CORROSION CHALLENGES WITH THE USE OF ALUMINIUM IN … o Most commercial alloys contain several types of intermetallic phases . Corrosion on aluminium alloys is essentially a microgalvanic

Norwegian University of Science and Technology

CORROSION CHALLENGES WITH THE USE OF ALUMINIUM IN OFFSHORE

AND MARINE ENVIRONMENTS

Presented by:Roy Johnsen, Professor Corrosion and Surface Protection

Department of Mechanical and Industrial Engineering (MTP)/[email protected]


THE PEARL, DOHA/QATAR



“STAINLESS STEEL” AND ALUMINIUM

Aluminium

“Stainless steel”

Norwegian University of Science and Technology 5

1986 - R&D PROJECTTITLE:Use of aluminium in connection with oil & gas exploration and productionoffshore

PARTICIPANTS:o Statoilo Raufoss Ammunisjonsfabrikkero Årdal og Sunndal Verk

BUDGET:o 50 MNOK

RESEARCH PARTNERS:o SINTEF AND NTNU


ALUMINIUM USED IN OIL AND GAS INDUSTRY TODAY

o Helicopter decko Living quartero Support for lifeboatso Prefabricated wallso Telescopic gangways

o Handrailso Ladderso Walkwayso Cable trayso Drill pipes


EXAMPLES – OIL AND GAS INDUSTRY

HelideckLiving quarter

Telescopic gangway

StairsHand rails

SpiralStairs


LILLE FRIGG – SUBSEAProtective cover made by an aluminium alloy

Protective cover

Sacrificial anode

Protective cover

Hinge in aluminium


OFFSHORE POWER GENERATIONNew opportunities for aluminium


BOATS CONSTRUCTED IN ALUMINIUM


BUOY made from AA5083 sheet and AA6082 extrusions – operated since 1978

Inspection in 2010:o No substantial wall thickness reduction on any partso No cracks in base material or weld seamso Joints with stainless steel screw (1.4571) fully intact without galvanic isolation

Source: [email protected]


WHY USE OF ALUMINIUM IN MARINE ENVIRONMENT?o Light weighto Easy to recycleo Corrosion resistance due to protective oxide layero Wide range of surface treatments possible

Buto Aluminium also have some negative properties compared

to alternative construction materials like carbon steel (galvanic corrosion, stiffness, availability, lack of design standards, lack of experience, …)


CORROSION OF ALUMINIUMo From a purely thermodynamical point of view aluminium is

active. However, in oxygen containing environment (air, water), aluminium is rapidly covered with a dense oxide layer.

o The oxide layer is essentially inert, and prevents corrosion. o The thickness of the layer may vary as a function of

temperature, environment and alloy elements. Oxide films formed in air at room temperature are 2-3nm thick on pure aluminium. Heating to 425°C gives films up to 20 nm.

o If the oxide film is damaged, e.g. by a scratch, new oxide will immediately form on the bare metal. In this way aluminium gives excellent corrosion protection.


GOOD CORROSION PROPERTIES:Development of a protective oxide film

o Al2O3 + H2O = 2 AlOOH (boehmite)o AlOOH + H2O = Al(OH)3 (bayerite)

Oxide film on aluminium – PhD candidate NTNU 2004

Water/humidity


CORROSION AS A FUNCTION OF pH FOR ALUMINIUM - SCHEMATIC

Pourbaix diagram


EFFECT OF pH ON CORROSION RATE IN DIFFERENT SOLUTIONS


CORROSION OF ALUMINIUM

Some factors may affect the stability of the aluminium oxide and thereby cause corrosion:

o The oxide is not stable in acidic (pH < 4) or alkaline (pH > 9) environments.

o Aggressive ions (chlorides, fluorides) may attack the oxide locally.o Certain elements (Ga, In, Sn, Pb) may become incorporated in the

oxide and destabilise it.


IMPORTANT CORROSION FORMS ON ALUMINIUM

Uniform or General corrosiono Overall and lateral corrosion – uniform

corr.rate.

Pitting corrosiono Localized attack – can cause hole through a

wall

Intergranular corrosiono Corrosion attacks on the grain boundaries

Galvanic corrosiono Two alloys/compositions in metallic contact

and exposed to an electrolyteo The un-noble alloy corrodeo Serious corrosion attack can occur

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CORROSION OF ALUMINIUMUniform corrosion in marine atmosphere

Carbon steel

Cast iron

Zinc

Copper

Stainless steels

Aluminium

Corrosion Rate (μm/year)


CORROSION OF ALUMINIUMPitting corrosion

o Pitting is a highly localized type of corrosion in the presence of aggressive chloride ions.

o Pits are initiated at weak sites in the oxide by chloride attack and propagate according to the reactionso Al = Al3+ + 3e-

o Al3+ + 3H2O = Al(OH)3 + 3H+

o Hydrogen evolution and oxygen reduction are the important reductionprocesses at the intermetallic cathodeso 2H+ + 2e- = H2

o O2 + 2H2O + 4e- = 4OH-

o As a pit propagates, the environment inside the pit (anode) changes, thepH will decrease.

o To balance the positive charge produced, chloride ions will migrate into thepit. The resulting HCl formation inside the pit causes accelerated pit propagation.


Al Al3+

e-

O2

O2O2

OH-

O2

Oxidation reaction

Reduction reaction

Aluminium alloy Seawater

PITTING CORROSION OF Al IN SEAWATER

Oxidation reaction (anodic reaction): Al Al3 + 3e-

Reduction reaction (cathodic reaction): O2 + 2H2O + 4e- 4OH-


CORROSION OF ALUMINIUMPitting corrosion of AA6082


PITTING CORROSION ON AlDifferent degree of attack in the same solution


INTERMETALLIC PARTICLESAluminum alloy (AA8006) rolled product


CORROSION OF ALUMINIUMPitting corrosion, cont.

o The reduction reaction will cause local alkalinisation around cathodicparticles (i.e. intermetallic particles).

o Aluminium oxide is not stable in such environment, and aluminium around the particles will dissolve (alkaline pits).


INTERMETALLIC PHASES, cont.Ca

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CORROSION OF ALUMINIUMMicrogalvanic couplingo Most commercial alloys contain several types of intermetallic phases.

Corrosion on aluminium alloys is essentially a microgalvanic process between these phases and the matrix alloy.

o The phases often act as local cathodes because of their Fe content, the surrounding aluminium matrix undergoes localized attack. In addition the following phenomena may become important:• An active phase(s) may corrode preferentially.• A corroding phase may serve as a sacrificial anode and provide cathodic

protection to the surrounding material.• Due to the electrochemical reactions at the corroding sites and the cathodes,

the composition and pH of the electrolyte adjacent to the reaction sites may become different from the bulk electrolyte.

• Active components of the matrix alloy and the intermetallic phases may corrode selectively (dealloying), resulting in changed corrosion properties.


GALVANIC CORROSIONOne big threat with aluminium


Electrochemical potential (mV vs. Ag/AgCl)

Met

all/A

lloy


GALVANIC CORROSIONInsulate the materialsIn a corrosive environment (e.g. in marine environment) aluminium needs to be insulated from all other metals in connection to avoid galvanic corrosion of the aluminium material!


CORROSION OF ALUMINIUMStress corrosion cracking (SCC)• Occurs on some alloys due to a combination of corrosion and

static mechanical load • Crack growth along grain boundaries• 1xxx, 3xxx and 6xxx normally not exposed to SCC• 5xxx with Mg > 3% special exposed to SCC and 7xxx

TIG weld on 6061-T6 with SCC


FISH FARMINGShip hull in Aluminium with corrosion attacks

Corrosion attacks probably caused by current leakage/”stray current”.


CORROSION PREVENTIONCathodic protection in practice

Propeller

Conductor/wire

Sacrificial anode


ALUMINIUM IN SEAWATERPourbaix diagram for AA5086 in 3.5% NaCl at RT

K.Nisancioglu, “Corrosion and protection of aluminum alloys in seawater”. European Federation of Corrosion Publications, Vol. 50 (2007): p. 145

Pourbaix diagram for pure Al


CORROSION PREVENTIONCathodic protection of Al

o Potential in the passive region (not the immune region as for CS)

o Cathodic reaction on intermetallic particles

o Surface around the particle becomes alkaline oxide destabilized and particles are detached called cathodic pitting

o -1160 mVSCE is often used as a critical potential (most acceptable negative potential)

Cathodic protection


WHY SURFACE TREATMENT OF ALUMINIUM?In many applications, in very tough environments, aluminium works very well without any surface treatment


PURE ALUMINIUM ON A ROOFInstalled in 1897


SO – WHY SURFACE TREATMENT?

o For aesthetic and decorative reasonso Colour, gloss etc.

o To improve corrosion resistanceo To improve wear resistance and/or hardnesso Control reflectivityo Pre-treatment for joining

o Weldingo Brazingo Adhesive bonding

o Improved uptake of lubricationo Prepare surface for printingo Electrical insulation or contact


CORROSION COATINGS ON Al

o Anodizing Build a thicker oxide layer

o Organic coating Prevent contact between

humidity and the substratemetal

BARRIERLAYER

PORE DIAMETER

CELL SIZE

ALUMINIUM

CELL WALLTHICKNESS

UNTREATED DEGREASING ETCHING

LIGHT ANODISING2 - 10 µm

NORMAL ANODISING10 - 25 µm

HARD ANODISING25 - 200 µm

Electrostaticpowder coating

Tribostaticpowder coating

Powdercoating

Spraypainting

Electrostaticpainting

Electrophoreticspaying

Coilcoating

Wetpainting

Organiccoating


IMPORTANT DOCUMENT FROM 2002


HOW TO AVOID CORROSION ON ALUMINIUM?1. Select the right material in the actual environment.2. Apply the “best” surface treatment.

3. Design– avoid galvanic couplings– with acceptable stress level– avoid “dead legs”

4. Apply cathodic protection (submerged conditions)


ALUMINIUMProperties in marine environment

o Aluminium alloys have been used in marine environment for many years with good results– Atmospheric conditions– In seawater – e.g. boat hull

o Two alloys have been most widely used– Alloy AA5083 – AlMg alloy– Alloy AA6082 – AlMgSi alloy– These alloys are aften called “Seawater resistant alloys”

o Few problems with corrosion has been seen, except– Galvanic corrosion– Pitting corrosion (?)– Over-protection by the CP system– Stray current corrosion (on ship hulls)


TWO IMPORTANT STANDARDS


Thank you – any questions?

Documents

CORROSION CHALLENGES WITH THE USE OF ALUMINIUM IN … o Most commercial alloys contain several types of intermetallic phases . Corrosion on aluminium alloys is essentially a microgalvanic