1. Ship Related Corrosion Topics

2. Galvanic Series of Metals in Sea Water
The next slide show a table of the reactivity of metals and various alloys in sea water
Base refers to the anodic properties of the metal or alloy. Anodic refers to the giving up of electrons.
Noble refers to the cathodic properties of the metal or alloy. Cathodic refers to the gaining of electrons.
The reference for this table is Reeds General Engineering Knowledge for Marine Engineers (Russell, Jackson, Morton), page 148

3. Magnesium Base end of the table (anodic, most reactive)
Aluminum
Zinc
Mild Steel
Cast iron
Manganese steel (active)
Admiralty brass
Copper
Aluminum bronze
Gunmetal
70/30 Cupronickel
Nickel (passive)
Inconel
Stainless steel (passive)
Monel metal
Graphite (carbon)
Titanium Noble end of the table (cathodic, least reactive)
Note: in Activity Series of Metals nickel is more reactive than copper

4. Corrosion of Metals in Sea Water
Galvanic action is most likely the main cause for corrosion.
Conditions for galvanic action:
dissimilar metals close to each other or in an alloy
strong electrolytes such as salts in sea water or air coming into contact with the metal surface
composition of the metal alloy

5. Dissimilar Metals and Galvanic Action
Referring to the table on slide 3, the relative position of the metals in the table determines the degree of galvanic action
If metals are close together, there is very little galvanic interaction.
If the metals are far apart, there is much more galvanic interaction.

6. Dissimilar Metals and Galvanic Action (con.)
In galvanic action, as we saw with galvanic cells, it is the more anodic (more reactive) metal that corrodes.
Looking at the table on slide 3
any metal or alloy is anodic to the metal or alloy below it

7. Galvanic Protection on Ship

8. Cathodic Protection from Corrosion
Sacrificial Anodes
A more anodic metal is placed near the metal requiring protection making it the cathode
the cathode gains electrons rather than giving them up.

9. Cathodic Protection from Corrosion
Impressed Current
A small voltage (potential difference) is applied to reverse the polarity of the galvanic action between two dissimilar metals.
the applied current flows through the anodes (simulating the anodes giving up electrons) to the cathode.

10. Other Types of Corrosion Action
Graphitization of Cast Iron
Velocity of Seawater
De-zincification, de-aluminification
Pitting Corrosion
Stress Corrosion
Fretting Corrosion

11. Graphitization of Cast Iron
Cast iron is up to 3.5% carbon, in the form of graphite
In seawater, the iron in the cast iron is more anodic than the graphite and the iron corrodes

12. Velocity of Seawater
As a ship moves faster through the water, the velocity of the seawater relative to the ship’s hull increases causing:
an increased supply of O2
erosion of protective oxide films
Both of these conditions increase the rate of corrosion

13. Stress Corrosion Most commonly found in brasses (zinc- copper alloy)
Found also in aluminum alloys and stainless steels
Corrosion of the anodic metal can lead to exposure of stresses such as inter or intra- crystalline cracks in the cathodic metal

14. De-zincification
General name for this type of process is “de-alloying”
De-zincification is the corrosion of zinc from zinc-containing alloys such as brass.
The zinc corrodes leaving a porous mass of copper
Adding arsenic and reducing zinc to under 37% content stops the corrosion

15. De-aluminification This is another de-alloying process
De-aluminification occurs in aluminum bronzes.
Adding 4 to 5% nickel reduces this corrosion

16. Pitting Corrosion Localized galvanic action
Occurs where there is a relatively large cathodic area and a small anodic area
Intensity of electron loss from the anodic area to the cathodic area is high

17. Pitting Corrosion (con.)
Causes:
loss of protective coating
acidic pockets of water
scale from salts
Dangerous form of corrosion
occurs in boilers also

18. Fretting Corrosion
Due to microscopic oscillatory motion between two surfaces in contact with each other
Causes removal of metal and metal oxide films
Formation of metal oxide powder can occur
Metal oxide powder, such as iron oxide, increases wear

19. Fretting Corrosion (con.)
Factors affecting fretting corrosion damage:
Increases with amplitude and frequency of the oscillatory movement
Increases with load on the surfaces
Decreases with low oxygen levels in the presence of moisture
Hardness of metals involved. Damage decreases as the hardness of the metals increase