Corrosion

What is corrosion?

In general • Corrosion is the deterioration of a material

by electrochemical reaction with its surrounding environment

• The resistance of metals to the corrosion depends upon the formation of a thin protective film on the surface, which is exposed to the environment. Depends on the characteristics of this film some metals are more or less resistant to the corrosion

• Provided the film is maintained intact, the corrosion rate of the metal will be a minimum for that particular metal.

• If the film is removed or disrupted, the exposed metal will corrode rapidly until the protective film is formed again.

• The corrosion resistance of metals is therefore a function of the nature of the protective film and the rate at which it is formed.

Protection by painting and coating

• It seems logical to protect metal from environment by painting or coating. Unfortunately any mechanical imperfection or damage will open the door to corrosion.

• Painting and coating do the job in general but still not enough to prevent corrosion completely

nature of the oxides bonding

• What is the nature of the oxides bonding so they are so persistent to form?

• In encyclopaedia Britannica I have found:

“Oxides of the transition metals with oxidation numbers of +1, +2, and +3 are ionic compounds consisting of metal ions and oxide ions. Those transition metal oxides with oxidation numbers +4, +5, +6, and +7 behave as covalent compounds containing covalent metal-oxygen bonds.”

• It seems once metal atoms are in close contact to oxygen they experience strong attraction to oxygen, stronger than bonding forces in the lattice.

• Formation of ions can relate to some electrical flow as long as metal shall release electrons and oxygen shall receive electrons i.e.:

M M2+ +2e-

O + 2e- O2-

• Actually it is so called dry corrosion or oxidation

Water is universal solvent! Most often oxygen is dissolved in water as well as number of salts. And dissolved in water salts are broken down in ions. Thus natural water becomes electrolyte. Sea water is even more concentrated electrolyte.

Behaviour of alloys in water when they are alone dictates by nature of its oxides : its integrity and ability to repair.

• Natural repair of the protective film occur only when oxygen is presented in the water.

• Depends on ability of metals to create and maintain firm oxides they exhibit different rate of corrosion in still water

Warren, N. 19801mil=0.039mm

In an environment where some deficiency of oxygen does exist

• The oxide layer can not be maintained and than severe corrosion occurs in deep pits

• Where can we meet this situation on the boat? Under deposits, accumulated dirt, places where water drops can be detained for long period of time – water pockets

Daivies, G. 2003

Typical seam used for door hem – traditional source of crevice attack

the velocity of the water • Another important factor

is the velocity of the water. Some oxides can be washed out by the water flow and to expose bare metal to oxygen again and again. It is erosion action of the water on metals. Where the flow change direction or simply is turbulent the erosion is even higher and it is called impingement attack.

Callister, Jr.,2003

Bradford, S., 1993

Flow speed factor – acceptable flow limit

• 3.00 m/s Galvanized steel• 0.6 m/s Cooper• 3.5 m/s 90/10 Cooper-nickel-iron• 5 m/s 70/30 Cooper-nickel• 3.00 m/s Aluminum brass

Nature of dissimilar metals• Electro potential - Actually it

is ability of the specific metal or alloy to produce electrical current at specific circumstances.

Each couple of metals immerged in electro conductive fluid can manifest its specific electrical potential

The difference in electro potential of metals when they are electrically connected creates electrical forces.

pump

lights

starter

• This phenomenon is used in batteries, where all materials are carefully chosen to control the processes

• Any couple of dissimilar metals submerged in electrolyte is known as a galvanic cell

Let’s go deeper!

What happen to the metals in a galvanic cell?

Giver(lowerEl. potential)

Receiver(Higher El. Potential)

Electrical current

In result of electrochemical reaction the metal with lower electrical potential is eaten and this with higher electrical potential is intact

Galvanic corrosion is a process where in result of electrochemical

reaction between electrically connected dissimilar metals and

electrolyte, where they are submerged, the metal with lower

electrical potential is steadily consumed

Conditions galvanic corrosion to occur:

• Presence of two dissimilar metals• Electrolyte• Electrical connection

• The greater the difference in electrical potential the higher the rate of galvanic corrosion

• The more concentrated the electrolyte the higher the rate of galvanic corrosion

Can we measure electrical potential of metals?

• Test

Zinc Cooper

• V 0.85V• What is measured?

difference in electrical potential

• If we keep one of the electrodes the same we can make a table of measured electro potential differences

• Such table is made using silver electrode in sea water. It is known as Galvanic series of metals in seawater

Galvanic series of metals in seawater

Cathode End, Most Noble

RANGE ALLOYS

+0.3 Graphite

+0.26 Platinum

+0.06 Titanium

0 Stainless Steel - Type 316, 317

(passive)

-0.15 Silver

-0.10 Stainless Steel -Types 302 304 321

347

-0.24 Copper-Nickel

-0.25 Lead

-0.34 Manganese Bronze

-0.37 Copper

-0.57 Stainless Steel - Type 410 416 430

(active)

-0.72 Mild Steel, Cast Iron

-1.00 Aluminium Alloys

-1.03 Zinc

-1.63 Magnesium

Givers, Anode End, Least Noble

Factors influencing rate of galvanic corrosion

• Surface area ratio• Distance between both metals • Polarization and ability of the metals to

self-maintain oxide protective coating

Other type of corrosion where galvanic mechanism is involved

• Dezincification of brass In result of galvanic corrosion within the alloy itself

Cooper-zinc mixture is loosing zinc and loosing the strength of the metal composition. That’s why brass should be avoided in boat application

Other type of corrosion where galvanic mechanism is involved

• Crevice corrosion of stainless steel

[Charles Wing,1993]

• Pitting corrosion -similar process in open area

Galvanic protection

• Test

[Charles Wing,1993]

Sacrificial anodes

[Charles Wing,1993]

Sacrificial anodes conditions to work

Sacrificial anode should be correctly located• Sacrificial anode should be correctly sized• Sacrificial anode should be in correct

number • Sacrificial anode should be electrically

connected to the metal detail to be protected (Resistance free connection – 11AWg/4mm2)

Electrolytic (stray-current) corrosion

• Test

• V = 0• I = 0

Aluminium Aluminium

• V = 6V (compare with galvanic voltage)

• I 0

Aluminium Aluminium

Intensive corrosion

Stress corrosion

• Some metals under combined tensile or torsional stress and corrosive environment fail unexpectedly.

• For example: stainless steels are susceptible to chlorides and caustic: copper alloys – to ammonia and SO2 in air plus water vapour

• To avoid this type of corrosion careful selection of the material should be done

Intergranular corrosion

• Corrosion along the grain boundaries for some alloys in specific environments

• Some stainless steels when welded is an example. Chromium carbide is formed (precipitation reaction) that deplete the zone adjacent to the boundaries of Chromium

• Weld decay

Callister, Jr.,2003

Corrosion prevention

• Material selection• Design – avoidance of water traps, drain, etc.• Painting and coating• Grease and oil • Galvanic protection• Use of inhibitors

Inhibitors

• Inhibitor is a chemical additive. Small amount of inhibitor in the corrosive environment reduces the overall rate of corrosion.

• Inhibitor usually reacts with the surface and create a protective film

• It decrease rate of overall corrosion