MARINE COROSSION AND PROTECTION (Part 1)

To understand how marine corrosion protection works, it’s necessary to look in more detail in to the corrosion process. In this undesired chemical effect, the material can reach with different chemicals in its surroundings. The reaction can be divided into:

  • Chemical reaction
  • Electro-chemical reaction

These reaction take place exclusively at the surface  of material. It’s possible the microscopic pits are formed by corrosion on the metal’s surface.The corrosion can also occur in existing crack.

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CHEMICAL REACTION

In chemical reaction, there’s a charge transfer between the reactions. If the exchange of charge is local effect, then the resulting ” Chemical corrosion “.

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An example of this reaction between Bare steel and air. A thin oxide layer formed at the surface rapidly. Then water come into contact to the oxide layer, the compound react (oxidation + reduction) to give the iron hydroxide (rust).

It is a very complex process which is completed in the following steps.

  • Oxidation of iron-the iron gets oxidized into ferrous ions [Fe (II)] with the loss of two electrons.
Fe  Fe+2 + 2 e
  • The ferrous ions again get oxidized into ferric ions [Fe(III)] in the presence of water and oxygen.
Fe+2  Fe+3 + e
  • These electrons from the above reactions are used to reduce oxygen.
O2(g) + 2 H2O + 4e  4 OH

  • The ferric ions interact with oxygen and constitution ferric oxide [iron (III) oxide]. This ferric oxide gets hydrated with water.

The mechanism for the rusting process is similar to the electrochemical cell. The electrons formed during the oxidation of iron is conducted through the metal. Thus, the iron ions diffuse from the water layer to the metal surface where oxygen is present. This is an electrochemical cell where iron acts as the anode and oxygen gas as the cathode.

ELECTRO-CHEMICAL REACTION

Many compounds have the tendency to dissolve charged particle (ions) into water. Ion can move freely in water. Compounds that is always behave is this way are Acids, Alkaline, Soluble-salt, Metal and some Gases.

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The lesser noble, The stronger tendency to generate these ions and become more negative (Anode)

Electro-chemical corrosion can occur in the following places:

  • Between Propeller & Surrounding Steel
  • Between copper contained part ( heat exchanger ) and the steel part of piping system.
  • Between aluminium/zinc parts and steel part of the ship.

Eliminating the corrosion current can prevent electro-chemical corrosion, it can be achieved in several way as below:

  • Insulating the metal in the water by Painting.

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  • Reversing the current by using sacrificial anode (Zinc/Aluminium)

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TABLE I – GALVANIC SERIES OF METALS IN SEA WATER WITH REFERENCE TO SILVER/SILVER CHLORIDE REFERENCE CELL [Sea water flowing at 8 to 13 ft./sec. (except as noted), temperature range 50°F (10°C) to 80°F (26.7°C)]
(ANODIC OR LEAST NOBLE) CORROSION-POTENTIAL RANGE IN MILLIVOLTS
Magnesium and Magnesium Alloys -1600 to –1630
Zinc -980 to –1030
Aluminum Alloys -760 to –1000
Cadmium -700 to –730
Mild Steel -600 to –710
Wrought Iron -600 to –710
Cast Iron -600 to –710
13% Chromium Stainless Steel, Type 410 (active in still water) -460 to –580
18-8 Stainless Steel, Type 304 (active in still water) -460 to –580
Ni-Resist -460 to –580
18-8, 3% Mo Stainless Steel, Type 316 (active in still water) -430 to –540
Inconel (78%Ni, 13.5%Cr, 6%Fe) (active in still water) -350 to -460
Aluminum Bronze (92% Cu, 8% Al) -310 to -420
Nibral (81.2% Cu, 4% Fe, 4.5% Ni, 9% Al, 1.3% Mg) -310 to –420
Naval Brass (60% Cu, 39% Zn) -300 to –400
Yellow Brass (65% Cu, 35% Zn) -300 to –400
Red Brass (85% Cu, 15% Zn)  -300 to –400
Muntz Metal (60% Cu, 40% Zn) -300 to –400
Tin -310 to –330
Copper  -300 to –570
50-50 Lead- Tin Solder -280 to –370
Admiralty Brass (71% Cu, 28% Zn, 1% Sn) -280 to –360
Aluminum Brass (76% Cu, 22% Zn, 2% Al) -280 to –360
Manganese Bronze (58.8% Cu,39%Zn,1%Sn, 1%Fe, 0.3%Mn) -270 to –340
Silicone Bronze (96% Cu Max, 0.80% Fe, 1.50%Zn, 2.00% Si, 0.75% Mn, 1.60% Sn) -260 to –290
Bronze-Composition G (88% Cu, 2% Zn, 10% Sn -240 to –310
Bronze ASTM B62 (thru-hull)(85%Cu, 5%Pb, 5%Sn, 5%Zn) -240 to –310
Bronze Composition M (88% Cu, 3% Zn, 6.5% Sn, 1.5% Pb) -240 to –310
13% Chromium Stainless Steel, Type 410 (passive) -260 to –350
Copper Nickel (90% Cu, 10% Ni) -210 to –280
Copper Nickel (75% Cu, 20% Ni, 5% Zn) -190 to –250
Lead -190 to –250
Copper Nickel (70% Cu, 30% Ni) -180 to –230
Inconell (78% Ni, 13.5% Cr, 6% Fe) (passive) -140 to –170
Nickel 200 -100 to –200
18-8 Stainless Steel, Type 304 (passive) -50 to –100
Monel 400, K-500 (70% Ni, 30% Cu) -40 to –140
Stainless Steel Propeller Shaft (ASTM 630:#17 & ASTM 564: # 19) -30 to +130
18-8 Stainless Steel, Type 316 (passive) 3% Mo 0.0 to –100
Titanium -50 to +60
Hastelloy C -30 to +80
Stainless Steel Shafting (Bar) (UNS 20910) -250 to +60
>Platimium +190 to +250
Graphite +200 to +300

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To be continued in MARINE COROSSION AND PROTECTION (Part 2)

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