Ship Related Corrosion Topics

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

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

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

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.

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

Galvanic Protection on Ship

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.

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.

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

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

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

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

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

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

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

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

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

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