CORROSION-WHY APPLY A COATING -
©Copyright1999 Mario S Pennisi

INTRODUCTION
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
• Erosion-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.

PITTING CORROSION
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:

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 the situation.

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.

CORROSION FATIGUE
Failure under repeated cycling stresses in a corrosive environment.

INTERGRANULAR CORROSION
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
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
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 corrodes rapidly.

FRETTING CORROSION
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 CORROSION
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.

RESTRICTING CORROSION
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 plating.
  - Electrolytically deposited metals, zinc for functional purposes or decorative nickel/chromium for decorative purposes.
  - Electroless deposited metals eg nickel.

SUBSTRATE PREPARATION
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 correctly.

THE FUTURE
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 is used.