1. Corrosion

2. 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

3. 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.

4. 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

5. 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.”

6. 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

7. Water is universal solvent
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.

8. 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. 1980
1mil=0.039mm

9. 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
Typical seam used for door hem –
traditional source of crevice attack
Daivies, G. 2003

10. 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

11. Bradford, S., 1993

12. 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

13. 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.
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

14. Let’s go deeper! What happen to the metals in a galvanic cell?
Giver
(lower
El. potential)
Receiver
(Higher
El. Potential)
In result of electrochemical reaction the metal with lower electrical potential is eaten and this with higher electrical potential is intact
Electrical
current

15. 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

16. 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

17. Can we measure electrical potential of metals?
Test
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
Cooper
Zinc
V V
What is measured?
difference in electrical potential

18. Galvanic series of metals in seawater

19. 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

20. 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

21. Other type of corrosion where galvanic mechanism is involved
Crevice corrosion of stainless steel
Pitting corrosion -similar process in open area
[Charles Wing,1993]

22. Galvanic protection
Test
[Charles Wing,1993]

23. Sacrificial anodes
[Charles Wing,1993]

24. 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)

25. Electrolytic (stray-current) corrosion
Test
Aluminium
Aluminium
Aluminium
Aluminium
Intensive corrosion
V = 6V (compare with galvanic voltage)
I 0
V = 0
I = 0

26. 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

27. 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

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

29. 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