Corrosion -Corrosion -Back to nature……Back to nature……

Silver turns black, copper green –and steel gets brown

Why on earth does everythingstart to corrode?

Corrosion costs:3-4% of western countries GNB

It has been stipulatedthat the costscan be reducedapprox. 15%by better know-ledge and skillof corrosion protection

Rust is corrosion of ironCorrosion (latin ”corrodere”)means – to tear away – due to chemical exposureCorrosion is tearing or destruc-tion of materials caused byreaction with their surroundingsConcrete og plastic materials are also corroding

Corrosion speed and humidity

Corrosion speed

Relative humidity Rh%

10080604020

Iron corrodeswhen exposedto air and water.

Most metalscorrode much faster in humid environment.

In contact withsea water thecorrosion speedis even faster.

IRON + OXYGEN + WATER = RUSTrust is iron oxide/iron hydroxide

All metals can corrode, evenfine metals likegold can dissolve in a mixture ofstrong acids

Fine metals like gold and silver are found naturally pure

Silver from Kongsberg mines Norway

Most metals do not appear in pure condition, however they are found as ores reacted with oxygen, carbon, sulphur aso.oxides, sulphides, sulphates, carbonates or chlorides.

Baviaans Wilderness South Africa:

iron oxide pigments

Why are so many constructionmaterials around us brokendown, and how does it happen?

Most metals areproduced by addinghuge amounts ofenergy to the rawmaterial – the ore

By smelting of iron ore, liquid iron is extractedfrom the slag. The iron solidifies, but getsprone to corrode.

During milling, the liquid steelReact with oxygen and a thinscale is formed on the surfacecalled ”millscale”.This is brittle andwill only protectthe steel as longas kept intact.

If steel is left exposed outdoors, the mill scale will crack and peeloff, and the steel starts to corrode.Rust has a chemical composition similar to that of iron ore. We are ”back to nature” – whatstartet out with ore has endedas rust. The iron is back to itsoriginal and stable, natural statewith a lower energy

Material energy levels during processing

Reaction between materials and surroundings (corrosion)

Water/humidity is neededfor steel to corrode

Adding energyduring manufactureand processing

RustIron ore

Corrosion theoryFor a metal to corrode, the surface must have presence of an electrical conductive liquid (electrolyte) and oxygen.

Metals are sending out electrical conductive atoms (iones) when in contact with water. With some metals this tendensy is weak, for example platinum og gold. Such metals are called fine metals.

Other metalls send out lots of ioner in contact with water, for example zinc and aluminium. Such metals are called base metals.

Corrosion theoryIf two different metals are placed in a conductive liquid (electrolyte), both will corrode at a given speed. If the metals are put in contact with each other, the corrosion speed will increase for the less noble of the two.

2 years 5 years 10 years

SteelAluminiumCopperZinc

51,1 32,8 20,7 0,48 0,76 0,35 1,8 1,1 0,71 3,6 2,6 1,7

Corrosion speed in µm/year for different metals after 2, 5 and 10 years exposure in marine athmosferic conditions

CORROSION CELL

AnodeZinc

CathodeSteel Zn = Zn2+ + 2e-

O2

O2 + H2O + 4e- = 4OH -

Torch battery

Zinc shield”anode”

ElectrolyteAmmonium chlorideZinc chloride

Carbon stick”cathode”

Corrosion theoryThe corrosion resistance of a metal is a way of rating the metals ability to resist corrosion under specific conditions.

All metals or alloys have a certain amount of energy load that can be mesured in volts, potential measurements.

We can rank metals and alloys according to their corrosions resistance.

Corrosion potensial in sea water

GraphitePlatinumTitanium....SteelAluminiumZincMagnesium

Precious metals

”Simple” metals

• sacrifice a metalZinc coatings can be used as ”anode”

Zinc anodes

Different types of corrosion:

• simple (even) corrosion• galvanic corrosion• selective corrosion• crevice corrosion• tension corrosion• pitting corrosion• corrosion fatigue• turbulence corrosion

Pit corrosionCarbon steel

If what we call ”rust” is assosiatedwith iron, what does corrosionof other metals look like?

• Copper turns green, - verdigris• Silver turns black Table silver must be polished

• Brass is susceptible to selective corrosion

Brass is an alloy of copper and zinc. When corroding, zinc will dissolve and what remains finally is a porous matrix of copper.

Some metals corrode in a waythat their corrosion productsprovide good protection againstfurther attack. A good example is

verdigris.

A thin copper plate can remain un- protected outdoorsin more than100 years withoutbeing damaged, like the Statue of Liberty.

Acid climate, with low pH, causeincreased corrosion

• normal rain water has an acidity of pH 5,4• acid rain can give pH- values below 4• not only fish suffers from acid rain, corrosion of materials is speeded up as well

How can corrosion be prevented?

• by painting barrier against air and water use the principles of galvanic corrosion