Corrosion - Back to nature……

Silver turns black, copper green – and steel gets brown Why on earth does everything start to corrode?

Corrosion costs: 3-4% of western countries GNB It has been stipulated that the costs can be reduced approx. 15% by better know- ledge and skill of corrosion protection

Rust is corrosion of iron Corrosion (latin ”corrodere”) means – to tear away – due to chemical exposure Corrosion is tearing or destruc- tion of materials caused by reaction with their surroundings Concrete og plastic materials are also corroding

Corrosion speed and humidity Corrosion speed Relative humidity Rh%

Iron corrodes when exposed to air and water. Most metals corrode much faster in humid environment. In contact with sea water the corrosion speed is even faster.

IRON + OXYGEN + WATER = RUST rust is iron oxide/iron hydroxide All metals can corrode, even fine metals like gold can dissolve in a mixture of strong 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 construction materials around us broken down, and how does it happen? Most metals are produced by adding huge amounts of energy to the raw material – the ore

By smelting of iron ore, liquid iron is extracted from the slag. The iron solidifies, but gets prone to corrode. During milling, the liquid steel React with oxygen and a thin scale is formed on the surface called ”millscale”. This is brittle and will only protect the steel as long as kept intact.

If steel is left exposed outdoors, the mill scale will crack and peel off, and the steel starts to corrode. Rust has a chemical composition similar to that of iron ore. We are ”back to nature” – what startet out with ore has ended as rust. The iron is back to its original and stable, natural state with a lower energy

Material energy levels during processing Reaction between materials and surroundings (corrosion) Water/humidity is needed for steel to corrode Adding energy during manufacture and processing Rust Iron ore

Corrosion theory For 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 theory If 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 Steel Aluminium Copper Zinc 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 Anode Zinc Cathode Steel Zn = Zn e - O2O2 O 2 + H 2 O + 4e - = 4OH -

Torch battery Zinc shield ”anode” Electrolyte Ammonium chloride Zinc chloride Carbon stick ”cathode”

Corrosion theory The 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 Graphite Platinum Titanium. Steel Aluminium Zinc Magnesium Precious metals ”Simple” metals

sacrifice a metal Zinc 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 corrosion Carbon steel

If what we call ”rust” is assosiated with iron, what does corrosion of 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 way that their corrosion products provide good protection against further attack. A good example is verdigris. A thin copper plate can remain un- protected outdoors in more than 100 years without being damaged, like the Statue of Liberty.

Acid climate, with low pH, cause increased 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