WHY APPLY A COATING -- CORROSION ?
Mario S Pennisi
Principal Consultant - Penlia & Co
Surface coatings such as powder coatings 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 functionally 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 clause 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.
For corrosion to proceed there must be an anode, a cathode and an electrolyte joined by an external current circuit.
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:
Corrosion scientists achieve this by coating the metal with another metal which either
There are eleven main corrosion mechanisms:
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.
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 environment.
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 corrodes rapidly.
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 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
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.
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.
Demetalification is the removal of one of the alloying elements in an alloy by the electrolyte. This results in a "spongy" metal. Typical example is the removal of zinc in chloride waters from brass.
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:
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.
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.
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