corrosion

1Frosio course hand-out: Section 05 Corrosion

Frosio 05 Corrosion slide # 1

Corrosion


Production and degradation of steel

Plates, pipes,

profiles, etc.

Ene

rgy

Man

ufac

turing

Water /hum

idity

Oxygen

Raw material

Iron ore Rust

Reaction between the

material and

the surrounding

environment takes place

The presence of water / humidity

and Oxygen is a pre-requisite for

corrosion of steel



Definition of Corrosion

Corrosion is a chemical reaction between

a metal

and

its surrounding environment

under the formation of corrosion products


The corrosion process.A galvanic cell

Materials exposed to seawaterwill obtain differentelectrochemical potentials

If two differently chargedmetals are brought in contact acurrent will start to flowbetween them

Electrons will transport currentbetween the metals and ions inthe electrolyte

CP and corrosion: Corrosion_cell1



Pre-requisites for corrosion

A Cathode: The noble metal / alloy (or part of metal)

An Anode: The less noble metal / alloy

An electrical connection between the two metals, conducting electrical current (by

electrons)

An electrolyte: Conducting electrical current (by ions)

A galvanic cell consists of:


What happens at the anodeand the cathode?

At the anode:The metal is dissolved in the electrolyte

At the cathode:Usually, oxygen and water is absorbed, and alkaliesare produced (OH -ions)

(Electrons are involved, they go from the anode

to the cathode via the metallic conductor)



Electrolytes

Water or moisture conducts electricity by way of chargedspecies (ions). Positive and negative ions

Examples: Seawater, soil, acids, alkalis, concrete, humidwood, salt solutions

A sugar solution is not an electrolyte

The more ions in the electrolyte, the better theconductivity


The corrosion rate depends on theelectrolyte conductivity

Steel corrodes atdifferent speed in freshwater and seawater

Seawater conducts thecorrosion currentbetter than freshwater, and gives morecorrosion



Energy Metal / Alloy Potential Corrosion

(volts) *

Least energy

required

for refining

High energy

required

for refining

Gold

Silver

Titanium

Stainless steel (316, active)

Ni-Al- Bronze

Copper

Carbon steel

Aluminium (pure)

Zinc (anode alloy)

Aluminium (anode alloy)

Magnesium (anode alloy

+0,500

- 0,205

- 0,225

- 0,235

- 0,380

- 0,435

- 0,600

- 0,800

- 1,080

- 1,140

- 1,550

Least corrosive

Very corrosive

*) Potential in seawater measured versus a

Copper / Copper Sulphate reference electrode

Galvanic Series in Sea Water


Information from the galvanic series

a) The degree of general corrosion

b) Prediction of galvanic corrosion:

The more negative metal will corrode The more positive metal will be protected

The corrosion rate depends on the driving force

( i.e. the difference in potential between the

two metals)

Rule of thumb:Differences < 50 mV will notgive galvanic corrosion problems



Parameters influencing the corrosion speed.Atmospheric corrosion

Humidity

Temperature

Concentration of salts

Amount of air pollution,

including acid rain, soot and dust particles


Atmospheric corrosioncorrosion rate depends on humidity

Relative Humidity, %

Corrosion rate

0 20 40 60 80 100



Neutral

Acidic

Alkaline

pH-scale


The pH of the solutionwill affect the corrosion speed

Steel

Corrosion increases in acidic solutions

Strong alkaline solutions prevent corrosion

(pH 10 and higher)

Zinc and Aluminium

Slow corrosion in near neutral solutions

Heavy corrosion in acidic and alkaline solutions



Passivity

Some metals stop to corrode after a short time,because a passive film of corrosion productsprevents further corrosion

Examples: Titanium, Stainless steel, Aluminium

For other metals a corrosion inhibitor may beadded to the electrolyte in order to obtainpassivation

Example: Cooling water systems for motors


Passivity cont d

However, passive films may be destroyed (byaggressive species in the electrolyte), so corrosionwill proceed

Examples: dissolved gases (hydrogen sulphide,chlorine) or salts (sodium chloride)

Seawater is corrosive both because of its electricconductivity and its content of aggressive salts



Types of corrosion

1. Uniform corrosion

2. Galvanic corrosion

3. Selective (preferential / dealloying) corrosion

4. Pitting corrosion

5. Crevice corrosion

6. Micro biological corrosion (bacteria)

7. Corrosion fatigue

8. Stress corrosion cracking

9. Erosion corrosion

10. Cavitation

11. Stray current corrosion


General corrosion

An example: Freely corroding steelAn example: Freely corroding steel

Here, small anodic andHere, small anodic and cathodiccathodic spots arespots are

formed due to slight differences in salt level,formed due to slight differences in salt level,

oxygen content, steel microstructure etc.oxygen content, steel microstructure etc.

The anodic andThe anodic and cathodiccathodic spots change placespots change place

all the timeall the time

10

Frosio course hand-out: Section 05 Corrosion


Uniform corrosion

A steel surface consists of noble and less noble areas This can be looked upon as small galvanic cells The anodic parts will corrode

Rust

++

++

-

-

-

+

-

-+

-

++-+ +


CD 4911-0004CD 0026-001

General corrosion

General corrosion is uniform by nature

Still, deep pits or uneven areas are found

11



Uniform attack is a form of

electrochemical corrosion

that occurs with equal

intensity of the entire

surface of the metal. Iron

rusts when exposed to air

and water due to exposure

to air. Potentially very

risky, this type of corrosion

is very easy to predict and

is usually associated with

"common sense" when

making material decisions.

Uniform corrosion


Steel with mill scale

Mill scale is more noble than steel.

After exposure:

The mill scale cracks

Corrosion will develop on the steel

Mill scale Corrosion

Steel Steel

12



Rr, delvis med

gldeskall/rust viktig

fjerne gl.sk. fr maling.

Kan ikke brstes vekk.

Bedre med lett rust enn g.l.

sk.

Mill Scale on Pipes

Corrosion on the mill scale

Rust and mill scale must beremoved prior to paintapplication.

Mill scale can be removedby blast-cleaning


Welds may corrode rapidly if thecorrect weld material is not used

Steel

NOTE

The weld material (filler) must always

be more noble than the base material

Corrosion attack on weld

Filler is less noble than the steel

Cathode Anode Cathode

Electrolyte

13



Galvanic (bimetallic) corrosion

Two different metals are connected

Increased corrosion rate (higher corrosion current)

Corrosion rate determined by the difference ingalvanic potential

Corroding metal and corrosion rate can be foundfrom the Galvanic Series

The galvanic potential depends on the electrolytecomposition


Galvanic corrosion is an electrochemical action of two dissimilar metals in the

presence of an electrolyte and an electron conductive path. It occurs when

dissimilar metals are in contact.

Galvanic corrosion

14



Galvanic corrosion

Screw of wrong material


Galvanic corrosion

Important parameters: Anode - Cathode area ratio. Electrolyte resistivity (Conductivity) Difference in galvanic potential

of the metals

Pre-requisites: Water (Electrolyte) Two metals with different potentials Electrical contacts between the metals

e -

Fe ++

Cathode

Stainless steel

Anode

Steel

15



Galvanic corrosion(or Bimetallic corrosion)

Unfavourable area ratio

Small anode will corrode rapidly

Anode Cathode Anode

Electrolyte with low conductivity

Attack close to cathode

Anode Cathode Anode

Cathode Anode Cathode

The severity of the attack depends on:

Conductivity of the electrolyte

Anode - Cathode area ratio

Electrolyte with good conductivity

Wide corrosion attack


This rainwater guttering is made of aluminium and wouldnormally resist corrosion well. Someone tied a copper aerial

wire around it, and the localised bimetallic cell led to aknife-cut effect.

Galvanic corrosion, aluminium - copper

16



Pitting corrosion

Happens both on steel and on metals witha protective oxide film, e.g. stainlesssteels and aluminium

Weak points in the oxide film are attacked

Initiated by aggressive ions (e.g.chlorides)

Can perforate metal sheet in a short timewithout notice


Pitting corrosion

Pitting corrosion is a localised attack on a materialnormally protected by a passive film

The passive film may be destroyed mechanically orby aggressive ions in an electrolyte

Severe corrosion may take place beneath the passive layer

Seen from above Cross section

Stainless steel

Passive layerPitting corrosion

17



In pitting corrosion the metal at the top of the pit has access to the

oxygen in the air and becomes the cathode. At the bottom of the pit

oxygen is depleted and the metal becomes the anode. The deeper the

pit is the less the oxygen available at the bottom and the corrosion rate

increases. Figure 2 shows the mechanism of pitting corrosion.

Pitting corrosion


Pitting of stainless steel: Stainless steel can corrode if conditions

are unfavourable. This is the shaft of a central heating pump for a

block of flats. Leakage at a flange led to permanently damp

conditions and evaporation produced concentration of dissolved salts

in the water. This produced pitting corrosion. The material is a

special high strength low-nickel steel (martensitic).

Pittingcorrosion

18



Stainless steel: Omega pit

Cross section through pits on two pipes



Cross section through pits on pipe (10x magnification)

19




Cross-section through pit showing micro structure (50x magn.)


This flange was in contact with a sealing gasket, and in the crevice between the

two conditions developed which encouraged pitting corrosion. Titanium

normally has a very strong protective oxide film, but the metal is very reactive

once the film is broken. Crevices frequently give rise to different conditions of

temperature, electrolyte concentration and pH and thus encourage localised

corrosion cells.

Pitting oftitanium

crevicecorrosion

20



Pitting corrosion is

localized corrosion that

occurs at microscopic

defects on a metal

surface. The pits are often

found underneath surface

deposits caused by

corrosion product

accumulation.

Pitting in cast iron


Pittings inside a pipe.Power plant

Severe corrosionhas developed

Such damages willincrease the frictionand thereby reducethe power of thewater fall.

Exchanging thepipes may be theresult

Cd-4930-88

21



CD 0026-004

Pitting corrosion on an aluminium pipe

Corrosion startedfrom the inside

Aggressive ionscaused pitting

corrosion to penetrate

the pipe wall

Pressure inside thepipe has blown away

the last thin bridge

over the pits

Photo taken from theoutside


Crevice Corrosion

Crevice corrosion is a consequence of concentrationdifferences of ions or dissolved gases in an electrolyticsolution.

A solution may become trapped between a pipe and aflange.

The stagnant liquid in the crevice eventually have alowered dissolved oxygen concentration and crevicecorrosion take over and destroy the flange.

In the absence of oxygen, the metal and/or its passivelayer begin to oxidize.

To prevent crevice corrosion, one should use weldsrather than rivets or bolted joints whenever possible.

Remove accumulated deposits frequently and designcontainment vessels to avoid stagnant areas as muchas possible.

22



Crevice corrosion

Due to differences in electrolytecomposition

Areas with stagnant water with lack ofoxygen will be anodic

Aerated water areas (i.e. with dissolvedoxygen) will be cathodic

Occurs often at stainless steel flanges,under rivet and bolt heads, under mud anddirt etc


Crevice corrosion occurs under paintspillage or plates

Stainless SteelStainless Steel

PaintPlate

Corroded areas

Ingress of seawater

Seawater

Crevice corrosion occurs in narrow gaps where the oxygen

concentration is lower than on the freely exposed part of the

aluminium, affecting the oxide layer and forming concentration-cells

23



Crevice corrosion


A crevice is created whenevertwo objects are broughttogether. Unless they areperfectly flat a crevice ispresent and oxygen cannoteasily enter the gap but isplentiful outside. Corrosionstarts in the crevice becauseof differential aeration.

Crevice corrosion

24



Cavitation and erosion corrosion

Combination of mechanical wear and corrosion

Cavitation:

On ships propellers and rudder, caused byrepeated impact on the same spot of air bubbles orvacuum bubbles

Erosion:

Inside copper pipes, typically where the fluidvelocity is high and combined with turbulence


Erosion corrosion

Erosion-corrosion arises from a combination of chemicalattack and the physical abrasion as a consequence of thefluid motion. Virtually all alloy or metals are susceptibleto some type of erosion-corrosion as this type ofcorrosion is very dependent on the fluid. Materials thatrely on a passive layer are especially sensitive toerosion-corrosion. Once the passive layer has beenremoved, the bare metal surface is exposed to thecorrosive material. If the passive layer cannot beregenerated quickly enough, significant damage can beseen. Fluids that contain suspended solids are oftentimes responsible for erosion-corrosion. The best way tolimit erosion-corrosion is to design systems that willmaintain a low fluid velocity and to minimize sudden linesize changes and elbows.

25



Systems with flowing water must be

designed to avoid turbulent flow

Sharp corners and intrusions creates turbulence,

leading to erosion corrosion


Erosion corrosion caused by uneven joint

Direction of flow

Severe corrosion attack on pipe wall caused by turbulence

Uneven joint

26



The photo above shows erosion-corrosion of a copper-nickel tube in a

seawater surface. An imperfection on the tube surface probably cause

an eddy current which provided a perfect location for erosion-corrosion.

Erosioncorrosion


Selective Corrosion(also called preferential corrosion or de-alloying)

Explanation:

An ignoble alloying element corrodes away from the restof the alloy. The shape or dimension of the object maynot necessarily change, but the colour and the propertieswill be altered.

Examples:

De-zincification of brass

Graphitization of cast iron

Appearance:

Brass: affected areas change from yellow to red colour(white zinc corrodes away, leaving behind red copper

Cast iron: affected areas change from grey to black

27



When one element or constituent of a metal is selectively corroded

out of a material it is referred to as selective leaching. The most

common example is the dezincification of brass. On the right,

nickel has be corroded out of a copper-nickel alloy exposed to

stagnant seawater. After leaching has occurred, the mechanical

properties of the metal are obviously impaired and some metal will

begin to crack.

Selective corrosion


Dezincification: This component was originally solid brass. The end

section shows that only a small thickness of yellow brass remains in the

centre, the outer zones consisting of red spongy copper. This damage

occurred in sea water.

Selectivecorrosion

28



Dealloying is a rare form of corrosion found in copper alloys, gray cast

iron, and some other alloys. Dealloying occurs when the alloy loses the

active component of the metal and retains the more corrosion resistant

component in a porous "sponge" on the metal surface.

Selective corrosion

Heat exchanger: colour

change from yellow

brass to red copper

when zinc corrodes away

from the brass alloy


Graphitic corrosion of cast iron: An unusual type of corrosion is

illustrated here. The pipe is of grey cast iron, which contains flakes of

graphite in the metal structure. These flakes bind together the corrosion

products which retain the original shape of the metal and make the

damage difficult to detect until it reaches an advanced stage. The inner

dark ring is a shadow, but black areas of graphitic corrosion can be seen

eating into the pipe wall from the exterior circumference.

Selectivecorrosion

29



Microbiological (Bacteria) corrosion

Anaerobic bacteria

Avoids oxygen

Sulphur Reducing Bacteria (SRB)

Aerobic bacteria Need oxygen

Sulphur Oxidizing Bacteria (SOB)

Will produce aggressive substances / ions, such assulphuric acid, which will accelerate corrosionand/or cause damages to metals in their own right

Can be found more or less everywhere, but maythrive in sewer, under mud (in WBT), bottom of oiltanks, in connection with slime, etc.


Microbial corrosion (also called

microbiologically -influenced corrosion

or MIC) is corrosion that is caused by

the presence and activities of microbes.

This corrosion can take many forms and

can be controlled by biocides or by

conventional corrosion control methods.

There exist many species of bacteria living in moist

environments that release acidic waste products or that can strip

out elemental components of a metal. If these bacteria grow on

pipe walls and metal surfaces they will cause corrosion. They

occur in both oxygenated (aerobic) and oxygen free (anaerobic)

conditions.

Bacterial Corrosion

30



Stress corrosion cracking

Causes premature cracking of metals in certainenvironments

Combined action of corrosion and surface tensilestress (applied or residual)

Copper: In ammonia solutions

Stainless steel: In seawater

Carbon steel: In nitrate solutions


Stress Corrosion Cracking

Stress corrosion can result from the combination of anapplied tensile stress and a corrosive environment.

Metal under tensile stresses can corrode at higher ratesthan normally expected.

The stressed areas have changed electrical potentials tothe neighbouring metal and are also more likely todevelop microscopic surface cracks.

Once the stress cracks begin, they easily propagatethroughout the material, which in turn allows additionalcorrosion and cracking to take place.

The tensile stress is often the result of expansions andcontractions that are caused by violent temperaturechanges or thermal cycles.

The best defence against stress corrosion is to limit themagnitude and/or frequency of the tensile stress.

31



Stress corrosion cracking (SCC) is caused by the simultaneous effects of

tensile stress and a specific corrosive environment. Stresses may be due

to applied loads, residual stresses from the manufacturing process, or a

combination of both.



Corrosion fatigue is a special case of stress corrosion caused by the

combined effects of cyclic stress and corrosion. No metal is immune from

some reduction of its resistance to cyclic stressing if the metal is in acorrosive environment.

Corrosionfatigue

32



Duplex Stainless Steels

A most significant contribution to the fight against corrosion offshore has been made by

duplex stainless steels. These have often been adopted on offshore structures in preference

to carbon steel or other stainless steels. The value of the duplex stainless steel is that it

combines the basic toughness of the more common austenitic stainless steels with the higher

strength and improved corrosion resistance of ferritic steels. The optimum chemical

composition of these steels provides a high level of corrosion resistance in chloride media

together with high mechanical strength and ductility. Other benefits include the ability of some

duplex stainless steels to be used at quite low sub-zero temperatures and be able to resist

stress corrosion cracking.

Metallographic structure

Duplex stainless steel is a stainless steel that

is composed of two types of compounds,

austenite (white part) and ferrite (gray part).

A significant feature of duplex stainless steel

is that its pitting and crevice corrosion

resistance is greatly superior to that of

standard austenitic alloys.


Occuring along grain boundaries for some alloys, intergranular corrosion

can be a real danger in the right environment. The heating of some

materials causes chromium carbide to form from the chromium and the

carbon in the metals. This leaves a chromium deficient boundary just shy

of the where the metal was heated for welding. To avoid this problem, the

material can be subjected to high temperatures to re-dissolve the

chromium carbide particles. Low carbon materials can also be used to

minimize the formation of chromium carbide. Finally, the material can be

alloyed with another material such as Titanium which forms carbides more

readily so that the chromium remains in place.

Intergranular corrosion

A piece of stainless steel (especially

suspectible to intergranular corrosion) has seen

severe corrosion just an inch from a weld.

33



Fire hydrant: Intergranular corrosion is an attack on or adjacent to the

grain boundaries of a metal or alloy.

Intergranular corrosion


Hydrogen embrittlement is a problem with high-strength steels, titanium,

and some other metals. Control is by eliminating hydrogen from the

environment or by the use of resistant alloys.

Hydrogen embrittlement

34



CORROSION OF STEEL



(volts) *

Least energy

required

for refining

High energy

required

for refining

Gold

Silver

Titanium


Ni-Al- Bronze

Copper

Carbon steel

Aluminium (pure)

Zinc (anode alloy)



+0,500

- 0,205

- 0,225

- 0,235

- 0,380

- 0,435

- 0,600

- 0,800

- 1,080

- 1,140

- 1,550

Least corrosive

Very corrosive




35



Most frequently occurringtypes of corrosion

On carbon steel

Uniform corrosion

Uneven corrosion (deep pits)

Galvanic corrosion



CORROSION OF ALUMINIUM

36




(volts) *

Least energy

required

for refining

High energy

required

for refining

Gold

Silver

Titanium


Ni-Al- Bronze

Copper

Carbon steel

Aluminium (pure)

Zinc (anode alloy)



+0,500

- 0,205

- 0,225

- 0,235

- 0,380

- 0,435

- 0,600

- 0,800

- 1,080

- 1,140

- 1,550

Least corrosive

Very corrosive





Does Aluminium corrode?

The corrosion resistance of aluminium varies widelydepending on alloy, environment, design and protectivemeasures taken

According to its electro-chemical potential, Aluminiumshould corrode more willingly than steel

A clean aluminium surface is reactive and will reactspontaneously with water or air and form aluminiumoxide

This oxide is very stable and has in addition a very goodadhesion to the metal surface and thus protectsaluminium from corrosion or further oxidation

This means that aluminium has good corrosionresistance in environments where the oxide layer isstable

37



Does Aluminium corrode (2)?

The oxide layer will deteriorate in environments with highor low pH or where aggressive ions are present

Below a pH of 4 (acidic conditions) and above a pH of 8.5(alkaline conditions), there will normally be an increase inthe corrosion rate of aluminium, but this also depends onwhich ions that are present in the environment

Aggressive ions will break down the oxide layer locally andstart local corrosion attacks

Among the aggressive ions, chloride (Cl-) is the one with

the most practical importance, because it is present inlarge amounts in both sea water, road salts and somesoils, and in lower concentrations in other natural sources


Most frequently occurring types ofcorrosion on Aluminium

Pitting corrosion Galvanic corrosion

Because of the stability of the oxide layer, general corrosion willrarely be a problem on aluminium, except in very alkaline oracidic environments

Aluminium may however experience local attacks in connectionwith formation of small anodic areas as a result of a localbreakdown of the oxide layer

Pitting corrosion starts from a weak point in the oxide layer andcan penetrate several mm during a short period if the conditionsare unfavourable

38



Bi-metallic (galvanic) corrosion

Since aluminium is less noble than most commonly usedconstruction materials, galvanic corrosion can be aserious corrosion form with aluminium

The protective effect from the oxide layer can beseriously deteriorated by the coupling to a noblermaterial. This is particularly dangerous in atmospheresor waters with high concentration of chlorides or otheraggressive species

The under-water hull of a work-boat made fromaluminium was poorly painted with a primer before anantifouling containing traces of metallic copper wasapplied. The copper / aluminium combination resulted inbi-metallic corrosion and the boat started leaking after afew weeks in seawater

Direct contact between aluminium and steel will also leadto bi-metallic corrosion, e.g. welding, bolts, screws, etc.


Paint system on Aluminium

1. Degreasing

2. Abrading: sweep-blasting (use non-metallic grit)or light grinding (abrasive pads, emery paper,etc.)

3. Penguard HB as tie-coat

4. Eventually a build-coat

5. Suitable topcoat

39



CORROSION OFSTAINLESS STEEL



(volts) *

Least energy

required

for refining

High energy

required

for refining

Gold

Silver

Titanium


Ni-Al- Bronze

Copper

Carbon steel

Aluminium (pure)

Zinc (anode alloy)



+0,500

- 0,205

- 0,225

- 0,235

- 0,380

- 0,435

- 0,600

- 0,800

- 1,080

- 1,140

- 1,550

Least corrosive

Very corrosive

*) Potential in seawater measured against a



40



Stainless Steel

Stainless steel is a group of alloys consistingof iron mixed with chromium (typically 12 18%) and often smaller amounts of othermetals

The basic resistance of stainless steel occursbecause of its ability to form a protectivecoating (chrome-oxide) on the metal surface.This coating is a passive film which isresistant to further oxidation or rusting.

The formation of this film is instantaneous inan oxidizing atmosphere such as air, water ormany other fluids that contain oxygen


Stainless Steel

Unlike aluminium, this passive film isinvisible in stainless steel. It is due to thecombining of oxygen with the chrome inthe stainless to form chrome oxide

Halogen salts easily penetrate this passivefilm and will allow corrosive arrack tooccur. Halogens are fluorine, chlorine,bromine, iodine and astatine.

Chlorides are one of the most commonelements in nature and are soluble, activeions, forming good electrolytes, formingthe best conditions for corrosion

41



Most frequently occurringtypes of corrosion on Stainless Steel

Galvanic corrosion

Pitting corrosion

Crevice corrosion


Paint system on Stainless Steel

Basically the same as for Aluminium:

1. Degreasing

2. Abrading: sweep-blasting (use non-metallic grit)

or light grinding (abrasive pads, emery paper,

etc.)



5. Suitable topcoat

42



CORROSION OF COPPER &ITS ALLOYS



(volts) *

Least energy

required

for refining

High energy

required

for refining

Gold

Silver

Titanium


Ni-Al- Bronze

Copper

Carbon steel

Aluminium (pure)

Zinc (anode alloy)



+0,500

- 0,205

- 0,225

- 0,235

- 0,380

- 0,435

- 0,600

- 0,800

- 1,080

- 1,140

- 1,550

Least corrosive

Very corrosive




43



Copper & its alloys

Copper Copper is commonly used for plumbing, heat

exchangers, process equipment, roofing, etc. (andelectrical wires)

Bronze Initially a copper alloy with tin, but the term is today

also used for copper alloyed with aluminium(aluminium bronze), nickel (nickel-aluminiumbronze), silicon (silicon bronze), etc., and maycontain several additional alloying elements to imbuespecific properties. They all have a light golden colourwith high corrosion resistance and excellentmechanical properties

Brass Group of alloys between copper and zinc, where the

zinc content may vary from about 5 to 45%


Corrosion

The resistance of all grades of copper toatmospheric corrosion is good, hence theirwide usage for roofing and for contact withmost waters

The metal develops adherent protectivecoatings, initially of oxide, butsubsequently thickening to give a familiargreen patina on roofs and the darkbrownish colour of bronze statues.

44



Patina on copper- The surface film whichforms on copper in theatmosphere contains basicsalts and is quiteprotective. The film has a

pleasant green colour andis used for architecturalapplications.

- The green patina takessome time to develop, anda freshly repaired zone atthe base of the spire maybe seen which contrastswith the rest of the roof.


Most frequently occurringtypes of corrosion on Copper

Copper based alloys Erosion corrosion Selective (de-alloying) corrosion

Copper and its alloys form a protective coating of oxidesand other salts in atmospheric exposure and is not

particularly troubled by further corrosion under normal

conditions

Copper pipes may suffer from erosion caused by turbulencein the flow inside the pipe

Brass may suffer from de-zincification

45



Paint system on Copper & Alloys

Basically the same as for Aluminium:

1. Degreasing

2. Abrading: sweep-blasting (use non-metallic grit)or light grinding (abrasive pads, emery paper,etc.)



5. Suitable topcoat


CORROSION OF ZINC(GALVANISED OBJECTS)

46



Zinc surface (galvanised)

A fresh zinc surface is quite reactive when exposed tothe atmosphere, a thin film of corrosion productsdevelops rapidly, greatly reducing the rate of furthercorrosion of the zinc

When exposed outdoor for longer periods, thegalvanising (zinc coating) will corrode and form zincoxide, seen as a white stain which become powderywhen it progresses and sometimes referred to as whiterust

Galvanised steel may be left un-treated, but may alsobe painted in order to protect the zinc coating (prolongits life). This may also provide a more suitable /pleasing appearance

Corrosion rate of zinc in urban and industrialatmosphere may be 1 - 10 micr./year, while in rural andmarine atmosphere only 0.5 2 micr./year


Galvanised: Pre-treatment

Weathered galvanising: Clean the substrate well, using a suitable detergent

Remove any loose white rust (zinc oxide) with water

jetting or use a stiff brush and rinse with water

Apply the paint

New galvanising: Always degrease the substrate

Lightly abrade the substrate, using sandpaper, abrasive

pads or a light blast-sweeping (non-metallic grit!)

Remove any dust from the abrading

Apply the paint

47



Etching: Not recommended by Jotun

It has been common practise to etch galvanisedsurfaces prior to applying the paint in order to removethe oxide layer and thus improve the adhesion. Typicalproducts used in this respect are:

Mordant solution (acid, e.g. phosphoric acid)

Wash Primer (also called Etch Primer), a vinyl-basedtranslucent primer containing phosphoric acid, that shouldbe applied in a very low DFT, typically 5 10 microns

DANGERS:

If any remaining acid is not neutralised before over-coating, it will interfere with the curing of two-packpaints

If the Wash Primer is applied in too high DFT, onemay experience a reduction in the adhesion insteadof an improvement

In submerged areas, use of etching / wash primermay lead to osmotic blistering


Painting zinc (galvanised) substrates

Zinc is an alkaline metal, meaning that in wet or moistconditions an alkaline environment is formed.

It is important that when over-coating zinc, an alkaline-resistant paint is used, particularly if the object isexposed outdoors or in a humid environment (wetrooms, etc.)

Alkyd paints are not alkaline resistant. The binder willbreak down (saponification) and the film will becomebrittle, start to crack and flaking will be experienced

The best adhesion will be obtained by using a pureepoxy tie coat, such as Penguard Special or PenguardHB, applied directly to the cleaned / degreased andabraded galvanised surface

48



CORROSION OF CONCRETE


Concrete is an Alkaline Material

The pH of new / fresh Concrete is in therange of 12 14(pH1 = strong acid; pH7 = neutral; pH14 = strong caustic /alkaline)

Steel will not corrode in an environmentwith such a high pH, which is why soundconcrete protects its own reinforcementbars so well

Concrete must not be exposed to acids orcomponents which may react to formacidic solutions, since this will neutralisethe alkalinity and react with the concrete,causing it to disintegrate

49



What do we mean byconcrete corrosion?

Concrete can corrode (deteriorate) like anyother type of material

Mechanism: A change in the chemistry.

The pH value drop from pH 12 to 8.

This makes the rebar corrode, resulting in pieces looseningfrom the remaining concrete.

For submerged conditions in salt water: The aggressive chlorides will increase the corrosion of the

rebars


Carbonizing

1. Carbon dioxide (CO2) reacts with Calcium Hydroxide inthe concrete (Ca(OH)2, very alkaline)

2. The reaction product is Calcium Carbonate (CaCO3,chalk)

3. The pH will become lower, increasing the danger forcorrosion to start

4. Sulphur dioxide (SO2) from the air will react and convertCalcium Carbonate to gypsum, which is water soluble.This encourages further carbonising

5. With the pH dropping from alkaline towards neutral andintroduction of aggressive ions, corrosion ofreinforcement steel-bars gains momentum

50



How rebars are damaged

COCO22

pH loweredThe pH will become lower,

leading to an increased danger of

corrosion

Corrosion startsSulphur dioxide (SO2) from the air will

react and convert Calcium Carbonate to

gypsum, which is water soluble. This

increases the possibilities for further

Carbonising.

Concrete is forced awayRebars starts corroding. When steel

converts to rust its volume increases and

the weakened concrete is forced away

COCO22

COCO22

SOSO22

Carbonisation startsCarbon dioxide (CO2) reacts with

Calcium Hydroxide in the concrete,

forming Calcium Carbonate


Consequences of rebar corrosion

The corrosion productsare very voluminous

When steel corrodes, itsvolume increasesbetween 4 to 6 times

An enormous pressurewill be exerted to thesurroundings.

The concrete will crack,loosen and fall off.

Rust

Steel

VOLUME

INCREASE4 6 TIMES

51



Concrete is a widely-used structural material that is frequently reinforced with carbon steel

reinforcing rods, post-tensioning cable or pre-stressing wires. The steel is necessary to

maintain the strength of the structure, but it is subject to corrosion.


Rebars & CorrosionCorroding rebars cause

chunks of concrete to fall off

52



Concrete repairs

Chipping damaged concrete Remove carbonated concrete


Concrete repair

Shaping the repair area Preparing the rebar

53



Concrete repair

Corrosion

protection of rebar

Filling repair area Cover and

carbonation-inhibition


Why Paint Concrete?

Protect structures against atmosphericconditions (CO2 / SO2 / pollution),otherwise leading to concrete corrosion

Protect concrete against exposure tochemicals, otherwise attack may takeplace while immersed or by spillage

Protect concrete subjected to mechanicalstresses, such as traffic, high loads, impactand abrasion

Waterproofing

Cosmetic

54



Cathodic protection


The principle of cathodic protection

In a corrosion cell steel willcorrode when coupled to amore noble material

The noble material is replacedwith a material being lessnoble than steel: A sacrificialanode

The direction of the currentwill change

The steel will be protectedwhile the anode corrodes

Cp and corrosion \ CP_protection cell1

55



Cathodic protection can be used toback up the paint system

No paint system is100 % perfect

Weak spots andholidays will exist

For submergedstructures a CPsystem will protectsuch areas

A calcareous depositprecipitates on thesubstrate and reducesthe corrosion rate

Cp and corrosion \ Paint_and_cp1


Example of sacrificial anodes

Many different sizes and shapes are available

Anodes can be welded or clamped on to the structure.

CD-4930-89

56



Sacrificial anodes on ships hullRudder area is important to protect

Anodes protectareas sufferingfrom mechanicaldamages

Alloy:Zinc/Aluminium

A CP system willonly work forsubmergedconditions

Sacrificial anodesincrease thefriction

CD-4930-90


Bracelet anodes can be installed onpipelines and platform legs

The anodes areclamped on to theconstruction

Welding to theconstructionensures goodelectrical contact

CD-4930-99

57



Cathodic ProtectionSteel protected by a sacrificial anode


Impressed Current Cathodic Protectionsystem (ICCP)

58



Impressed current anode mounted ona ships hull

Protective current and polarisation level controlledvia a rectifier (onboard the ship / structure)

Will give lower friction and lower fuelconsumption than sacrificial anodes

CD-4930-92


ICCP system PT/TI anode

Permanent electrodes

may be made from

platinum, graphite or

other suitable materials

59



Electrolytic corrosion & fouling protection

Iron or aluminium anode to help corrosion protection

Copper anode to prevent fouling inside the sea-chests and pipes

Will release metal ions which travel with the water insidethe pipes, e.g. in cooling system, fire fighting system, etc.

CD-4930-96


Objects protected by CP:

Ships (hull and w.b.t.)

Offshore platforms and rigs

Sub-sea installations

Harbour facilities

Buried tanks & foundations

An electrolyte (water or soil)must connect the anode and the cathode

60



CORROSION PROTECTION

WITH COATINGS


Corrosion protection by paints:The three principles

Barrier effect.Barrier effect.Builds a barrier that prevents seawater or othercorrosive agents from coming into contact with thesubstrate.Example: Epoxy paints.

Inhibitor (Inhibitor (passivatingpassivating) effect) effectThe paint passivates the surface of the steel.Example: Paints incorporating rust preventing pigments. (Redlead, Chromates and Phosphates.

CathodicCathodic effect.effect.The paint itself acts as a sacrificial anode (Galvaniceffect).Example: Zinc-rich paints.

61



Bi-metallic:

To reduce possible galvanic corrosiondamage: Paint the noble metal (cathode)

If you paint the ignoble metal,concentrated anodic attacks may occurwhere paint defects occur

When you paint the noble metal, onlygeneral corrosion will occur on the ignoblemetal


Most paints protect the steel by forming a barrier tothe surrounding environment

A barrier willincrease theresistance in thegalvanic circle

A thick barrierwill give betterprotection thana thin

CP and corrosion: Cell_paintprotection1

62



Barrier Effect

An imperviouscoating serves as aninert barrier toprotect the substrate


Inhibitor Effect

In inhibitive coatings,moisture may penetrateto reach the inhibitiveprimer where thereactive pigments areactivated, which in turnpassivates the metalsubstrate at thecoating/metal interface

Examples: Red Lead Primer Zinc Chromate Primer Zinc Phosphate Primer

63



Galvanic Effect

A zinc-rich primer reacts to protect thesteel substrate when the topcoat is damaged.


What film-thickness shouldcorrosion protecting paint systems have ?

Deciding factor is the environment:

In a dry inland atmosphere and indoor 120 microns willbe sufficient. In an aggressive industrial environmentand along the coast and at sea, 250 - 300 microns willbe necessary.

The number of coats is also of importance, not only thefilm thickness

100 microns in 2 coats (50 + 50 microns) are givingbetter protection than 100 microns in 1 coat. Specialcoatings may be applied in thick coats.

64



STRUCTURAL DESIGN


Apart from using paint and CP:How to protect against corrosion ?

Good design

Use the right materials (avoid galvanic corrosion)

Avoid corrosion traps

Keep water and moisture away by good design

Improved accessibility - maintenance

Proper materials selection

Insulate between dissimilar materials

Change the surrounding environment

Remove water / humidity

Apply metallic coatings

Use corrosion inhibitors (closed systems)

Good maintenance routines

Corrosion protection can be achieved in many waysCorrosion protection can be achieved in many ways

65



Avoid stagnant water at

bottoms of tanks and containers

Unfortunate Better Best solution

x

x

x


Example: Avoid designs where dust,

moisture and water collect

no good better best

Entrapped water

66



Access


Isolate between different metals

67



When evaluating a structure forvulnerability to corrosion, check for:

Sharp edges & corners

Rough welding seams / Blow holes / Weld spatter

Bi-metallic situations

Drainage / Stagnant water / Accumulation of water

Access for paint application and maintenance work:Notch radius / Stitch welds / Crevices / Narrow gaps /Design of structure / etc.

Flow: Turbulence / Cavitation / Crevices

Exposure: In-/ Out-doors / Aggressive ions / Chemicals

Environment: Temperature / Humidity / Stress (appliedor residual or Cyclic)


Stainless steel

Copper alloys

Cracks in the metal due to

stress in the metal (Seawater,

caustic soda, ammonia)

Stress

corrosion

cracking

Brass, cast ironLocal or uniform attack on

brass or cast iron in seawaterDe-alloying

Copper alloys

Holes, grooves, rough surface

in the metal by mech. Means or

due to high velocity.

Turbulence

AllLocal attack. Small/large and

deep holes in the metalPitting

Stainless Steel

Aluminium

Between two metals. The metal

is locally attacked under

deposits

Crevice

All

On the weakest of two metals.

Rate of attack depends on area

ratio of cathode and anode.

Galvanic

AllEven attack all over the surfaceUniform

MetalAppearanceCorrosion

Attack

Common corrosion types at a glance

corrosion

Documents

corrosion slide

corrosion process

protectedthe corrosion

positive metal

negative metal

noble metal alloy

seawater seawater

cell13frosio course