corrosive environment, cracking can also be
accelerated in the form of stress corrosion failure.The electrons reach the surface of the cathode material and neutralize positively
charged hydrogen ions that have become attached to the cathode surface.Corrosion can also take the form of erosion in which the protective film, usually
an oxide film, is removed by a rapidly moving atmosphere or medium.In the case of the ship, the bronze
propeller acts as a cathode, the steel hull as an anode and the sea water as an
electrolyte.Other metals revert to sulphides and oxides or
carbonates.Direct oxidation corrosion is often seen in the scaling that takes
place when a piece of metal is left in a furnace for a length of time.In this process, part or all of the metal
becomes transformed from the metallic state to the ionic state and often forms a
chemical compound in the electrolyte.In the corrosion of iron, each iron
atom releases two electrons and then becomes a ferrous iron carrying two positive
charges.Two of
these ions will now become neutral atoms, and are released generally in the form
of hydrogen gas.When this process is taking place, it can be observed that hydrogen bubbles are
forming at the cathode only.Depolarization can also take place, for example, on the propellers of ships
because of the movement through the water, which is the electrolyte.Severe corrosion can occur on the hull as a result of galvanic action.Both the hull and propeller become cathodic and consequently do not
deteriorate.On the surface of some metals such as
copper or aluminium, the corrosion product sometimes exists as a thin film that
resists further corrosion.Metallic atoms form positive ions while non-metals form
negative ones.An electrolyte is any solution that conducts electric current and
contains negative or positive ions.Corrosion requires the presence of an
electrolyte to allow metal ions to go into solution.The electrolyte may be fresh or
salt water and acid or alkaline solutions of any concentration.This causes
an increased corrosion rate of the anodic steel ship's hull.Pitting corrosion is localized and results in small holes
on the surface of a metal caused by a concentration cell at that point.Cathodic protection is often used to protect steel ships hulls and buried steel
pipelines.