.L.

Because, this part becomes an

Underwater Sea bottom

anode of oxidation cell.

Seabed

→ Corrosion rate（mm/yr） -４-

-５-

-６-

-７-

Stray Current Corrosion Wet Corrosion Natural Corrosion

Corrosion

Dry Corrosion

Due to stray current from DC transit system Local cell action Micro cell Corrosion Bacteria Galvanic Corrosion Macro cell Corrosion Corrosive Gas

-８-

Dry battery

Electron e−

Current i Cathode Copper

Carbon

Anode (Zinc)

H2 2e−

Zn→ Zn2+＋2e−

2H+ Current i

SO4 Electrolyte Zinc

2−

2e− Zn2+

Dilute sulfuric acid solution

H2SO4→2H+＋SO42− 2H+＋2e−→2H2 Zn2+＋SO42− →ZnSO4

Metal, high ionization, corroded

Current flows out from anode to cathode through electrolyte and corroded Cathode is protected vice versa. -９-

+200

±0

-200

-400

-600

-800

-1000

Aluminum Alloy Anode Zinc Aluminum Mild Steel Cast Iron Steel Lead Brass Copper Bronze ３０４ SS ３１６ SS Potential ［mV vs. SCE] -１０-

-1200

（Cathodic Reaction）

（Anodic Reaction）

H2O＋1/2O2+2e→2OH-

Fe→Fe++＋2e‐

（Corrosive Reaction） Fe++＋2OH−→Fe（OH）2

Electrolyte(Seawater)

Anode part Fe++ Fe++

Cathode part OHSteel

2e

2e

Steel surface, both of steel itself and environment is un-even, it causes potential different by dissolve oxygen, remaining stress, temperature different, impurity, deformation and fouling, then anode part is corroded and cathode part is protected. -１１-

-１２-

【A】

【B】

【C】

Most Popular

M.L.W.L L.W.L

Coating

Tidal Zone

H.W.L

Long Bridge or Gate

Coating

Splash Zone

Covering

Atmospheric Zone

L.W.L−1.0m

Source: Corrosion Protection & Repair Manual for Port & Harbor Steel Structures, Issued by OCDI -１３-

Corrosion Allowance

Seabed

Cathodic Protection

Sea bottom

Cathodic Protection

Underwater

Shallow and Not Important Structure

Protection method with covering system for the splash and tidal zone and cathodic protection for underwater and seamud by aluminum alloy anode is most common method and has high reliability. Following covering system will be applied, New structure: Heavy duty（Applied at factory) Existing: Petrolatum lining or underwater epoxy lining). -１４-

-１５-

Upper Structure

M.L.W.L Covering

Aluminum Alloy Anode Steel Pile Sea Bottom

Protective Current

-１６-

Protected Piece

Test Piece

Non Protected Piece Um-protected

Fitting level

Surface of test piece

-１７-

Protected

Technical Criteria and Description for Harbor Structures(Issued by Japan Port Association) Protective Potential：-780mV（vs Ag/AgCl）

Characteristics of Aluminum Alloy Anode ALAP-K Potential and Current capacity, etc; (1) Specific gravity比重 ： 2.7

Potential & Current Capacity; (1) Specific Gravity:2.6∼2.8

(2) Driving potential to steel： 0.25（Ｖ）

(2) Closed Potential： -1.05(V)

(3) Theoretical capacity:2.9（A・hr/g）

(3) Driving potential to steel:0.25(V) (4) Theoretical capacity:2.87（Ahr/g)

(4) Current capcity： ≧2.6（A・hr/g） （Average of supplied anodes：More than2.7 (5) Anode efficiency： ≧90％

(5) Current efficiency: More than 90% (6) Current capacity:2.6（Ahr/g）

-１８-

-１９-

Petrolatum Lining

Specification

Thickness Underwater work

Workability Environment Pollution

1. ISO St2 2. Petrolatum tape or equivalent 3. Protect with FRP cover

15mm Applicable Need much work process for sheet piles

Non

Underwater Lining

1. ISO Sa2.5 2. Coat epoxy resin

Mortar Lining 1. ISO St2 2. Fit FRP cover 3. Pour mortar

5mm(design)

50 to 60mm

Applicable

Applicable

Need much time for surface preparation Spread pitch during blasting

Design Life

>20 years

10 to 15 years

Application

Many

Many

Evaluation

○

○ -２０-

Possibility of sink during mortar pouring Possibility of mortar seepage >20 years

Not many △

PTC Covering

PTC Application

FRP Cover

Foamed PE Petrolatum tape or sheet

Steel Pile

Petrolatum paste Steel

-２１-

《PTC APPLICATION ① Surface Preparation FLOW》 Remove shell, fouling, rust and disbonded coating film from steel surface（ISO St2) ② Apply Paste Apply petrolatum paste which is purified during process of vacuum distillation of crude oil. This is inertness agent and has proof to acid, alkaline and seawater. ③ Lapping Tape After applied paste, lap petrolatum tape with 55% lapping tae. ④ Setting Protective Cover Setting FRP cover on the tape and fix by anti-corrosive bolts and nuts.

-２２-

-２３-

ƒ ƒ

《Working flow of underwater application Lining Method》 ①Temporary facility & curing

ƒ

Due to apply sand blast, prepare for strong scaffold and anti- marine pollution as safety countermeasure。

ƒ

②Surface Preparation

ƒ

Use marine blast machine. Finish surface condition is SIS Sa2.5 or equivalent

ƒ

③Epoxy resin mixing

ƒ

Use dedicated mixer due to consists of base and hardener resin and mix equally.

ƒ

④Coating

ƒ

Mixed resin coat on the structure within 30min. After mixed. (Curing time varies according to seasonal condition)

ƒ

ƒ

⑤Inspection

ƒ

Coating thickness shall be checked during and after coating by thickness gauge to control designed thickness.

-２４-

-２５-

Structures to be protected Performance Check of CP System

・Potential measurement ・Anode consumption heck

・Visual

Investigation of coating deterioration

Non protected structures Corrosion Evaluation

Investigation of Repairing method

・Peeling off ・Strength

・Visual ・Thickness gauging

・Detail visual inspection ・Detail thickness gauging -２６-

・Investigation for ensuring that CP system works properly to the structure to be protected. ・Investigation for ensuring that coating works properly to the structures to be protected.

・Investigation for current corroded conditions and evaluation of soundness of the existing harbor structures.、

・Investigation of detail corroded conditions for planning of repair method for existing harbor structure

START Planning of repairing investigation

Planning Yes

Budgeting

Investigation or site survey for repairing

Evaluation of Soundness

Necessity of Repairing

Budgeting of repairing investigation cost

Investigation Corrosion

No 『Planning of Countermeasure of Corrosion Control』 ・Above seawater：Coating or covering

『Planning of Repairing』

・Underwater：Cathodic Protection

Budget of repairing and corrosion control countermeasure

Summary of repairing & corrosion control method

Execution of protection and repairing work

END -２７-

Evaluation method of soundness of structures ・Evaluation of soundness in future from remaining thickness of steel ・Evaluation of soundness based of actual corrosion and corrosion rate

Un-soudness

Soundness

・Structural calculation, repairing planning & corrosion control planning

・Planning of corrosion control system to keep current conditions of structures -２８-

Structural yield strength

Non protected period

After apply protection

Designed section Design corrosion curve

Corrosion curve after apply protection

Required section

Actual corrosion curve

Remaining life

Initial Design Life -２９-

Elapse year

Structural yield strength

Non Protected period

After apply protection

Design section

Design corrosion curve

Corrosion curve after apply protection

Required Section Actual corrosion curve

Repairing

Remaining life Original design life -３０-

Elapse year

-３１-

Repair method that weld steel plate on the corroded pie pile or sheet piles This can be kept sufficient welding bead by using slit plate

-３２-

Covering method which applies reinforced concrete on the corroded pipe pile or sheet pile Keep necessary strength and protected by high alkaline concrete Reinforcement and protection of sheet piles

Reinforcement and protection steel pile

-３３-

Corrosion , Protection and Repairing of Harbor Steel Structure

THE END -３４-