© Sarah Don, Australia, 2008

TITANIC – Resting or Reacting?

The famous Titanic shipwreck has been submerged 4km below the surface of the North

Atlantic Ocean for nearly 100 years. Scientists believed that a number of factors typical of a deep

ocean environment would keep the Titanic in relatively good condition until the technology required

to salvage it was developed. However, in 1985 when the technology became available and the Titanic

wreck was first explored, scientists were surprised at what they discovered. The Titanic had corroded

far beyond what they had imagined, contrary to their predictions. The scientists didn’t yet know about

one other factor that heavily contributed to the corrosion of the Titanic. All the subsequent research

concerning the Titanic has contributed to explaining why the scientists’ predictions were incorrect.

The hull of the Titanic is made of steel which is an alloy of mostly iron and about 0.2-1.7%

carbon for strength (Broad, 1998; Wikipedia, 2008). Iron is an active metal, which means that when it

corrodes, the by-products of corrosion do not form a protective coating on the metal to prevent further

corrosion. Instead, when iron rusts, the iron oxide (Fe2O3) forms a porous crust that is permeable to

water. This allows more water to come in contact with the un-corroded iron underneath the rust layer,

causing it to continue to rust. Passive metals such as copper and aluminium form a protective layer of

corrosion that prevents the metal from further rusting. However because of porous rust, iron can

continue to rust until there is no pure metal left.

The most rusting occurred where the iron was under mechanical stress in places such as joins,

bends and rivets. When iron is under stress, the Fe atoms are less closely bonded so it becomes easier

for them to be oxidised by the dissolved oxygen in the water. In the following redox reaction (the

rusting of iron where oxygen is present), iron is the anode and oxygen is the cathode – electrons are

transferred from the iron to the oxygen – oxidising the iron and reducing the oxygen.

Fe(s) Fe2+

(aq) + 2e-

O2(g) + 2H2O(l) + 4e- 4OH

-(aq)

2Fe(s) + O2(g) + 2H2O(l) 2Fe2+

(aq) + 4OH-(aq)

2Fe2+

(aq) + 4OH-(aq) Fe(OH)2(s)

4Fe(OH)2(s) + O2(g) 2Fe2O3(s) + 4H2O(l)

(Charles Sturt University, 2004)

This sort of rusting occurs most easily in warm temperatures where oxygen, water and salts or

impurities are present. It is universally accepted that ocean temperature decreases with depth as

pressure increases. Also, gas solubility increases while salt solubility decreases as temperature

decreases. At 4km below the ocean’s surface, with the water temperature being near freezing and little

oxygen or salt dissolved in the water, any chemical reactions would be hindered. Based on this

information, scientists in the 1910’s predicted that the Titanic would corrode only very slowly.

However, the one factor they were not aware of at the time was “iron-eating” bacteria (Charles Sturt

University, 2004).

At great depths of the ocean (below 1000m), lives anaerobic sulfate-reducing bacteria (also known as

“iron-eating” bacteria). The bacteria is said to be “iron-eating” because it reduces sulfate as the first of

several chemical reactions that contribute to the corrosion of iron. Sulfate ions are plentiful in sea

water which the abundant sulfate-reducing bacteria feed on and produce hydrogen sulfide (H2S) as a

Iron oxidation half-reaction

Oxygen reduction half-reaction

Redox reaction

Synthesis reaction

Second oxidation of iron

corrosion product is rust

© Sarah Don, Australia, 2008

result. The equation below shows the reduction reaction caused by the sulfate-reducing bacteria. The

oxidation number of sulfur has changed from +6 to -2, hence sulfur has been reduced.

+6 -8=-2 +2 -2=0

SO42-

+ 10H+ + 8e

- H2S + 4H2O

The pH of seawater is typically 8, however as ocean depth increases, the solubility of carbon dioxide

(CO2) increases, causing the water to become more acidic as shown in the following chemical

equation.

H2O + CO2 H2CO3

The corrosion of iron occurs more rapidly in acidic water, which counteracts the effect that the depth

and low temperature of the ocean has on slowing the corrosion of the Titanic wreck. Some areas of

the water surrounding the Titanic was found to be of a pH of as low as 4. Also, the higher

concentration of hydrogen ions (made available by the dissolved hydrogen sulfide in the water)

contributes to the corrosion of iron as shown in the following equations.

Fe(s) Fe2+

(aq) + 2e-

2H+

(aq) + 2e- H2(g)

Fe(s) + 2H+

(aq) Fe2+

(aq) + H2(g)

The sulfate-reducing bacteria are then able to

convert the hydrogen gas (H2) into hydrogen ions

(H+) which then chemically react to reduce the

sulfate ions in the water, and more iron is

consequently ionised.

On the Titanic, researchers have found what

are called rusticles – icicle-shaped rust formations as

pictured in figure 1. These rusticles provide an ideal

environment for the sulfate-reducing bacteria to live.

Also, the hydrogen sulfide (H2S) produced by the

sulfate-reducing bacteria dissociates into its ions in

the sea water and the sulfide ions (S2-

) form a

precipitate with the iron ions (Fe2+

) in the water as

shown in the following equations.

H2S(aq) 2H+ + S

2-

Fe(s) + 2H+ Fe

2+ + H2(g)

Fe2+

+ S2-

FeS(s)

The resulting iron sulfide (FeS) forms insoluble deposits similar to the rusticles, although they have a

black and less porous appearance than rust. The precipitation of iron sulfide isolates more H+ ions,

making them available for involvement in the oxidation of more iron. This, in turn, increases the rate

at which the steel hull of the Titanic rusts.

Two half-reactions of galvanic corrosion of iron

Figure 1 – Rusticles on the bow of the Titanic

(Gould, 2006).

Hydrogen sulfide dissociating into ions

Oxidation of iron

Synthesis reaction of iron and sulfide ions

(Charles Sturt University, 2004)

© Sarah Don, Australia, 2008

One other relatively minor – however still considerable – factor in the increased corrosion rate

of the Titanic is due to organic material aboard the ship. Because the Titanic was a luxury vessel, it

was largely fitted out with interior wooden panelling. As the cellulose in the wood on the titanic

shipwreck breaks down, the oxygen released further contributes to the corrosion of the iron hull. The

released oxygen fuels normal rusting reactions and also feeds the aerobic bacteria which produce by-

products that decrease the pH of the water surrounding the Titanic. (Charles Sturt University, 2004)

Sulfur-reducing bacteria was not the only extra contributing factor involved in the faster-than-

expected rate of corrosion of the Titanic, though it was the most significant. Normal rusting in the

presence of oxygen and dissolved salts in the seawater still played a large role in the corrosion of the

Titanic, however slowed. At the Titanic’s current rate of decay, scientists have predicted that the

wreck will have completely rusted away all of the steel of the hull within the next 75-90 years (Gould,

2006). The discovery of the anaerobic bacteria has allowed scientists to adjust their methods of

predicting the corrosion rate of shipwrecks depending on their associated environmental conditions

far below the ocean’s surface. For the era in which the Titanic sank, with the established scientific

knowledge of marine corrosion, scientists made logical predictions. However the unknown factor of

the anaerobic sulfur-reducing bacteria caused their predictions to be found inaccurate. Therefore,

because the environmental conditions at 4km below the surface of the ocean favour spontaneous

corrosion, the Titanic is rusting at a much faster rate than was first thought.

© Sarah Don, Australia, 2008

BIBLIOGRAPHY

Broad, W. (1998) Faulty Rivets Emerge as Clues to Titanic Disaster, The New York Times,

http://query.nytimes.com/gst/fullpage.html?res=980CE1DB133AF934A15752C0A96E958260&sec=

&spon=&pagewanted=all (18/05/08)

Charles Sturt University (2004) Shipwrecks, Corrosion and Conservation: 1. The ocean as an

electrolyte, http://www.hsc.csu.edu.au/chemistry/options/shipwrecks/ (18/05/08)

Charles Sturt University (2004) Shipwrecks, Corrosion and Conservation: 2. Corrosion of iron and

its alloys, http://www.hsc.csu.edu.au/chemistry/options/shipwrecks/2728/ch961Dec2_03.htm

(18/05/08)

Charles Sturt University (2004) Shipwrecks, Corrosion and Conservation: 3. Electrolytic cells,

http://www.hsc.csu.edu.au/chemistry/options/shipwrecks/2730/ch963Dec2_03.htm (18/05/08)

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protection, http://www.hsc.csu.edu.au/chemistry/options/shipwrecks/2731/ch964Dec2_03.htm

(18/05/08)

Charles Sturt University (2004) Shipwrecks, Corrosion and Conservation: 5. Rate of decay and

corrosion, http://www.hsc.csu.edu.au/chemistry/options/shipwrecks/2732/ch965Dec2_03.htm

(18/05/08)

Charles Sturt University (2004) Shipwrecks, Corrosion and Conservation: 6. Corrosion at great

depths, http://www.hsc.csu.edu.au/chemistry/options/shipwrecks/2733/ch966Dec2_03.htm (18/05/08)

DeAngelis, M. (2005) Deep divers explore Titanic shipwreck, CDNN - Cyber Diver News Network,

http://www.cdnn.info/news/industry/i050906.html (29/04/08)

Gould, M. Et.Al (2006) Chemistry in Use – Book 2, McGraw Hill Education, Australia.

Miles, K.A. & Peters II, C.F (2001) The Science of Titanic, and How to Wreck the Shipwreck,

http://starryskies.com/Artshtml/dln/11-00/titanic.pt1.html (05/05/08)

National Geographic Society Press Release (2004) Scientists Return to the Titanic,

http://usinfo.state.gov/journals/itgic/0404/ijge/gj09b.htm (12/05/08)

Vector Corrosion Technologies (2005) Impressed Current Cathodic Protection, http://www.vector-

corrosion.com/impcurrent_main.html (14/05/08)

Vector Corrosion Technologies (2005) Corrosion Basics: Understanding the Different Types of

Corrosion that Affect Concrete, http://www.vector-corrosion.com/corr_basics.html (01/05/08)

Wikipedia, (2008) Cathodic protection, http://en.wikipedia.org/wiki/Cathodic_protection (30/04/08)

Wikipedia, (2008) Carbonic acid, http://en.wikipedia.org/wiki/Carbonic_acid (18/05/08)

Wikipedia, (2008) RMS Titanic, http://en.wikipedia.org/wiki/RMS_Titanic (18/05/08)

Wikipedia, (2008) Steel, http://en.wikipedia.org/wiki/Steel (18/05/08)

Wikipedia, (2008) Rust, http://en.wikipedia.org/wiki/Rust (18/05/08)