CORROSION-WHY APPLY A COATING
Mario S Pennisi
Surface coatings such as electroplating, powder coatings,
galvanizing, etc are applied to metallic surfaces to improve
the aesthetic appearance of the item, and so that it functions
better. We try to improve both the aesthetics and functionality
by impeding the corrosion habit that is inherited by most
commercial metals used in construction and fabrication. Corrosion
may be defined as a destructive phenomena, chemical or electrochemical,
which affects the aesthetic appeal or an object; and in extreme
cases may cause structural failure. The mechanism is based
on anode and cathode reactions in an electrolyte. Corrosion
takes place at the anode with the release of hydrogen gas
or the formation of hydroxyl ions at the cathode. These hydroxyl
ions may react with metal ions dissolved at the anode and
form metal hydroxides or hydrated oxides. If these are insoluble
they will deposit on the metal surface and may reduce the
rate of corrosion.
THE REQUIREMENTS FOR CORROSION TO PROCEED
For corrosion to proceed there must be an anode, a cathode
and an electrolyte, all joined by an external current circuit.
PREVENTING CORROSION To prevent corrosion we have
to break this triangle, by removing one of the legs.The main
technique available for reducing corrosion is to eliminate
the electrolyte, either by:
- Drying out the environment eg reduce the humidity to
well below 60% such as at a desert destination. or
- Place a barrier between the metal and the electrolyte.
Corrosion scientists produce this barrier by coating the
metal with another material which either
- protects it by dissolving in preference to it, eg zinc
on steel, or
- places a physical barrier between the electrolyte and
the electrodes eg paint.
TYPES OF CORROSION
There are eleven main corrosion mechanisms:
- General or uniform corrosion
- Pitting corrosion
- Galvanic or bi-metallic corrosion
- Stress-corrosion cracking
- Corrosion fatigue
- Intergranular corrosion
- Filiform corrosion
- Crevice corrosion
- Fretting corrosion
- Selective leaching or de-metalification.
GENERAL OR UNIFORM CORROSION
Differences in electrical potential occur on the surface
of a piece of metal due to small differences in chemical composition,
phase differences, amount of cold work, etc. These differences
set up small corrosion cells each with an anode and cathode.
Corrosion continues until the metal is consumed or the film
of rust formed on the surface sets up a barrier to the electrolyte.
In pitting corrosion the surface of the metal is attacked
in small-localised areas. Organisms in water or breaks in
a passive film can initiate corrosion. Halides such as chlorides
- the main constituent of common salt, fluoride, etc stimulate
pitting. In pitting corrosion very little metal is removed
from the surface but the effect is marked.
GALVANIC OR BI-METALLIC CORROSION
Galvanic corrosion takes place between two different
metals, or coatings, which are joined together in the presence
of an electrolyte. Each metal has a potential different from
any other metal when placed in an electrolyte. A series can
be built up of all the metals relative to each other. In seawater,
the series, or table is:
Aluminium 5xxx series
Aluminium 3xxx series
Aluminium 1xxx series
Aluminium 6xxx series
Aluminium 2xxx series
Mild steel - low carbon steel
410 Stainless steel - active
50/50 lead-tin solder
304 Stainless steel - active
316 Stainless steel - active
Nickel - active
Copper - 30% nickel
Nickel - passive
Stainless steel - passive
Tend to form
voluminous, fluffy lightly adherent
tend to form
tenaceous, usually colourless,
NB: The table is often drawn upside down with the cathodic
metals at the top.
The metals at the top of the table are more anodic than
those below them and when in electrical contact in an electrolyte
will corrode in preference to the metal below them on the
table. The further apart the metals, the faster will be the
corrosion rate. Because of this relationship, zinc is applied
to steel to protect it. When a holiday occurs in the zinc
coating, the zinc will become the anode in the steel/zinc/electrolyte
circuit and will corrode before the steel will. While zinc
is available the steel will not corrode.
You may ask if this really works. The following series of
experiments with a steel nail, zinc and copper wire in an
electrolyte illustrates this quite well.
When a nail is produced, the head and point are heavily cold
worked and act as anodes to the remainder of the nail when
in a corrosion cell.
Why do you think that corrosion took place in the centre
of the nail (where indicated)?
When the agar was stripped away from the nail, it was found
that a nick had been placed in the nail at that location so
that a small work hardened area had been generated.
In this experiment we have wrapped a piece of zinc wire around
the shank of the steel nail. If you look at the table, you
will notice that zinc is above steel and will corrode in preference
to steel when in a galvanic couple. The experiment shows the
zinc becoming the anode while the head and point of the nail
are now acting as cathodes. The white colour of the corrosion
product (when blue was expected) is due to the volume of white
zinc salts that were produced and diluted the blue colouration.
In this final experiment we have wrapped a piece of copper
wire around the head and part of the shank of the steel nail.
The table tells us that the steel in a galvanic couple with
copper will corrode in preference to the copper. This is clearly
STRESS CORROSION CRACKING
Failure is due to the simultaneous influence of static
tensile stresses and a corrosive environment and this is specific
to a particular metal. The stresses may be internal such as
those caused by cold work, welding, heat treatment or external
forces caused by mechanical stresses set up by assembly practices.
A good example of this form of corrosion is 316 stainless
steel in marine environments. 316 was developed to withstand
attacks in chloride environments - but if stressed the steel
will fail by stress corrosion cracking.
Failure under repeated cycling stresses in a corrosive
Corrosion occurs at the grain boundaries due to a difference
in potential between the anodic grain boundaries and the cathodic
grains. "Sensitised " stainless steels, where carbides have
been precipitated in the grain boundaries during improper
heat treatment or in the heat-affected zone of a weld, are
particularly susceptible to intergranular corrosion.
Filiform corrosion appears as a network of corrosion
trials, of a wormlike structure, particularly beneath thin
organic coatings. Salts containing chlorides, which have been
left on the surface prior to coating are suspected.
Crevice corrosion occurs when there is a difference in
ion, or oxygen, concentration between the metal and its surroundings.
Oxygen starvation in an electrolyte at the bottom of a sharp
V-section will set up an anodic site in the metal that then
Fretting corrosion occurs when two or more parts rub
against each other. The rubbing action removes the corrosion
products and exposes new metal to the electrolyte.
Erosion is the removal of metal by the movement of fluids
against the surface. The combination of erosion and corrosion
can provide a severe rate of corrosion.
SELECTIVE LEACHING OR DEMETALIFICATION
Demetalification is the removal of one of the alloying
elements in an alloy by the electrolyte. This results in a
"spongy" metal. A typical example is the removal of zinc from
brass taps in chloride containing waters. T prevent this arsenical
brass is used in suspect locations.
INCIDENCE OF CORROSION
Evidence is available to show that the majority of metal
failures due to corrosion occur through general, or uniform,
modes. The next most common cause is stress corrosion cracking,
followed by pitting corrosion and intergranular corrosion.
These four modes account for about 80% of the failures examined.
In this survey no failures due to galvanic corrosion were
reported so the results are somewhat skewed.
Corrosion can be retarded by any of a number of techniques.
In some cases it is not feasible to eliminate even one of
the three basic requirements for corrosion, ie an anode, a
cathode and an electrolyte electrically connected to the electrodes.
Techniques available include;
- Alter the environment
- Use more corrosion resistant materials such as Monel
rather than brass for components rotating in seawater.
- Alter design to optimise geometry.
- Employ "cathodic" protection.
- Use organic coatings such as paints or powder coatings.
- Use inorganic coatings such as zinc rich paints or phosphates.
- Use conversion coatings such as chromates or phosphates.
- Use metallic coatings:
- Mechanically applied zinc as in sheridizing or mechanical
- Electrolytically deposited metals, zinc for functional
purposes or decorative nickel/chromium for decorative purposes.
- Electroless deposited metals eg nickel.
When coatings are used as the means of reducing corrosion,
it is essential that the coating adhere very tightly to the
surface. For maximum adhesion, the substrate must be prepared
As it is unlikely that metal corrosion will go away,
the future for coating specialists is assured, whether it
be metallic, organic or inorganic coating technology that