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Corrosion

The document discusses corrosion of steel in marine environments. It explains that corrosion is an electrochemical reaction where electrons flow from one metal location (the anode) to another (the cathode), depositing rust at the cathode. For this process to occur, the two locations must be in a closed circuit, which is provided by the surrounding seawater electrolyte. The presence of chloride in seawater accelerates corrosion. Factors that influence corrosion include the pH of seawater, with more acidic or alkaline conditions increasing corrosion, and higher salt content, as chloride increases the corrosion reaction rate.

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Prof S Nallayarasu
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27 views1 page

Corrosion

The document discusses corrosion of steel in marine environments. It explains that corrosion is an electrochemical reaction where electrons flow from one metal location (the anode) to another (the cathode), depositing rust at the cathode. For this process to occur, the two locations must be in a closed circuit, which is provided by the surrounding seawater electrolyte. The presence of chloride in seawater accelerates corrosion. Factors that influence corrosion include the pH of seawater, with more acidic or alkaline conditions increasing corrosion, and higher salt content, as chloride increases the corrosion reaction rate.

Uploaded by

Prof S Nallayarasu
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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3.4. Corrosion Chapter 3.

MATERIALS AND CORROSION

the air to contact the metal and it eventually grows so thick that the movement of electrons
and ions across it stop. Provided the oxide layer does not crack, or is not removed, the
metal is protected from further corrosion. But if the bare metal is exposed to the oxygen,
it will again react to form the oxide. In this case the presence of oxygen benefits the metals
protection. Removal of the oxygen removes the metals ability to create its own protective
corrosion barrier.

Corrosion of steel in marine environmental is basically a electro-chemical reaction. This is


caused by flow of electrons from one location to another location results in recombined metal
or rust deposited in the receiving location called Cathode. The location from where the
metal is being taken away is called Anode.

The essential requirement of such metal transfer to occur is closed circuit between the two
locations. This is provided by the surrounding seawater which is a best electrolyte. The
presence of chloride, accelerates the process of corrosion.

This process is schematically showing in Figure 3.7.

In the above process the electrons from the corroding anode metal move to the connected
cathode where they recombine with the atoms of oxygen and water in the electrolyte to make
a new hydroxyl ion (OH-). This new negatively charged ion then reacts to make a stable
compound with the positively charged metal ions (F 2 +) that originally lost the electrons.
In this case, the electrons have a continuous pathway to escape the parent metal and the
parent metal, which cannot develop a protective barrier, disassociates or falls apart. Once
corrosion starts it continues until the ingredients are all used up.

3.4.2 Factors influencing corrosion

The factors influencing or accelerating the sea water corrosion process are listed below.

• PH value of sea water- The electrolyte in wet corrosion can be neutral, acidic or
alkaline. For corrosion in near neutral solutions (pH 6 8) under oxygenated conditions
the predominant cathodic reaction is the oxygen absorption reaction (O2 + 2H2O +
4e- = 4OH-) shown in Figure 1. If instead the bimetallic cell has no oxygen present
in the electrolyte the hydrogen evolution reaction (H+ + e- = H followed by H +
H = H2 gas) becomes the cathodic process and the anode continues to corrode. This
reaction is a much slower reaction (the H+ ion has a very low concentration in solution)
than the oxygen absorbing reaction. In acidic solutions (pH 0 - 6) the hydrogen ion
concentration is higher and the hydrogen evolution reaction is the predominant one.
Corrosion rates become extreme as the pH drops (acid gets stronger).

• Salt Content - The presence of chloride in sea water increase the chemical reaction
that takes place during the corrosion process.

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